Tellurium in Organic Synthesis

Foreword There is a vast and often bewildering array of synthetic methods and reagents available to organic chemists to...

Author: Nicola Petragnani | HÃ©lio A. Stefani

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Foreword There is a vast and often bewildering array of synthetic methods and reagents available to organic chemists today. Many chemists have their own favoured methods, old and new, for standard transformations, and these can vary considerably from one laboratory to another. New and unfamiliar methods may well allow a particular synthetic step to be done more readily and in higher yield, but there is always some energy barrier associated with their use for the first time. Furthermore, the very wealth of possibilities creates an informationretrieval problem. How can we choose between all the altematives, and what are their real advantages and limitations? Where can we find the precise experimental details, so often taken for granted by the experts? There is therefore a constant demand for books on synthetic methods, especially the more practical ones like Organic Syntheses, Organic Reactions and Reagents for Organic Synthesis, which are found in most chemistry laboratories. We are convinced that there is a further need, still largely unfulfilled, for a uniform series of books, each dealing concisely with a particular topic from a practical point of view - a need, that is, for books full of preparations, practical hints and detailed examples, all critically assessed, and giving just the information needed to smooth our way painlessly into the unfamiliar territory. Such books would obviously be a great help to research students as well as to established organic chemists. We have been very fortunate with the highly experienced and expert organic chemists, who, agreeing with our objective, have written the first group of volumes in this series, Best Synthetic Methods. We shall always be pleased to receive comments from readers and suggestions for future volumes. A. R. K., O. M.-C., C. W. R.

Preface (1994 edition) Tellurium is the fourth element of the VIA family of the periodic table, which starts with oxygen. Since tellurium exhibits an electronic configuration similar to that of selenium and sulphur, the chemical behaviour of these elements is obviously closely related. This similarity was a hindrance to the greater development of tellurium chemistry. During several decades, research was restricted to an extrapolation of well-established reactions for the preparation and use of organic sulphur compounds to selenium, and mainly from selenium to tellurium. Although over a quarter of a century ago it would not have been predicted that the importance of the VIA family of the periodic table would exceed that of its second element, sulphur, the development of organoselenium chemistry has been so explosive that little comment is necessary, as illustrated by the impressive number of papers as well as by several books 1-3 on the field. Tellurium compounds have taken an even longer time to rise from being considered an exotic and perverse element to a useful tool in organic chemistry. In the mid-1950s, the German chemist Heinrich Rheinboldt reviewed the preparation and reactivity of the organic tellurium compounds by comparison with selenium analogues in Houben-Weyl-Methoden der Organischen Chemie, Vol. IX (1955). During the following years, two books and several review articles were published on the organic chemistry of tellurium, but these were still within the areas of preparation and reactivity. It was in 1971 that the first "Symposium on the Selenium and Tellurium Chemistry" took place, and since then symposia have been held every 4 years. In the last few years tellurium compounds have begun to be employed in organic synthesis, giving rise to a significant number of publications and to several review articles. Finally, in 1990, a Houben-Weyl volume of more than 1000 pages was published coveting all aspects of tellurium chemistry (author, K.Y. Irgolic). At present the author feels that the remarkable development attained by the organotellurium chemistry is a clear reason for a monograph from the point of view of the organic chemist. The aim of this monograph is to provide a comprehensive overview of the preparation and synthetic applications of tellurium compounds. It will focus on the preparation of selected inorganic tellurium compounds and on the main classes of organotellurium compounds. The major interest of the volume probably resides in the use of these inorganic and organic compounds as reagents in organic syntheses as well as the conversion of organotellurium into free organic compounds. In this regard it is sufficient to emphasize that extremely useful reactions achieved with tellurium reagents, such as selective reductions and oxidations of a large variety of organic functionalities, have not been mentioned until now in textbooks specifically devoted to organic synthetic methodology. This monograph is appropriate for chemists who, although not experts in tellurium chemistry, have had a basic grounding to graduate level in organic chemistry and with sufficient experience of typical experimental operations. vii

viii

PREFACE (1994 EDITION)

A considerable number of "experimental procedures" have been included. These have been described in accordance with the original papers, and therefore the reader will find significant differences in the description of these procedures, some rich and others very poor in detail. In the last case, the practitioner who is used to overcoming the usual laboratory difficulties will need to use a certain degree of initiative. The author will feel very gratified if this volume helps some chemists to become familiar with tellurium which up till now they may have considered only as a useless and unpleasant dement. N. Petragnani

REFERENCES 1. Klayman, D. L.; Gunther, W. H. H. (eds.). Organic Selenium Compounds: Their Chemistry and Biology. Wiley, New York, 1973. 2. Paulmier, C. Selenium Reagents and Intermediates in Organic Synthesis. Pergamon Press, Oxford, 1986. 3. Liotta, D. (ed.). Organoselenium Chemistry. Wiley, New York, 1987.

Preface Over a decade after the publication of the first edition of this book, it is unnecessary to emphasize once more the remarkable developments attained in organic tellurium chemistry. Almost a thousand papers have been published in this last period, and numerous research groups all over the world have consolidated their international status. Undoubtedly tellurium chemistry can participate in several areas of organic chemistry, as well as in others, and ever-more research chemists introduce tellurium to solve a wide variety of problems. All this seems to us to be a good reason to write this new edition of the book. It does not differ in its general approach from the first one. Obviously it is not a compendium coveting exhaustively all the aspects of organic tellurium chemistry. For this purpose the Irgolic E12b volume of Houben-Weyl is yet incomparable although 16 years old. This book was devised in accordance with the intention of the Academic Press series "Best Synthetic Methods" of the first edition, which is clearly focused in the foreword and in the preface. Several chapters such as vinylic tellurides, transmetallation reactions, coupling reactions and free-radical chemistry have been enriched and brought up to date with new information related to their increased involvement in the most varied synthetic manipulations. A chapter has been dedicated to heterocyclic compounds, covering the preparation and reactivity of the most familiar members of the class. Toxicology and pharmacology have also been briefly considered, and we are greatly indebted with C. R. Nogueira, G. Zeni and J. B. T. Rocha for allowing us to use their recent review (Chem. Rev. 2004, 104, 6255) as a source for our new chapter. Probably some contributions have escaped our judgement and not been introduced in this book. We apologize to any authors for the omission. We would be gratified if this new edition, with its broad level of information, can disclose basic knowledge to the beginner in tellurium chemistry as well as furnish salient features for opening new perspectives to advanced researchers. Nicola Petragnani and H61io A. Stefani

ix

Abbreviations AIBN CTAB DIBAL-H DME DMSO ee ESR GC-MS HMPA LICA LiTMP m-CPBA MPLC NBSP NMP NOESY PTC TBTH TCNQ TEMPO TFA TLC TMEDA TUDO

2,2'-Azo bisisobutyronitrile Cetyl trimethylammonium bromide Diisobutylaluminium hydride

1,2-Dimethoxyethane Dimethyl sulfoxide Enantiomeric excess Electron spin resonance Gas chromatography-mass spectrum Hexamethylphosphoric acid triamide Lithium isopropylcyclohexylamide Lithium 2,2,6,6-tetramethylpiperidide meta-Chloroperbenzoic acid Medium pressure liquid chromatography para-Nitrobenzenesulphonyl peroxide N-methyl-2-pyrrolidinone Nuclear overhauser enhancement spectroscopy Phase transfer catalyst Tributyltin hydride 7,7,8,8-Tetracy ano-pa ra-quinodimethane 2,2,6,6-Tetramethyl-l-piperidinyloxy free radical Trifluoroacetic acid Thin layer chromatography N,N,N',N'-tetramethyl- 1,2-ethanediamine Thiourea dioxide

XXV

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–1– Introduction

1.1

SOME PHYSICAL PROPERTIES OF TELLURIUM

The Pauling electronegativities of carbon and tellurium are, respectively, 2.5 and 2.1. This, in addition to the large volume of the tellurium atom (atomic radius 1.37, ionic radius 2.21), promotes easy polarization of Te–C bonds. The ionic character of the bonds increases in the order C(sp3)⫺Te⬎C(sp2)⫺Te⬎C(sp)⫺Te, in accordance with the electronegativity of carbon accompanying the s character (Table 1.1). The Te⫺C bond therefore exhibits a high reactivity, as demonstrated typically by the easy heterolytic cleavage towards nucleophilic reagents.

1.2

RELEVANT MONOGRAPHS AND REVIEW ARTICLES

1. Rheinboldt, H. in Houben-Weyl-Methoden der Organischen Chemie (ed. E. Muller). 4th edn, Vol. IX. Georg Thieme, Stuttgart, 1955. 2. Petragnani, N.; Moura Campos, M. Organomet. Chem. Rev. 1967, 2, 61. 3. Cooper, W. C. (ed.). Tellurium. Van Nostrand Rheinhold, New York, 1971. 4. Irgolic, K. J.; Zingaro, R. in Organometallic Reactions (eds. E. Becker; M. Tsutsui). Wiley, New York, 1971. 5. Irgolic, K. J. The Organic Chemistry of Tellurium. Gordon and Breach, New York, 1974. 6. Irgolic, K. J. J. Organomet. Chem. 1975, 103, 91. lrgolic, K. J. J. Organomet. Chem. 1977, 130, 411. Irgolic, K. J. J. Organomet. Chem. 1978, 158, 235; Irgolic, K. J. J. Organomet. Chem. 1980, 189, 65. Irgolic, K. J. J. Organomet. Chem. 1980, 203, 368. Table 1.1 Some physical properties of the VIA family of elements Element Atomic number

Atomic Electronic mass configuration

O S Se Te

15.99 32.06 78.96 127.6

8 16 34 52

1s22s22p4 2s22p63s23p4 3s23p63d104s24p4 4s24p64d105s25p4 1

Pauling Ionization electronegativity potential

Ionic radius

Atomic radius

3.5 2.5 2.4 2.1

1.40 1.84 1.98 2.21

0.66 1.04 1.17 1.37

13.61 10.36 9.75 9.01

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7. Uemura, S. Nippon Kagaku Kaishi 1981, 36, 381. 8. Petragnani, N.; Comasseto, J. V. Proceedings of the 4th International Conference of the Organic Chemistry of Selenium and Tellurium (eds. F. Y. Berry; W. R. McWhinnie), pp. 98–241. The University of Aston in Birmingham, Birmingham, 1983. 9. Uemura, S. J. Synth. Org. Chem. Jpn. 1983, 41, 804. 10. Engman, L. Acc. Chem. Res. 1985, 18, 274. 11. Petragnani, N.; Comasseto, J. V. Synthesis 1986, 1. 12. Suzuki, H. Synth. Org. Chem. Jpn. 1987, 45, 603. 13. Sadekov, D.; Rivkin, B. B.; Minkin, V. Y. Russ. Chem. Rev. 1987, 56, 343. 14. Patai, S.; Rappoport, Z. (ed.). The Chemistry of Organic Selenium and Tellurium Compounds, Vols I and II. Wiley, New York, 1986, 1987. 15. Engman, L. Phosphorus Sulfur 1988, 38, 105. 16. Irgolic, K. Y. Houben-Weyl Methods of Organic Chemistry (ed. D. Klamann). 4th edn, Vol. E12b. Georg Thieme, Stuttgart, 1990. 17. Petragnani, N.; Comasseto, J. V. Synthesis 1991, 793. 18. Petragnani, N.; Comasseto, J. V. Synthesis 1991, 897. 19. Petragnani, N. Tellurium in Comprehensive Organometallic Chemistry II (ed. A. McKillop). Vol. 11, Chapter 14. Pergamon, Elsevier, 1995. 20. Comasseto, J. V.; Lo, W. L.; Petragnani, N.; Stefani, H. A. Synthesis 1997, 4, 373. 21. Petragnani, N.; Stefani, H. A. Tetrahedron 2005, 61, 1613.

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–2– Preparation of the More Important Inorganic Tellurium Reagents

2.1

TELLURIUM TETRACHLORIDE

Tellurium tetrachloride is prepared directly from the respective elements.1 Te + 2Cl2

TeCl4

Experimental procedure1. The apparatus in Figure 2.1 is charged with 100–150 g of finepowdered Te (the apparatus and the Te have been previously heated overnight at 110°C to ensure dryness). A stream of Cl2 (dried by bubbling in concentrated H2SO4) is slowly introduced by means of a Tygon tube; meanwhile, the apparatus is heated with a small burner flame. After a few minutes the solid Te begins to be converted into a black liquid. The reaction is exothermic (and proceeds spontaneously), but the absorption of Cl2 is accelerated by heating the mixture from time to time. After a while the mixture becomes clearer, giving finally an amber-coloured liquid, which by increased heating forms a vapour of the

Figure 2.1 Apparatus to prepare TeCl4. 3

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2. PREPARATION OF THE MORE IMPORTANT INORGANIC TELLURIUM REAGENTS

same colour. At that time the absorption is complete and all the Te has been converted into TeCl4. By vigorous burner heating, the product is transferred by distillation, under a continuous Cl2 stream, into the tubes, where it solidifies. The tubes are detached from the apparatus by melting. The yield is 90%. Tellurium tetrachloride is a pale yellow crystalline solid, melting point (m.p.) 225°C, highly hygroscopic, soluble in dioxane, acetone, ether, methanol and ethanol but less soluble in benzene and chloroform. Tellurium tetrachloride is instantaneously hydrolysed by water, giving tellurium oxochloride, which is soluble in concentrated hydrochloric acid, forming HTeCl5 and H2TeCl6. TeCl4 + H2O

TeOCl2 + 2HCl

The Raman data suggest the ionic structure for TeCl3⫹ Cl⫺ in both the solid and liquid states.2

2.2

TELLURIUM DIOXIDE

Tellurium dioxide is prepared by oxidation of elemental tellurium with concentrated nitric acid.3

Te

conc. HNO3

2TeO2 . HNO3

400°C

TeO2

Experimental procedure.3 Commercial Te (20 g, finer than 60 mesh) is weighed into a 1000 mL beaker, covered with H2O (200 mL) and treated by the slow addition of 95 mL of concentrated HNO3 (95 mL, specific gravity 1.42). The reaction is allowed to continue for 5–10 min with occasional agitation. Insoluble impurities are removed immediately by filtration on a Büchner funnel. The filtrate is transferred to a 600 mL beaker. Concentrated HNO3 (65 mL) is then added and the solution boiled until oxides of nitrogen have been expelled. Basic nitrates of antimony and bismuth precipitate at this point if these substances are present as impurities. These are removed by filtering through asbestos, after which the clear liquor is evaporated gently on a water bath under a hood in an open 600 mL beaker. Basic tellurium nitrate is deposited. The evaporation is continued until the solution volume has been reduced to 100 mL. The solution is then cooled. The crystalline deposit is filtered, washed with H2O on a suction filter and air dried. The dry crystals are placed in a 400 mL beaker, covered by inverting a 1000 mL beaker over the smaller beaker, and heated at a hot-plate temperature of 400–430°C for 2 h. The TeO2 (21 g, 84%) is cooled and bottled immediately to avoid slow darkening due to reduction by organic matter from the atmosphere.

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ALKALI METAL TELLURIDES (Te2⫺Cat2⫹)

5

Tellurium dioxide is a white solid existing in two crystalline forms. It is soluble in water and in both aqueous sodium hydroxide and hydrochloric acid.

2.3

⫺ ⫹ ALKALI METAL TELLURIDES (Te2⫺ Cat2⫹ )

These reagents are usually prepared in situ. The preparative methods are outlined in sequence and representative experimental procedures are described in later chapters. 2.3.1

From the elements (2Na + Te → Na2Te)

2.3.1.1

In liquid ammonia4

2.3.1.2

In DMF5

2.3.1.3

In the presence of naphthalene6

2.3.2

From tellurium and reducing agents (Te → Te2−−)

2.3.2.1

Rongalite (HOCH2SO2Na/NaOH )7

2.3.2.2

Thiourea dioxide (TUDO, HN=C(NH2)S(O)OH)/NaOH8

2.3.2.3

Hydride transfer reagents

KBH4/NaOH9 Me4NBH4 (giving tetramethylammonium telluride)10 NaBH4/DMF11 NaBH4/H2O12 LiHBH313 2.3.2.4

Tin(II) chloride/KOH/DMSO14

2.3.2.5

Hydrazine hydrate/NaOH15

2.3.3

From tellurium and non-reducing bases

2.3.3.1

NaH/DMF16

2.3.3.2

NaH/DME17

2.3.3.3

NaH/N-methyl-2-pyrrolidone18

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2. PREPARATION OF THE MORE IMPORTANT INORGANIC TELLURIUM REAGENTS

2.4 2.4.1

ALKALI METAL DITELLURIDES (Na2Te2)

From the elements (2Na + 2Te → Na2Te2)

2.4.1.1

In liquid ammonia19

2.4.1.2

In HMPA20

2.4.1.3

In the presence of naphthalene21

2.4.2

red

⫺ From tellurium and reducing agents (2Te → Te22⫺ )

2.4.2.1

Rongalite22

2.4.2.2

TUDO/NaOH23

2.4.2.3

Hydride transfer reagents

NaBH4/EtONa/EtOH24 NaBH4/DMF25 2.4.2.4

Hydrazine hydrate/NaOH26

2.5

HYDROGEN TELLURIDE (H2Te)

Hydrogen telluride is prepared in situ by hydrolysis of aluminium telluride.27 Al2Te3 + 3H2O → 3TeH2 + Al203

2.6

SODIUM HYDROGEN TELLURIDE (NaHTe)

Sodium hydrogen telluride is prepared by reduction of tellurium with NaBH4 under several conditions. The original procedure uses ethanol as the solvent, adding, after complete reduction of the tellurium, an appropriate amount of acetic acid (see Section 4.1.2, ref. 10; Section 4.1.7, ref. 29). REFERENCES 1. 2. 3. 4.

Suttle, Y. F.; Smith, C. R. F. Inorg. Synth. 1956, III, 140. Gerding, H.; Houtgraff, H. Recl. Trav. Chim. 1954, 73, 737. Marshall, H. Inorg. Synth. 1950, III, 143. Section 3.1.1.1 (a), ref. 1.

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7

5. Section 3.1.1.1 (a), ref. 8. 6. Section 3.1.1.1 (a), refs. 10, 11. 7. Section 3.1.1 1 (b), refs. 14, 15; Section 4.1.6, ref. 19; Section 4.1.9, ref. 33; Section 4.1.11, ref. 41; Section 4.2.5, ref. 6. 8. Section 3.1.1.1 (b), ref. 23. 9. Section 3.1.1.1 (b), ref. 24. 10. Section 3.1.1.1 (b), ref. 15. 11. Section 3.1.2.2, ref. 8; Section 4.1.11, ref. 40; Section 4.3.1.1, ref. 2. 12. Section 3.1.3.1, ref. 1; Section 4.1.13.1, ref. 49. 13. Section 5.1, ref. 2. 14. Section 3.1.1.1 (b), ref. 26. 15. Section 3.1.1.1 (b), ref. 27. 16. Section 3.1.2.1, ref. 5; Section 3.16.1.3, ref. 27; Section 4.1.5, ref. 17; Section 4.2.1, ref. 1; Section 4.3.2.2, ref. 5. 17. Section 4.2.5, ref. 5. 18. Section 3.1.2.1, ref. 7. 19. Section 3.2.1.1, ref. 1. 20. Section 3.2.1.2, ref. 10. 21. Section 3.2.1.1, ref. 3. 22. Section 3.2.1.1, ref. 6. 23. Section 3.2.1.1, ref. 7. 24. Section 3.2.1.1, ref. 5. 25. Section 4.3.1.1, ref. 2. 26. Section 3.2.1.1, ref. 9. 27. Section 4.1.1.1, ref. 1; Section 4.1.4, ref. 14; Section 4.1.6, ref. 18.

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–3– Preparation of the Principal Classes of Organic Tellurium Compounds

This chapter is devoted mainly to the preparation of those classes of organotellurium compounds that have been more systematically investigated in past years, owing to their peculiar role as reagents or intermediate in organic synthesis, including compounds of structural, biological or theoretical interest. Sections 3.1–3.3 outline the principal preparative methods of diorganyl tellurides and ditellurides, organyltellurium trichlorides and diorganyltellurium dichlorides, which were the first classes of compounds investigated at the beginning of tellurium organic chemistry. The physical properties and stability of the compounds are also described briefly.

DIORGANYL TELLURIDES (SECTION 3.1) The main routes to symmetrical diorganyl tellurides involve the direct reaction of nucleophilic telluride dianions (usually as Na2Te) with alkylating or arylating reagents. Otherwise the electrophilic tellurium tetrahalides react with arylmagnesium reagents, giving diaryl tellurides. Unsymmetrical tellurides are obtained as follows: •

• •

starting from nucleophilic tellurolate anions (easily generated by telluration of Grignard and lithium reagents or by reduction of diorganyl ditellurides) by reaction with alkylating or arylating reagents, or by addition to acetylenes; by cleavage of diaryl ditellurides with arylmagnesium reagents, diazonium salts, or arenesulphonylazo compounds; by the reaction of electrophilic tellurenyl halides with Grignard reagents.

9

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3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM

2RX

RTeR

2ArX Te2-

ArMgX (exc.)

ArTeAr

[2ArN2+]XRX.R1X

R1X

RTeR1

RTe-

RX

Ar-

ArTe-

R-

Te

R(Ar)Te

R(Ar)

ArTeR

TeX4

Te

NaBH4 R-

Na (Li) [ArTeX] Ar1X

R(Ar)

ArTeTeAr X2

Ar1ArTeAr1 [Ar1N2+]XAr1N = NSO2Ar

Diorganyl tellurides have low molecular mass and are colourless or yellowish liquids with an unpleasant and penetrating odour. Dimethyl telluride is a metabolite of tellurium and tellurium compounds in a variety of living organisms, including humans. Higher dialkyl tellurides and most diaryl tellurides are solids with low melting points (diphenyl telluride is a liquid). Diorganyl tellurides are soluble in common organic solvents. Because of the above-mentioned organoleptic properties, it is recommended that contact between dialkyl tellurides and the skin is avoided, and that, in general, all work involving diorganyl tellurides is performed under a well-ventilated hood. The thermal stability and sensitivity to the atmosphere of diorganyl tellurides is dependent on the organic moiety. Many aliphatic, cycloaliphatic, as well as vinylic and acetylenic tellurides are reported to be decomposed by light, and therefore some authors recommend preparation of these compounds in the dark or under a red light.1 These tellurides are also sensitive to exposure to the open atmosphere, easily undergoing oxidation to mixtures containing the corresponding telluroxides in addition to other products. These oxidations are especially effective in solution, where the oxidation products separate as amorphous insoluble solids. Benzyl groups in tellurides are characterized by a peculiar lability, as demonstrated by the oxidative cleavage of aryl benzyl tellurides leading to aromatic carbonyl compounds.2 Diaryl tellurides are generally more stable and can be handled in the open atmosphere.

DIORGANYL DITELLURIDES (SECTION 3.2) Diorganyl ditellurides are prepared by three routes: • •

alkylation or arylation of the ditelluride dianion (usually as Na2Te2); oxidation of tellurolate anions;

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ORGANYLTELLURIUM TRICHLORIDE AND DIORGANYLTELLURIUM DIHALIDES

•

11

reduction of the corresponding organyltellurium trichlorides. red. 2RX

RTeTeR

Te222ArX

ArTeTeAr

RTeCl3 R-

ox.

RTe-

ox.

ArTe-

red.

ArTeCl3

Ar-

Te

The Te2 group is a chromophore, and aliphatic and aromatic ditellurides exhibit a characteristic orange to pale or dark red colour (absorption maximum at ~400 nm). The short-chain aliphatic ditellurides are liquids with a pungent odour, whereas the higher members (more than 10 C chains) are solids with low melting points. Diaryl ditellurides, which are more important as synthetic intermediates, are solids, highly soluble in solvents such as petroleum ether, benzene, chloroform, ether, and tetrahydrofuran (THF), but are less soluble in methanol and ethanol. Dibenzyl ditelluride exhibits the peculiar lability of the tellurium–benzyl bond already mentioned for the corresponding tellurides. By heating the solid at 120°C, or by exposure in solution to ordinary incandescent lighting or to a Hanovia lamp, a rapid decomposition into elementary tellurium and dibenzyl telluride occurs.3

ORGANYLTELLURIUM TRICHLORIDE AND DIORGANYLTELLURIUM DIHALIDES (SECTIONS 3.5 AND 3.9) Organyltellurium trichlorides and diorganyltellurium dichlorides are prepared starting from the electrophilic tellurium tetrachloride (or aryltellurium trichlorides) by: • • • •

condensation reactions with active methylene compounds; addition to a C⫽C bond (or a C⬅C bond, not shown in the scheme); electrophilic substitution in aromatic hydrocarbons; and reaction with organomercury chlorides.

Organyltellurium trichlorides and the not directly accessible tribromides and triiodides are obtained by the halogenolysis of the Te–Te bond of the corresponding ditellurides. Diorganyltellurium dihalides can also be prepared by the addition of halogens to the parent tellurides. R

TeCl3 R

Y

R

or

Y

Y

Cl

Cl

or

)2 TeCl2

TeCl3

TeCl4 ArH ArHgCl

ArTeCl3

)2 TeCl2

ArH

Ar2TeCl2

ArHgCl

Ar1HgCl

ArAr1TeCl2

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3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM

ArTeCl2 R

ArTeCl3

Ar1H

Y

R

Y

ArAr1TeCl2 Cl ArTeCl2

ArTeTeAr

3X2 red

ArTeAr

X2

2ArTeX3 X = Cl, Br, I Ar2TeX2

red

The tri- and dihalides are crystalline compounds. The chlorides are colourless (or yellow such as some aryltellurium trichlorides), the colour changing to orange and red (or deep red) for the bromides and iodides. The aryltellurium trihalides are generally more stable than the alkyltellurium trihalides (alkyltellurium trichlorides, produced by the addition of TeCl4 to olefins, easily liberate elemental tellurium). The reactivity of aryltellurium trihalides decreases on going from the chlorides to the iodides, the same trend occurring for hydrolysis. Aryltellurium trichlorides are very sensitive to water and moisture and are easily hydrolysed, the tribromides being more stable, while the triiodides are unaffected by cold water and can be prepared even by aqueous procedures. Diaryltellurium dihalides are stable in water, and ionic exchange reactions allow the conversion of dichlorides into dibromides and diiodides. Aryltellurium trichlorides are highly soluble in methanol and ethanol but less soluble in benzene. Diaryltellurium dichlorides exhibit inverse solubilities, being more soluble in benzene than in methanol or ethanol. These properties allow an easy separation of diaryl tellurides from diaryl ditellurides (frequently formed as by-products in the preparation of tellurides): the mixture is treated with SO2Cl2 and the obtained mixed di- and trichlorides are separated by the appropriate solvents, and reduced back into the pure tellurides and ditellurides.

REFERENCES 1. Clive, D. L. J.; Chittattu, G. J.; Farina, V.; Kiel, W. A.; Menchen, S. M.; Russell, C. G.; Singh, A.; Wong, C. K.; Curtis, N. J. J. Am. Chem. Soc. 1980, 102, 4438. 2. Ferreira, J. T. B.; Oliveira, A. R. M.; Comasseto, J. V. Tetrahedron Lett. 1992, 33, 915. 3. Spencer, H. K.; Cava, M. P. J. Org. Chem. 1977, 42, 2937.

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DIORGANYL TELLURIDES

3.1

13

DIORGANYL TELLURIDES

Diorganyl tellurides, compounds with two organic groups linked to a tellurium atom, constitute the most abundant and familiar class of organic tellurium compounds. The organic groups, of the most differentiated types, can be identical or different, giving rise to symmetrical or unsymmetrical tellurides. Since symmetrical dialkyl and diaryl tellurides are the most employed in organic synthesis, and often exhibit structural or biological interest, their preparation will be examined in detail, focusing on the methods and procedures that are considered as the most popular. 3.1.1 3.1.1.1

Symmetrical dialkyl tellurides From alkali tellurides and alkylating agents Te2- + 2RX

RTeR

Alkali tellurides, among which sodium telluride is the most widely employed, are powerful nucleophilic reagents and therefore react easily with alkylating agents. (a) From sodium telluride prepared from the elements (i) Sodium/liquid ammonia method

Te + 2Na

NH3 liquid

Na2Te

RX

RTeR

This method has been applied successfully to n- and s-alkyl halides.1–4 Dialkyl tellurides (general procedure).1 Elemental Te is added in ~0.5 g portions to a wellstirred solution of Na in liquid NH3 until the solution decolourizes, forming a colourless suspension (2 g-atom Na/1 g-atom Te). The quantities of the materials are chosen to give a suspension of ~0.7 M. The alkylating agent is added dropwise in 10% excess to the suspension of Na2Te. The reaction mixture is stirred until the NH3 evaporates. H2O is then added, the mixture extracted with ether and the ethereal solution worked up in the usual manner. Diethyltelluride: yield 80%; b.p. 38°C/14 torr, Diisopropyltelluride: yield 80%; b.p. 49°C/14 torr. When the alkylating agent is insoluble in liquid ammonia, as in the case of long-chain compounds, an organic solvent is added to the sodium telluride residue after evaporation of the ammonia. Some cyclic and steroidal tellurides have been prepared from sodium telluride in ethanol and the appropriate dihalides.5–7

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Sodium/DMF method 8,9 Te + 2Na

DMF

Na2Te

RX

RTeR

O O Bis(2-phenyl-1,2-dioxolan-2-yl)methyl telluride (R⫽Ph–C–CH2) (typical procedure).8 Finely powdered Te (0.30 g, 2.3 mmol) and Na (0.22 g, 9.6 mmol) are stirred under N2 in dry DMF (15 mL) at 110°C until all the Te has disappeared (15 min). 2-(Bromomethyl)2-phenyl-1,3-dioxolane (1.15 g, 4.7 mmol) in dry THF (10 mL) is added to the resulting yellowish suspension. After 2.5 h at room temperature the reaction mixture is poured into H2O and extracted with ether. Chromatography on SiO2 (eluent: CH2Cl2/petroleum ether at 40–60°C, 1:1) furnishes the telluride (0.65 g (60%); recrystallized from EtOH, m.p. 67–68°C). Additional examples: R ⫽ n-C12H25 (66%), PhCH⫽CH- (41%).9 (iii) Sodium/naphthalene method Sodium telluride can be prepared under mild conditions from the elements in the presence of catalytic amounts of naphthalene10 or by treating tellurium with sodium naphthalide.11 Te

Na / naphthalene RX Na2Te RTeR THF (60-90%)

R = Et, n- Pr, n - Bu, MeOCH2CH2

Dialkyl tellurides (general procedure).10 Te powder (3.58 g, 28 mmol), Na chips (1.29 g, 56 mmol), and naphthalene (0.72 g, 5.6 mmol) in THF (25 mL) are refluxed under N2 and stirred for 3 h. The heterogeneous white mixture is cooled at 0°C and the alkyl halide (56 mmol) is added slowly, stirring for 30 min. After 30 min of additional stirring, the mixture is filtered, the solution evaporated and the residue distilled under vacuum, giving the telluride. Diallyl telluride (typical procedure).11 In a 25 mL Schlenk flask, Te pieces (6.1 g, 48 mmol, from a Te ingot), Na (2.2 g, 96 mmol) and naphthalene (0.1 g, 0.8 mmol) in THF (20 mL) are stirred under argon at 25°C for 4 days. The mixture is filtered and the filter cake washed with THF (20 mL) and dried under vacuum to give Na2Te (7.6 g, 92%). Alternatively, Te powder (60 mesh) and sodium naphthalide (2 mol equiv) are stirred in THF for 4 h. Na2Te is isolated as an amber-coloured solid. The Na2Te (62.9 g, 0.362 mmol) is slurried in absolute EtOH (400 mL). The slurry is cooled at 0°C and allyl bromide (93.3 g, 0.77 mmol) is added dropwise during a 1-h period while stirring. The mixture is stirred for an additional hour at 20°C and then filtered. The grey filter cake is washed with EtOH (400 mL). The combined washing and filtrate are evaporated at atmospheric pressure. The residue is distilled, giving diallyl telluride as a pungent, air-sensitive, yellow liquid (56.7 g (75%); b.p. 70–72°C/13 torr).

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(b) From alkali tellurides prepared from tellurium and reducing agents (i) Rongalite (sodium formaldehyde sulphoxylate) method This method was first described at the beginning of the past century,12 and continues to find a wide application.

Te

HOCH2SO2Na NaOH / H2O

Na2Te

RX

RTeR

n-Alkyl halides, benzyl chloride, ethyl sulphate, and s-alkyl bromides give the expected tellurides in medium yields whereas t-butyl chloride is not converted into the telluride.13–15 Owing to the insolubility of the alkyl halides, ethanol must be added to the reaction mixture. Dibenzyl telluride (typical procedure).15 Rongalite (18.0 g, 0.5 mol) is added under N2 at 80°C to a suspension of Te (2.56 g, 20 mmol) in a solution of NaOH (12 g, 0.3 mol) in 125 mL of H2O. After stirring for 1 h, a solution of benzyl chloride (1.26 g, 10 mmol) in a small volume of EtOH is added dropwise at room temperature to the almost colourless telluride solution. After stirring for an additional hour, the mixture is extracted with ether, and the ethereal solution dried (MgSO4) and evaporated. The residue is recrystallized from petroleum ether (40–60°C) under red light, giving the telluride as yellow needles (1.09 g (70%); m.p. 49–57°C). Cyclic tellurides, 16–18 including some with a steroidal structure,19,20 have been prepared by the Rongalite method. Te ( )n

Te

Te

Y Y = 0, S

n = 1, 2

(ii) Sodium dithionite and thiourea dioxide method The use of sodium dithionite (Na2S2O4) and thiourea dioxide (TUDO; HN⫽C(NH2)S(O)OH) has been introduced later as a reducing agent for the preparation of sodium telluride in an aqueous medium, followed by reaction with n-alkyl halides to give dialkyl tellurides.21,22 The TUDO method has been reinvestigated and has found wide use because of the simplicity of the experimental conditions.23 TUDO RX Na2Te THF (CTBA) RTeR NaOH / H2O / THF (72 - 85%) R = n- C12H23, n- C8H17, i- C3H7CH2CH2, THPO(CH2)6 Te

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Dialkyl tellurides (general procedure).23 A mixture of TUDO (0.2 g, 2 mmol), NaOH (0.112 g, 2.6 mmol), Te powder (0.128 g, 1 mmol) in H2O (0.75 mL) and THF (0.75 mL) is refluxed for 1 h. The alkyl halide (2 mmol) and (cetyl trimethyl ammonium bromide) CTBA (0.004 g, 1.1 ⫻ 10⫺5 mmol) in THF (0.5 mL) are added to the pale pink solution. After 1 h of additional reflux the mixture is worked up in the usual manner and the residue purified by column chromatography on SiO2 (elution with petroleum ether 40–60°C). (iii) Hydride transfer reagent method A useful method for the reductive conversion of elemental tellurium into Te2⫺ anions employs complex hydrides such as sodium or potassium borohydride and tetraalkyl ammonium borohydride as reducing agents.

Te

cat+BH42solvent / reflux Te

cat = Na

+,

K+,

RX

RTeR

N+

Me4

Di-n-octyl telluride (typical procedure).24 Elemental Te (2.54 g, 20 mmol) is heated at reflux in 20% aqueous NaOH (43 mL) containing KBH4 (2.7 g, 50 mmol), under argon. After 1.5 h the mixture is deep purple, but after an additional 30 min of reflux turns pale yellow and no Te metal remains. Bromooctane (7.72 g, 40 mmol) in MeOH (50 mL) is added, the solution refluxed for 30 min, cooled, poured into H2O (100 mL) and extracted with ether (3⫻). The ether extracts are washed with H2O, dried (Na2SO4) and evaporated. The oily residue is distilled under vacuum, giving the telluride (5.5 g (78%); b.p. 154°C/0.7 torr). Dibenzyl telluride (typical procedure).15 Elemental Te (1.27 g, 10 mmol) and Me4NBH4 (1.78 g, 20 mmol) are heated in H2O (100 mL) in a steam bath under N2 until the initially produced purple colour is discharged. After cooling at room temperature, benzyl chloride (2.53 g, 20 mmol) in EtOH (20 mL) is added slowly with stirring and rigorous exclusion of air. After 2 h of stirring, the mixture is extracted with ether, dried and evaporated. The residue is recrystallized from petroleum ether (under red light), giving the telluride as yellow needles (2.56 g (82%); m.p. 49–57°C). In an interesting one-pot procedure the borohydride is used as both the reducing and alkylating agent.25 Te + 2R4NBH4

RTeR + 2R3NBH3 + H2

Di-n-butyl telluride (typical procedure).25 Elemental Te (0.63 g, 5 mmol) and n-Bu4NBH4 (3.87 g, 15 mmol, 50% excess) are refluxed in toluene (80 mL) under N2 and stirring. The initial red solution gradually turns colourless, giving a clear solution after 3 h. The cooled solution is washed with H2O (50 mL) and 1 M HCl (50 mL) (evolution of H2). The organic phase is dried (CaCl2) and evaporated, giving an oil (mixture of the telluride and Bu3NBH3). The telluride is separated by column chromatography on SiO2 (eluent: CH2Cl2/petroleum ether at 40–60°C, 1:1) (1.20 g (95%); nD20⫽1.5165).

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17

(iv) Tin(n)/potassium hydroxide method 26 Te

SnCl2 / KOH DMSO, 120°C, 10h

K2Te

MeI

MeTeMe

Dimethyl telluride (typical procedure).26 KOH (100 g, 1.78 mol) in dimethyl sulfoxide (DMSO) (180 mL) is heated on a steam bath. SnCl2 (10 g, 53 mmol) and Te powder (60 g, 0.47 mmol) are added and the mixture heated at 120°C for 20 h. After cooling at 40°C, methyl iodide (157 g, 1.1 mol) is added dropwise for 2 h. The mixture is then heated at 70°C for 2 h and then distilled until the condensing vapour reaches a temperature of 100°C. The organic layer of the residue is separated, dried and distilled (71.6 g (97%); b.p. 93–94°C). (v) Hydrazine hydrate method 27 N2H4 / NaOH

RX Na2Te RTeR (55 - 73%) DMF R = Et, n- Bu, n- pentyl, s- Bu, i - pr - CH2CH2, n - octyl

Te

Dibutyl telluride (typical procedure).27 Hydrazine hydrate 80% (0.50 mL, 7.1 mmol) is added dropwise using a syringe to a stirred mixture of finely ground Te (0.64 g, 5 mmol) and powdered NaOH (0.40 g, 10 mmol) in DMF (10 mL) at 50–60°C. After 3 h stirring, n-butyl bromide (1.4 g, 10 mmol) in DMF (2 mL) is added, the mixture is heated at 60°C for a further 30 min, cooled at room temperature and extracted with petroleum ether (40–60°C). The organic phase is separated, washed with H2O, dried (CaCl2) and evaporated, giving the pure telluride (0.69 g (57%); b.p. 111–114°C/13 torr). 3.1.1.2

From bis(triphenylstannyl) telluride and alkylating reagents

The title reagent (prepared by the reaction of sodium hydrogen telluride with chlorotriphenylstannane)28 reacts easily with the more active halides such as benzyl bromides whereas common halides need to be activated by cesium fluoride.29 Alkyl iodides and bromides react satisfactorily under these conditions whereas alkyl chlorides and aryl halides are nonreactive.

Ph3SnTeSnPh3 + RX

MeCN THF

PH3Sn TeSnPh3

R X

-Ph3SnF

F.. PH3Sn TeR

R X

40 - 100%

RTeR + Ph3SnF

F.. RX = BzBr, n - C10H21I, n - C10H21Br, i - PrI, EtO2CCH2Br, PhCOCH2Br, Cl(CH2)6Br (giving [Cl(CH2)6]2Te),

Br Br

giving

Te

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Dialkyl tellurides (general procedure).29 Bis(triphenylstannyl) telluride (1 equiv), alkyl halide (2 equiv), excess CsF (4 equiv) and the solvent (MeCN or MeCN/THF) are mixed, kept under N2 and monitored by thin layer chromatography (TLC) (or 1H NMR). Usual work-up of the mixture yields the telluride. REFERENCES 1. Brandsma, L.; Wijers, H. E. Recl. Trav. Chim. Pays-Bas 1963, 82, 68. 2. Sukhai, R. S.; Jong, R. A.; Verkruijsse, H. D.; Brandsma, L. Recl. J. R. Neth. Chem. Soc. 1981, 100, 368. Sukhai, R. S.; Jong, R. A.; Verkruijsse, H. D.; Brandsma, L. Chem. Abstr. 1982, 96, 104204. 3. US 2398414 (1946), Cal Research Corporation; Denison, G. H.; Condit, P. C. Chem. Abstr. 1946, 40, 3598. 4. Bogolyubov, G. M.; Shlyk, Y. N.; Petrov, A. A. J. Gen. Chem. USSR 1969, 39, 1768 (in English). 5. Buchta, E.; Greiner, K. Chem. Ber. 1961, 94, 1311. 6. Zanati, G.; Wolf, M. E. J. Med. Chem. 1972, 15, 368. 7. Zanati, G.; Gaare, G.; Wolf, M. E. J. Med. Chem.1974, 17, 561. 8. Engman, L. Organometallics 1986, 5, 427. 9. Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. Nippon Kagaku Kaishi 1987, 1469. 10. Bhasin, K. K.; Gupta, V.; Gautam, A.; Sharma, R. P. Synth. Commun. 1990, 20, 2191. 11. Higa, K.; Harris, D. C. Organometallics 1989, 8, 1674. 12. Tschugaeff, L.; Chlopin, W. Ber. Dtsch. Chem. Ges. 1914, 47, 1274. 13. Balfe, M. D.; Nandi, K. M. J. Chem. Soc. 1941, 70. 14. Balfe, M. P.; Chaplin, C. A.; Phillips, H. J. Chem. Soc. 1938, 344. 15. Spencer, H. K.; Cava, M. P. J. Org. Chem. 1977, 42, 2937. 16. Farrar, W. V.; Gulland, J. M. J. Chem. Soc. 1945, 11. 17. McCullough, J. B. Inorg. Chem. 1965, 4, 862. 18. Holliman, F. G.; Mann, F. G. J. Chem. Soc. 1945, 37. 19. Knapp Jr., F. F. J. Labelled Compd. Radiopharm. 1980, 17, 81. 20. Suginome, H.; Yamada, S.; Wang, J. B. J. Org. Chem. 1990, 55, 2170. 21. Brigov, B. G.; Bregadze, V. I.; Golubinskaya, L. N.; Tonoyan, L. G.; Kozyzkin, B. I. USSR 541, 851, Chem. Abstr. 1977, 87, 5394. 22. Kozyzkin, B. I.; Salamtin, B. A.; Ivanov, L. L.; Kuzovlev, I. A.; Gribov, B. G.; Federov, V. A. Poluch. Anal. Veshchestv Osoboi Chist [Dokl. Vses. Konf.] 1976, 5, 142, Chem. Abstr. 1979, 91, 140278. 23. Ferreira, J. T. B.; Oliveira, A. R.; Comasseto, J. V. Synth. Commun. 1989, 19, 239. 24. Kirsch, G.; Goodman, M. M.; Knapp Jr., F. F. Organometallics 1983, 2, 357. 25. Bergman, J.; Engman, C. Synthesis 1980, 569. 26. Voronkov, M. G.; Stankevich, V. K.; Rodkniko, P. A.; Korchevin, N. A.; Deryagina, E. N.; Trofimov, B. A. J. Gen. Chem. USSR 1987, 57, 2398. 27. Lue, P.; Chen, B.; Yu, X.; Chen, J.; Zhou, X. Synth. Commun. 1986, 16, 1849. 28. Einstein, F. W.; Jones, C. H. W.; Jones T.; Sharma, R. D. Can. J. Chem. 1983, 61, 2611. 29. Li, C. J.; Harpp, D. N. Tetrahedron Lett. 1990, 31, 6291.

3.1.2 3.1.2.1

Symmetrical diaryl tellurides From sodium telluride and non-activated aryl halides

The normal low reactivity of aryl halides towards nucleophilic reagents is not generally observed in their reaction with alkali tellurides. The observed reactivity seems to be

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19

dependent on the method employed to prepare the alkali telluride as well as on the experimental conditions and solvents. Non-activated aryl halides react only moderately with sodium telluride prepared from the elements in inert solvents (DMF, N-methyl-2-pyrrolidone (NMP), hexamethylphosphoric acid triamide (HMPA)).1–4 Te + 2Na

solvent

Na2Te

ArX solvent, 130 -170°C, 16 -24h (35 - 50%)

ArTeAr

solvent = DMF, NMP, HMPA ArX = PhI, 2 - bromonaphthalene

Better results are achieved with tellurium and sodium hydride in DMF.5

Te

NaH DMF, 140°C

Na2Te

ArI DMF, 130°C, 24 h (41 - 70%)

ArTeAr

Ar = 1 - naphthyl and derivatives, 2 - fluorenyl

Diaryl tellurides (general procedure).5 A mixture of powdered Te (0.128 g, 1.0 mmol), NaH (0.053 g, 2.2 mmol, 60% suspension in oil, washed with hexane) and dry DMF (3 mL) is heated at 140°C for 1 h. Within 0.5 h the initial deep red colour is lost, and a pale yellow suspension is obtained. After cooling at room temperature, the aryl iodide (2.0 mmol) in dry DMF (3 mL) is added and the mixture heated at 130°C for 24 h. After cooling at room temperature, the mixture is quenched with 10% aqueous NH4HSO4 (10 mL) and extracted with ether (10 mL). The ethereal extract is washed with H2O, dried (Na2SO4) and evaporated, giving the telluride, which is purified by column chromatography on SiO2 (eluent/hexane). The Rongalite method (see Section 3.1.1.1b,i) can be successfully applied to the preparation of diaryl tellurides from aryl iodides.5,6 This method seems to be advantageous compared to the preceding one because of the higher yields and milder experimental conditions.

Te

HOCH2SO2NA NaOH / H2O

Na2Te

ArI DMF, 60°C, 10 h (55 - 94%)

ArTeAr

Ar = Ph and (alkyl and methoxy) derivatives, 1-naphthyl, 2 -naphthyl and derivatives, 9 -anthryl, 9 -phenanthryl, 1-pyrenyl

Some concurrent reactions sometimes observed in the above methods (dehydrohalogenation with the tellurium/Rongalite method and formation of tellurocarbamates RTeC(O)NMe2 with the tellurium/NaH/DMF method) that decrease the yields of the desired tellurides can be avoided by a modified procedure, where sodium telluride is generated by heating elemental tellurium and NaH in NMP.7 The deep purple solution of the reagent prepared under these conditions can be stored for many days (in contrast to the use

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of DMF as the solvent) without appreciable degradation. NaH / NMP 100 -110°C, 1 h

ArI ArTeAr NMP, 50 -110°C, 2 -24 h (42-77%) Ar = Ph and (alkyl and methoxy) derivatives, 1-naphthyl, 2-naphthyl, 1-(2-methyl)naphthyl

Te

Na2Te

Di(1-naphthyl) telluride (typical procedure).7 Finely ground Te (0.140 g, 1.1 mmol) and NaH (2.2 mmol, 60% suspension in oil, washed with hexane) are heated in NMP (3.5 mL) for 1 h. 1-Iodonaphthalene (0.310 g, 1.2 mmol) is added in small portions over 1.5 h to the obtained solution of Na2Te at 100°C under argon. The resulting mixture is kept at this temperature for 19 h. During this period the colour of the solution gradually changes from deep purple to black. The progress of the reaction is monitored by TLC. After complete disappearance of the starting material the reaction is quenched by addition of saturated aqueous NH4Cl (1 mL) followed by benzene (5 mL). The Te deposit is filtered off and the filtrate is partitioned between EtOAc (30 mL) and H2O (20 mL). The organic phase is separated and dried (Na2SO4), the solvent is evaporated and the residue chromatographed over SiO2 (eluent/hexane), giving the telluride. The product is recrystallized from hexane/ CHCl3 (1:1) as yellow crystals (0.160 g (76%); m.p. 123–126°C). 3.1.2.2

From sodium telluride or sodium O,O-diethyl phosphorotellurolate and arenediazonium fluoroborates

Sodium telluride and sodium O,O-diethyl phosphorotellurolate, prepared respectively by the Te/NaBH4DMF method and by the reaction of elemental tellurium with NaH and O,O-diethyl phosphate in ethanol, react with arenediazonium fluoroborates, giving good yields of diaryl tellurides.8 Te

(method A) NaBH4 DMF, 100°C

(method B)(EtO)2P(O)H NaH / EtOH, r.t.

Na2Te

[ArN2+]BF4DMF, 0-5°C, 30min (59 - 72%)

(EtO)2P(O)TeNa

ArTeAr

[ArN2+]BF4DMF, r.t. (64 - 94%)

A = Ph and (alkyl, methoxy, halo and acyl) derivatives

Di(p-tolyl) telluride (typical procedure).8 Method A. Elemental Te (0.65 g, 5 mmol) and NaBH4 (0.45 g, 12 mmol) in DMF (15 mL) are heated in a Schlenk reactor at 100°C with stirring and under N2 until disappearance of the Te powder. The solution is cooled at 0°C and p-toluenediazonium fluoroborate (2.06 g, 10 mmol) in DMF (5 mL) is added dropwise. The mixture is stirred at 0°C for 30 min, quenched with H2O (20 mL) and then stirred continuously for 30 min. The mixture is filtered, the filtrate extracted with ether (3⫻15 mL) and the extracts washed with H2O (3⫻15 mL) and dried (MgSO4). The solvent is evaporated and the residue recrystallized from MeOH, giving the telluride (1.05 g (68%); m.p. 67°C). Method B. Elemental Te (0.64 g, 5 mmol), NaH (0.30 g, 10 mmol, 80% oil suspension, washed with hexane) and (EtO)2P(O)H (1.41 g, 10 mmol, 97%) in anhydrous EtOH

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(20 mL) are stirred in a Schlenk apparatus at room temperature under N2 until disappearance of the Te powder. p-Toluenediazonium fluoroborate (2.27 g, 11 mmol) in DMF (5 mL) is added slowly and the whole stirred for 30 min. The solution is diluted with H2O, extracted with ether (3⫻15 mL) and the extract dried (MgSO4) and evaporated. The residue is recrystallized from MeOH, giving the telluride (1.38 g (89%); m.p. 67°C). 3.1.2.3

From potassium tellurocyanate and arenediazonium fluoroborates

Another method that uses arenediazonium fluoroborates to prepare diaryl tellurides is the reaction with potassium tellurocyanate.9 Aryl tellurocyanides are postulated as intermediates.

2KTeCN + 2ArN2+]BF4-

DMSO, r.t. -N2, -2KBF4

[2ArTeCN] (41- 47%)

ArTeAr + Te(CN)2 Te + (CN)2

Ar = alkyl, methoxy, nitro, cyano, halophenyl derivatives, 2 - biphenyl

Diaryl tellurides (general procedure).9 Finely ground Te (1.6 g, 12.5 mmol) and KCN (0.82 g, 12.5 mmol) in dry DMSO (20 mL) are heated at 100°C, for 1 h under N2. After all the Te has dissolved, the mixture is cooled in an ice bath until most of the solvent has solidified. The diazonium fluoroborate (12.5 mmol) is added rapidly while a brisk stream of N2 is passed into the system. When the initial violent gas evolution has ceased, the ice bath is removed and stirring is continued at room temperature for 3 h. The mixture is diluted with CH2Cl2 (250 mL), filtered from the dark insoluble material (Te) and washed several times with H2O. The solution is dried (CaCl2) and evaporated, giving an oil or a pasty-solid, which is dissolved in CH2Cl2 and filtered through a short SiO2 column. (ln the few cases where aryl tellurocyanides are formed as by-products, more careful chromatography is required with CH2Cl2/hexane, 1:1 as the eluent.) The small amounts of diaryl ditellurides formed frequently as by-products can be converted into tellurides by treatment with copper powder in refluxing dioxane.10 For Ar⫽2HO2CC6H4, the crude product is extracted with aqueous Na2CO3 and precipitated with acid (yield 8%). 3.1.2.4

From tellurium(IV) halides and arylmagnesium halides

In this method, the electrophilic tellurium tetrachloride is employed as starting material to introduce tellurium in organic substrates, in contrast to the preceding methods using nucleophilic tellurium species. Tellurium tetrachloride (as well as tellurium tetrabromide and tellurium tetraiodide) reacts with 4 mol equiv of arylmagnesium bromide, giving rise to diaryl tellurides in high yields.11,12

TeX4 + 4ArMgBr

ether / benzene 0°C, reflux (>90%)

ArTeAr + ArAr + 4MgBrX

Ar = Ph; X = Cl, Br, I Ar = 1-naphthyl, o -MeC6H4, p -MeC6H4; X = Br

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The intermediates in these reactions are the corresponding triaryltelluronium halides (which react with an additional equivalent of Grignard reagent) or the unstable tetraaryltellurium derivatives. TeX4 + 3ArMgX

[Ar3Te+]X-

TeX4 + 4ArMgX

[Ar4Te]

ArMgX

ArTeAr + ArAr

This method is an efficient modification of the early method of Lederer,13 employing tellurium dihalides (unstable compounds which disproportionate in elemental tellurium and tellurium(IV) halides). To make the isolation and purification of the products easier, the obtained tellurides are converted in situ into the corresponding dichlorides, dibromides or diiodides by treating, respectively, with sulphuryl chloride, bromine or iodine.11,12 Diphenyl telluride (typical procedure).12 To an ethereal solution of a Grignard reagent (100 mL), prepared from bromobenzene (39.3 g, 0.25 mol) and Mg (6.1 g, 0.25 mol), benzene (100 mL) is added. The solution is cooled at 0°C and TeCl4 (13.5 g, 0.05 mol) in benzene (200 mL) is added slowly with vigorous stirring. The reaction mixture is refluxed for 2 h, then cooled at 0°C and quenched with saturated aqueous NH4Cl (300 mL). The organic layer is separated, washed with H2O (3⫻), dried (Na2SO4) and evaporated in a rotatory evaporator, giving the crude telluride. Treatment of the crude telluride with Br2 gives diphenyltellurium dibromide (20 g (91%); m.p. 197°C). 3.1.2.5

From elemental tellurium and diarylmercury compounds

The reaction of diarylmercurials with elemental tellurium under heating, one of the oldest methods for the preparation of diaryl tellurides,14–16 has been later employed in some specific cases.17–19 Ar2Hg + 2Te

3.1.2.6

>200°C

ArTeAr + TeHg

From diaryl ditellurides by extrusion of a tellurium atom

Diaryl ditellurides are relatively thermolabile compounds and eliminate one tellurium atom by heating at ~300°C.20,21 In the presence of copper metal, however, the extrusion of tellurium is achieved during reflux with toluene or dioxane.10,22–24 ArTeTeAr

~300°C -Te

Cu toluene, dioxane, reflux (70 - 90%)

ArTeAr (Ar = Ph, p -MeOC6H4, p -EtOC6H4) ArTeAr

Ar = p -MeC6H4, m -MeOC6H4, p -FC6H4, m -BrC6H4, p -NO2C6H4, p -Me3SiC6H4, 4 - biphenyl, [2 - (2 - quinolylphenyl)]

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23

Bis(2,4,6-trimethylphenyl) telluride (typical procedure).24 A solution of bis(2,4, 6-trimethylphenyl) ditelluride (4.94 g, 10 mmol) in toluene (150 mL) is refluxed in the presence of electrolytic Cu (1.40 g, 22 mmol) for 12 h. The mixture is filtered and the filtrate evaporated to give the pure telluride (m.p. 123–125°C). 3.1.2.7

Bis-(phenylethynyl) telluride as Te2+ equivalent

On the basis that bis-organyl tellurides undergo Te/Li exchange by treatment with an organolithium reagent, if a thermodynamically more stable organolithium moiety is released,25 bis-(phenylethynyl) telluride26 has been employed as starting material for the synthesis of diaryl tellurides.27 2ArBr

t - BuLi

2ArLi

Ph CTeC Ph ArTeAr -2PhC CLi (75 - 100%) THF, -78°C

Ar = Ph, p - Me2NC6H4, p - MeOC6H4, p - HOC6H4, p -MeC6H4, m - MeC6H4, o,m-Me2C6H3, o,p,o - Me3C6H2, 2 - thienyl, p - F3CC6H4, 2 - thianapftenyl

REFERENCES 1. Sandman, J. D.; Stark, J. C.; Acampora, L. A; Gagne, P. Organometallics 1983, 2, 549. 2. Sandman, D. J.; Stark; J. C.; Rubner, M.; Acampora, L. A.; Samuelson, L. A. Mol. Cryst. Liq. Cryst. 1983, 93, 293. 3. Acampora, L. A.; Dugger, D. L.; Emma, T.; Mohammed, J.; Rubner, M. F.; Samuelson, L.; Sandman, D. J.; Tripathy, S. K. Polym. Electron. 1984, 36, 461. 4. Sandman, D. L.; Stark, J. C.; Acampora, C. A.; Samuelson, C. A.; Allen, G. W. Mol. Cryst. Liq. Cryst. 1984, 107, 1. 5. Suzuki, S.; Padmanabhan, S.; Inouye, M.; Ogawa, T. Synthesis 1989, 468. 6. Suzuki, H.; Inouye, M. Chem. Lett. 1985, 389. 7. Suzuki, H.; Nakamura, T. Synthesis 1992, 549. 8. Li, J.; Lue, P.; Zhan, X. Synthesis 1992, 281. 9. Engman, L. J. Org. Chem. 1983, 48, 2920. 10. Sadekov, T. D.; Bushkov, A. Y.; Minkin, V. T. J. Gen. Chem. USSR 1973, 43, 815. 11. Rheinboldt, H.; Petragnani, N. Chem. Ber. 1956, 89, 1270. 12. McWhinnie, W. R.; Patel, M. G. J. Chem. Soc. Dalton Trans. 1972, 199. 13. (a) Lederer, K. Bericht 1915, 48, 1345, 2049. (b) Lederer, K. Bericht 1916, 49, 334, 345, 1071, 1076, 2002, 2532, 2663. (c) Lederer, K. Bericht 1917, 50, 238. (d) Lederer, K. Ber. Dtsch. Chem. Ges. 1919, 52, 1989. (e) Lederer, K. Bericht 1920, 53, 712. 14. Kraft, F.; Lyons, R. E. Bericht 1896, 27, 1769. 15. Zeiser, F. Bericht 1895, 28, 1760. 16. Lyons, R. E.; Bush, G. H. J. Am. Chem. Soc. 1908, 30, 834. 17. Hellwinkel, D.; Farbach, G. Tetrahedron Lett. 1965, 1823. 18. Cohen, S. C.; Reddy, M. L. N.; Massey, A. G. J. Organomet. Chem. 1968, 11, 563. 19. Jones, C. W. H.; Sharma, R. D.; Naumann, D. Can. J. Chem. 1986, 64, 987. 20. Farrar, W. V. Research 1951, 4, 177. 21. Petragnani, N.; Moura Campos, M. Chem. Ber. 1961, 94, 1759. 22. Haller, W. S.; Irgolic, K. J. J. Organomet. Chem. 1972, 38, 97.

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23. Sadekov, I. D.; Bushkov, A. Y.; Minkin, V. I. J. Gen. Chem. USSR 1977, 47, 576. 24. Akiba, M.; Lakshmikantham, M. V.; Jen, K. Y.; Cava, M. C. J. Org. Chem. 1984, 49, 4819. 25. Hiiro, T.; Kambe, N.; Ogawa, A.; Miyoshi, N.; Murai, S.; Sonoda, N. Angew. Chem. Int. Ed. Engl. 1987, 26, 1187. 26. The synthesis of bis(phenylethynyl) telluride, isolated as the corresponding diiodide, was described at first by Moura Campos, M.; Petragnani, N. Tetrahedron 1962, 18, 527. And later by Dabdoub, M. J.; Comasseto, J. V.; Braga, A. L. Synth. Commun. 1988, 18, 1979. See Section 3.17.2.2. 27. Engman, L.; Stern, D. Organometallics 1993, 12, 1445.

3.1.3

Unsymmetrical tellurides

Unsymmetrical tellurides comprise a large class of organotellurium compounds characterized by two different alkyl groups, one alkyl and one aryl group, or two different aryl groups linked to a tellurium atom. These compounds find only a minor use as reagents in organic synthesis, but because of structural, and in some cases biological interest, their preparation will be discussed in the following sections.

3.1.3.1

From sodium telluride and two different alkyl halides

Te

NaBH4 H2O

Na2Te

(1) RX, (2) R1X EtOH / THF

RTeR1 (+ RTeR + R1TeR1) (25%) (25%) (25%)

R = n -Bu, n - octyl, p - IC6H4(CH2)9, CH C(CH2)3, ICH CH(CH2)3 R1 = (CH2)nCO2Me

Unsymmetrical telluro-substituted fatty esters (of biological interest) are obtained in about 40% yield after chromatographic separation from the symmetrical tellurides.1 n-Octyl (7-carbomethoxy)heptyl telluride (typical procedure).1 To a suspension of elemental Te (0.127 g, 1 mmol) in H2O (3 mL) heated at 80°C is added a solution of NaBH4 (0.100 g, 2.64 mmol) in H2O (1 mL), under red light, after careful deaeration and argon purge. The obtained clear solution of Na2Te is cooled at room temperature and a solution of methyl 8-bromooctanoate (0.223 g, 0.95 mmol) and 1-bromooctane (0.202 g, 1.05 mmol) in THF/EtOH (1:1, 15 mL) is added under argon. The solution is stirred for 1 h, poured into H2O (150 mL) and then extracted with ether (3⫻50 mL). The ether extracts are washed with H2O (3⫻50 mL), dried (Na2SO4) and evaporated under vacuum. The crude product is dissolved in petroleum ether (30–60°C) and purified by column chromatography on SiO2 slurried with petroleum ether (elution with 10⫻25 mL aliquots of petroleum ether followed by 10⫻25 mL aliquots of benzene, monitoring the fractions by TLC). By combination of aliquots 13–16 the telluride is obtained as an oil (0.155 g (39%)).

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25

Aryl alkyl tellurides are prepared in medium yields by sequential arylation/alkylation of Na2Te generated by the described Te/NaH/NMP system (see Section 3.1.2.1).2 Te

NaH NMP, 100 - 110°C, 1 h

Na2Te

ArI (0.5 equiv) NMP, 120°C, 2 - 4 h

[ArTe-]

Ar = o - MeC6H4, 1-naphthyl; R = n - hepthyl Ar = 2- naphthyl; R = Me

RX NMP, 50°C, 2 h (52 - 67%)

ArTeAr

Methyl 2-naphthyl telluride (typical procedure).2 2-Iodonaphthalene (0.150 g, 0.59 mmol) is added dropwise to a stirred solution of Na2Te (1.3 mmol) in NMP (4 mL) and the mixture is heated at 120°C for 5 h. An excess of MeI (0.40 mL, 4.3 mmol) is then added and the resulting mixture is stirred at 50°C for 2 h. Usual work-up gives the telluride as pale yellow crystals (0.110 g (67%); m.p. 59–60°C). 3.1.3.2

From organyl tellurolates and alkylating agents

(a) From organyl tellurolates generated by tellurium insertion in organomagnesium or organolithium reagents The magnesium aryl tellurolates described earlier 3 have seldom been employed for the preparation of unsymmetrical tellurides.4–6 Te + ArMgBr

ArTeMgBr

RX

ArTeR

Ar = Ph; R = Et, H2C-C CH Ar = o-MeS-, o-Me2N, o-ClC6H4; R = Me (68-92%)

Elemental tellurium also inserts easily into a C(sp2)–Mg bond of a vinyl or styryl Grignard reagent (see Section 3.16.1.4), but is unaffected by alkylmagnesium halides.7 In contrast, tellurium insertion in alkyl- or aryllithium compounds followed by alkylation is a useful method for the synthesis of unsymmetrical tellurides. (For a tabulation of unsymmetrical tellurides prepared by alkylation of organyl tellurolates, see ref. 8.) Te + RLi Te + ArLi

1 RTeLi R X

ArTeLi RX

RTeR1 ArTeR

Methyl (3-hydroxy)propyl telluride (typical procedure).9 MeTeLi. A suspension of Te powder (12.8 g, 0.1 mol) in THF (100 mL) is frozen in a liquid N2 bath, and then MeLi (66.7 mL of a 1.5 M solution in ether, 0.1 mol) is injected into the flask. The mixture is allowed to thaw and is stirred magnetically for a further 30 min at room temperature. The resulting yellow-orange solution is stored under N2 and used within 2 h. MeTe(CH2)3OH. 3-Bromopropan-1-ol (6.95 g, 50 mmol) is injected into a frozen (by liquid N2) solution of MeTeLi (50 mmol). The mixture is allowed to thaw and is stirred magnetically at room temperature for about 1 h. Aqueous NaCl (100 mL) is then added and the aqueous layer extracted several times with ether. The combined organic phases are dried

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(MgSO4) for 16 h, and evaporated. The residue is chromatographed on an SiO2 column (elution with petroleum ether 40–60°C), giving the telluride (8.3 g (83%)) in acceptable purity. Methyl phenyl telluride (typical procedure).10 To a well-stirred suspension of Te powder (3.0 g, 24 mmol) in anhydrous THF (36 mL) under argon is added dropwise an equimolar amount of PhLi (1.5–2.0 M in ether). The mixture is then stirred for 2 h at room temperature, and for 1 h under reflux, and then MeI (3.4 g, 24 mmol) is added. After refluxing for 0.5 h the mixture is poured into three volumes of H2O, the aqueous layer is extracted with CH2Cl2, and the combined organic phases are washed with H2O, dried (Na2SO4) and evaporated. The oily brown residue is distilled under vacuum, giving the telluride (2.1 g (43%); b.p. 57–58°C/0.7 torr). (b) From organyl tellurolates generated by the cleavage of ditellurides with alkali metals or reducing agents The reductive fission of diaryl ditellurides with sodium in liquid ammonia reported earlier11 has seldom been employed.12,13 More recently, aprotic solvents have been substituted for ammonia.14–16 A practical and successful method employs lithium in THF.17 ArTeTeAr

Li THF

ArTeLi

RX

ArTeR

Ar = Ph; R = Me, Et, n -Pr, n -Bu, n -C14H29 (62-84%) R = i -prop (46%)

Alkyl phenyl tellurides (general procedure).17 Li metal (1.4 g, 0.2 mol) in small pieces is added under N2 to a solution of diphenyl ditelluride (4.1 g, 10 mmol) in dry THF (100 mL). The mixture is stirred at room temperature for 6 h, unreacted lithium is removed using a spatula and the alkyl halide (20 mmol) neat or in THF is added dropwise to the stirred yellowish-brown solution. The solution is stirred at room temperature for 30 min, and under reflux for an additional 30 min. The solvent is evaporated, and H2O (10 mL) and ether (25 mL) are added to the residue, mixing thoroughly. The ethereal phase is separated and evaporated. The residue is fractionally distilled under vacuum (yields 62–79%). Actually, the method of choice for the preparation of organyl tellurolate anions is the reduction of diorganyl ditellurides with reducing agents. NaBH4 is the more widely used reagent owing to the mild experimental conditions. The choice of solvent, or solvent mixture, and of neutral or basic media is governed by the further use of the tellurolate solution. Typical media are benzene/ethanol/aqueous NaOH,18 MeOH or EtOH.19–21

ArTeTeAr (RTeTeR)

NaBH4 EtOH (MeOH)

ArTeNa (RTeNa)

R 1X

ArTeR1 (RTeR1)

Medium-to-high yields of the expected tellurides are obtained from diaryl or dialkyl ditellurides and n- and s-alkyl halides (and steroidal tosylates).

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27

Reactions with epoxides are also successful, giving the corresponding hydroxy tellurides.19 ArTeNa + R

O

R R

OH

ArTe R

Phenyl dodecyl telluride (typical procedure).19 To a solution of diphenyl ditelluride (1.21 g, 2.96 mmol) in absolute EtOH (100 mL) is slowly added powdered NaBH4 (0.23 g, 6.08 mmol) under N2 at room temperature. At the end of the addition the red colour is discharged and 1-bromododecane (1.5 g, 6.02 mmol) is added by a syringe. The mixture is refluxed for 3.5 h (during this period the progress of the reaction is monitored by TLC; the plates are developed in the dark) and then evaporated. The residue is purified by chromatography on SiO2 (eluting with CCl4), giving the telluride (1.73 g (76%)). Diaryl ditellurides, like diaryl disulphides and diselenides,22 undergo disproportionation into arene tellurolates and tellurinates by alkaline treatment. 3ArTe- + ArTeO2- + 2H2O

2ArTeTeAr + 4OH-

The tellurolate anion formed can be alkylated in situ, giving alkyl aryl tellurides, if the above reaction is effected in the presence of a phase transfer catalyst.23 Otherwise, performing the reaction in the presence of the reducing agent TUDO a telluride yield near to quantitative is generated, owing to the reduction of the aryl tellurinate anion to aryl tellurolate.24 2ArTeTeAr + 4OH-

PTC

3ArTe- + ArTeO2- + 2H2O TUDO

ArTe- + RX

ArTeR (81-96%)

Ar = Ph, 2-naphthyl, p -MeOC6H4, m-F3CC6H4 R = n- and s-alkyl, benzyl

Alkyl aryl tellurides (general procedure).24 To a solution of diaryl ditelluride (1.0 mmol) in THF (7.5 mL) are added TUDO (0.108 g, 1.0 mmol), 2HT (0.030 g) and the alkyl halide (2.0 mmol). Then, 50% NaOH (7.5 mL) is added and the mixture is stirred vigorously for several hours at room temperature. The phases are separated, and the organic phase is extracted with EtOAc (3⫻20 mL). The combined organic phases are washed with H2O, dried (MgSO4) and evaporated. The residue is purified by column chromatography on SiO2 (eluting with EtOAc). Aryl ditellurides are cleaved by samarium diiodide (SmI2) to generate the new nucleophilic species diiodosamarium aryl tellurolate which reacts smoothly with alkyl and acyl

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halides, and even with aryl halides to afford the corresponding tellurides and telluroesters.25–27 ArTeTeAr + 2SmI2

THF / HMPA r.t., 0.5 h, 1 h

2 "ArTeSmI2"

ArTeR

RCOCl r.t., 2 h

CN X C C H Y THF, r.t., 3h

ArTe

RX r.t., THF, 3 h

ArTeCOR (77, 72%)

CN C C H Y (64-85%)

Ar = Ph X = Br, I R = Me, Et, prop, n -Bu i -propyl, PhCH2

Ar = Ph, p -MeC6H4 R = Ph

Ar = Ph, p -MeC6H4 X = Cl, Br Y = CN, CO2Et

Reaction of diphenyl ditelluride with organic halides mediated by SmI2.25 Samarium powder (0.20 g, 1.3 mmol) was placed in a well-dried, two-neck, round-bottom flask containing a magnetic stirrer bar. The flask was flushed with nitrogen several times. Methylene diiodide (0.32 g, 1.2 mmol) in THF (7 mL) was added through a rubber septum by a syringe. The mixture was stirred at room temperature until the solution became deep green and homogeneous (0.5–1 h). HMPA (0.7 mL) was then added and the solution became deep purple; the THF-HMPA solution of SmI2 was ready for subsequent use. To the THF-HMPA (7–0.7 mL) solution of SmI2 (1.2 mmol), prepared as described above, was added a mixture of ditelluride (0.21 g, 0.5 mmol) and organic halide (1 mmol) in THF (7 mL) at room temperature, and the solution was stirred for 5 h, during which time its colour changed into brownish-yellow. The reaction solution was poured into diluted HCl, and the mixture was extracted with ether (2⫻15 mL). The ethereal solution was treated with aqueous Na2S2O3, washed with brine, and dried (MgSO4). Evaporation of the solvent left a yellow oil that was subjected to preparative TLC on silica gel (petroleum ether as eluent), giving the phenylalkyl tellurides. Diisobutylaluminium benzenetellurolate, generated in situ from diphenyl ditelluride and diisobutylaluminium (DIBAL), reacts with acetals, alkyl sulphonates and oxiranes, giving the expected tellurides in high yields.28

(i-Bu)2AlTePh

RCH(OMe)2

CH2Cl2

re

OMe TePh R = H, C11H23 (42-80%)

RCH

r.t. flu

x

C11H23CH

TePh TePh

(50%) C6H13OMs(OTs)

OMs ( )11

same condition

same condition

C6H13TePh (72%) TePh ( )11 (46%)

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29

OH

O

same condition

RCHCH2TePh (70-88%)

R R = Me, Et, C12H25 O ( )n

OH

same condition

( )n TePh

n = 4, 10

(c)

(80%)

From organyl telluroesters generated by the reduction of organyltellurium trichlorides with NaBH4

The reaction of organyltellurium trichlorides with NaBH4 followed by the in situ treatment of the generated tellurolate with alkyl halides is a useful method to prepare several types of unsymmetrical telluride.29 NaBH4 RX ArTeR ArTeNa THF/H2O THF/N2 (88-95%) 0°C Ar = Ph, p -MeOC6H4, p -C6H5OC6H4 RX = MeI, EtBr, Me(CH2)2CH(Me)Br ArTeCl3

Transformation of aryltellurium trichlorides into alkyl aryl tellurides (typical procedure).29 To a solution of the aryltellurium trichloride (5 mmol) and the appropriate alkyl halide (7.5 mmol) in THF (60 mL) under a nitrogen atmosphere, was slowly added a solution of NaBH4 (0.93 g, 25 mmol) in water (30 mL) at 0°C. The colour changed from yellow to red when the addition of NaBH4 started and to yellow again after the addition was completed. After the addition, the mixture was stirred for 20 min at room temperature and then treated with saturated aqueous solution of NH4Cl (100 mL) and extracted with ethyl acetate (3⫻20 mL). The extracts were washed with brine (3⫻20 mL) dried with MgSO4 and the solvents were evaporated. The residue was purified by column chromatography on silica gel, eluting with hexane, to give the aryl–alkyl tellurides. Vinylic30 and cyclic organyltellurium trichlorides31 undergo similar reactions. R Cl

R1

1) NaBH4

TeCl3

THF/ H2O 2) EtBr / N2

Cl

R = Ph; R1 = Ph, Me R2 R1 ( )n R

O

R1

R

TeEt

(78, 75%)

1) NaBH4 TeCl3 THF/ H O 2 2) n-BuBr

n=1

R, R1, R2 = H R = i -prop; R1, R2 = Me

n=2

R, R1, R2 = H

R2 R1 ( )n R

TeBu-n O (90-95%)

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By addition of aryl tellurolates to electrophilic alkenes

Aryl tellurolates add to alkenes bearing electron-withdrawing groups in a typical 1,4-Michael addition, giving -telluro-substituted derivative.32 R Y

PhTeNa / EtOH (PhTe)2 / NaBH4 (31-71%)

R

PhTe Y

R = H, Me; Y = CO2Et, CN

THF Te + RLi r.t., N2

EtOH [RTeLi] r.t.

[RTeH]

R1 R2

EWG RTe R1 2 R

R = Bu, s- Bu EWG = CN, CHO, COR, CO2R R1, R2 = H R1 = H; R2 = Me, cyclohexenone, 4,4 - dimethyl cyclohexenone

EWG

(98%)

3.1.3.4 From organyl tellurolates and arylating agents The arylation of organyl tellurolates, restricted first to aryl halides activated by electronwithdrawing groups,24 or requiring special conditions such as heating in HMPA in the presence of CuI,33 or photostimulation in liquid ammonia,12,13 has later been achieved successfully with non-activated aryl halides under normal conditions.6 PhTeTePh

1) NaBH4 / HMPA, 70 - 80°C

2) CuI, 3) ArI (80 - 90%) Ar = nitroaryl

PhTeTePh

RLi + Te

Na/ NH3

ArX UV

PhTeNa

RTeLi

ArX

PhTeAr

[ref. 33]

PhTeAr

(ref. 12,13)

RTeAr (R = Me, Ph)

(ref. 6)

Methyl o-methoxyphenyl telluride (typical procedure).6 To a frozen solution of MeTeLi (50 mmol) is added o-bromoanisole (9.3 g, 50 mmol). The mixture is allowed to warm at room temperature, stirred for 1.5 h and then quenched with deoxygenated H2O (100 mL). The organic product is extracted with ether (3⫻50 mL), the organic extracts dried (MgSO4) for 16 h and evaporated, and the residue is distilled under vacuum, giving the telluride as a pale yellow oil (9.3 g (74%)). Arenediazonium salts34,35 as well as arenesulphonylazo compounds36 have also found a use as arylating agents for arene tellurolates. ArTeTeAr

NaBH4 EtOH/THF/NaOH aq.

PhTeLi + ArN = NSO2C6H4CH3

ArTeNa

[Ar1N2+]X-

18-crown-6 MeCN, r.t.

PhTeAr

ArTeAr1

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Reaction of diarylditellurides with arenediazonium salts (typical procedure). p-Methoxy phenyl p-tolyl telluride.35 With heating and stirring in an atmosphere of nitrogen, sodium tetrahydroborate was added in small portions to a solution of 9.4 g of bis(p-methoxyphenyl) ditelluride in a mixture of 50 mL of ethanol and 15 mL of benzene until the solution was decolourized completely (1.5 g was required). Then 8.24 g of p-toluenediazonium fluoroborate was added rapidly and the mixture was stirred for 1 h and poured into dilute HCl. The oil formed was extracted with ether, and the extract was washed with water and dried over CaCl2. Ether was evaporated, and the residue was dissolved in benzene and chromatographed on alumina (eluent hexane). After the evaporation of hexane, 4.8 g (36%) of the telluride was isolated, m.p. 64–64.5°C (hexane). Trimethylsylilmethyl telluride generated in situ from methyllithium and trimethylchlorosilane can be used instead of the tellurolate in the latter reaction.36 MeTeLi + Me3SiCl

3.1.3.5

MeTeSiMe3

ArN = NSO2C6H4CH3

MeTeAr

From diorganyl ditellurides or arenetellurenyl halides and organometallic reagents

A different approach to unsymmetrical diorganyl tellurides, in which an electrophilic tellurium species is used, involves the nucleophilic attack of organomagnesium or organolithium reagents to diorganyl ditellurides. RTeTeR + R1Met R aryl aryl, alkyl alkyl

RTeR1 + RTeMet

R1

Met

aryl alkyl alkyl

MgX, Li MgX, Li Li

(ref. 37, 39) (ref. 39, 40) (ref. 38, 6)

Only half of the tellurium of the starting ditelluride is converted into telluride. The other half, converted into tellurolate, can be oxidatively recovered as the starting ditelluride. If the diaryl ditelluride is previously converted in situ into the corresponding tellurenyl halide, both the electrophilic tellurium moieties are used.41 (The stable crystalline 2-naphthyltellurenyl iodide42 (see Section 3.7) reacts similarly.) The reaction is of general application, giving high yields of the expected tellurides with aromatic, aliphatic, cycloaliphatic, vinylic43 and acetylenic Grignard reagents,44 under mild conditions. ArTeTeAr

X2 THF / benzene

[ArTeX]

RMgX (>90%)

ArTeR

Ar = Ph, p -Me, p -MeO, p -EtOC6H4 R = Ph, n-Bu, cyclohexyl, PhC C ; X = Br, I

Unsymmetrical diorganyl tellurides (general procedure).41 A solution of the ditelluride (2 mmol) in THF (30 mL) is treated dropwise at 0°C under N2 with Br2 (0.32 g, 3 mmol)

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in benzene (4 mL). The Grignard reagent is then added dropwise by means of a syringe. Gradual disappearance of the red colour of the solution is observed. Finally, the solution becomes almost colourless after the addition of a 10% excess of the Grignard reagent. After stirring for 30 min at room temperature the solution is diluted with petroleum ether 30–60 C (30 mL) and treated successively with aqueous NH4Cl and brine. The organic layer is dried (MgSO4) and evaporated, giving the telluride. 3.1.3.6

Additional methods

(a) From dialkyl ditellurides and arenediazonium fIuoroborates45

RTeTeR

[ArN2+]BF4-

ArTeR

CHCl3(CH2Cl2), KOAc, 18-crown-6 (25 - 48%) R = Et Ar = 2-halC6H4, 2-MeC6H4, 3-MeC6H4, 4-MeC6H4, 2-AcC6H4, 2-CO2HC6H4

(b) From diaryl ditellurides and dialkylmercury reagents46,47

ArTeTeAr + R2Hg

dioxane / reflux -Hg (70 -80%)

2 ArTeR

Ar = Ph R = n -C4H9, i -C4H9, Bz

(c) From phenyl tellurocyanate and alcohols48

PhTeTePh

1) NaBH4 2) BrCN

PhTeCN

ROH Bu3P / CH2Cl2

PhTeR (41-78%)

R = n -C8H17, n -C11H23, n -C12H25, n -C14H29, n -C16H33, Ph(CH2)2, Ph(CH2)3, n -C12H25CHCH3

(d) From diphenyl ditellurides and trialkyl boranes49

PhTeTePh

R3B O2 / THF

RTePh (64 -95%)

Selected examples: R = n -C6H13, n -C16H33, c-hex, Br(CH2)5, PhCH2OCO(CH2)4 R3B = n -C6H13B(c-hex)2

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(e) From trimethylsylilphenyl tellurium and organyl halides50 Me3SiTePh + RX

MeCN

RTePh + Me3SiX (60-100%)

X = Br R = sec - C10H21, PhCH2, o -ClC6H4CH2, m -ClC6H4CH2, Ph allyl, cinnamyl, c - hex, t - But (36%),

(48%), N

CH2

CH2 ,

N N N

( f ) From arylethynyl tellurides and aryllithium The protocol shown in Section 3.1.2.7 is also useful to prepare unsymmetrical diaryl tellurides.51 ArTe-C

Ph + Ar1Li

THF -78°C

ArTeAr1 + PhC CLi (44-85%)

Ar = p -Me2NC6H4, p -MeOC6H4 Ar1 = Ph, p -MeOC6H4, p -MeC6H4, p -FC6H4, p -F3CC6H4

(g) From aryltellurium tribromides and arylboronic acids52 ArB(OH)2 + Ar1TeBr3

MeNO2 reflux, 30min

Ar Te Ar1 Br Br

NaHSO3

ArTeAr1 (46 - 54%)

Ar = o -NO2C6H4, m -NO2C6H4 Ar1 = m, m -Me2C6H3

(h) From diaryl ditellurides and ethyldiazoacetate53 PhTeTePh

N2CHCO2Et PhTeCH2CO2Et CuSO4, benzene (70%) reflux

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

Goodman, M. M.; Knapp Jr., F. F. Organometallics 1983, 2, 1106. Suzuki, H.; Nakamura, T. Synthesis 1992, 549. Giua, M.; Cherchi, F. Gazz. Chim. It. 1920, 50, 362. Bowden, K.; Braude, A. E. J. Chem. Soc. 1952, 1076. Pourcelot, G.; Lequan, M.; Simmonin, M. P.; Cadiot, P. Bull. Soc. Chim. Fr. 1962, 1278. Kemmitt, T.; Levanson, W. Organometallics 1989, 8, 1303. Haller, W. S.; Irgolic, K. J. J. Organomet. Chem. 1972, 38, 97. Irgolic, K. J. in Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, p. 389. Georg Thieme, Stuttgart, 1990.

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9. Hope, E. G.; Kemmitt, T.; Levanson, W. Organometallics 1988, 7, 78. 10. Seebach, D.; Beck, A. K. Chem. Ber. 1975, 108, 314. 11. Lederer, K. Ber. Dtsch. Chem. Ges. 1915, 48, 1345; see also Liesk, J.; Schultz, P.; Klar, G. Z. Anorg. AlIg. Chem. 1977, 435, 98. 12. Pierini, A. B.; Rossi, R. A. J. Organomet. Chem. 1979, 168, 163. 13. Pierini, A. B.; Rossi, R. A. J. Org. Chem. 1979, 44, 4667. 14. Uemura, S.; Fukuzawa, S. I.; Yamauchi, T.; Hattori, K.; Mizutaki, S.; Tamaki, K. J. Chem. Soc. Chem. Commun. 1984, 426. 15. Engman, L. Organometallics 1986, 5, 427. 16. Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. J. Organomet. Chem. 1987, 326, 35. 17. Irgolic, K. J.; Busse, P. J.; Grigsby, R. A.; Smith, M. R. J. Organomet. Chem. 1975, 88, 175. 18. Piette, J. L.; Renson, M. Bull. Soc. Chim. Belg. 1970, 79, 353. 19. Clive, D. L.; Chittattu, G. J.; Farina, V.; Kiel, W. A.; Menchen, S. M.; Russel, C. G.; Sing, A.; Wong, C. K.; Curtis, N. J. J. Am. Chem. Soc. 1980, 102, 4438. 20. Knapp, F. F.; Ambrose, K. R.; Callahan, A. P. J. Med. Chem. 1981, 24, 794. 21. Uemura, S.; Fukuzawa, S. J. Am. Chem. Soc. 1983, 105, 2748. 22. Rheinboldt, H. in Houben-Weyl-Methoden der Organischen Chemie (ed. E. Muller). 4th edn, Vol. IX, p. 1102. Georg Thieme, Stuttgart, 1955. 23. Comasseto, J. V.; Ferreira, J. T. B.; Fontanellas, J. A. J. Organomet. Chem. 1984, 277, 261. 24. Comasseto, J. V.; Lang, E. S.; Ferreira, J. T. B.; Simonelli F.; Correia, V. R. J. Organomet. Chem. 1987, 334, 329. 25. Fukuzawa, S. I.; Niimoto, Y.; Fujinami, T.; Sakai, S. Heteroatom. Chem. 1990, 1, 491. 26. Zhang, Y.; Yu, Y.; Lin, R. Synth. Commun. 1993, 23, 189. 27. Bao, W.; Zhang, Y. Synth. Commun. 1995, 25, 1913. 28. Sasaki, K.; Mori, T.; Doi, Y.; Kawachi, A.; Aso, Y.; Otsubo, T.; Ogura, F. Chem. Lett. 1991, 415. 29. Chieffi, A.; Menezes, P. H.; Comasseto, J. V. Organometallics 1997, 16, 809. 30. See Section 3.16.2.1. 31. See Section 4.5.1.1. 32. (a) Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, M. Nippon Kagaku Kaishi 1987, 1469. (b) Zinn, F. K.; Righi, V. E.; Luque, S. C.; Formiga, H. B.; Comasseto, J. V. Tetrahedron Lett. 2002, 43, 1625. 33. Suzuki, H.; Abe, H.; Ohmasa, N.; Osuka, A. Chem. Lett. 1981, 1115. 34. Piette, L.; Thibaut, P.; Renson, M. Tetrahedron 1978, 34, 655. 35. Sadekov, J. D.; Ladatko, A. A.; Minkin, U. I. J. Gen. Chem. USSR 1977, 47, 2194. 36. Evers, M. J.; Christiaens, L. E.; Renson, M. J. J. Org. Chem. 1986, 51, 5196. 37. Petragnani, N. Chem. Ber. 1963, 86, 247. 38. Herberhold, M.; Leitner, P. J. Organomet. Chem. 1987, 336, 153. 39. O⬘Brien, D. H.; Dereu, N., Huang, C. K.; Irgolic, K. J.; Knapp, F. F. Organometallics 1983, 2, 305. 40. Dereu, N.; Piette, I. L. Bull. Soc. Chem. Fr. 1979, 623. 41. Petragnani, N.; Torres, L.; Wynne. K. J. J. Organomet. Chem. 1975, 92, 185. 42. Vicentini, G. ; Giesbrecht E.; Pitombo, L. R. M. Chem. Ber. 1959, 92,40. 43. Dadboub, M.; Dabdoub, V. M.; Comasseto, J. V.; Petragnani, N. J. Organomet. Chem. 1986, 308, 211. 44. Moura Campos, M.; Petragnani, N. Tetrahedron 1982, 18, 527. 45. Luxen, A.; Christiaens, L. Tetrahedron Lett. 1982, 3905. 46. Okamoto, Y.; Yano, T. J. Organomet. Chem. 1971, 29, 99. 47. Vychkova, T. I.; Kalabin, G. A.; Kushnarev, D. F. J. Org. Chem. Russia 1982, 17, 1179. 48. Ogura, F.; Yamaguchi, H.; Otsubo, T.; Chikamatsu, K. Synth. Commun. 1982, 12, 131.

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49. 50. 51. 52. 53.

35

Abe, T., Aso, Y.; Otsubo, T.; Ogura, F. Chem. Lett. 1990, 1671. Yamago, S.; Lida, K.; Yoshida, J. I. Tetrahedron Lett. 2001, 42, 5061. Engman, L.; Stern, D. Organometallics 1993, 12, 1445. Clark, A. R.; Nair, R.; Fronczek, F. R.; Junk, T. Tetrahedron Lett. 2002, 43, 1387. Dabdoub, M.; Guerrero, P. G.; Silveira, C. C. J. Organomet. Chem. 1993, 460, 31.

3.1.4 Diorganyl tellurides by reduction of diorganyltellurium dihalides Diorganyltellurium dihalides are often the primary products in the synthesis of organic derivatives of tellurium and are therefore immediate precursors of the corresponding tellurides. The most employed reducing agents in the “old” tellurium chemistry were alkali sulphites or hydrogen sulphites, methylmagnesium iodide and sodium sulphide hydrate. The last reagent is still used, together with the more recently introduced sodium borohydride, TUDO, hydrazine, samarium diiodide and sodium ascorbate.1 red

RTeR

RTeR

X X

Often the sequence R2Te

X2

R2TeX2

red

R2Te

is employed for isolation and purification purposes, since the dihalides are generally solid, easily recrystallizable compounds, and the two steps cause only a little loss in the telluride yield. Reduction of diorganyltellurium dihalides With sodium sulphide hydrate (general procedure).2 The diorganyltellurium dihalide is mixed with a 15 times molar excess of Na2S·9H2O and the mixture heated at 95–100°C for 10 min or more, until all the solid has melted. Sufficient H2O is added to dissolve the sulphide and then the mixture is filtered if the obtained telluride is a solid, or extracted with a solvent (ether or petroleum ether) if the telluride is a liquid. The products are purified by crystallization or distillation. Yields are high or quantitative (except for diphenyl telluride or di-p-tolyl telluride). With sodium borohydride – tetrahydrotellurophene (typical procedure.3 NaBH4 is added to a boiling methanolic solution of diiodotetrahydrotellurophene until the orange colour disappears. The solution is filtered and the filtrate is poured into H2O (1 L). Extraction with ether, followed by drying (CaCl2) and evaporation, gives the product as a yellow oil with a persistent odour (b.p. 165–167°C/760 mmHg; no yield was given). With TUDO (general procedure).4 A mixture of diorganyltellurium dihalide (2 mmol) and 2 N NaOH (10 mL) is stirred at room temperature for 15 min. TUDO (0.432 g, 4 mmol)

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and petroleum ether (30–60°C, 10 mL) are then added and the two-phase system is stirred at room temperature until the mixture turns clear. The phases are separated and the aqueous phase is extracted with ether (3⫻). The combined organic phases are dried (MgSO4) and the solvent is evaporated. The residue is placed on an SiO2 column and eluted with petroleum ether giving the telluride (dibutyl, didodecyl, butyl phenyl, and di-4-methoxyphenyl telluride; yields 77–90%). With hydrazine – di-p-anisyl telluride (typical procedure).5 p-Anisyltellurium dichloride (8.3 g, 0.02 mol) suspended in EtOH/H2O (150:15) is heated under reflux, and hydrazine (3.2 g, 0.1 mol) is added dropwise (vigorous evolution of N2). When further addition of hydrazine no longer causes evolution of N2, the mixture is poured into H2O (700 mL), and extracted with ether (2⫻300 mL). The extracts are washed with H2O, dried and evaporated, giving the telluride. The product is recrystallized from MeOH (at ⫺25°C) (5.2 g (77%); m.p. 57°C). 3.1.5

Additional methods

Sodium ascorbate6 and samarium diiodide7 have also been used as reducing agents. Reduction of diaryltellurium dichlorides with sodium ascorbate (typical procedure).6 Bis (p-methoxyphenyl)tellurium dichloride (0.20 g, 0.48 mmol) dissolved in acetone (10 mL) was added to a stirred solution of sodium ascorbate (0.20 g, 1.0 mmol) in water/methanol (2⫹8 mL). After 24 h, water (50 mL) and CH2Cl2 (50 mL) were added and the two phases separated. The organic phase was dried (CaCl2) and the solvent evaporated in vacuo. Flash chromatography yielded 0.14 g (84%) of bis(p-methoxyphenyl) telluride. Reduction of diaryltellurium dichlorides with samarium diiodide (typical procedure).7 Diaryl tellurium dichloride (1 mmol) was added to the deep blue solution of SmI2 (2.2 mmol) in THF (22 mL) at room temperature under nitrogen with stirring. The deep blue colour of the solution disappeared immediately and became yellow. The resulting solution was stirred at room temperature under nitrogen for 30 min. To the solution was added dilute hydrochloric acid, and the mixture was extracted with ether. The ethereal solution was washed with brine and dried over MgSO4. The solvent was evaporated in vacuo, and the residue was purified by preparative TLC on silica gel (petroleum ether–methylene dichloride as eluent).

REFERENCES 1. For an extensive coverage on the reduction of diorganyltellurium dichloride see: Rheinboldt, H. in Houben-Weyl-Methoden der Organishen Chemie (ed. E. Muller). 4th edn, Vol. IX, p. 1068. Georg Thieme, Stuttgart, 1955. Irgolic, K. J. in Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. El2b, p. 426. Georg Thieme, Stuttgart, 1990. 2. Reichel L.; Kirschbaum, E. Chem. Ber. 1943, 76, 1105. 3. AI-Rubaie, A. Z.; Alshirayda, H. A. Y. J. Organomet. Chem. 1985, 287, 321.

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Lang, E. S.; Comasseto, J. V. Synth. Commun. 1988, 18, 301. Bergman, J. Tetrahedron 1972, 28, 3323. Engman, L.; Persson, J. Synth. Commun. 1993, 23, 445. Jia, X.; Jin, P.; Zhang, Y.; Zhou, X. Synth. Commun. 1995, 25, 253.

3.2

DIORGANYL DITELLURIDES

Three main routes have been well established for the preparation of diorganyl ditellurides: (1)

The reaction of sodium ditelluride with alkylating or arylating agents. NaTeTeNa + RX

(2)

The oxidation of tellurols or tellurolate anions. RTeH (RTe-)

(3)

RTeTeR

ox.

RTeTeR

The reduction of organyltellurium trichlorides. RTeCl3

red.

RTeTeR

R = alkyl, aryl

3.2.1 3.2.1.1

From sodium ditelluride From sodium ditelluride and alkylating agents

This is the most direct route to diorganyl ditellurides and therefore parallels the route leading to diorganyl tellurides, substituting sodium telluride for sodium ditelluride. Sodium ditelluride is prepared employing, with the appropriate ratio of the elements, methods analogous to those described for sodium telluride. (a) Liquid ammonia method Dimethyl ditelluride (typical procedure).1 Clean Na metal (3.2 g, 0.14 mol) is added to 100 mL of liquid NH3 at ⫺78°C. After stirring for 1 h, high-purity powdered Te (18.2 g, 0.14 mol) is added in 0.5 g portions. Methyl iodide (24 g, 0.17 mol) is then added dropwise for 20 min, with stirring, to the dark green solution. The NH3 is evaporated, H2O is added to the residue and the mixture extracted with ether (4⫻50 mL). The combined deep red extracts are dried overnight (CaCl2), and evaporated under vacuum. The residue is then distilled (7.6 g (38%); b.p. 97°C/9 torr). Diethyl ditelluride and dibenzyl ditelluride are prepared similarly2 in yields of 71% and 82%, respectively.

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(b) Sodium naphthalene method Dialkyl ditellurides (general procedure).3 A mixture of powdered Te (3.58 g, 28 mmol), Na chips (0.65 g, 28 mmol) and naphthalene (0.36 g, 2.8 mmol) in anhydrous THF (25 mL) is refluxed under N2 and stirred for 1 h. During this time all the sodium is consumed and the mixture turns a light brown colour. The solution is stirred for an additional 3 h to ensure the complete reduction of Te, the temperature is then lowered to 10°C and the alkyl halide (28 mmol) is added dropwise for 30 min with stirring. After an additional hour of stirring at room temperature, the reaction mixture is filtered, the solvent evaporated and the residue distilled under vacuum, giving the pure ditelluride (R⫽Et (85%), n-Pr (90%), n-Bu (90%), MeOCH2CH2 (60%)). (c) Sodium hydride/DMF method Bis(4-carbomethoxy)butyl ditelluride (typical procedure).4 Powdered Te (1.27 g, 10 mmol), NaH (0.44 g, 11 mmol) and dry DMF (50 mL) are stirred at 70°C under argon for 3 h. The purple Na2Te2 solution is cooled at room temperature, and methyl 5-bromovalerate (2.15 g, 11 mmol) in dry argon-purged DMF (10 mL) is added. The mixture is stirred at room temperature for 1 h, cooled and poured into H2O (100 mL) and extracted several times with ether. The combined extracts are washed thoroughly with H2O, dried (Na2SO4) and evaporated under vacuum, giving a dark orange oil. The crude ditelluride is chromatographed on SiO2 (basic, 125 g eluting with CHCl3). Dark orange oil (1.38 g (53%)); single component on TLC (Rf ⫽0.56 in CHCl3). (d) Rongalite method Dialkyl ditellurides (general procedure)5 Rongalite (9.24 g, 0.08 mol) and NaOH (9.0 g, 0.23 mol) are dissolved in distilled H2O (200 mL). The apparatus is flushed with N2 and finely powdered Te (15.3 g, 0.12 mol) is added. The mixture is stirred for 5 h and then the alkyl bromide (0.12 mol) is added dropwise for 30 min with cooling and stirring. The solution is extracted with CCl4 (3⫻100 mL), and the extract is dried (CaCl2) and distilled under vacuum, giving the dark red, foul-smelling ditelluride (R⫽Et, n-Bu, n-pentyl, n-dodecyl).5,6 (e) TUDO method Dialkyl ditellurides (general procedure).7 A mixture of powdered Te (0.128 g, 1 mmol) and cetyltrimethylammonium bromide (CTAB, 0.004 g, 1.1⫻10⫺5 mmol) in THF (0.75 mL) and DMSO (0.5 mL) is purged with a flux of deoxygenated N2 for 15 min and then heated at 80°C. TUDO (0.1 g, 1 mmol) and NaOH (0.112 g, 2.6 mmol) in H2O (0.75 mL) are added. The mixture is refluxed for 1 h and then the purple solution is cooled at 15°C. The alkyl halide (2 mmol) is added and the mixture is stirred at room temperature for 1 h. Normal work-up and filtration through a pad of Celite® (using CH2Cl2 as the mobile phase) give the ditellurides as dark red oils (R⫽n-C11H23, n-C8H17, Me2CHCH2CH2, 2-heptyl, THPO(CH2)6 (93–98%)).

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( f ) Sodium borohydride method Dioctyl ditelluride (typical procedure)8 Na (0.10 g, 4 mmol) in small pieces is added to a suspension of Te (0.50 g, 4 mmol) in absolute ethanol (50 mL), under N2 and with stirring. When the Na is dissolved, NaBH4 (0.075 g, 2 mmol) is added and the mixture heated under N2 for 2 h, using a heating mantle. The mixture is cooled at room temperature and neat octyl bromide (0.72 g, 4 mmol) is slowly added while stirring and cooling in an ice bath. After stirring for 30 min, the mixture is poured into distilled H2O (100 mL), extracted with ether (2⫻40 mL) and the combined extracts are dried. The solvent is evaporated under vacuum and the residue is recrystallized from ether (by cooling at ⫺78°C) (0.77 g (85%); m.p. 9°C). (g) Hydrazine hydrate method Dialkyl ditellurides (general procedure).9 Powdered Te (6.35 g, 50 mmol) is added to a stirred solution of NaOH (3.0 g, 75 mmol) in deoxygenated H2O (20 mL). The mixture is cooled in a water bath, and 100% hydrazine hydrate (2.5 g, 200 mmol) is added over a period of 30 min and stirring is continued for an additional hour at room temperature. The alkyl halide (50 mmol) is then added dropwise over a period of 2–3 h. During the addition, the temperature is maintained at 15–20°C. The end-point of the alkylation is indicated by a sharp colour change from dark brown to nearly colourless. The mixture is then extracted with ether, the organic layer is washed with H2O, dried (Na2SO4) and the solvent removed by slow distillation. The residue is distilled under vacuum. 3.2.1.2

From sodium ditelluride and aryl halides

The low reactivity of aryl halides towards nucleophilic reagents makes their reaction with sodium ditelluride unattractive for the preparation of diaryl ditellurides. Low yields are obtained, like the similar reaction with sodium telluride (see Section 3.1.2.1). Na2Te2 + 2ArX

HMPA ArTeTeAr DMF (5 - 40%)

Ar = Ph, 1- naphthyl, 2 - naphthyl, 9 -antracenyl

Di-2-naphthyl ditelluride (typical procedure).10 2-Chloronaphthalene (2 mol equiv) is added to Na2Te2 (prepared by heating equimolar amounts of Te and Na in HMPA under N2). The mixture is heated at 130–170°C for 16–24 h (yield 20%; m.p. 116–118°C, recrystallized from hexane). This method was later revised replacing sodium ditelluride by potassium ditelluride.11 K2Te2

RTeTeR Te/ KOH/ H2NNH2 45, 38% 80- 90°C, 2 h R = Et, Me

RX

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By oxidation of organotellurols or organotellurolates

Organyl tellurolates, obtained by the insertion of elemental tellurium in organyllithium (or magnesium) reagents (used as starting materials to prepare unsymmetrical tellurides as shown in Section 3.1.3.2), can be oxidatively converted into the corresponding ditellurides. This general method can be performed by direct oxidation of the tellurolates or through a previous hydrolysis to the tellurols. RMet

Te

R = alkyl, aryl Met = Li(MgX)

RTeMet H+

air RTeH

air

RTeTeR

The Grignard route to diorganyl ditellurides suffers from lack of generality and the mechanism of the oxidation seems to be uncertain. Alkylmagnesium halides demonstrate lack of reactivity towards elemental tellurium,12 whereas arylmagnesium halides in ether as the solvent furnish a mixture of ditellurides and tellurides.13 Satisfactory results are obtained by tellurium insertion in arylmagnesium halides in THF followed by oxidation before or after aqueous work-up.12,14,15 Bis(m-methoxyphenyl) ditelluride (typical procedure).14 To a stirred solution of m-bromoanisole (3.74 g, 20 mmol) in dry THF (80 mL) under N2 are added Mg turnings (0.49 g, 20 mmol) at room temperature. The mixture is heated under reflux until most of the Mg is dissolved. The Grignard reagent solution is cooled to room temperature and well-powdered Te metal is added. After 0.5 h most of the metal will have dissolved. The reaction mixture is then saturated with dry air at room temperature. The m-methoxyphenyl ditelluride is obtained after routine work-up as a red oil (2.96 g (63.0%)). Bis(p-methoxyphenyl) ditelluride (typical procedure).15 To a solurion of p-methoxyphenylmagnesium bromide (prepared from p-bromoanisole (5.82 g, 0.0311 mol) and Mg (1.0 g, 0.042 mol) in THF (20 mL)) is added Te shot (3.81 g, 0.0300 mol) at room temperature. The mixture is stirred under reflux for 3 h and then cooled to 0°C and treated with a saturated solution of NH4Cl (20 mL; vigorous evolution of gas). The mixture is filtered through Celite® and the solids washed with saturated solution of NH4Cl and ether. The organic phase is washed with brine and dried with Na2SO4. Evaporation of the solvent and recrystallization from CHCl3/petroleum ether affords the pure product (5.16 g (75%); m.p. 57–59°C). Early experiments to prepare diaryl ditellurides, employing aryllithium reagents generated by lithium–halogen exchange using ether as the solvent, gave only modest yields.16,17 At present the most successful and general route to dialkyl and diaryl ditellurides employs the telluration of organyllithium compounds in THF. Di-n-butyl ditelluride (typical procedure).18 n-Butyllithium (8.0 mL, 2.15 M in hexane; 17.2 mmol) is added dropwise at room temperature under N2 to a stirred suspension of finely ground elemental Te (1.9 g, 14.9 mmol) in dry THF (50 mL). After 15 min all the Te has been consumed, and the resulting yellowish solution is diluted with H2O (200 mL).

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Air is then bubbled through it to oxidize the lithium tellurolate. The red oily ditelluride that separates is extracted with ethyl ether (200 mL). The organic phase is washed with H2O (2⫻50 mL), dried (CaCl2) and evaporated to give 2.83 g of a viscous red oil. Distillation of 5.66 g of the crude material prepared in this manner affords 4.87 g of pure di-n-butyl ditelluride (89% based on Te; b.p. 103–105°C/0.8 torr). Di-t-butyl ditelluride is prepared in the same manner. Diphenyl ditelluride (typical procedure).19 To a suspension of powdered Te (25.5 g, 0.2 mol) in anhydrous THF (300 mL), 2.0 M phenyllithium (100 mL, 0.2 mol) is added dropwise, with stirring and under argon. The resultant mixture is stirred at room temperature for 2 h and under reflux for 1 h. The reaction mixture, which should contain only a small amount of unreacted Te, is allowed to cool to 20°C, and is poured into 1 L of H2O. Oxygen (or air) is bubbled for 1 h through the mixture, which is then extracted with benzene (100 mL). The benzene solution is washed three times with H2O, dried (Na2SO4) and evaporated. The residue is recrystallized from EtOH (77%; m.p. 63.8–65.0°C). Di-2-thienyl and di-2-furyl ditelluride are prepared from thiophene and furan by lithiation with n-buthyllithium, tellurium insertion and oxidative work-up.17 Di-2-thienyl ditelluride (typical procedure).17 n-Butyllithium (11.0 mL, 2.2 M, 24.2 mmol) is added to an ice-cooled stirred solution of thiophene (2.0 g, 23.8 mmol) in dry THF (50 mL). After 10 min at 0°C and 50 min at room temperature, elemental Te (2.9 g, 22.7 mmol) is added rapidly. All Te is completely dissolved after 30 min, when the yellowish solution is poured into a beaker containing H2O (300 mL). CH2Cl2 (200 mL) is then added and air passed through the two-phase system for 1 h. To effect complete oxidation, the beaker is left overnight in the open air. The organic phase is separated and the aqueous phase extracted several times with CH2Cl2. The combined organic extracts are dried (CaCl2) and evaporated to give a red solid. Recrystallization from EtOH affords 3.54 g (74%) of di-2-thienyl ditelluride (m.p. 89–90°C). The preparation of aryllithium reagents can also be performed by using t-butyllithium in a halogen–metal exchange, and aqueous potassium ferricyanide as an oxidant.17,20 Bis(p-methoxyphenyl) ditelluride (typical procedure).20 t-Butyllithium (10 mL, 1.7 M, 17.0 mmol) is added dropwise to a stirred solution of p-bromoanisole (1.59 g, 8.5 mmol) in THF (40 mL) at ⫺78°C under argon. After 1 h the cooling bath is removed and the mixture allowed to warm to room temperature for 30 min. Finely ground Te (1.08 g, 8.5 mmol) is then added rapidly while a brisk stream of argon is passed through the open system to prevent admission of air. After 1 h, when only traces of Te remain, the mixture is poured into a separating funnel containing K3Fe(CN)6 (2.80 g, 8.5 mmol) in H2O (150 mL). The ditelluride product is extracted with CH2Cl2 (100 mL ⫹ 2⫻50 mL). After drying of the combined extracts (CaCl2), evaporation on SiO2 (10 g) and flash chromatography (SiO2; hexane/CH2Cl2, 8:2) give the ditelluride (0.99 g (50%); m.p. 58–59°C). The material is recrystallized from EtOH. The following diaryl ditelluride can be prepared in the same manner as in the foregoing procedure: Ar⫽2-pyridyl, Ph and (halogen and Me) derivatives, p-(Me2N)C6H4, 1-naphthyl, 4-biphenylyl (50–84%).

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The direct oxidation of lithium aryl tellurolates into the ditellurides without previous aqueous work-up is exemplified by the preparation of di(2,4,6-tri-t-butylphenyl) ditelluride.21 The arylation of sodium telluride in NMP (see Section 3.1.2.1) followed by air oxidation gives rise to diaryl ditellurides in medium to good yields.22 Te

NaH NMP, 100 -110°C, 1 h

Na2Te

ArI (0.5 equiv)

air ArTeTeAr benzene, r.t. (51 -71%)

ArTe-

NMP, 120°C , 5 h

Ar = Ph and (methyl and methoxy) derivatives, 1- naphthyl, 2 - naphthyl

Diphenyl ditelluride (typical procedure).22 Iodobenzene (130 mg, 0.64 mmol) is added dropwise to a stirred solution of Na2Te (1.3 mmol) in NMP (4 mL) and the mixture is heated at 120°C for 5 h. After cooling, the mixture is diluted with benzene (5 mL), and a stream of air is bubbled into the dark solution. Free Te which separates is removed by filtration and the filtrate is worked up as usual to give the crude ditelluride, which is purified by recrystallization from a mixture of hexane and EtOAc (3:1) to give the pure product (90 mg (66%)). 3.2.3

By reduction of organotellurium trichlorides

The reduction of organyltellurium trichlorides, which are the primary products of several reactions involving tellurium tetrachloride (see Section 3.5.1), is a useful and general method for the preparation of diorganyl ditellurides.

TeCl4

organic substrate or metallo - organic reagent

red

RTeCl3

RTeTeR

Some examples of these reductions are given below. 3.2.3.1

Reduction of -carboxyalkyltellurium trichlorides

-Carboxylalkyltellurium trichlorides, obtained by the reaction of tellurium tetrachloride with carboxylic acid anhydrides, are reduced to the corresponding ditellurides with potassium hydrogen sulphite.23,24 R TeCl4 + R

Cl3Te

CO)2O R HO2C

R

Te CO2H Cl Cl

R KHSO3 CO2H H2O KHSO3 H2O

HO2C R

HO2C

)2Te

R Te Te

CO2H

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3.2.3.2 Reduction of -alkoxyalkyltellurium trichlorides The trichlorides obtained by the alkoxytellurination of alkenes (see Section 3.5.1.2) or by the cyclotellurination of hydroxyalkenes (see Section 4.6.2.4) are reduced in situ, without isolation, with sodium metabisulphite.25 OR1 OR1 TeCl3 Na2S2O5 Te Te R R R R H2O R (yield %): n - Bu (58), s - Bu (22), i - Bu (29), n - hexyl (72), n - decyl (72), n-C14H29 (50), HOC9H15 (45), HOC10H20 (41), Bz (80), Ph (18); R1 = Me, R = n - decyl (55), Bz (65); R1 = Et, R = n - decyl (23); R1 = i - Pr OR1

TeO2 / R1OH / HCl

R

R

R

R

same conditions OH

R

O

TeCl3

62, 83 % R1

R O

Te Te

O

R1

R1 = R = H,

Bis(2-methoxy-3-phenylpropyl) ditelluride (typical procedure).25 TeO2 (2.0 g, 12.5 mmol) is dissolved in concentrated HCl (10 mL) and the yellow solution diluted with MeOH (40 mL). Allyl benzene (1.75 g, 14.8 mmol) is added and the mixture heated under reflux for 24 h and then allowed to cool to room temperature. The yellowish solution (containing a trace of elemental Te) is shaken in a separating funnel with H2O/CH2Cl2 (100 mL/100 mL) containing Na2S2O5 (5 g). The red CH2Cl2 phase is separated, dried and evaporated, giving a red oil which is purified by flash chromatography (SiO2/CH2Cl2) to give the ditelluride (2.78 g (80%)). 3.2.3.3

Reduction of aryltellurium trichlorides

Aryltellurium trichlorides (see Sections 3.5.1.3 and 3.5.1.4) are easily reduced by means of several reducing agents. The most popular reduction procedures are described in sequence. (a) By hydrated sodium sulphide This reagent, together with potassium hydrogen sulphite,25 has been used to reduce aryltellurium trichlorides26,27 since the early period of tellurium chemistry, and continues to find wide application. Diaryl ditellurides (general procedure).27 The aryltellurium trichloride is poured into 15 mol equiv of Na2S·9H2O heated (and melted) at 95–100°C in a beaker, with strong manual stirring. An exothermic reaction occurs. After 10 min at the same temperature, excess H2O is added. By cooling the ditelluride solidifies, and is collected by decantation or filtration. The yields are near to quantitative.

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The following ditellurides can be prepared by the foregoing method28–31: Ar⫽Ph, p-MeOC6H4, p-EtOC6H4, p-PrOC6H4, p-BuOC6H4, p-PhOC6H4, p-PhSC6H4, p-ClC6H4, p-BrC6H4, m-FC6H4, p-Me02C, p-Et02CC6H4, 3,4-(MeO)2C6H3, 1-naphthyl, 2-naphthyl. (b) By TUDO Diaryl ditellurides (general procedure).32 A mixture of the aryltellurium trichloride (2.0 mmol) and 2 N NaOH (5 mL) is vigorously stirred at room temperature for 15 min (at that time the trichloride has been converted into the corresponding sodium tellurinate). TUDO (0.43 g, 4.0 mmol) and petroleum ether (30–60°C, 5 mL) are added to the mixture, and stirring at room temperature is continued for 15 min. The phases are separated and the organic layer is dried (MgSO4). The solvent is evaporated, giving the ditelluride (Ar⫽Ph (92%), p-MeOC6H4 (94%)). The corresponding aryltellurium tribromides are reduced similarly. (c) By hydrazine hydrate Di-p-tolyl ditelluride (typical procedure).33 A solution of hydrazine hydrate (3.2 g, 0.1 mol) in EtOH (20 mL) is added slowly dropwise to a refluxing solution of p-tolyltellurium trichloride (3.25 g, 10 mmol) in EtOH until evolution of N2 ceases. The hot mixture is filtered, and the ditelluride crystallizes by cooling (0.96 g (44%; m.p. 52°C). In the same manner the following ditellurides can be prepared: Ar⫽Ph, p-MeC6H4, p-EtOC6H4, p-Br-C6H4, m-MeC6H4, p-MeOC6H3 (45–84%). (d) Additional methods Sodium ascorbate34 has also successfully employed as reducing agents. Reduction of aryltellurium trichlorides with sodium ascorbate (typical procedure).34 p-Methoxyphenyltellurium trichloride (0.30 g, 0.88 mmol) was added to a stirred solution of sodium ascorbate (0.53 g, 2.7 mmol) in water/methanol/acetone (1 mL + 5 mL + 5 mL). After 5 h, water (50 mL) and CH2Cl2 (50 mL) were added and the two phases separated. The organic phase was dried (CaCl2) and the solvent evaporated in vacuo. Flash chromatography (SiO2; CH2Cl2) yielded 0.20 g (97%) of bis(p-methoxyphenyl) ditelluride. 3.2.4

Diaryl ditellurides from aryl boronic acids

Aryl boronic acids treated with TeCl4 generate aryltellurium trichlorides which are reduced to diaryl ditellurides without prior isolation.35 TeCl4

NaHSO3 ArTeCl3 ArTeTeAr MeNO2 (60 - 95%) reflux, 30 min Ar = Ph, o -NO2C6H4, o -ClC6H4, m -NO2C6H4, p -MeC6H4

ArB(OH)2

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45

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

Chen, M. T.; George, J. W. J. Organomet. Chem. 1968, 12, 401. Bogolyubov, G. M.; Slyk, Y. N.; Petrov, A. A. J. Gen. Chem. USSR 1969, 39, 1768. Bhasin, K. K.; Gupta, V.; Gautam, A.; Sharma, R. P. Synth. Commun. 1990, 20, 2191. Goodman, M. M.; Knapp, F. F. J. Org. Chem. 1982, 47, 3004. Davies, J.; McWhinnie, W. R.; Dance, N. S.; Jones, C. H. W. Inorg. Chim. Acta 1978, 29, L217. Uemura, S.; Takahashi, M.; Ohe, K.; Sugita, N. J. Organomet. Chem. 1989, 361, 63. Ferreira, J. T. B.; Oliveira, A. R. M.; Comasseto, J. V. Synth. Commun. 1989, 19, 239. Lee, W.; M. S. Thesis. Texas A&M University (1986). Irgolic. K. J. in Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, p. 259. Georg Thieme, Stuttgart, 1990. Bhasin, K. H.; Gautam, A. Phosphorus Sulfur 1988, 38, 211. Sandman, D. J.; Stark, J. C.; Acampora, L. A.; Gagne, P. Organornetallics 1983, 2, 549. Vvedenskii, V. Yu.; Deryagina, E. N.; Trofimov, B. A. Russ. J. Gen. Chem. 1996, 66, 1539. Haller, W. S.; Irgolic, K. J. J. Organomet. Chem. 1972, 38, 97. Petragnani, N.; Moura Campos, M. Chem. Ber. 1963, 96, 249. Akiba, M.; Lakshmikantham, M. V.; Jen, K.; Cava, M. P. J. Org. Chem. 1984, 49, 4819. Detty, M. R.; Murray, B. J.; Smith; D. J.; Zumbulyadis, N. J. Am. Chem. Soc. 1983, 105, 875. Piette, J. L.; Renson, M. Bull. Soc. Chim. Belg. 1970, 79, 353. Engman, L.; Cava, M. P. Organometallics 1982, 1, 470. Engman, L.; Cava, M. P. Synth. Commun. 1982, 12, 163. Seebach, D.; Beck, A. K. Chem. Ber. 1975, 108, 314. Engman, L.; Persson, J. J. Organomet. Chem. 1990, 388, 71. Lange, L.; Du Mont, W. W. J. Organomet. Chem. 1985, 286, C1. Suzuki, S.; Nakamura, T. Synthesis 1992, 549. Morgan, G. T.; Drew, H. D. K. J. Chem. Soc. 1925, 531. Morgan, G. T.; Kellett, R. E. J. Chem. Soc. 1926, 1080. Engman, L. Organometallics 1989, 8, 1997. Reichel, L.; Kirschbaum, E. Annals 1936, 523, 211. Reichel, L.; Kirschbaum, E. Ber. Dtsch. Chem. Ges. 1943, 76, 1105. Petragnani, N.; Vicentini, G. (unpublished results). Petragnani, N. Tetrahedron 1960, 11, 15. Vicentini, G.; Giesbrecht, E.; Pitombo, L. R. M. Chem. Ber. 1959, 92, 40. Sadekov, I. D.; Sayapina, L. M.; Bushkov, A. Y.; Minkin, V. I. J. Gen. Chem. USSR 1971, 41, 2747. Lang, E. S.; Comasseto, J. V. Synth. Commun. 1988, 18, 301. Bergman, J. Tetrahedron 1972, 28, 3323. Engman, L.; Persson, J. Synth. Commun. 1993, 23, 445. Clark, A. R.; Nair, R.; Fronczek, F. R.; Junk, T. Tetrahedron Lett. 2002, 43, 1387.

3.3

ORGANYL TELLUROLS

Organyl tellurols are very unstable compounds owing to their extreme sensitivity to oxygen, giving the corresponding ditellurides. The first short-chain alkyltellurols (C1–C4) have been isolated as yellow liquids with an obnoxious odour, from the reaction of aluminium telluride and hydrogen telluride, respectively, with alcohols and alkyl bromides.1 Aryltellurols seem not to have been isolated. As shown in Sections 3.1.3.2 and 3.2.2, aryl tellurolates are

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starting materials for the preparation of aryl tellurides and diaryl ditellurides and, consequently, aryltellurols find wide use in the manipulation of several organic functionalities. Because of their intractability, these reagents are generated in situ, the most appropriate methods being described in several sections in the text. REFERENCE 1. (a) Irgolic, K. J. in Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, p. 152. Georg Thieme, Stuttgart, 1990. (b) Rheinboldt, H. in Houben-Weyl-Methoden der Organishen Chemie. Vol. IX, p. 970, Georg Thieme, Stuttgart, 1955.

3.4

BIS-ORGANYL TELLUROMETHANES

Bis-organyl telluromethanes or organyl telluroketals, can be prepared by the conventional method involving the reaction of 2 equiv of organyl tellurolates with dihalomethanes. 2RTeMet + XCH2X

RTeCH2TeR

R = Me; Met = Li; X = Cl (ref. 1) R = Ph; Met = Li; X = I (ref. 1, 2) R = p -EtOC6H4; Met = Na; X = I (ref. 3) R= S Met = Na; X = Cl (ref. 4)

N Ts

A more convenient method due to the mild experimental conditions and high yields involves the reaction of diaryl ditellurides with diazomethane.5

ArTeTeAr

CH2N2 ether, 0°C (100%)

ArTeCH2TeAr

Ar = p -MeOC6H4, p -EtOC6H4

Diaryl diselenide and diaryl disulphide react analogously. Reaction of diaryl ditellurides with diazomethane-formation of diaryl telluroketals (typical procedure).5 Di-p-methoxyphenyl ditelluride (2.34 g, 5 mmol) in ether (30 mL) is treated dropwise, at 0°C and while stirring, with an ethereal solution of diazomethane until disappearance of the red colour of the ditelluride. Evolution of N2 ensues. The product is obtained by evaporation of the solvent. Recrystallization from hexane or EtOH gives colourless crystals (2.41 g (100%); m.p. 98–99°C). Telluroketals exhibit synthetic interest since they easily generate methyllithium derivatives by treatment with alkyllithium compounds (see Section 4.8.1.3).

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47

REFERENCES 1. 2. 3. 4. 5.

Hope, E. G.; Kemmitt, T.; Levanson, W. Organometallics 1988, 7, 78. Seebach, D.; Beck, A. B. Chem. Ber. 1975, 108, 314. Pathirane, H. M. K. K.; McWhinnie, W. R. J. Chem. Soc. Dalton Trans. 1986, 2003. Engman, L.; Cava, M. P. Organometallics 1982, 1, 470. Petragnani, N.; Schill, G. Chem. Ber. 1970, 103, 2271.

3.5

ORGANYLTELLURIUM TRIHALIDES

Organyltellurium trichlorides, tribromides and triiodides are well-known compounds, the trichlorides being the most familiar and useful for synthetic purposes. Organyltellurium trichlorides are easily accessible, exploiting the electrophilic character of tellurium tetrachloride by the direct introduction of the –TeCl3 moiety in organic substrates. Otherwise, organyltellurium trichlorides as well as the tribromides and triiodides, seldom generated by the preceding method, can be obtained by the halogenolysis of the corresponding ditellurides.

TeCl4

organic substrate

RTeTeR

X2

Organic substrate

TeCl3

RTeX3

X = Cl, Br, I

3.5.1 Organyltellurium trichlorides from tellurium tetrachloride and organic substrates 3.5.1.1

From tellurium tetrachloride and ketones and carboxylic anhydrides

Reactions involving the attack of a nucleophilic carbon atom activated by an -carbonyl group at the electrophilic tellurium atom of tellurium tetrachloride belong to the oldest methods of preparative organic tellurium chemistry. Depending on the structure of the ketone, different regiosubstituted trichlorides are obtained, in addition to the corresponding dichlorides.1,2 O R O

O R

TeCl3

TeCl3

+ TeCl4 R R

O Cl Cl O Te (and isomers)

R

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Acetic and propionic anhydrides furnish compound of type RCH(TeCl3)CO2H (R⫽H, Me) 3.5.1.2

From tellurium tetrachloride and alkenes

The addition of tellurium tetrachloride to alkenes produces chloroalkyltellurium trichlorides. Butenes, 1-decene, cycloalkenes, and 3-substituted 1-propenes give mixtures of syn and anti additions.3–6 R

R

H

H

Cl + TeCl4

CHCl3

R

TeCl3 H H

R

Cl +

R

TeCl3 H R

H

-Chloroalkyltellurium trichlorides (general procedure).5 Freshly sublimed TeCl4 (1.67 g, 6.2 mmol) and the olefin (6.4 mmol) are stirred in an ice bath for 3 h in dry, ethanol-free CHCl3 (20 mL), until almost all the TeCl4 has disappeared. Filtration and evaporation yield the adduct as a mixture of isomers (percentage yield, sin/anti ratio): (Z)-2-butene (67, 20/80), (E)-2-butene (94, 42/58), cyclopentene (98, 50/50), cyclohexene (99, ⬎97% anti), cyclooctene (83, >97% syn), 1-decene (98%), (E)-1-D-1-decene (96%). A regiospecific concerted syn addition (path (a)) competing with a radical chain reaction (path (b)) has been proposed to rationalize the addition of TeCl4 to olefins. R

a)

R

R

+ TeCl4

R H Cl

R Cl TeCl3

b)

TeCl3 . + Cl.

TeCl4 R

R H TeCl3

R

R

+ TeCl3 .

R .

TeCl4

R Cl

TeCl3

R TeCl3

p-Benzoquinone, a radical scavenger, has been shown to be highly effective to promote the syn addition in the above reactions. Depending on the TeCl4/alkene ratio and the polarity of the solvent, bis(chloroalkyl)tellurium dichlorides can be formed (see Section 3.9.2.2). The alkoxytrihalotellurination of alkenes or cycloalkenes is afforded by treatment with tellurium dioxide/hydrochloric acid/alcohol7 or with tellurium tetrachloride (or tetrabromide)/alcohol in carbon tetrachloride.8 The addition is regiospecific, the tellurium moiety bonding to the less hindered carbon atom, and stereospecific as demonstrated by the anti addition to cycloalkenes. By the first procedure the trichloride is reduced without isolation to the corresponding ditelluride (see Section 3.2.3.2). OR1 R

TeO2 / HCl /R1OH

R

OR1

OR1 TeCl3

red.

R

Te Te

R

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( )n

+ TeX4

49

TeX3

ROH CCl4

( )n OR

X = Cl, Br n = 1, 2

Trans-2-ethoxycyclohexyltellurium trichloride (typical procedure).8 Cyclohexene (1.6 g, 19.5 mmol) is added to TeCl4 (5.0 g, 18.6 mmol) in CCl4 (50 mL) and EtOH (2.76 g, 60 mmol). The mixture is heated under reflux for 2 h, cooled, filtered to remove traces of elemental Te, shaked with charcoal and refiltered. Evaporation under vacuum and recrystallization from petroleum ether (30–60°C) give the pure product ((63%); m.p. 97–98°C). The following trans-2-alkoxycycloalkyltellurium trihalides are prepared similarly: n⫽1, 2; X⫽Cl, Br; R⫽MeO, n-Pr, i-Pr. It must be pointed out, however, that the addition of tellurium tetrachloride to olefins lacks generality, and undefined results have been mentioned such as formation of tarry products, reduction to elementary tellurium, or no reaction at all.9 3.5.1.3

From tellurium tetrachloride and arenes

The condensation of tellurium tetrachloride with aromatic compounds is of general application and has long been used as an efficient route for the preparation of aryltellurium trichlorides. In accordance with a typical electrophilic aromatic substitution, this reaction is successfully accomplished by simply treating aromatic compounds activated by electrondonating groups, such as alkoxy, phenoxy, hydroxyl and thiophenoxy moieties, with tellurium tetrachloride in refluxing chloroform, carbon tetrachloride or toluene.10–15

R

+ TeCl4 solvent, 85 - 96%

TeCl3 R

+ HCl

representative examples: R = p - MeO, p -EtO, p -PhO, p -HO

p-Methoxyphenyltellurium trichloride (typical procedure).13 Anisol (5.4 g, 50 mmol) and TeCl4 (13.5 g, 50 mmol) in CCl4 (40 mL) are heated under reflux for 2 h. Evolution of HCl occurs and the heavy crystalline TeCl4 is progressively converted into yellow flakes of trichloride. The product is filtered off and washed with CCl4. (15.5 g (91%)). The crude product is recrystallized from glacial HOAc as yellow needles (m.p. 182°C). Non-activated arenes react only under more severe conditions,16,17 or in the presence of a Lewis acid.18 In the last case, however, the reaction does not stop at the trichloride step, and is more appropriate for the preparation of diaryltellurium dichlorides (see Section 3.9.2.3).

TeCl4 +

110°C (1:3)

TeCl3

2-Naphthyltellurium trichloride (typical procedure).16 TeCl4 (7.5 g, 27.8 mmol) is added to molten naphthalene (11.0 g, 86 mmol) kept at 110°C under N2. After 12 h at this temperature the greenish solid obtained is pulverized in a mortar and washed thoroughly with hot toluene (15.4 g (54%)). The product is recrystallized from benzene (m.p. 200–202°C).

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The above-described solventless procedure has recently been promoted to a general method to prepare aryltellurium trichloride.19 The crude trichlorides can be converted in situ into the corresponding aryl butyl tellurides by sequential treatment with aqueous sodium borohydride and n-butyl bromide (see Section 3.1.3.2). R1 R 1a-i

R1

TeCl4 no solvent 120°C

R

TeCl3

THF,BuBr NaBH4/H2O

R1 TeBu

0°C to r.t., 1 h R 2a-i

1a R = H, R1 = 4 -MeO 1b R = H, R1 = 4 -EtO 1c R = 3-MeO, R1 = 4 - MeO 1d R = 3-Me, R1 = 4 -MeO 1e R = H, R1 = 4 -Me

1f R = H, R1 = 4-Ph 1g R = R1 = Ph 1h R = H, R1 = 4-PhO 1i R = H, R1 = 4-HO

2a (73%); 2b (76%); 2c (72%); 2d (82%); 2e (69%); 2f (79%); 2g (68%); 2h (55%); 2i (72%).

General procedure for preparation of aryltellurium trichlorides and aryl butyltellurides (typical procedure).19 A one-necked, round-bottomed flask containing tellurium tetrachloride (5.38 g, 20 mmol) was heated to 120°C and the corresponding aromatic compound (20 mmol) was added at once, dissolving the tellurium tetrachloride promptly. An evolution of HCl was observed and a yellow solid was formed. The system was cooled to room temperature and the trichloride was used crude in the reduction/alkylation procedure. The trichloride prepared previously was dissolved in THF (80 mL) and bromobutane (5.48 g, 40 mmol) was added. The mixture was cooled to 0°C and a solution of sodium borohydride (3 g, 80 mmol) in water (80 mL) was added dropwise. The mixture turned to dark with addition of the first drops and then to light yellow after completion of the addition. The reaction was stirred for a further 20 min at room temperature and quenched with a saturated solution of NH4Cl (50 mL). The mixture was extracted with ethyl acetate (2⫻80 mL) and washed with brine (2⫻60 mL). The organic phases were combined, dried over MgSO4 and concentrated at reduced pressure. The desired telluride was purified by silica gel column chromatography using hexanes as eluent. 3.5.1.4

From tellurium tetrachloride and arylmercury chlorides

Arylmercury chlorides are valuable reagents for the preparation of aryltellurium trichlorides because they are used when the aromatic substrate is not sufficiently reactive for an easy direct condensation with tellurium tetrachloride.20 TeCl4 + ArHgCl

dioxane -HgCl. dioxane

ArTeCl3

representative examples (ref. 14, 15, 21-24) Ar = Ph (ref. 20), 2 - naphthyl (ref. 23),o - PhSC6H4 (ref. 14), p - Cl, p - Br, m -F, p -NO2, m -NO2C6H4 (ref. 15), 2 - thienyl (ref. 24); yields 56 - 96%.

The formed mercury dichloride is separated as a crystalline complex with dioxane. Since the starting arylmercury chlorides are easily prepared from diazonium salts, this method allows the conversion of anilines into aryltellurium trichlorides.

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1-Naphthyltellurium trichloride (typical procedure).14 A solution of 1-naphthyl mercury chloride25 (7.2 g, 20 mmol) and TeCl4 (5.4 g, 20 mmol) in pure dioxane (40 mL) is heated under reflux for 2 h. The crystalline HgCl2 dioxane that precipitates on cooling the solution at 10°C is removed by filtration. The dark yellow solution is evaporated, giving a viscous oil. By cooling and adding petroleum ether at 30–60°C the oil crystallizes. The crude trichloride is crystallized from glacial HOAc as yellow crystals (m.p. 175–180°C; 6.9 g (96%)). The crude product can also be purified by reduction to the corresponding ditelluride with Na2S·9H2O (see Section 3.2.3) and subsequent chlorinolysis with SO2Cl2 (see next section). 3.5.2

By chlorinolysis of diorganyl ditellurides

Dialkyl and diaryl ditellurides are easily converted into the corresponding tellurium trichlorides by means of the fission of the Te–Te bond by a chlorinating reagent. This route is very suitable for the preparation of the alkyl derivatives in view of the easy accessibility of dialkyl ditellurides via the alkylation of sodium ditelluride (see Section 3.2.1). The chlorinolysis can be effected by employing chlorine26 in solvents (chloroform, carbon tetrachloride, dichloromethane and benzene) or, more conveniently, avoiding the use of gaseous chlorine, by treatment with thionyl chloride27–28 or sulphuryl chloride.7,29–31

RTeTeR

Cl2 or SOCl2 or SO2Cl2

RTeCl3

R = alkyl, aryl

Organyltellurium trichlorides (general procedure).7,30 A solution of the ditelluride in benzene or CCl4 is treated dropwise at 0°C with a slight excess of SO2Cl2 in the same solvent. The crude precipitate of the trichloride is separated by filtration. The yield is close to 100%. 3.5.3 Organyltellurium tribromides and triiodides by halogenolysis of the corresponding ditellurides Like chlorinolysis, the brominolysis and iodinolysis of diorganyl ditellurides offer a facile route to tellurium tribromides and triiodides.14,31 RTeTeR

X2 solvent

2RTeX3

X = Br, I

The aryl derivatives, which are not easily accessible from tellurium tetrabromide and tetraiodide, are therefore prepared by this route. p-Methoxyphenyltellurium tribromide (typical procedure).14 Bromine (0.48 g, 3 mmol) dissolved in CCl4 (5 mL) is added dropwise, with stirring, to a solution of di-p-methoxyphenyl

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ditelluride (0.47 g, 1 mmol) in CCl4 (5 mL) at 0°C. The tribromide precipitates as an orange crystalline product and is separated by filtration (0.95 g (100%)). It is recrystallized from glacial HOAc (m.p. 188–190°C (dec.)). p-Methoxyphenyltellurium triiodide (typical procedure).14 The triiodide is prepared following the previous procedure, using the corresponding ditelluride (1 mmol) in CCl4 and iodine (3 mmol) in benzene. The yield is quantitative. The product is recrystallized from benzene (m.p. 131–133°C (dec.)). The following tribromides can be prepared using the same procedure as described above: Ar⫽p-EtOC6H4, p-PhOC6H4, p-PhSC6H4,14 1-naphthyl,30 2-naphthyl.14 The following triiodides can also be prepared: Ar ⫽ p-EtoC6H4, p-PhOC6H4, p-PHSC6H4, 1-naphthyl.14 (2-naphthyl-Te)2 forms exclusively 2-naphthyl-Tel, the triiodide being unknown30 (see Section 3.7). Dibenzyl ditelluride on treatment with bromine undergoes cleavage of the Te–benzyl bond, giving benzyl bromide and tellurium tetrabromide.32 Ph

Te Te

Ph

Br2 -Te

2 Ph

Br + TeBr4

3.5.4 Additional method: preparation of organyltellurium trichlorides and tribromides by the reaction of tetraorganyltin compounds with tellurium tetrachloride and tetrabromide33 SnR4 + TeX4

benzene 20°C

RTeX3 + R3SnX

R = Me, Et, C3H7, Ph X = Cl, Br

REFERENCES 1. Morgan, G. T.; Elvins, O. C. J. Chem. Soc. 1925, 127, 2625. 2. For compounds reported in the old literature see: (a) Rheinboldt, H. in Houben-Weyl-Methoden der Organischen Chemie. Vol. IX, p. 1060, 1152. (b) Irgolic, K. J. in The Organic Chemistry of Tellurium. p. 71. Gordon and Breach, New York, 1974. 3. Moura Campos, M.; Petragnani, N. Tetrahedron Lett. 1959, 11. 4. Ogawa, M.; Ishioka, R. Bull. Chem. Soc. Jpn. 1970, 43, 496. 5. Backwall, J. E.; Bergman, J.; Engman, L. J. Org. Chem. 1983, 48, 3918. 6. Uemura, S.; Fukuzawa, S. J. Organomet. Chem. 1984, 268, 223. 7. Engman, L. Organometallics 1989, 8, 1997. 8. Ali, M. E. S.; Malik, M. A.; Smith, B. C. Inorg. Chim. Acta 1989, 162, 157. 9. Irgolic, K. J. in Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, p. 304. Georg Thieme, Stuttgart, 1990. 10. Morgan, G. T.; Kellet, R. E. J. Chem. Soc. 1926, 1080. 11. Morgan, G. T.; Drew, H. D. J. Chem. Soc. 1925, 127, 2307.

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53

Drew, H. D. J. Chem. Soc. 1926, 223. Reichel, L.; Kirschbaum, E. Liebigs Ann. Chem. 1936, 523, 211. Petragnani, N. Tetrahedron 1960, 11, 15. Sadekov, I. D.; Sayapina, L. M.; Bushkov, A. Y.; Minkin, V.I. J. Gen. Chem. USSR 1971, 41, 2747. Bergman, J.; Engman, L. Tetrahedron 1980, 36, 1275. Bergman, J. Tetrahedron 1972, 28, 3323. Gunther, W. H. H.; Nepywoda, J.; Chu, J. Y. C. J. Organomet. Chem. 1974, 74, 79. Cunha, R. L. O. R.; Omori, A. T.; Castelani, P.; Toledo, F. T.; Comasseto, J. V. J. Organomet. Chem. 2004, 689, 3631. Irgolic, K. J. in Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, p. 311. Georg Thieme, Stuttgart, 1990. Farrar, W. V. Research 1951, 4, 179. Campbell, I. G. M.; Turner, E. E. J. Chem. Soc. 1938, 39. Rheinboldt, H.; Vicentini, J. Chem. Ber. 1956, 89, 624. Chia, L. Y.; McWhinnie, W. R. J. Organomet. Chem. 1978, 148, 165. (a) Nesmejanov, N. Ber. Dtsch. Chem. Ges. 1929, 62, 1010. (b) Nesmejanov, N. Org. Synth. Coll. Vol. II, p. 432. Wiley, New York, 1943. (a) Wynne, K. J.; Pearson, P. S. Inorg. Chem. 1970, 9, 106. (b) Wynne, K. J.; Pearson, P. S. Inorg. Chem. 1971, 10, 1871. Rheinboldt, H. in Houben-Weyl-Methoden der Organischen Chemie (ed. E. Muller). 4th edn, Vol. IX, p. 1156. Georg Thieme, Stuttgart, 1955. Sadekov, I. D.; Bushkov, A. Y.; Yureva, U. S.; Minkin, V. I. J. Gen. Chem. USSR 1977, 47, 2321. Petragnani, N.; Moura Campos, M. Chem. Ber. 1963, 96, 249. Vicentini, G.; Giesbrecht, H.; Pitombo, L. R. M. Chem. Ber. 1959, 92, 40. Irgolic, K. J. in Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, p. 315. Georg Thieme, Stuttgart, 1990. Spencer, H. K.; Cava, M. P. J. Org. Chem. 1977, 42, 2937. Schumann, H.; Magerstadt, M. J. Organomet. Chem. 1982, 232, 147.

3.6 THE PRODUCTS OF THE HYDROLYSIS OF ARYLTELLURIUM TRIHALIDES Aryltellurium trihalides are sensitive to water and can be converted into products at different stages of hydrolysis. Oxohalides, aryltellurinic acids and aryltellurium anhydrides have been known for a long time, the oxohalides being the products of partial hydrolysis. ArTeX3

H2O

ArTe(O)X

H2O

ArTe(O)OH

ArTe(O)-O-Te(O)Ar

The ease of hydrolysis follows the order ArTeCl3 ⬎ ArTeBr3 ⬎ ArTel3. Aryltellurium trichlorides and tribromides are converted into the oxohalides by simple treatment with cold water. The triiodides are unaffected under these conditions, and on treatment with boiling water give mixtures or undefined products in most cases.1

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Aryltellurium triiodides can be prepared by treating aryltellurium trichlorides, tribromides, oxohalides or aryltellurinic anhydrides with aqueous sodium iodide.1 Aryltellurinic acids were described in the early period of tellurium chemistry as the products of the alkaline hydrolysis of the trichlorides.2 However, the structure of these compounds was later argued about, and the corresponding anhydrides are today accepted as the unequivocal products.1,3,4 The anhydrides are stable, colourless, solids with high melting points and have recently attained a relevant role as synthetic reagents. Aryltellurium anhydrides (general procedure).1 The aryltellurium trichlorides (or tribromides) are treated by heating them with 10% aqueous Na2CO3 or NaOH until dissolution (a few minutes are sufficient). The cooled solution is treated with 10% HOAc. The anhydrides precipitate as white solids and are separated by filtration and dried under vacuum (in the presence of CaCl2) (m.p. >200°C; Ar⫽Ph, p-HOC6H4, p-MeOC6H4, p-EtOC6H4, p-PhOC6H4, 2-naphthyl (yields 90–100%)). Mixed anhydrides are prepared by reacting tellurinic anhydrides with carboxylic acids or anhydrides.5 2ArTe(O)OCOR + H2O

(ArTeO)2O + 2RCO2H Ar = Ph; R = Me, F3C

Arenetellurinic anhydrides are formed from the mixed anhydrides by hydrolysis, but a more convenient one-pot procedure for their preparation involves the reaction of phenyliodine(III) dicarboxylate with diaryl ditellurides in the two-phase system CH2Cl2/H2O 10%.6

ArTeTeAr + PhI(OCOR)2

CH2Cl2 / H2O r.t.

O ArTe)2O (90 - 96%)

Ar = p- MeC6H4, p-MeOPh, Ph R = CF3, CH3, CH2Cl

Arenetellurinic mixed anhydrides (general procedure).1 A solution of diaryl ditelluride (0.5 mmol) in methylene chloride (5 mL) was added to a stirred solution of phenyliodine(III) dicarboxylate (1.5 mmol) in the same solvent (15 mL) at room temperature. The colour of diaryl ditelluride faded quickly. After stirring about 15 min the solvent was removed in vacuo and petroleum ether (15 mL) was added to the residue. The solid was collected and recystallized from methylene chloride–petroleum ether or benzene–petroleum ether to give pure anenetellurinic mixed anhydride.

REFERENCES 1. Petragnani, N.; Vicentini, G. Universidade de São Paulo; Faculdade de Filos. Cienc. Letras. Biol. Quím. 1959, 5, 75. Chem. Abstr. 1963, 11256, 58.

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Reichel, L.; Kirschbaum, E. Annals 1936, 523, 211. Thavornyutikarn, P.; Whinnie, W. R. J. Organomet. Chem. 1973, 50, 135 and references therein. Barton, D. H. R.; Finet, J. P.; Thomas, M. Tetrahedron 1986, 42, 2319. Hu, N. X. ; Aso, Y.; Otsubo, T.; Ogura, F. J. Org. Chem. 1989, 54, 4398. Chen, D. W.; Chen, Z. C. Synth. Commun. 1995, 25, 1605.

3.7

ARYLTELLURENYL HALIDES

Halogenated derivatives of tellurium in the oxidation state ⫹2, namely tellurenyl halides, are unfamiliar compounds compared to the well-known selenium analogues.1,2 These compounds are in principle easily accessible by the controlled halogenolysis of the parent ditellurides. However, the first compound obtained in 1959, 2-naphthyltellurenyl iodide,3 remained for a long time the only stable representative of this class until the preparation of other members of this class by a successful use of the same method.4 ArTeTeAr + X2

2ArTeX

Ar = 2-naphthyl; X = I Ar = Ph, p - MeOC6H4, o - PhC6H4, p - PhC6H4, 2 -byphenylyl; X = Br, I Ar = m,p - (MeO)2C6H3; X = I

Several aryltellurenyl halides have been prepared, bearing an ortho carbonyl, nitro or another electron-attracting group, exhibiting a stabilizing effect by coordination to tellurium.5–8 With the exception of 2-naphthyltellurenyl iodide, the unsubstituted members are, however, thermally unstable in the solid state. Therefore, to exploit their highly electrophilic character, aryltellurenyl halides are not isolated, instead being generated in situ and used directly for further conversion.9,10 A recent report11 describes the synthesis of Te-anisyl phosphorotellurolate esters by the reaction of anisyltellurium trichloride with trialkylphosphites. The intermediacy of the anisyltellurenyl chloride, formed by the reduction of the trichloride at the expense of the excess of the trialkylphosphonite, rationalizes the result. ArTeCl3 + RO)2POR1

ArTeCl]

RO)2POR1

R = Et, n-Bu, Ph, PhCH2 R1 = H, Me, Et, n - Bu, PhCH2

RO)2P(O)-TeAr (76- 95%)

REFERENCES 1. Irgolic, K. J. in Houben-Weyl Methods of Organic Chemistry. 4 edn, Vol. E, p. 238. Georg Thieme, Stuttgart, 1990.

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2. For an exhaustive survey see Sadekov, I. D.; Minkin, V. I. Russ. J. Org. Chem. 1999, 35, 953. 3. Vicentini, G.; Giesbrecht; E. L.; Pitombo, R. M. Chem. Ber. 1959, 92, 40. 4. Schulz, P.; Klar, G. Z. Naturforsch. B 1974, 30, 40, 43. 5. Piette, J. L.; Renson, M. Bull. Soc. Chim. Belg. 1971, 80, 669. 6. Piette, J. L.; Lysy, R.; Renson, M. Bull. Soc. Chim. Fr. 1972, 3559. 7. Wiriyachitra, P.; Falcone, S. J.; Cava, M. P. J. Org. Chem. 1979, 44, 3957. 8. Engman, L. J. Org. Chem. 1989, 54, 2964. 9. See Section 3.1.3.5. 10. Representative examples: Section 3.1.3.5, ref. 41; 3.16.1.2, ref. 26; 3.16.2.1, ref. 32; 3.16.2.2, ref. 12; 3.16.5.1, refs. 55–58; 3.16.5.2, ref. 59; 3.16.6, ref. 62; 3.16.7, refs. 66–69, 71, 73; 3.17.2.1, refs. 7, 9; 3.18, ref. 18; 4.5.1.4, ref. 8. 11. Hayashi, M.; Miura, T.; Matsuchika, K.; Watanabe, Y. Synthesis 2004, 1481.

3.8

ARYL TELLURENYL PSEUDOHALIDES: ARYL TELLUROCYANATES

Aryl tellurocyanates are easily prepared by reacting aryltellurenyl bromides (generated in situ by reduction of the corresponding tribromides) with potassium cyanide.1

ArTeBr3

Na2S2O5 / H2O / MeOH

[ArTeBr]

KCN 93, 86%

ArTeCN

Ar = p -MeOC6H4, Ph

p-Methoxyphenyl tellurocyanate (typical procedure).1 p-Methoxyphenyl tellurium tribromide (5.0 g, 14.2 mmol) is added to a solution of NaHCO3 (2.45 g, 29.2 mmol) in H2O (50 mL) and MeOH (3 mL). The mixture is stirred for 30 min and then KCl (3.70 g, 58.4 mmol) followed by Na2S2O5 (11.0 g, 58.4 mmol) are added. The formed mixture was stirred for 8 min and KCN (3.70 g, 58.4 mmol) was added. A white precipitate forms. The reaction mixture was diluted with H2O, extracted with CH2Cl2, and the organic phase was dried (Na2SO4) and filtered. The filtrate was diluted with hexane and the resultant solution was concentrated by distilling some of the solvent. The formed precipitate was filtered (3.60 g (93%); m.p. 77°C). The stable (o-nitrophenyl) tellurenyl bromide reacts directly with potassium cyanide to give the corresponding tellurocyanate.

o-NO2C6H4TeBr

KCN MeOH

o -NO2C6H4TeCN

REFERENCE 1. Falcone, S. J.; Fava, M. P. J. Org. Chem. 1980, 45, 1044.

(ref. 1)

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57

DIORGANYL TELLURIUM DIHALIDES

From elemental tellurium

The reaction of elemental tellurium with alkyl iodides giving dialkyltellurium diiodides is a 100-year-old method,1 but still finds some use2 even if the low yields make this method unattractive. Te + 2RI

R2TeI2

In an improved procedure, employing alkyl chlorides and bromides in the presence of sodium iodide, tellurium diiodides are obtained in increased yields.3 Te + 2RX

NaI solvent

R2TeI2

X = Cl, Br

1,3-Dihydro-2-benzotellurophene diiodide (typical procedure).3 A mixture of ,⬘dichloro-o-xylene (3.5 g, 20 mmol), finely powdered Te (2.55 g, 20 g-atom) and NaI (12.0 g, 80 mmol) in 2-methoxyethanol (100 mL) is stirred and gently boiled in an open beaker. After 1 h the mixture contains a heavy orange precipitate and a small amount of unreacted Te. After the addition of deionized H2O (200 mL), which causes additional precipitation, the precipitate is filtered off, washed with H2O, rinsed with acetone and air dried (8.5 g (83%)). The product is recrystallized from 2-methoxyethanol (two crystalline forms) (m.p. 222–225°C). 3.9.2

From tellurium tetrahalides (or tellurium dioxide)

Employing appropriate ratios of reagents and experimental conditions, some of the reactions that convert tellurium tetrachloride into organyltellurium trichlorides (see Section 3.5.1) can be applied to the synthesis of diorganyltellurium dichlorides. 3.9.2.1

With ketones and carboxylic acid anhydrides O + TeCl4

R

CHCl3 reflux

O R

)2TeCl2 Cl

O

O

R

R + TeCl4 R

R

(ref. 4)

R

Cl Te R

O

O

(ref. 5)

R R

CHCl3 reflux

O

O

R

R R

R

)2TeCl2

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O

O + TeCl4

O

3.9.2.2

3:58 PM

HO2C

)2TeCl2

(ref. 6)

With alkenes Cl + TeCl4

2R

)2TeCl2

R

(ref. 7 -10)

Cyclotelluroetherification reaction, such as shown in the accompanying equation, is discussed in Section 4.6.2.4.11 ( )n

OH

TeO2 HOAc / LiCl

n(

) O

Cl Cl Te

( )n O

n = 1, 2

3.9.2.3

With arenes

When the reactions of tellurium tetrachloride with activated arenes (such as alkoxybenzenes) are performed using a large excess of the organic substrate and/or under prolonged heating at high temperatures, diaryltellurium dichlorides are formed in high yields.12–15 Cl Cl Te + TeCl4

2 R

R

R

Di-(p-methoxyphenyl)tellurium dichloride (typical procedure).14 Anisole (64.8 g, 0.6 mol) and TeCl4 (27.0 g, 0.1 mol) are heated at 160°C for 6 h. The mixture is allowed to crystallize under vacuum (30.6 g (90%)). The product is recrystallized from MeCN as colourless crystals (m.p. 181–182°C). In the presence of Lewis acids such as aluminium trichloride, diaryltellurium dichlorides are formed even with non-activated arenas.14,16 Diphenyltellurium dichloride (typical procedure).14 TeCl4 (27.0 g, 0.1 mol) and benzene (150 mL) are heated under reflux for 0.5 h. The solution is then cooled at 60°C and AlCl3 (1.0 g) is added. Evolution of HCl ensues. The mixture is then refluxed for 12 h, cooled, filtered to remove the formed trichloride and inorganic impurities. After evaporation, the residue is recrystallized from MeCN (12.6 g (36%); m.p. 160–161°C). 3.9.2.4

With arylmercury chlorides TeCl4 + 2ArHgCl

dioxane -HgCl2. dioxane

ArTe(Cl)2Ar

Diaryltellurium dichlorides (general procedure).17 TeCl4 (10.8 g, 40 mmol) and arylmercury chloride (80 mmol) are heated under reflux in anhydrous dioxane (100 mL) for 2 h.

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The solution is cooled at 10°C and the HgCl2.dioxane precipitate filtered off and washed with cold dioxane (15 mL). The filtrate is poured while stirring into 1% ice-cold HCl (400 mL). The dichlorides precipitate as crystalline solids (yields >90%), and are recrystallized from benzene/petroleum ether 30–60°C (Ar⫽Ph, m.p. 160–162°C; p-MeC6H4, m.p. 165–166°C). It must be pointed out that the reaction of tellurium tetrachloride with aryl Grignard or lithium reagents is inappropriate for the preparation of diorganyltellurium dichlorides owing to the difficulty of stopping the reaction at the 1:2 stage. 3.9.2.5

With arenediazonium salts

TeCl4 in concentrated hydrochloric acid reacts with arenediazonium chlorides giving bis(arenediazonium) hexachlorotellurates that are decomposed by copper powder in acetone to give diaryltellurium dichlorides in low to medium yields.18

TeCl4

HCl H2O

H2TeCl6

[ArN2+]Cl-

[ArN2+]2TeCl62-

Ar = Ph, p -MeO, p -Me, p -ClC6H4

Cu acetone (21- 55%)

Ar2TeCl2

Bis(p-methoxyphenyl)tellurium dichloride (typical procedure).18 TeCl4 (27 g, 0.1 mol) or TeO2 (16 g, 0.1 mol) dissolved in concentrated HCl (75 mL) is added to a stirred solution of (p-MeOC6H4N2)⫹Cl⫺ (0.2 M, from aminobenzene (24.6 g, 0.23 mol), half-concentrated HCl (150 mL) and NaNO2 (14 g. 0.21 mol) in H2O (20 mL)) at ⫺5°C. The mixture is stirred at ⫺5°C for 20 min and then filtered. The filter cake is washed successively with acetone (3⫻50 mL), ether and petroleum ether. The diazonium hexachlorotellurate product is dried in air (52.8 g (86%)). Of the above product, 18.3 g (30 mmol) is carefully ground and suspended in acetone (95 mL). The suspension is vigorously stirred at ⫺15°C while Cu powder (11.5 g, 180 mmol) is added over 40 min (the temperature must not exceed ⫺10°C). The mixture is stirred for a further 20 min and is then allowed to warm at 20°C. The dark-coloured mixture is filtered to remove CuCl, the filter cake is washed with acetone and the filtrate and washings are combined. The acetone solution is warmed at 20°C until a grey precipitate forms. The precipitate is collected and extracted repeatedly with boiling toluene. Activated charcoal is added to the toluene extracts, the mixture is heated at reflux and filtered and most of the toluene is distilled from the filtrate. Petroleum ether is added to precipitate the product (p-MeOC6H4)2TeCl2, which is recrystallized from benzene (6.8 g (52%); m.p. 182°C). 3.9.3

From organyltellurium trihalides

Like tellurium tetrachloride, aryltellurium (and alkyltellurium) trichlorides (and tribromides) undergo condensation reactions with reactive organic substrates or metalloorganic reagents as well as addition reactions to C⫽C bonds, giving rise to tellurium dichlorides.

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With ketones O

O

(ref. 19 - 22)

ArTeCl3 +

ArTeCl2 Ar = p -MeOC6H4, p -EtOC6H4, p -PhOC6H4 ketone = acetone, acetophenone, pinacolone, cycloalkanones

The reaction can be performed by simply maintaining a mixture of the trichloride and excess ketone (1:3) at room temperature, or by refluxing equimolar amounts of the reagents in benzene. Alkyltellurium trichlorides give only low yields of the products. Aryltellurium tribromides are generally less reactive towards ketones. MeOC4H4Te(Br2)CH2COPh was prepared by this method.19 Improved reaction times and yields are achieved employing ketone silyl enol ether.20–22 OSiMe3 Ar(R)TeCl3 +

benzene, reflux

TeCl2Ar(R)

[Ar = p -MeOC6H4] OMe R = Ph

O

OMe , n -C6H13

ketone = acetone, pinacolone, cycloalkanones

(p-Methoxyphenyl)phenacyltellurium dichloride (typical procedure).19 p-Methoxyphenyltellurium trichloride (0.34 g, 1 mmol) and acetophenone (0.36 g, 3 mmol) are intimately mixed and allowed to stand for 48 h. During this time the product crystallizes and is then washed thoroughly with ether and successively with a small volume of cold methanol. The crude dichloride is recrystallized from benzene/petroleum ether 30–60°C (0.25 g (60%); m.p. 136–137°C). (p-Methoxyphenyl)pinacolyltellurium dichloride (typical procedure).21 A mixture of p-methoxyphenyltellurium trichloride (1.020 g, 3 mmol) and pinacolone trimethylsilyl enol ether (0.516 g, 3 mmol) in benzene (10 mL) is heated under reflux. The reaction is monitored by TLC. After 10 h the mixture is treated with MeOH/H2O (1:1, 100 mL) and then extracted with CHCl3 and the organic layer is then dried (MgSO4) and evaporated. The residual oil is filtered on an SiO2 pad (eluting with CHCl3). The product is recrystallized from CHCl3/petroleum ether 30–60°C (0.97 g (80%); m.p. 90–92°C). 3.9.3.2

With alkenes

The addition of aryltellurium trichlorides to alkenes occurs with a high stereospecificity, giving exclusively anti-2-chloroalkyltellurium dichlorides,23–26 in contrast to the addition of tellurium tetrachloride which gives mixtures of syn and anti adducts

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(see Section 3.5.1.2). R

R

Cl R H

+ ArTeCl3

H H R H Te(Cl2)Ar Ar = 2 - naphthyl; olefin = propene, (E) - and (Z) - 2 - butenes, 1- decene, cyclopentene, cyclohexene

2-Chloroalkyl-2-naphthyltellurium dichloride (general procedure).24 2-Naphthyltellurium trichloride (2.98 g, 0.83 mmol) is heated under reflux with the olefin (1.2 mmol) in dry, ethanol-free CHCl3 (15 mL) until the trichloride has dissolved (0.5–1 h). Filtration from a small amount of elemental Te and evaporation gives an oil or a semi-solid that is recrystallized from a large amount of petroleum ether at 40–60°C. The reactions with propene and 2-butenes are performed in a sealed tube at 80°C. The yields are in the range 70–95%. In the presence of alcohols the addition of aryltellurium trihalides to alkenes gives rise to alkoxytelluration products.27–30 The addition is highly anti-stereospecific and regiospecific for terminal alkenes, tellurium moieties attacking exclusively the terminal carbon. The reaction is usually performed with aryltellurium tribromides, generated in situ by the treatment of the corresponding ditellurides with bromine (although trichlorides, ditellurides/CuCl2 and tellurocyanates/CuCl2 are also effective). R

R PhTeBr3 /R1OH

H

H

Cl R H R H Te(Br2)Ar

olefin = 1-hexene, 1-octene, 1-decene, styrene, α-methylstyrene 2-methyl-1-pentene, isobutylene, cyclopentene, cyclohexene, cycoheptene, cyclooctene

(2-Methoxycyclohexyl)phenyltellurium dibromide (typical procedure).28 To a solution of phenyltellurium tribromide (from diphenyl ditelluride (1.02 g, 2.5 mmol) and Br2 (1.20 g, 7.5 mmol) in MeOH (5 mL) is added cyclohexene (0.49 g, 6.0 mmol) and the mixture is heated under reflux for 1 h. By cooling the yellow solution at room temperature, the dibromide precipitates (1.45 g (61%); m.p. 169–171°C). 3.9.3.3

With arenes

Aryltellurium trichlorides and tribromides condense with aromatic compounds bearing electron-donating groups, giving unsymmetrical diaryltellurium dihalides.19 This electrophilic substitution therefore parallels that described for tellurium tetrachloride. ArTeX3 +

R1

r.t., -HX

X2TeAr

R1

R R representative examples: X = Cl, Br; Ar = p -MeOC6H4, p -EtOC6H4, p -PhOC6H4; R = H; R1 = NMe2; R = R1 = OH

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p-Methoxyphenyl-p-dimethylaminophenyltellurium dichloride (typical procedure).19 p-Methoxyphenyltellurium trichloride (0.34 g, 1 mmol) and N,N-dimethylaniline (0.36 g, 3 mmol) are intimately mixed. After 24 h at room temperature, the mixture is extracted several times with MeOH leaving a yellow crystalline residue (0.32 g (75%)). Recrystallization from MeOH or benzene/MeOH gives the product as yellow prisms (m.p. 170–172°C (dec.)). 3.9.3.4

With organylmercury chlorides

The reaction of aryltellurium trichlorides with aryl- or alkylmercury chlorides is a wellestablished and general method for the preparation of unsymmetrical diaryl- or alkylaryltellurium dichlorides.31,32 Ar1HgCl, dioxane, -HgCl2.dioxane

ArTeCl2Ar1

Ar = p - MeO, p - PhOC6H4 Ar1 = Ph, 1-naphthyl, 2 -naphthyl

ArTeCl3 RHgCl, dioxane, -HgCl2.dioxane

ArTeCl2R

Ar = p -MeOC6H4; R = Et, PhCH2 Ar = 2 - naphthyl; R = cyclohexyl, PhCH2

Unsymmetrical diaryltellurium dichlorides (general procedure).32 The aryltellurium trichloride (20 mmol) and the arylmercury chloride (20 mmol) are heated under reflux in dry dioxane (30 mL) for 1.5 h. The solution is cooled at 10°C and the HgCl2 dioxane precipitate so formed is filtered off and washed with a small volume of cold dioxane. The dioxane solution is then poured into 1% HCl (100 mL) with vigorous stirring. The formed oily product crystallizes by friction and is separated by filtration and then washed with cold EtOH. The reaction with ethyl- and cyclohexylmercury chlorides requires heating for 24 h. 3.9.4

By addition of halogens to diorganyl tellurides

Diorganyl tellurides, which as shown in Section 3.1 are the primary products of several methods for the synthesis of organotellurium compounds, can easily be converted into the corresponding dihalides by the simple addition of halogens.

RTeR

X2

X X RTeR

X = Cl, Br, I

This reaction has a general character and can be applied to the different types of telluride.33 The halogenation is accomplished in inert solvents (ether, benzene, dichloromethane and chloroform) and the dihalides are normally easily crystallizable products. The chlorination of tellurides is more conveniently done by using, instead of chlorine (gaseous or dissolved in solvents), chlorinating reagents such as sulphuryl or thionyl chloride (like the chlorinolysis of ditellurides, see Section 3.5.2).

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Diaryltellurium dihalides (general procedure).32 The diaryl telluride dissolved in a small amount of benzene or CHCl3 is treated dropwise, while cooling (ice bath) and stirring, with a solution of an equimolar amount of SO2Cl2, Br2 or I2 in the same solvent (in the case of SO2Cl2, evolution of SO2 is observed). By addition of a large excess of petroleum ether at 30–60°C (50 mL for 1 mmol) the dihalide precipitates as a crystalline solid (the dichlorides are colourless, the dibromides yellow and the diiodides red). The yields are quantitative. Representative examples of diaryltellurium dihalides prepared by the above method are ArTeX2Ar1 X = Cl, Br, I Ar = Ph; Ar1 = p -MeOC6H4, p -EtOC6H4, 1- and 2-naphthyl Ar = p -MeOC6H4, p -EtOC6H4; Ar1 = 1- and 2-naphthyl Ar = 1-naphthyl; Ar1 = 2 - naphthyl

Dibenzyl and arylbenzyl tellurides (like dibenzyl ditelluride, see Section 3.5.3) exhibit an unusual behaviour towards halogens, undergoing cleavage of the tellurium–benzyl bond.34,35

Ph

Te Ph

Ph

TeAr

Br2 X2

2 Ph Ph

Br + TeBr4 X + ArTeX3

X = Cl, Br, I

3.9.5 3.9.5.1

Additional methods Reaction of elemental tellurium with arenediazonium salts36 [2ArN2+]Cl- + Te

3.9.5.2

Ar2TeCl2

Reaction of TeO2/LiCl with aryl hydrazines37 ArTeCl2 + 2 N2 + 2 H2O + 2 LiOH + Te

2 ArNHNH2 + 2 TeO2 + 2 LiCl

3.9.5.3

-N2

Reaction of diaryl ditelluride with arenediazonium salts/CuX238 (ArTe)2 + 2 [Ar1N2+]X-

X = Cl, Br

-N2

2 ArTeAr1

CuX2

2 ArTeX2Ar1

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REFERENCES 1. Rheinboldt, H., in Houben-Weyl-Methoden der Organischen Chemie (ed. E. MulIer). 4th edn, Vol. IX, p. 1066. Georg Thieme, Stuttgart, 1955. 2. Thayer, J. S.; Smith, K. V. Synth. Inorg. Met. Org. Chem. 1973, 3, 101; see also Irgolic, K. J. Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, pp. 525–526. Georg Thieme, Stuttgart, 1990. 3. Ziolo, R. F.; Gunther, W. H. H. J. Organomet. Chem. 1978, 146, 245. 4. Morgan, G. T.; Elvins, O. C. J. Chem. Soc. 1925, 127, 2625. 5. Rheinboldt, H. in Houben-Weyl-Methoden der Organischen Chemie (ed. E. Muller). 4th edn, Vol. IX, p. 1061. Georg Thieme, Stuttgart, 1955. 6. Morgan, G. T.; Drew, H. D. K. J. Chem. Soc. 1925, 127, 531. 7. Funk, H.; Weiss, W. J. Prakt. Chem. 1954, 1, 33. 8. Arpe, H. J.; Kuckertz, H. Angew. Chem. (Engl.) 1971, 10, 73. 9. Ogawa, M.; Ishioka, R. Bull. Chem. Soc. Jpn. 1970, 43, 496. 10. Ogawa, M. Bull. Chem. Soc. Jpn. 1968, 41, 3031. 11. Bergman, J.; Engman, L. J. Am. Chem. Soc. 1981, 103, 5196. 12. Morgan, G. T.; Kellett, R. E. J. Chem. Soc. 1926, 1080. 13. Morgan, G. T.; Drew, R. D. J. Chem. Soc. 1925, 127, 2301. 14. Bergman, J. Tetrahedron 1972, 28, 3323. 15. Irgolic, K. J. Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, p. 529, Table 16. Georg Thieme, Stuttgart, 1990. 16. Gunther, W. H. H.; Nepywoda, J.; Chu, J. Y. C. J. Organomet. Chem. 1974, 74, 79. 17. Petragnani, N.; Comasseto, J. V.; Varella, N. H. J. Organomet. Chem. 1976, 120, 315. 18. Sadekov, L. D.; Minkin, V. I. Dokl. Akad. Nauk. USSR 1971, 197, 316. 19. Petragnani, N. Tetrahedron 1961, 12, 219. 20. Sadekov, I. D.; Maksimenko, A. A.; Rivkin, B. B. J. Org. Chem. USSR 1978, 14, 810. 21. Stefani, H. A.; Comasseto, J. V.; Petragnani, N. Synth. Commun. 1987, 17, 443. 22. Stefani, H. A.; Chieffi, A.; Comasseto, J. V. Organometallics 1991, 10, 1178. 23. Moura Campos, M.; Petragnani, N.Tetrahedron 1962, 18, 521. 24. Backwall, J. E.; Bergman, J.; Engman, L. J. Org. Chem. 1983, 48, 3918. 25. Bergman, J.; Engman, L. Tetrahedron 1980, 36, 1275. 26. Uemura, S.; Fukuzawa, S. J. Organomet. Chem. 1984, 268, 223. 27. Uemura, S.; Fukuzawa, S.; Toshimitsu, A.; Okano, M. Tetrahedron Lett. 1982, 23, 1177. 28. Uemura, S.; Fukuzawa, S.; Toshimitsu, A. J. Organomet. Chem. 1983, 250, 203. 29. Uemura, S.; Fukuzawa, S. J. Am. Chem. Soc. 1983, 105, 2748. 30. Uemura, S.; Fukuzawa, S. J. Chem. Soc. Perkin Trans. 1985, 1, 471. 31. Petragnani, N. Tetrahedron 1961, 12, 219. 32. Rheinboldt, H.; Vicentini, G. Chem. Ber. 1956, 89, 624. 33. For a large literature coverage upon addition of halogens to organyl tellurides, see Irgolic, K. J. Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. EI2b, Section 5.1, p. 553. Georg Thieme, Stuttgart, 1990. 34. Vicentini, G. Chem. Ber. 1958, 91, 801. 35. Spencer, H. K.; Cava, M. P. J. Org. Chem. 1977, 42, 2937. 36. Taniyama, H.; Miyoshi, F.; Sakakibara, E.; Uchida, H. Yakugaku Zasshi 1957, 77, 191; Chem. Abstr. 1951, 51, 10407. 37. Bergman, J.; Engman, L. Z. Naturforsch. B 1980, 35, 882. 38. (a) Sadekov, I. D.; Maksimenko, A. J. Org. Chem. USSR 1978, 14, 2411. (b) Sadekov, I. D; Maksimenko, A.; Rivkin, B. B. J. Org. Chem. USSR 1983, 19, 541.

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3.10 DIORGANYL TELLUROXIDES Diorganyl tellurides are easily oxidized to the corresponding telluroxides. Dialkyl tellurides are especially sensitive to oxidation, and their exposure to air, either neat or in solution, causes the formation of dialkyl telluroxides. Diaryl tellurides are much more stable and are oxidized only by chemical methods. Diaryl telluroxides have recently attained outstanding importance as mild and selective oxidizing reagents (see Section 4.4.1). Two methods are well established and generally employed to prepare diaryl telluroxides. 3.10.1

Hydrolysis of diaryltellurium dihalides

This method, which dates from the early German tellurium chemistry1 and is today widely employed,2 uses aqueous sodium hydroxide or ammonia as hydrolytic reagents. Ar2TeX2

-OH H2O

Ar2TeO

Di-p-anisyl telluroxide (typical procedure).2 Di-p-anisyltellurium dichloride (8.00 g, 23 mmol) is stirred at 95°C in 5% aqueous NaOH (100 mL) for 1 h. After cooling at 0°C the white solid is separated by filtration, washed with cold H2O (3⫻10 mL) and dried under vacuum over P2O5 giving the telluroxide (5.60 g (81%); m.p. 187–189°C). 3.10.2

Oxidation of diaryl tellurides

Sodium periodate3 or positive halogenanting species such as N-chlorosuccinimide and t-butyl hypochlorite4 are useful oxidizing reagents. NaIO4 solvent, reflux

Ar2TeO

Ar2Te NCS or t -BuOCl

Ar2TeCl+

H2O

Ar2TeO + NCS

Di-p-anisyl telluroxide (typical procedure).3 Di-p-anisyl telluride (0.80 g, 2.3 mmol) and NaIO4 (0.50 g, 2.3 mmol) in CH3CN/MeOH/H2O (100 mL, 1:1:1) are heated under reflux for 2 h. The mixture is then cooled, treated with H2O (100 mL) and extracted with CHCl3 (3⫻30 mL). The extracts are evaporated and the residue chromatographed on SiO2 (elution with CHCl3/MeOH, 9:1). The crude product is recrystallized from benzene (0.26 g (22%); m.p. 189–191°C). Di-(2,4,6-Me3C6H2)2TeO is also prepared by the above procedure (yield 95%).

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REFERENCES 1. (a) Lederer, K. Ber. Dtsch. Chem. Ges. 1916, 49, 345, 2532, 2663. (b) Lederer, K. Ber. Dtsch. Chem. Ges. 1920, 53B, 1674. 2. Ley, S. V.; Meerholz, C. A.; Barton, D. H. R.Tetrahedron 1981, 37, 213. 3. Akiba, M.; Lakshmikantham, M. V.; Jen, K. Y.; Cava, M. P. J. Org. Chem. 1984, 49, 4819. 4. Detty, M. R. J. Org. Chem. 1980, 45, 274.

3.11

TELLUROESTERS

Telluroesters (acyltellurides), a class of compounds of promising synthetic utility1 have been synthesized in good to high yields by the following methods. (a)

Reaction of sodium,2–4 bromomagnesium5 or lithium2 organyl tellurolates with aroylchlorides and (in the case of sodium tellurolates) with aliphatic acid anhydrides.2

ArTeTeAr

NaBH4 MeOH or EtOH

ArMgBr + Te

THF

RCO)2O

ArTeNa

ArTeCOR R = Me, Et

Ar1COCl

ArTeCOAr1

ArTeMgBr

Ar, Ar1 = Ph; Ar = 4- and 2-substituted Ph; Ar1 = Napht

BuLi + Te

(b)

ether

BuTeLi

PhCOCl

BuTeCOPh (20%)

Reaction of acylchlorides with sodium telluride6 followed by treatment of the formed sodium acyltellurolates with alkyl iodides. RCOCl + Na2Te

THF

1 RCOTeNa R I

RCOTeR1 (60-84%)

R = Ph, p -MeOC6H4, p -MeC6H4, PhCH2, 1-Napht, 2-Napht, C17H35 R1 = Me, i -pr, n -Bu

(c)

Reaction of phenyltrimethylsilyl telluride with aroylchlorides.7 PhTeLi + Me3SiCl

PhTeSiMe3

Ar = Ph and p -substituted Ph

Ar1COCl -Me3SiCl

PhTeCOAr (62-91%)

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Reaction of aldehydes with i-Bu2AlTeBu, prepared in situ from dibutyl telluride and diisobutyl aluminium hydride in hexane.1 O R

i -Bu2AlTeBu H hexane,1h

R

TeBu

R = Ph, 2-Naphthyl (71, 73%) R = n -C8H17, t -Bu (36, 27%)

Butyl telluroesters are obtained satisfactorily from aromatic aldehydes, the yield being improved appreciably by the addition of Et2AlCl, whereas aliphatic aldehydes are less efficient and Et2AlCl is ineffective. The reaction probably proceeds via the addition of i-Bu2AlTeBu to aldehyde to form an adduct which then undergoes an intramolecular hydride shift giving the telluroester. The role of the Et2AlCl is unclear.

O Ph

OAlBu2

i-Bu2AlTeBu H

Ph

Ph BuTe

O

AlBu2 H

O PhCHO

TeBu-i

Ph BuTe

AlBu2 O H H

Ph

O

O Ph

Ph

TeBu

+

Ph

OAlBu2

Formation of telluroesters (typical procedure).4 Bis(p-fluorophenyl) ditelluride (450 mg, 1.0 mmol) was dissolved in THF (8 mL) with stirring at room temperature and treated with NaBH4 (150 mg, 4.0 mmol). MeOH (~1 mL), was added dropwise to this suspension which was then stirred vigorously until a light yellow solution formed. Benzoyl chloride (300 mg, 2.2 mmol) was then added dropwise and the resulting mixture stirred for a further 30 min. The reaction was quenched by addition of degassed water (5 mL), and extracted into benzene (3⫻30 mL), and the combined extracts dried (Na2SO4), and concentrated to give a pale yellow oil which solidified on standing in the freezer (660 mg, 100%) and which was sufficiently pure for use in subsequent experiments. An anaytical sample was obtained by recrystallization from hexanes: m.p. 60–61°C. Preparation of aryl telluroesters (general procedure).5 Aryl Grignard reagent was prepared in 20 mL anhydrous THF from magnesium (0.24 g, 10 mmol) and aryl bromide (10 mmol) according to standard method. Tellurium powder (1.02 g, 8mmol) was added to the Grignard reagent solution in one portion under N2. The mixture was refluxed for 2h, then cooled to room temperature. The solution of acyl chloride (10 mmol) in 10 mL anhydrous ethyl Et2O was added in drops at room temperature, then stirred for 4 h at the same temperature. Filtered away the unreacted tellurium powder and washed it with Et2O (20 mL), then added NH4Cl solution (40 mL) to the filtrate. Separated the organic phase, the water phase was extracted with Et2O (3⫻20 mL). The combined organic phase was washed with water (2⫻50 mL), dried with Na2SO4. The solvent was removed in vacuo and the residue was recrystallized from Et2O–petroleum ether (30–60°C), giving the yellow solid product.

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Typical procedure.7 Phenyltellurotrimethylsilane (2.10 mL, 10.9 mmol) was added into astirred solution of benzoyl chloride (1.42 g, 10.1 mmol) in 30 mL of dry tetrahydrofuran under an argon atmosphere. The mixture was stirred for 3 h at room temperature and concentrated under a reduced pressure. The residual Te-phenyl tellurobenzoate was purified by recrystallization from pentane to give yellow needles. Yield: 2.71 g (87%). Reaction of benzaldehyde with iBu2AlTeBu (typical procedure).1 In a flame-dried flask equipped with an Ar inlet and a rubber septum was placed n-BuTeTeBu-n (185 mg, 0.5 mmol). After iBu2AlH (1 N in hexane, 1 mL) was added under Ar, the solution was stirred at 25°C for 1 h and then 2 mL of THF was added. The solution was cooled to ⫺23°C and 2 mmol of benzaldehyde and 0.5 mL of Et2AlCl (1 N in hexane) were injected. The mixture was gradually warmed to 25°C, stirred for another 1 h and poured into saturated NH4Cl solution. Products were extracted with Et2O (3⫻30 mL), dried over MgSO4 and concentrated in vacuo. Medium pressure liquid chromatography (MPLC) of the residue gave pure Te-butyl tellurobenzoate in 71% yield (205 mg) in a hexane/Et2O (100:1) fraction. REFERENCES 1. 2. 3. 4.

Inoue, T.; Takeda, T.; Kambe, N.; Ogawa, A.; Ryu, I.; Sonoda, N. J. Org. Chem. 1994, 59, 5824. Piette, J. L.; Renson, M. Bull. Soc. Chim. Belg. 1970, 70, 383. Gardner, S. A.; Gysling, H. J. J. Organomet. Chem. 1980, 197, 111. Crich, D.; Chen, C.; Hwang, J. T.; Yuan, H.; Papadatos, A.; Walter, R. I. J. Am. Chem. Soc. 1994, 116, 8937. 5. Zhao, C. Q.; Huang, X. Synth. Commun. 1997, 27, 249. 6. Kanda, T.; Nakaiida, S.; Murai, T.; Kato, S. Tetrahedron Lett. 1989, 30, 1829. 7. Sasaki, K.; Aso, Y.; Otsubo, T.; Ogura, E. Chem. Lett. 1986, 977.

3.12

ARYL TELLUROFORMATES

Aryl telluroformates have been prepared by the following methods: (a) (b)

Reaction of aryl tellurols with alkyl chloroformates. Reaction of aryl tellurols with alkyl chloroformates prepared in situ from alcohols and COCl2.1 O ArTeTeAr

1) NaBH4, THF

[ArTeH]

2) MeOH

ROH + COCl2

THF

a) RO

O Cl

RO

TeAr

b) ROCOCl

a) Ar = Ph, p -FC6H4, p-MeOC6H4 R = Me, i -Bu 83-93%

b) Ar = Ph, p -FC6H4 R = c-hex, n -C8H17, 3-β-cholesteryl, 3-β-cholestanyl 70-98% Ar = Ph; R = PhCH2 (60%)

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Method a (typical procedure).1 Sodium borohydride (113 mg, 3.0 mmol) was added with stirring to a solution of diphenyl ditelluride (420 mg, 1.0 mmol) in THF (30 mL). The reaction vessel was purged with N2, and methanol (~250 mL) was added dropwise until the red solution turned colourless (30 min). When the evolution of hydrogen had ceased (~60 min), methyl chloroformate (208 mg, 2.2 mmol) was added via syringe, and the resulting mixture was stirred for a further 60 min. Water (10 mL) was added, and the mixture was extracted into ether (3⫻50 mL). The combined ether extracts were dried (MgSO4) and the solvent removed in vacuo to afford a yellow oil. Pure product (480 mg, 91%) was obtained after Kügelrohr distillation (50–60%/0.075 mmHg). (c)

Reaction of aryl tellurotris(trimethylsilyl)silane, generated in situ from the corresponding aryl tellurocyclohexane and tris(trimethylsilyl)silane with chloroformates in the presence of tetrakis-(triphenylphosphine) palladium. The same procedure, performed with acyl chlorides, affords telluroesters.2,3 ArTec - hex + HSi(SiMe3)3

benzene AIBN

[ArTeSi(SiMe3)3] (Ph3P)4Pd

RCOCl

O RO

O

Cl

O

TeAr R 59-87% R = Me, Ph

ArTe OR (61-79%) R = Me, i -Bu, c-hex, Ph

Method c (typical procedure).3 Tris(trimethylsilyl)silane (110 L, 89 mg, 360 mol) and 2,2⬘-azobisisobutyronitrile (AIBN) (ca. 1 crystal) were added to a solution of (phenyltelluro)cyclohexane (103 mg, 360 mol) in benzene (900 L). The reaction vessel was sealed (septum), purged with nitrogen and heated at 80°C, for 2 h while it was shielded from background light. After the mixture cooled to room temperature, tetrakis(triphenylphosphine)palladium (21 mg, 18 mol) and methyl chloroformate (28 mg, 23 L, 358 mol) were added, and the mixture was briefly shaken vigorously and allowed to stand at room temperature (25°C), while it was shielded from light, for 6 h. At which time TLC analysis revealed the presence of a dominant product. The solvent was removed in vacuo and the residue separated by flash chromatography (30:1, petroleum ether/ethyl acetate) to afford the telluroformate as a yellow oil which exhibited properties identical to those reported previously (61 mg, 80%).

REFERENCES 1. Lucas, M. A.; Schiesser, C. H. J. Org. Chem. 1996, 61, 5754. 2. Schiesser, C. H.; Skidmore, M. A. J. Chem. Soc. Perkin Trans. 1 1997, 2689. 3. Schiesser, C. H.; Skidmore, M. A. J. Org. Chem. 1998, 63, 5713.

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3.13

TELLUROGLUCOPYRANOSIDES

The reaction of protected ,D-glucopyranosyl bromides with sodium arene tellurolates gives rise to the corresponding telluro -D-glucopyranosides.1 OY

OY O

PhTeNa PhTeTePh / NaBH4 OYBr EtOH

YO YO

YO YO

O OY TePh (93%)

Y = Ac, PhCO

REFERENCE 1. Stick, R. V.; Tilbrook, D. M. G.; Williams, S. J. Aust. J. Chem. 1997, 50, 233.

3.14

WATER-SOLUBLE DIORGANYL TELLURIDES

Water-soluble diorganyl tellurides with thiolperoxidase and autoxidant activity have been prepared as depicted in the following scheme.1 HO

Te

OH

1) MOH (2 equiv) 2) HO(CH2)3SO3H(2,3 equiv)

Te2)

O (CH2)3SO3H

M = Li, Na, K, NMe4

REFERENCE 1. Kanda, T.; Engman, L.; Cotgreave, I. A.; Powis, G. J. Org. Chem. 1999, 64, 8161.

3.15

DIHALOARYLTELLURO CYCLOPROPANES

Dihalocarbenes generated under phase transfer catalysis add to vinylic tellurides to give the corresponding gem-dihaloaryltelluro cyclopropanes.1 R H

TeAr CHX3 / MOH TEBA, r.t. H

X

X

R TeAr (75 - 87%) R = Ph Ar = Ph, p -MeC6H4, 1-naphthyl X = Cl; M = Na X = Br; M = K

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REFERENCE 1. Huang, X.; Jiang, S. H. Synth. Commun. 1993, 23, 431.

3.16

VINYLIC TELLURIDES AND DITELLURIDES

These classes of tellurium compounds are treated separately because of their peculiar behaviour as synthons and intermediates of fundamental organic transformations.1 Leading examples of such performance are the transmetallation reactions of (Z)-vinylic tellurides, easily generated by the hydrotelluration of alkynes, leading to (Z)-vinyl organometallic reagents, which are difficult to obtain by other currently available methods. It is worth mentioning that the sequence hydrotelluration of acetylenes/transmetallation complements the classical sequence hydrostannillation of acetylenes/transmetallation, which lead to (E)-vinylic organostannates. 3.16.1 3.16.1.1

Starting from nucleophilic tellurium Addition of alkali tellurides to acetylenes

Elemental tellurium, treated with strong aqueous alkali, undergoes an oxido-reduction disproportionation giving the corresponding telluride ion which reacts with acetylenes to give divinylic tellurides in modest yields.2,3

4Te°

Base H2O

Te6+ + 3Te2-

HC CH 54%

Te

Simple systems such as KOH/H2O/DMSO and KOH/H2O/HMPA at 100–120°C under pressure have been used. The yields were markedly increased by the addition of a reducing agent such as SnCl2.4 The reaction of elemental Te, phenylacetylene, KOH in hydrazine hydrate/ water/toluene was performed under three-phase catalytical system (in the presence of AlK3N⫹MeCl⫺) giving (Z,Z)-distyryl telluride in 50% yield.5,6 In the presence of alkyl halides under pressurized acetylene, alkylvinyl tellurides in medium to good yields are obtained along with variable amounts of divinyl and dialkyl tellurides.7,8

Te + HC CH + RX

KOH / H2O / SnCl2 105 -115°C

RTe

+

Te

+ RTeR

(65-70%)

Reaction of acetylene with Te-KOH-HMPA triad.3 A mixture of Te (19.2 g), KOH (16.8 g), H2O (8 mL) and HMPA (165 mL) was heated (110–120°C) in a 1 L steel autoclave under initial acetylene pressure of 10 atm (21 L of C2H2 in total) for 6 h. The reaction mixture was

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distilled to give 11.1 g (54.3%) of divynyl telluride, based on the equation: 4Te°→3Te2⫺⫹Te4⫹, b.p. 131–132°C. Reaction of vinylacetylene with Te-KOH-DMSO triad.3 A mixture of Te (25.9.), KOH (20 g), H2O (10.9 mL) and DMSO (100 mL) was heated (110°C) with vinylacetylene (31.7 g) in 1 L steel-rotating autoclave for 3 h. The reaction mixture was poured into water, and extracted with Et2O. The extracts were washed with H2O. The solvent was stripped off and the residue was distilled in vacuum to collect the run with b.p. 80–86°C (2.2 g) consisting of bis(1,3-butadienyl) telluride (1.1 g). Reaction of phenylacetylene with Te/KOH/SnCl2.6 A mixture of 6.4 g of tellurium, 42 g of potassium hydroxide, 22.6 g of SnCl2·2H2O, 10.2 g of phenylacetylene, 20 mL of toluene, 60 mL of water and 1.2 g of Adogen 464 was heated (80–97°C) with vigorous stirring for 7 h. The organic layer was separated, and the aqueous layer was extracted with benzene. The benzene was removed under vacuum. The residue was dissolved in a small amount of ether, poured into 200 mL of isopropyl alcohol, and placed in the refrigerator for 3 days. The yellow crystals of distyryl telluride (2.5 g, yield 15%) and the red needle crystals of distyryl ditelluride (0.7 g, yield 6%) were separated. Reaction of phenylacetylene with Te/KOH/N2H4⫻H2O.5 (Z,Z)-distyryl telluride. A mixture of Te (12.8 g), phenylacetylene (20.4 g), KOH (84 g), N2H4⫻H2O (120 mL), H2O (24 mL), toluene (40 mL), and AlK3N⫹MeCl⫺ (1 g) was heated (100–110°C) with a stirring of 6 h. The mixture was diluted with water and extracted with benzene. The organic layer was washed with water, dried over K2CO3 and evaporated. The residue was dissolved in ether, the ether solution was poured into ethanol and put into refrigerator. After several days yellow cristals of the telluride (16.7 g, 50% yield) were separated and dried. Preparation of alkylvinyl tellurides by reaction of Te/KOH/alkyl iodides, SnCl2 in HMPA/H2O with pressurized acetylene. Methylvinyl telluride.8 12.8 g of Te, 1.42 g of methyl iodide, 48.7 g KOH, 24.7 g SnCl2, 80 mL HMPA and 70 mL H2O were heated (105–115°C) in a rotating autoclave 1 L under acetylenic pressure (initial pressure 14 atm, residual pressure 7 atm, absorbed acetylene 0.8 mol) tor 5 h. The reaction mixture is evacuated at 1 mmHg. The fraction (10.5 g) collected into a cooled trap (⫺70°C) contains, according to GLC data, methylvinyl telluride (0.34 g, yield 20%), divinyl telluride (10 g, yield 55%) and dimethyl telluride (0.12 g, yield 8%); b.p. 115–116°C. Actually, the greatest practical facilities, the most general applicability and the highest yield to prepare bisvinylic tellurides from elemental Te and acetylenes are achieved by using NaBH4 to generate the telluride anion.9 Te

1) NaBH4 / H2O / EtOH / THF reflux 2) 2 RC CH

R = alkyl, vinyl, aryl

R

Te R (64-98%)

Divinyl tellurides (typical procedure).9 Sodium borohydride (0.34 g, 9 mmol) was added in small portions to a stirred suspension of tellurium (0.383 g, 3 mmol) in ethanol (15 mL)

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kept under nitrogen. The mixture was heated. Sodium hydroxide (0.32 g, 8 mmol), water (15 mL) and tetrahydrofuran (5 mL) were added to the hot mixture, which was then refluxed for 15 min. The reaction mixture turned dark violet. The mixture was refluxed until all the tellurium had disappeared. The heat source was removed and the acetylene (7.8 mmol) added. The mixture was then refluxed and monitored by TLC. When the mixture had turned yellow, it was cooled to room temperature, diluted with ethyl acetate (50 mL), and washed first with a saturated solution of ammonium chloride (3⫻30 mL) and then with brine (3⫻30 mL). The organic phase was separated, dried (MgSO4), and filtered. The solvent was evaporated from the filtrate in a rotatory evaporator at 20 mmHg and the residue purified by flash chromatography on silica gel with hexane/ethyl acetate. Alkyl vinyl tellurides can also be prepared by a sequential reaction of divinylic tellurides with lithium and alkyl halides in liquid ammonia.8,10 Te

1) Li 2) RX, NH3 liq.

RX = MeI, EtBr, PrBr

3.16.1.2

RTe 30 - 60%

From organyl tellurols or tellurolates and terminal acetylenes

Alkyl- and aryltellurols generated in situ by the well-established reduction of ditellurides with NaBH4/EtOH add to terminal acetylenes, giving (Z)-vinylic tellurides.9,11–13

RTeTeR

NaBH4 EtOH

1 [RTeH] R

H EtOH (36-93%)

RTe H

R1 H

R = n -Bu; R1=Ph, CH2OTHP, H2CN O R = p -MeOC6H4; R1 = n-pentyl R =Ph; R1 = Ph, CH2OH, CO2Et R = Ph; R1 = Ph, p -MeC6H4, p -MeOC6H4, p -ClC6H4, o -ClC6H4, m -H2NC6H4

The addition of the tellurium occurs exclusively at the terminal carbon of phenylacetylene, whereas a minor amount (11–28%) of the 2,2-disubstituted adduct (RTeR⬘C⫽CH2) is formed in addition to alkylacetylenes. 1-Phenyltelluro-2-phenylethene (typical procedure).11 To a solution of diphenyl ditelluride (2.05 g, 5 mmol) in EtOH (10 mL) is added a solution of NaBH4 (0.50 g, 13.2 mmol) in EtOH (10 mL) at room temperature under N2. The orange colour of the solution changes to pale yellow. Phenylacetylene (1.1 g, 10.8 mmol) is then added to the resulting solution and the mixture is stirred under reflux for 20 h. After cooling, the mixture is treated with brine and extracted with CHCl3, (3⫻50 mL) and the extract dried (MgSO4). Evaporation of the solvent gives a yellow-orange oil, which is purified by column chromatography on SiO2 (elution with hexane/EtOAc, 10:1), giving the crude product (R⫽R1⫽Ph) as a yellow solid (2.15 g (70%)). It is recrystallized from EtOH (m.p. 43–44°C).

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Useful systems for the reduction of ditellurides to the corresponding tellurolates are formamidine sulphinic acid (HN⫽C(NH2)SO2H), TUDO, in 50% aqueous NaOH under phase transfer catalysis,12 KOH/H2O/DMSO/SnCl214 or KOH/H2O/DMSO/N2H4.15 In this context, a valuable improvement has been introduced by substituting the tedious generation of sodium butyl tellurolate from the non-commercially available n-dibutylditelluride for n-butyl tellurolate easily generated in situ from n-butyllithium and elemental tellurium.16 n -BuLi + Te

THF r.t., 5 min

n -BuTeLi

R H EtOH, reflux

R

TeBu-n

Typical procedure for the hydrotelluration with n-BuLi/Te.16 n-BuLi (0.66 mL, 1 mmol of a 1.5 M solution in hexane) is added dropwise to a stirred suspension of elemental tellurium (0.127 g, 1 mmol) in THF (5 mL) at room temperature under N2. A clear solution is formed after 5 min of stirring. Then phenylacetylene (0.122 g, 1.2 mmol) in deoxygenated ethanol (10 mL) is added and the solution is refluxed for 4 h, monitoring by TLC. After this time the mixture is diluted with ethyl acetate (20 mL) and washed with brine (2⫻20 mL). The organic layer is separated, dried (MgSO4) and the solvent is evaporated. The residue is purified by silica gel column chromatography eluting with hexane to give 0.215 g (75%) of the vinylic telluride. When the reaction is performed in a 40 mmol scale, the yield rises to 88%. The above-described hydrotelluration have been successfully applied to conjugated enines and diines,9,17 functionalized alkynes and diines,18–22 providing a useful method for the synthesis of organotelluro 1,3-butadienes, organotelluro 1,3-enines and functionalized vinylic tellurides with the Z configuration at the newly formed C⫽C bond.

R1 R

R = n -Bu

RTeTeR

R2 n-BuTeTeBu-n R3 NaBH4, EtOH reflux

n -BuTe

R1

R

R2 R3

R1 = R2 = H; R3 = MeO, TeBu-n R2 = H; R1 = Me; R3 = CH2OH R2 = CH2OH; R1 = H; R3 = TeBu-n R3 = H; R1 = R2= (CH2)4

NaBH4 EtOH

[RTeH]

R1 R1 EtOH (68-93%)

(1)

(ref. 17)

R1 RTe

(2) R1

R1, R2 = H, CH2OH, Me, Ph, p -MeC6H4, p -MeOC6H4 R = n -Bu R1 = H, Me, CH2OH, Me2COH; R2 = Ph R1 = H, CH2OH; R2 = C6H13

(ref. 18,19)

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( )n

Te

n -BuLi THF, 25°C

[n -BuTeLi]

OR H EtOH, reflux

( )n OR +

n -BuTe

main product R = THP n = 1-9

R

SPh

R1TeTeR1

R1Te

R H (82-85%)

NaBH4, EtOH reflux 1 = n -Bu, Ph R, R

Me3Si

Et3Si

SPh 1) n -BuLi

SPh

2) H2O

n -BuTeTeBu-n NaBH4, EtOH reflux

OH n -BuTeTeBu-n NaBH4, EtOH reflux

R1Te

( )n OH main product R = H, n = 1

H

SPh

R

H

SPh

R H (79%)

Et3Si

TeBu-n

(3) (ref. 20)

(4) (ref. 21)

(5) (ref. 21)

OH TeBu-n (84%)

(6) (ref. 22)

Noteworthy is that by treating tellurothioalkene with n-BuLi, the phenylthio group remains untouched, showing the greater reactivity of vinyltellluride towards vinylsulphide (eq. (4)). The silylsubstituted thioacetylene gives only the butyltellurophenylthioethene resulting from a desilylation – hydrotelluration (eq. (5)). 1-Organyltelluro-1,3-butadienes (general procedure).9 To a solution of the conjugated enyne (4.0 mmol) and the ditelluride (1.0 mmol) in EtOH (6.0 mL) at room temperature under N2 is added NaBH4 (0.09 g, 2.5 mmol) in small portions. Towards the end of the addition, when the red colour has disappeared, the mixture is refluxed for 5 h, successively cooled at room temperature, treated with H2O (0.5 mL) and then with 10% aqueous NaOH (0.5 mL). The resulting mixture is diluted with ether (50 mL) and washed with brine (3⫻30 mL). The organic layer is separated, dried (MgSO4) and evaporated in a rotary evaporator (20 torr). The residue is purified by SiO2 flash chromatography (elution with heptane or heptane/EtOAc). (Z)-1-butyltelluro-1,4-(bis-hydroxymethyl)but-1-en-3-yne (typical procedure).18 To a solution of (1,6-dihydroxy)hexa-2,4-diyne (0.11 g, 1.0 mmol) and dibutyl ditelluride (0.184 g, 0.5 mmol) in 95% EtOH (20 mL) under N2 is added NaBH4 (0.045 g, 1.25 mmol) at room temperature. After the disappearance of the red colour, the yellow solution is refluxed for 15 min, allowed to reach room temperature, diluted with EtOAc (40 mL) and washed with brine (3⫻20 mL). After drying the organic phase (MgSO4), the solvent was evaporated under vacuum and the residue purified by SiO2 flash chromatography (elution with hexane/EtOAc, 7:3) to give the product (0.21 g (71%)). Another application of the above hydrotelluration protocol involves the synthesis of unsymmetrical divinyl tellurides by the addition of sodium vinylic tellurolates, generated

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by the reaction of the corresponding ditelluride with sodium borohydryde in aqueous ethanolic solution of sodium hydroxide to terminal acetylenes.9 R1

NaBH4 NaOH / EtOH Te2 R1

R1

R1

R

H

TeNa EtOH, reflux (65-77%)

Te

R1

R = H; = p-ClC6H4 R = Me; R1 = Ph, p-BrC6H4

In neutral conditions, the reduction of the divinyl ditellurides to the corresponding dialkyl ditellurides occurs preferentially to the hydrotelluration reaction.23 R1 R1 R

R Te Te

R = R1 = H R = H; R1 = Me R = Me; R1 = H

1) NaBH4 / EtOH reflux, N2 2) air (67 - 83%)

R1 R1 R

R Te Te

The actual reducing agent of the C⫽C bond seems to be the intermediate tellurol, present if the hydrogenolysis of the Te–Te bond is effected in a neutral medium. In the presence of OH⫺ the tellurol is converted into tellurolate, which does not react with the double bond. Telluroesters have also been employed as starting materials in telluro vinylation reactions. Thus, by treatment of telluroesters with aryl propiolates in the presence of K2CO3, (Z)--acyltellurocinnamates are formed in high yields. This procedure involves the addition of potassium aryltellurolate anions derived from the telluroesters.24 ArTeCOPh + Ar1C CCO2Me Ar = Ph, p -MeOC6H4, p -MeC6H4 Ar1 = Ph, p -ClC6H4

K2CO3 THF.H2O

Ar1 ArTe

H CO2Me (76 - 95%)

K2CO3 90% EtOH

Ar1 ArTe

H CO2Me

Otherwise, telluroacylation of terminal alkynes is achieved by treatment of alkynes with telluroesters in the presence of CuI and triethylamine.25 R H CuI Et3N, Me3NHCl ArTe COAr1 (64 -91%)

R

H + ArTeCOAr1

R = Ph

Ar = Ph, p -BrC6H4, p -ClC6H4, p -MeOC6H4; Ar1 = Ph Ar = Ar1 = p -ClC6H4

R = Ph, EtOCH2; Ar = p -ClC6H4; Ar1 = p -MeOC6H4

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As a further development of the hydrotelluration of alkynes the addition of diisobutylaluminium tellurolate to terminal acetylene was investigated. Regiochemistry of the obtained product leading to 1,1-disubstituted ethylenes is contrary to that observed in the addition of sodium butyltellurolate to aryl acetylenes, which produces exclusively the Z isomer resulting from an anti addition.26

RTeTeR

DIBAL-H

RTeAl(i-Bu)2

Toluene / Hexane reflux

R1

R1C H

(45 - 75%) BuTe R = n -Bu R1 = alkyl, vinyl, aryl

General procedure for the synthesis of 1-(tellurobutyl)-1-(organyl)-ethenes.26 To a solution of dibutyl ditelluride (0.738 g, 2.0 mmol) in dry hexane (4.0 mL) contained in a two-neck, round-bottomed flask, diisobutylaluminum hydride (DIBAL-H) (4.0 mL, 4.0 mmol, sol. 1.0 M in toluene) was added and the mixture refluxed. The corresponding alkyne (12.0 mmol) was added at once to the resulting yellow solution and the reaction mixture refluxed for 4 h. After this time the reaction was cooled to 0°C, water was added (4.0 mL) and the product extracted with hexane (3⫻50 mL) and ethyl acetate (3⫻50 mL). The organic phases were dried over anhydrous MgSO4 and the solvents evaporated under reduced pressure. The alkyl tellurium compounds were removed by distillation using a Kügelrohr apparatus. The residue is the vinylic telluride which was purified by flash chromatography using hexane as eluent. Vinyltellurides with E configuration were obtained by the reaction of alkynes with DIBAL-H followed by the addition of an organotellurenyl bromide in the presence of dry LiCl.26

R

H

DIBAL-H

R

H

Toluene / Hexane reflux

H

Al(i-Bu)2

BuTeBr LiCl (50 - 60%)

R

H

H TeBu major product

R = alkyl, vinyl, aryl

General procedure for the synthesis of (E)-1-(tellurobutyl)-2-(organyl)ethenes. To a 25 mL, round-bottomed flask, containing a solution of the corresponding alkyne (4.0 mmol) in dry hexane (4.0 mL) at 0°C, DIBAL-H (4.0 mL, 4.0 mmol, 1.0 M in toluene) was added at once. The mixture was slowly warmed for 45 min and then refluxed for 3 h. The reaction was cooled to room temperature and dry hexane (8.0 mL) and dry toluene (8.0 mL) were added. Concurrently, the butyltellurenyl bromide was obtained by the addition of a solution of bromine (0.16 g, 2.0 mmol) in benzene (~10 mL) to the solution of dibutylditelluride (0.738 g, 2.0 mmol) in hexane cooled at 0°C under a nitrogen atmosphere. The mixture was stirred at 0°C for 10 min, then LiCl (0.196 g, 4.2 mmol) was added, the dark solution turned clear red and stirring was continued until LiCl was dissolved (10 min). The resulting solution was transferred to the flask containing the vinyl alane. The reaction was stirred for 2 h at room temperature and then a mixture of ice and water (~60 mL) was added. The solids were filtered and the products extracted with hexane (3⫻70 mL) and ethyl acetate

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(2⫻70 mL). The organic layers were combined and after drying over anhydrous MgSO4, the solvents were removed under reduced pressure. To obtain analytically pure samples of the vinylic tellurides, the crude mixture was dissolved in 95% ethanol (20 mL) and NaBH4 (~0.18 g) was added. The resulting yellow solution was stirred for 10 min extracted with petroleum ether (2⫻70 mL), and washed with water (5⫻70 mL). After drying the organic phase over anhydrous MgSO4, the solvent was removed under reduced pressure and the residue distilled in a Kügelrohr apparatus to remove the alkyl tellurides. The residue is the vinylic telluride that was purified by column chromatography using hexane as eluent. The Z isomers were achieved by the addition of DIBAL-H to acetylenic tellurides. R

H

DIBAL-H Toluene / Hexane reflux

H

H

R

TeBu

+ (n -BuTe)2

(30 - 51%)

R = alkyl, vinyl, Ph

General procedure for the synthesis of (Z)-1-(tellurobutyl)-2-(organyl) ethenes. To a 25 mL, round-bottomed flask, containing a solution of the corresponding butyltelluro alkyne (2.0 mmol) in dry hexane (2.0 mL), DIBAL-H (2.0 mmol, 2.0 mL, sol. 1 M in toluene) was added at once. The reaction mixture was refluxed for 2 h, then cooled to room temperature and water added (2.0 mL). The product was extracted with hexane (3⫻50 mL) and ethyl acetate (2⫻50 mL). The organics were washed with water (3⫻50 mL), dried over anhydrous MgSO4 and the solvents evaporated. The product was purified by column chromatography using hexane as eluent for R⫽vinyl and Ph and as above for others. 3.16.1.3

From organyl tellurolate (and telluride) anions and vinyl bromides Br AlLiH4 R1 RTeLi RTeTeR THF / HMPA (83 - 86%)

TeR

(ref. 12)

R1

R = n -Bu, p -MeC6H4; R1 = Ph Br Na / HMPA 120°C, 1h R = R1 = Ph

RTeNa

TeR

R1 120°C, 2h (34%)

(ref. 27)

R1

The reactions of lithium n-butyl and p-methoxyphenyl tellurolate with trans--bromostyrene require, respectively, a 15 min and a 3 h reflux in THF, indicating a greater nucleophilicity of the aliphatic tellurolate towards the aromatic tellurolate. Vinylic tellurides (general procedure).12 A solution of the ditelluride (2.0 mmol) and trans-bromostyrene (0.73 g, 4.0 mmol) in THF/HMPA (10 mL) is added dropwise at room temperature and under N2 to AlLiH4 (0.17 g, 4.5 mmol) in THF (2 mL). The dark red colour of the ditelluride disappears at once. The resulting pale green solution is refluxed (R⫽n-Bu, 15 min; R⫽p-MeOC6H4, 3 h). After cooling to room temperature the reaction mixture is

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treated with H2O (1.0 mL) and 10% NaOH (1.0 mL) and then extracted with ether. The extract is washed with brine, dried (MgSO4) and evaporated. The residue is purified by recrystallization or Kügelrohr distillation. Sodium telluride, obtained by heating elemental tellurium with sodium hydride in DMF, reacts similarly, leading to symmetrical divinylic tellurides28 (compare the similar reaction with aryl iodides, see Section 3.1.2.1).

Te

3.16.1.4

NaH DMF, 140°C

Br Na2Te Ph DMF / THF (41%)

Ph

Te

Ph

From vinylic tellurolate anions and alkyl halides

R

R1 MgBr

+ Te

THF reflux

R = H, Me, Ph; R1 = H R = H; R1 = Me R2 = n -Bu

R

R1

R2Br TeMgBr (73 - 79%)

R

R1 TeR2

(ref. 12, 29)

This reaction is an application of the general method for the preparation of unsymmetrical tellurides (see Section 3.1.3). Vinylic tellurides (general procedure).12 Elemental Te (0.6 g, 5.0 mmol) is added to a solution of the vinylic magnesium bromide (5.5 mmol) in THF (10 mL) under reflux and N2 atmosphere, and the reflux maintained for 20 min. The mixture is allowed to reach room temperature and then treated with n-butyl bromide (0.7 g, 5.0 mmol). After stirring for 10 min, the reaction mixture is cooled at 0°C, treated dropwise with saturated aqueous NH4Cl, extracted with ether, dried (MgSO4) and then evaporated. Kügelrohr distillation of the residue under vacuum gives the vinyl alkyl tellurides as yellow liquids. By treating the vinylic magnesium tellurolates with air while stirring, instead of with alkyl bromides, divinylic ditellurides are formed.23 R1 2R

1) air TeMgBr 2) H2O (51-68%)

R1 R1 R

R Te Te

R = Ph, Me, H; R1 = H R = H; R1 = Me

Divinylic ditellurides (general procedure).23 Elemental Te (12.7 g, 100 mmol) is added at once to a stirred solution of the vinylic magnesium bromide (0.11 mol) in THF (150 mL) under N2. The mixture is refluxed for 30 min with stirring, allowed to reach room temperature, then stirred for 1 h in the presence of air (by opening the apparatus). Deposition of some Te is observed during the oxidation process. The mixture is treated with brine.

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The ditelluride is extracted with ether (3⫻50 mL), and the extract dried (MgSO4) and evaporated. The residue is purified by SiO2 flash chromatography. If the vinylic magnesium tellurolate is hydrolysed before the oxidation, a complex mixture of products is formed. The divinylic ditellurides, obtained as described, are converted into alkyl vinyl tellurides by reduction with AlLiH4 followed by alkylation.23 1 R1 R

R

Te Te

R

1) AlLiH4 / THF 2) R2X (82, 85%)

R1

R

TeR2

R = H; R1 = Me R = Me; R1 = H, R2 = n -Bu

Alkyl vinyl tellurides (general procedure).23 A solution of the divinylic ditelluride (1.0 mmol) in THF (6 mL) is added dropwise, at room temperature under N2, to AlLiH4 (0.09 g, 2.5 mmol) in THF (3 mL). The dark red colour of the ditelluride disappears. The resulting mixture is treated with butyl bromide (0.3 g, 2.2 mmol) and stirred for 15 min. Then H2O (0.5 mL), 10% aqueous NaOH (0.5 mL) and H2O (0.5 mL) are added in sequence. The product is extracted with ether (3⫻10 mL). The extract is washed with brine, dried (MgSO4) and evaporated, giving the product as a yellow oil. 3.16.1.5

From organotellurolate anions and activated vinylic halides

-Aryltelluro vinyl aldehydes and ketones have been prepared by treating -chlorovinyl carbonyl compounds or -acylvinyl triethyl ammonium chlorides with arene tellurolate anions.30

X

COR2

R1

R

R = H, alkyl R1 = H, Ph R2 = H, Ar

ArTe-(a or b or c) MeTe-(d)

Ar(Me)Te R1

COR2 R

X = Cl

X = N+Et3]Cl-

a) ArLi + Te / THF b) ArTeTeAr + Li / napht cat

c) ArTe)2 / NaBH4 / MeOH (70 - 86%)

THF. (37- 65%) R, R1 = (CH2)4 R2 = H

d) MeLi + Te / THF (54%)

Using TeLi2, bis(-acylvinyl) tellurides have been prepared by a similar protocol. 2

Cl

COR2 TeLi2

R1

R

R = R1 = H; R2 = Aryl R, R1 = (CH2)4; R2 = H

R

R1

R2OC

Te

COR2

R1 R (39 - 60%)

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1-(4-Bromophenyl)-3-(4-ethoxyphenyltelluro)-2-propen-1-one (typical procedure).30 Powdered NaBH4 was added in small portion to a suspension of di(4-ethoxyphenyl) ditelluride (4.47 g, 0.009 mol) in methanol (30 mL) and stirred under argon until full decolouration of the solution. A solution of the ammonium salt (X⫽N+(Et3)Cl⫺; R⫽R1⫽H; R2⫽4-BrC6H4) (6.25 g, 0.018 mol) in methanol was added under stirring. Deposition of the telluride occurred, starting when ~2/3 of the ammonium salt solution was added. The reaction mixture was refluxed under stirring for 30 min, cooled at room temperature and filtered off. Water (100 mL) was added to the filtrate, and the telluride remaining in solution was extracted with benzene (3⫻25 mL). The solvent was removed in vaccum to give an additional amount of the product – total yield 7 g (85%). Yellow crystals (from benzene/hexane, 1:1), m.p. 143–144.5°C. Similar substitutions at activated vinylchlorides have been performed using aluminium tellurolates.31 O BuTeTeBu + Te

DIBAL-H

[BuTeAl(i -Bu)2]

R

Cl

R1 0°C to r.t.

O

TeBu R1

R

This protocol has been applied to the coupling of arene tellurolates with (E)-2-iodo-1alkenyl sulphones, enolphosphonates, tosilates and triflates of -dicarbonyl compounds. R

SO2pTol ArTeNa

I

H

R

SO2pTol

ArTe

(ref. 32)

H Br2

R = n-Bu, C5H11, C6H13, Ph

R

Ar = Ph, p -FC6H4

SO2pTol

ArHTe Br Br

H (57- 72%)

THF

n -BuTe

O

O n-BuLi + Te

n -BuTeLi +

X

R2 R1

R

0°C,1-20 min

(ref. 31)

R = Me, R1 = H; R2 = Me, OEt a) X = O-P(O)(OEt)2 (65 - 80%)

R = Ph; R1 = H; R2 = Me R, R1 = (CH2)4; R2 = OMe, 1

R , R = (CH2)3;

R2

R

R2 R1

Ph

= Me

b) X = OAc; c) OTs

R = R2 = Me; R1 = H

d) X = Tf (70 - 80%)

R = Me; R2 = OEt; R1 = H

General procedure for the substitution reactions between activated enols and lithium n-butyltellurolates.31 To a suspension of elemental tellurium (0.38 g, 3 mmol) in THF (4 mL) under nitrogen at 0°C was slowly added n-butyllithium (from a 1.4 M solution in hexane, 2.1 mL, 3 mmol). A clear yellow solution was formed. Then the appropriate enol

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was added (2 mmol) and the mixture was stirred at 0°C monitoring by TLC, until the consumption of the enol. The reaction mixture was diluted with ethyl acetate (50 mL) and washed with brine (3⫻50 mL). The organic phase was dried with magnesium sulphate and the solvents were evaporated. The residue was purified by silica gel column chromatography eluting with hexane/ethyl acetate (9:1). Important remarks are that starting from mixtures of E/Z enol derivatives, only the (Z)-vinylic tellurides are obtained, and comparative experiments demonstrate that alkyl tellurolates (n-, s- and t-BuLi) react faster than the aromatic (PhMgBr, 2-ThLi), and that the reaction time is not influenced by the nature of the leaving group (phosphate, acetate, tosylate and triflate). 3.16.1.6 From organyl tellurolates and electrophilic acetylenes Organyl tellurolate anions effect 1,4-additions to acetylenes bearing electron-withdrawing groups such as acetylenic ketones,33 aldehydes, esters, diacetylenic ketones,33–36 as well as acetylenic phosphonates and sulphones32,37,38 giving 2-substituted vinyl tellurides with Z configuration. O R

COR1

ArTeNa / EtOH [(ArTe)2 / NaBH4] (68 - 88%)

ArTe R

H

R1

R = H, n -Bu, Ph; R1 = Ph, H, OEt, OMe; Ar = Ph, p -MeOC6H4, p -MeC6H4 R = Ph; R1 = Ph

RTeTeR

NaBH4

EWG = P(O)(OEt)2

RTeNa

R = n-Bu,Ph

R1

R1

EWG

RTe

R1 = n -Bu, Ph , R1 = H, n -Bu, Ph

R = Ph, p -ClC6H4, R1 = Ph, C5H11 p -FC6H4, p - Tol

EWG Yield % 42-69 (ref. 37) 72-95 (ref. 38) 57 - 67 (ref. 32)

EWG = PhSO2 EWG = p-TolSO2

R, R1 = Ph, n -Bu R = Ph, p -FC6H4 R1 = H, Ph, n -C6H13

93 - 94 (ref. 38) 71 - 82 (ref. 32)

EWG = -CHO

R = n -Bu, Ph; R1 = H

80 - 84 (ref. 36)

Addition of aryl tellurolates to acetylenic ketones (general procedure).34 NaBH4 (0.55 g, 15 mmol) is added in small portions, under N2, to a solution of the diaryl ditelluride (5 mmol) in EtOH (20 mL) at room temperature. A solution of the acetylenic ketone (10 mmol) in EtOH (10 mL) is then added to the reaction mixture, with stirring. The resulting precipitate of the 2-carbonylvinyl telluride is filtered off and recrystallized from EtOH.

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The obtained telluroacroleins have been submitted to a Wittig methylenation giving the expected tellurodienes.36 RTe

CHO Ph3P=CH2

RTe

By treating alkynes with diaryl ditellurides and sodium arylsulphonates in AcOH/H2O, an anti-tellurosulphonation takes place. Some of the obtained tellurosulphones have been converted into the corresponding dibromides. In the case of R⫽Me3Si a desilylate product is formed.32

R

H

ArTeTeAr / NaSO2Ar1 AcOH / H2O,80°C

ArTe

H

PhTeTePh / NaSO2p - Tol AcOH / H2O, 80°C

ArTe H Br Br (74 - 85%)

H

Me3Si

SO2p-Tol

PhTe

SO2Ar1

Ar

(51-70%)

R = n -Bu, C5H11, C6H13, Ph Ar = Ph, p -FC6H4 Ar1 = Ph, p -Tol, p -ClC6H4 Me3Si

SO2Ar1 Br2

R

SO2p - Tol

AcOH PhTe

H

26%

Anti-tellurosulphonation of 1-alkynes.32 A solution of alkyne (1.1 mmol), sodium arylsulphinate (5 mmol) and diorgano ditelluride (0.5 mmol) in 5 mL of 80% aqueous acetic acid was heated at 80°C for 24 h (R⫽Ph, 5.2 h). The reaction mixture was diluted with 20 mL of AcOEt, washed with saturated aqueous sodium bicarbonate, dried over Na2SO4 and concentrated in vacuum. Crude products, were concentrated under vacuum, and the residues were purified by preparative TLC on silica gel (AcOEt/hexanes, 1:10), treated with 1 equiv of Br2 in AcOEt and concentrated under vacuum. The residues were purified by recrystallization with CH3OH/HCCl3. 3.16.1.7

Tanden vicinal difunctionalization of alkynes

A tanden anti-vicinal difunctionalization of alkynes, involving the addition of a lithium organotellurolate to an activated alkyne, with subsequent trapping of the vinyllithium intermediate with electrophiles, was recently reported and named electrotelluration.39

R

EWG + R1TeLi

THF -20 to -30°C 5 - 20 min

R1Te

R = H, Me; EWG = CO2Me, SO2Ph, SOPh R1 = n -Bu, Ph E = PhCHO, C3H7CHO, Ph2CO 43 - 84% (Z main or only product) R = H; EWG = CO2Me2; R1 = n -Bu E = Me3SiCl 45% (E only product)

R

EWG Li

E+

R1Te R

EWG E

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Employing alkinyl esters bearing an aldehyde group with different chain length, an intramolecular reaction occurs giving five- to eight-membered cyclic systems. OH O ( )n

O

RTeM O O

H n = 2- 5

O( )n

TeR (31 - 80%)

RTeM = PhTeLi, t -BuTeLi, PhTeMgBr,

3.16.1.8

TeMgBr

Telluroacylation of terminal alkynes

The treatment of terminal alkynes with telluroesters in DMF in the presence of CuI and triethylamine, followed by addition of trimethylamine hydrochloride, gives rise to (Z)-(-aryltelluro)-,-enones.40 H + ArTeCOAr1

R

CuI R Et3N, Me3NHCl ArTe

H

COAr1 64-91% Ar = Ph, p -BrC6H4, p -ClC6H4, p -MeOC6H4; Ar1 = Ph Ar = Ar1 = p -ClC6H4

R = Ph

R = EtOCH2; Ar = p -ClC6H4; Ar1 = p -MeOC6H4

3.16.2

Starting from electrophilic tellurium

3.16.2.1 By addition of tellurium tetrahalides and aryltellurium trihalides to acetylenes Tellurium tetrachloride and aryltellurium trichloride,41–44 as well as tellurium tetrabromide45 and aryltellurium tribromides45,46 add to acetylenes to produce, respectively, 2-halovinyl tellurium trihalides and dihalides, which can be submitted to further manipulations.

R X = Cl

R1

CCl4 (77-97%)

TeX4

R1

R Cl

TeCl3

R = Ph, CH2OH; R1 = H, alkyl, Ph

TeCl4 + H

AcOH

Ph

R = Ph; R1 = H

Cl

)2TeCl2 Ph

Na2S.9H2O

Ph

R = R1 = Ph

Cl

1) NaBH4

R

2) R2X R2 = alkyl

X = Br H

R

(70, 89%)

Br

2R

R NaBH4 Br Te Br Br

R = Ph, C5H11

TeR2

Cl

R Br

Te)2 R1

R Te

Br

Na2S.9H2O

Ph

H

Cl

)2Te

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X = Cl Ar(R)TeX3 X = Br

R

R H benzene reflux (60-85%)

Te(Ar)Cl2

Cl

85

Cl

NaBH4 THF /H2O, r.t.

H

TeAr

R

H

OH ;

R = alkyl, CH2OH, aryl, Ar = p-MeOC6H4

R1

R1

H

Br

solvent (60-85%)

R1

NaBH4 TeAr(R) Br Br

H

Br

TeAr(R)

Ar, R = Ph, C5H11, HOCH2, CH3OCH2 R1 = Ph, p -tolyl, α -naphtyl, Bu

The addition of TeCl4, aryltellurium trichlorides and TeBr4 is stereoselective giving only Z adducts. The addition of aryltellurium tribromides exhibits Z or E configuration depending on the solvent, benzene or ethanol used in the reaction. 2-Chloro-2-phenylethenyl-1-p-methoxyphenyltellurium dichloride (typical procedure).43 p-Methoxyphenyltellurium trichloride (1.7 g, 5 mmol) is added to phenylacetylene (0.52 g, 5 mmol) in benzene (50 mL). The mixture is refluxed for 6 h and the reaction is monitored by TLC (elution with CHCl3). The mixture is washed with MeOH/H2O (1:1, 3⫻30 mL). The organic phase is dried (MgSO4) and the solvent evaporated in a rotary evaporator. The residual oil is filtered through a column of SiO2, eluting first with CCl4 and then with CHCl3/MeOH (9:1). The product is recrystallized from CHCl3/petroleum ether at 30–60°C (1.7 g (77%); m.p. 134–135°C). 2-Chloro-2-phenylethenyl-1-p-methoxyphenyl telluride (typical procedure).43 The dichloride from the preceding preparation (1.55 g, 3.5 mmol) in THF (30 mL) is treated dropwise with NaBH4 (0.14 g, 3.5 mmol) in H2O (17 mL). An immediate reaction occurs with gas evolution. After stirring for 15 min at room temperature the mixture is diluted with ether (30 mL) and washed in sequence with H2O, saturated aqueous NH4Cl and brine. The organic phase is dried (MgSO4), and evaporated in a rotary evaporator. The residual oil is filtered through a column of SiO2 (elution with petroleum ether at 30–60°C) (1.07 g, (84%)). In the case of 3-hydroxyalkynes the OH group promotes anti addition. Depending on the steric hindrance at the propargylic position, four- or five-membered cyclic tellurium oxychlorides are formed.47 ArTeCl3 + HO

O

ArTe Cl Cl ArTeCl3 +

Cl

Cl

HO

Te Cl Ar

Cl HO

HO ArTe Cl Cl

O Te Cl Ar

Cl

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Addition reaction of p-methoxyphenyltellurium trichloride to 3-hydroxy alkynes.47 A solution of the 3-hydroxy alkyne (11 mmol) in dry benzene (10 mL) was added to a suspension of p-methoxyphenyltellurium trichloride (3.40 g, 10 mmol) in dry benzene (40 mL). The reaction mixture was refluxed for 8 h, during which the p-methoxyphenyltellurium trichloride was consumed, forming a clear yellow solution. The resulting solution was cooled to room temperature, diluted with ethyl acetate and washed with a saturated NH4Cl solution and brine. The solvent was evaporated under reduced pressure, and the residue was quickly chromatographed on SiO2, using CCl4 and CHCl3/MeOH (5:1) as eluents. The resulting oil was crystallized from CHCl3/hexane. A more recent report describes the regio- and stereoselective addition of aryltellurenyl iodides (prepared in situ from the corresponding ditellurides and iodine) to alkynes to afford the (E)-1-iodo-2-aryltelluro-1-alkenes, which treated with bromine give the corresponding dibromides.32

R

H

ArTeI ArTe)2 + I2 THF, 0°C

I Br2

R ArTe

H

R = n -Bu, C5H11, C6H13 Ar = Ph, p -ClC6H5

R

I

ArTe H Br Br (37-53%)

Anti-addition of ArTeI with 1-alkynes.32 A mixture of 1 mmol of ArTeI (prepared by the reaction of a diarylditelluride (0.5 mmol) with I2 (0.55 mmol) in THF at 0°C) and 1 mmol of an alkyne was stirred at room temperature for 12 h followed by the treatment with saturated Na2S2O3. After stirring for 30 min, the mixture was extracted with hexane, dried over CaCl2 and concentrated in vacuum. The residues were purified by preparative TLC on silica gel eluted with hexanes and then reacted with Br2 (1 equiv). 3.16.2.2

From organyltellurenyl halides and vinylic Grignard reagents R

THF R1 + R2 TeY (71-86%) MgX

R

R1 TeR2

R = R1 = H

R2 = Ph; Y =I (ref. 48) R2 = p -MeOC6H4, n -Bu; Y = Br (ref. 12)

R = Ph; R1 = H R = H; R1 = Me

R2 = p -MeOC6H4; Y = Br (ref. 12)

This reaction is an application of the general method for the preparation of unsymmetrical tellurides (see Section 3.1.3.5). The tellurenyl halide is prepared in situ by halogenolysis of the corresponding ditelluride.12 Vinylic tellurides (general procedure).12 A solution of the organotellurenyl bromide, prepared by adding Br2 (0.40 g, 2.5 mmol) in benzene (5.0 mL) to the corresponding ditelluride (2.5 mmol) in THF (10 mL) at 0°C under N2, is added dropwise to a solution of the Grignard reagent (7.0 mmol) in THF (10 mL) at 0°C under N2. A gradual disappearance of the red colour of the reaction mixture is observed. After stirring the mixture for 1 h at

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0°C, the solvent is evaporated, and the residue is treated with saturated aqueous NH4Cl, extracted with ether, dried (MgSO4) and evaporated. The product is purified by SiO2 column chromatography (elution with benzene/hexane, 9:1). The obtained ethenylphenyltelluride can be deprotonated with strong hindered bases (LDCA) giving the lithium derivatives which react with electrophiles.48 TePh OH

1) PhCHO 2) H2O (88%) TePh H

Ph

TePh

LDCA THF

TePh

D2O

Li

D

3.16.2.3

Me3SiCl

TePh

(80%)

SiMe3

From vinyltellurenyl iodides and Grignard reagents

The following scheme is self-explanatory.12 RMgX +R1

THF R2 TeI 0°C -r.t., 1 h (64-71%)

R1

R2 TeR

R = p -MeOC6H4, n -Bu; R1 = R2 = H R = p -MeOC6H4; R1 = H; R2 = Me

3.16.3

Via radical reactions

Diorganyl tellurides add to alkynes via a radical mechanism in the presence of a catalytic amount of a radical initiator such as AIBN or 2,2⬘-azobis(4-methoxy-2,4-dimethylvaleronitrile) (ln), as well as under irradiation by visible light (tungsten lamp), affording vinylic tellurides in a variable Z/E ratio.49 RTeR1 + R2

R3

AIBN (0.1 mmol) benzene(1 mL), reflux

R2

R3

RTe

R1

Telluride

Alkyne (percentage yield, (E) / (Z) ratio)

PhTePr -i

R3 = H; R2 = Ph (97,56 / 44), Ac (99,19 / 81), MeO2C (93,29/ 71) R2 = R3 = CO2Et (95, 37 / 63)

PhTeBu-n n -BuTeBu-t

Ac H (73, 20 / 80) R3 = H; R2 = Ac (84, 88 / 12), n -hexyl (42, 80 / 20) Me3Si (77, 0 / 100), Ph (93, 100 / 0a) R2,R3 = (CH2)6 (52) (E)

n -BuTeCH2Ph

Ph

aCatalyst

H (91, 56 / 44)

= In; the adduct is also obtained by irradiation (tungsten lamp), 50°C, 3h, neat (84%, 100/0)

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The examination of the structure of the formed products revealed that the fission of the telluride bonds occurs exclusively at C(sp3)–Te rather than C(sp2)–Te bonds, at tertiary C–Te rather than primary C–Te bonds, and at PhCH2–Te rather than n-Bu–Te bonds. The following scheme shows that (2-propargyloxyphenyl)acyltellurides and (2-propargyloxyalkyl)aryltellurides undergo an intramolecular version of the above radical addition, promoted respectively by photolysis (tungsten lamp) and irradiation in the presence of (Bu3Sn)2 or Bu3SnH/AIBN giving chromanone and furan vinyltelluro derivatives (see also Section 5.7). O

O hν, 8°C

TeAr O

(ref. 50, 51)

TeAr

(100%)

O

Ar = p -FC6H4 R1 TeAr

O R

R

(Bu3Sn)2 or Bu3SnH / AIBN hν, bz, reflux (40, 46%)

R1

TeAr (ref. 52) O

R, R1 = (CH2)4 R = H; R1 = CH2CH2CH=CH2, Et, Ph, CH2Oallyl, CH2OPh, CH2OBz Ar = p -FC6H4

Diphenyl ditelluride, on irradiation with visible light, effects the vic-phenyltelluration of acetylenes, giving the corresponding bis-phenyltelluroalkenes in good yields.53

R

R1 + PhTeTePh

hυ (68-90%)

a) R = n -hexyl, HOCH2, H2NCH2, Ph,

PhTe R

R1 TePh ; R1 = H

R1

b) R = Ph; = CHO R = R1 = CO2Et

With non-activated acetylenes (R–≡–R1 (a)) the best results are provided by irradiation with visible light (400 nm) at 40–70°C, giving exclusively the E isomers. On irradiation in the near-ultraviolet (300 nm), lower yields are obtained, probably because of the reverse reaction to the starting materials. The addition to activated acetylenes (R–≡–R1 (b)) proceeds even upon irradiation at 300 nm. In this case, the intermediate vinyl radical is stabilized by the carbonyl, vinyl or aryl groups, and the reverse reaction to the starting materials is suppressed. . R

R1 TePh

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(E)-1,2-bis(phenyltelluro)-3-hydroxyprop-1-ene (typical procedure).53 A mixture of propargyl alcohol (0.14 g, 0.25 mmol) and diphenyl ditelluride (1.02 g, 0.25 mmol), in a Pyrex glass tube sealed under reduced pressure (after filling with argon), is irradiated at 70°C for 24 h through a filter (400 nm) with a tungsten lamp (500 nm) situated ~20 cm from the tube. The mixture is purified by preparative TLC on SiO2 (elution with hexane), giving the pure product (0.90 g (78%)).

3.16.4

Reduction of acetylenic tellurides

The reduction of acetylenic tellurides to the vinylic ones is achieved by treatment with NaBH4 in EtOH.54 The formal unusual reduction of the carbon triple bond by NaBH4 can be rationalized involving the attack of a hydride ion to the tellurium atom producing a tellurol and an acetylenic anion followed by the addition of the tellurolate anion to the acetylene, DIBAL-H has later been employed as a reducing agent.26

Ph

TeR

NaBH4 EtOH, refl., N2

R = n -Bu, C12H25

Ph

C- + RTeH-

Ph

CH + RTePh

TeBu

Ph

TeR

H

H

R

TeBu

TeR -

DIBAL-H toluene / hexane reflux, N2 R = alkyl, vinyl, Ph

R

EtOH (80-93%)

H

R

TeBu

H

Al(i-Bu)2

H2O

H H (30-51%)

Preparation of vinylic tellurides by the reaction of phenyl(alkyltelluro)acetylenes with NaBH4 under reflux.54 NaBH4 (0.093 g, 2.5 mmol) in EtOH (3.0 mL) is added to a solution of the phenyl(alkyltelluro)acetylene (2.0 mmol) in EtOH (10 mL) under N2. The solution is stirred under reflux for 3 h (the colour turns a pale yellow). Heating is stopped, and the pale yellow solution is treated with H2O (0.5 mL) and 10% NaOH (0.5 mL) and diluted with ether (30 mL). The mixture is washed with brine, the organic layer is dried (MgSO4) and the solvent is evaporated. The residue was chromatographed on SiO2 (eluting with hexane) to give the vinylic telluride. 3.16.5 3.16.5.1

Vinylic tellurides via olefination reactions Horner-Emmons route

-Phenyltelluro phosphonates, prepared from the electrophilic or nucleophilic reagents RTeI and RTeLi, undergo subsequent treatment with base and aldehydes to provide vinylic telluride with E configuration.55 Ketene telluroketals have been prepared by a

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similar olefination by one-pot, or by a step-by-step procedure.56 Ketones are almost unreactive.

(EtO)2P(O)CH2R R = H, Me, Ph R1 = Ph

1) LDA / THF, -78°C 2)R1TeI (65-90%) (EtO)2P(O)CH(R)TeR1 (EtO)2P(O)CH(R)I

R1TeLi (51-77%) R=H R1 = n -Bu, Ph, Me3SiCH2, BuC C-

R1Li + Te

R2 1) NaH / THF, r.t. 2CHO / THF, refl. 2) R H R = H; R1 = Ph R2 = vinyl, aril

H TePh

Preparation of vinyl tellurides.55 To a mixture of NaH (1 mmol) (80% suspension in oil) and (phenyltelluromethyl)-phosphonate (1 mmol) in THF (5 mL) at reflux under N2, aldehydes (1 mmol) were added dropwise. Following 3 h at reflux, the solution was cooled to room temperature. Then, saturated aqueous NH4Cl was added and the mixture was extracted with diethyl ether. The organic extract was dried (MgSO4) and evaporated to give vinyl tellurides, which were purified by column chromatography on silica gel with ethyl acetate/hexane (1:20) or by preparative TLC.

(EtO)2P(O)CH3 1) LDA 2) PhTeBr (EtO)2P(O)CH2TePh R=H

1) 3.1 LDA

R

TePh

2) 2PhTeBr 3) RR1CO

R1

TePh

1) 3.1 LDA 2) PhTeBr

(EtO)2P(O)C(TePh)2

RR1CO

(41-94%)

R1 = alkyl, vinyl, aryl

1,1-Bis(phenyltelluro)-1-pentene (typical procedure).56 To a solution of LDA (3.1 mmol) in THF (4 mL) cooled to ⫺78°C, under nitrogen, was added a solution of (phenyltelluromethyl)phosphonate (0.71 g, 2 mmol) in THF (1 mL) dropwise. The reaction was warmed up to 0°C and stirred 30 min at 0°C, then cooled to ⫺78°C and the PhTeBr (1 mmol) in THF (2 mL) was added. The temperature was again raised to 0°C for 30 min, butyraldehyde (1 mmol) was then added and the reaction mixture stirred at room temperature for 2 h. Usual work-up and column chromatography (SiO2, hexane) gave 0.45 g (94%). -Thiovinyl tellurides (telluro(thio)ketene acetals and -cyanovinyl(telluro acrylonitriles)) have been synthesized via Horner reaction, by treating respectively thiomethyl or cyanomethyl diethylphosphonates (easily accessible via Arbusov reaction of triethylphosphonate with chloromethyl sulphides or chloromethyl acrylonitrile) with LDA, organotellurenyl bromides and aldehydes.

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These olefination reactions are not stereoselective, giving mixtures of Z and E isomers.57,58 O (EtO)2 P

X

LDA THF, -78°C

O (EtO)2P

X

R1TeBr

LDA

Li

R2CHO X 1 0°C to r.t. TeR

R2HC

X TeR1

Li O (EtO)2 P

O (EtO)2P

X TeR1

(X = SR, CN)

X = SMe, SPh

45-75% yields R1 = H 38% R1 = Ph, n -Bu 2 R = Ph, p -Tol, p -ClC6H4, 2-Furyl, n -C3H7, i -C3H7 X = CN 78-93% R1 = Ph; R2 = n -C3H7, i -C3H7 yields R2 = H 23%

General procedure for the synthesis of ketene (S,Te) acetals.57 To a solution of LDA (3.1 mmol) in THF (4 mL) cooled to ⫺78°C, under nitrogen, was added dropwise a solution of thiomethylphosphonate (2 mmol) in THF (1 mL). The reaction was stirred at this temperature for 30 min after which PhTeBr (1 mmol) in THF (2 mL) was added. At the beginning the tellurium consumption was fast but by the end of the addition a slightly red solution remained, which became yellow in ca. 20 min. The temperature was raised to 0°C for 30 min, the aldehyde (1 mmol) was then added and the reaction mixture stirred for 1 h at 0°C and for 30–90 min at room temperature. The reaction was treated with water and extracted with ethyl acetate (3⫻25 mL). The organic layer was dried over MgSO4 and the solvent removed under vacuum. The residue was purified by column chromatography and eluted with hexane, yielding the product (mixture of isomers). General procedure for the synthesis of -phenyltelluro acrylonitriles.58 A solution of cyanomethylphosphonate (0.177 g, 1 mmol), THF(1 mL) was added dropwise to a solution of LDA (2.1 mmol) in THF (3 mL) at ⫺78°C under nitrogen. The reaction was stirred at this temperature for 20 min, then a solution of C6H5TeBr (2 mmol) in THF (2 mL) was added at 78°C. The temperature was raised to 0°C for 30 min, the aldehyde (1 mmol) was then added and the stirring continued for 30 min at 0°C and for an additional 30–90 min at room temperature. The reaction mixture was treated with water and extracted with ethyl acetate (3⫻25 mL). The organic layer was dried over MgSO4 and the solvent removed in vacuo. The residue was purified by column chromatography (SiO2) using hexane/ethyl acetate (99:1) as eluent. 3.16.5.2

Wittig route

Tellurophosphoranes, obtained through a transylidation reaction between tellurenyl halides and phosphoranes, react with aldehydes to give the expected vinylic tellurides as an E/Z isomeric mixture (method a). One other methodology involves the treatment of equimolar amounts of phenyl tellurenyl bromide and phosphonium salts with t-BuOK followed by an aldehyde (method b). Under these lithium-salt-free conditions, (Z)-vinylic tellurides are the main products.59

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Method a

2(Ph3P+CH2R)lR=H

n -BuLi THF, r.t.

2Ph3P=CHR

R1 = i -Pr, Ar

PhTeBr THF, -78°C -(Ph3P+CH2R)Br-

Ph3P=CRTePh

R1CHO THF, r.t. -Ph3PO (30-76%)

R

R1 TePh E+Z

Method b 1/2 PhTeTePh

1/ 2 Br2 solvent

[PhTeBr]

Ph3P+CH2R]X-

R = H, Me X = Br, I R1 = i -Pr, aryl

THF, r.t.

t - BuOK [Ph3P+CH(R)TePh]Xr.t. t -BuOK 1 R CHO 1 R CH = C(R)TePh Ph3P=C(R)TePh THF, r.t. E:Z 7-48% 1:7 to 8.5

[Ph3P+CH2R]PhTe-XBr

Method a. (-Styryl) phenyl telluride.59 To a solution of methyltriphenylphosphonium iodide (0.405 g, 1 mmol) in dry THF (4 mL) at room temperature was added dropwise n-BuLi (2 mmol). After stirring at room temperature for 20 min the solution was cooled to ⫺78°C and a solution of PhTeBr (0.28 g, 1 mmol in 2 mL THF) was added, followed by benzaldehyde (0.16 g, 1.5 mmol). The temperature was raised to room temperature and stirred for 3 h. The solvent was removed under vacuum and the residue incorporated on SiO2 and purified by flash column chromatography (SiO2/hexane) giving an oil. Yield: 0.185 g (60%). E/Z⫽2. Method b. (2-Furylvinyl) phenyl tellurides. To methyltriphenylphosphonium iodide (0.405 g, 1 mmol) dissolved in dry THF (4 mL) was added a solution of PhTeBr (0.28 g, 1 mmol in 2 mL THF) and after 15 min stirring was added t-BuOK (0.25 g, 2.2 mmol). The mixture was stirred for 20 min and 2-furaldehyde (0.14 g, 1.5 mmol) was added. After stirring again at room temperature for 3 h, work-up and purification as above gave an oil. Yield: 0.143 g (48%), Z/E⫽8:1. Symmetrical divinyl tellurides VI have been prepared via a ylidation reaction involving the treatment of bis-phosphonium halotellurate I with excess base and then with aldehydes in THF at ⫺78°C (method c).60 Method c TeCl4 + 2Ph3P+CH3]I-

2Ph3P+CH3]2Te Cl4I2 LICA excess I

[Ph3P=CHTe(Cl2)CH=PPh3] III X = Cl, I

[Ph3P+CH2Te(Cl2)CH2P+Ph3]X2II

1/2 Ph3P=CH-Te-CH=PPh3 + Ph3P-CH(X)-Te(Cl2)-CH=PPh3 IV V RCHO 1/2 RCH=CH-Te-CH=CHR VI

The disproportionation of III into IV and V can be rationalized on the basis of the known reaction of phosphoranes with halogen sources giving -haloalkyl-phosphonium salts.61

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In an alternative methodology, the bis-tellurophosphorane IV is prepared in situ by means of a transylidation reaction between II and excess of methylene triphenyl phosphorane (generated from the corresponding phosphonium salt and n-BuLi) (method d).60 Method d Ph3P+CH3]I-

n -BuLi

Ph3P=CH2

Ph3P=CH-Te(Cl2)-CH=PPh3 III R = Ph, p -MeC6H4, p -ClC6H4, 2-furyl

TeCl4

2Ph3P=CH2 Ph3P+-CH2-Te(Cl2)-CH2-P+Ph3]Cl2+ -2Ph 3P -CH3]Cl II

1/2Ph3P=CH-Te-CH=PPh3 + 1/2 Ph3P-CH(X)-Te(Cl2)CH=PPh3 IV V RCHO 1/2 RCH=CH-Te-CH=CHR VI

Method c.60 To a solution of tellurophosphonium salt Ph3P+Me]2Te2-Cl4I2 (0.539 g, 0.5 mmol) in dry THF (5 mL) at ⫺78°C under nitrogen, a solution of lithium dicyclohexylamide (LICA) (3 mmol) in THF (3 mL) was added dropwise. After 3 h of stirring at this temperature, benzaldehyde (0.159 g, 1.5 mmol) was added. The temperature was raised to room temperature, the mixture was stirred for 2 h and then treated with water and extracted with ethyl acetate (3⫻25 mL). The organic layer was dried over MgSO4 and the solvent removed under vacuum. The residue was purified by column chromatography over silica gel, eluting with hexane, to afford the bis-(2-phenylethenyl) telluride (mixture of isomers). Yield: 0.073 g, 44%. Method d. To a solution of methyltriphenylphosphonium iodide (2.43 g, 6 mmol) in dry THF (25 mL) under nitrogen, at room temperature, n-BuLi (6 mmol) was added dropwise. After stirring at room temperature for 20 min the solution was cooled to ⫺78°C and a solution of TeCl4 (0.270 g, 1 mmol) in dry THF (10 mL) was added. The mixture was stirred for 20 min and isobutyraldehyde (0.216 g, 3 mmol) was added. The temperature was raised to room temperature, the mixture was stirred for 3 h and then treated with water and extracted with ethyl acetate (3⫻25 mL). The organic layer was dried over MgSO4 and the solvent removed under vacuum. The residue was purified by column chromatography over silica gel, eluting with hexane, to afford the divinyl telluride (mixture of isomers). The reaction with aromatic aldehydes (R⫽Ph, p-Tol, p-ClC6H4, 2-furyl) is not strongly stereoselective, the E geometry being preferred. In the presence of HMPA (30%) the Z isomers become predominant. In the case of aliphatic aldehydes (R⫽i-C3H7, C3H7) the Z geometry is preferred in both solvent systems. The yields are in the same medium range for both systems (15–48%). Considering that the described reaction is feasible for both aromatic and aliphatic aldehydes, that the experimental procedure is very easy, that the yields, in spite of moderate, are not far from the theoretical, the described method is certainly a useful contribution for the synthesis of symmetrical divinyl tellurides. 3.16.6

Vinylic tellurides via borane chemistry

Internal vinyl tellurides, which are not accessible via hydrotelluration of alkynes, have been prepared from alkynes through a vinyl borane route.

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The sequential treatment of 1-alkynes with n-BuLi, trialkyl borane and a tellurenyl bromide reaches the (Z)-borosubstituted vinyl telluride which is easily hydrolyzed to the disubstituted vinyl tellurides. The reaction proceeds with high regio- and stereocontrol.62 R

H

R

1) n -BuLi, THF, -20°C BR1

R2Te

2) 3, 0°C 3) R2TeBr

R1 BR1

2

AcOH r.t.

R1

R R1Te

H (52, 72%)

R = R2 = n -Bu R = n-C5H11; R2 = Ph R1 = Et

It was observed that the obtained telluroborane fails to react under the Suzuki coupling (PhI/Pd(PPh3)4/Na3PO4/DMF) to give trisubstituted vinyl tellurides. Acetylenic tellurides can also be submitted to hydroboration reactions. R

TePh

R = n -C5H11

1) c -Hex)2BH

R

TePh

2) PhI, Pd(PPh3)4 NaOH, THF

H

Ph

+

R Ph

TePh

+

H

R

Ph

H

TePh

60 : 14 : 26 (25%)

An additional protocol involves the reaction of vinyl boranes with diorganyl ditellurides.63 R

H

1) c -Hex)2BH

R

THF, -10 to 0°C

H

R = Ph, MeOCH2, MeCH2OCH2 R1 = Ph, p -Tol, n -Bu

R1TeTeR1 Pd(PPh 3)4 cat. B(c -hex)2

H

R

H

H

TeR1

(59-68%)

Vinylic tellurides from vinylboranes.63 To a solution of cyclohexene (10 mmol) in THF (10 mL) was added a solution of diborane (5 mmol) in THF at 0°C with stirring; the precipitate thus formed [(c-Hex)2BH] was stirred at 0°C for 1 h. The reaction mixture was diluted with a solution of a terminal acetylene (5 mmol) added at ⫺10°C, and the mixture was kept at 0°C for 2 h. After the precipitate had dissolved, the resulting solution was treated with 3 M NaOH (2 mL), diorgano ditelluride (4 mmol) and 3% Pd(PPh3)4, then was refluxed for 3 h under N2. After the reaction was complete normal work-up was performed. Vinyltellurides were isolated and purified by TLC with petroleum ether (30–60°C) as developer.

3.16.7 Telluro(seleno)ketene acetals, 1-seleno-2-telluro-ethenes, telluro ketene acetals, telluro(stannyl)ketene acetals and telluro(thio) ketene acetals Vinylic tellurides linking another chalcogeno group at the - or -position deserve special synthetic interest since the tellurium moiety can be removed selectively by means of several methods, furnishing new reactive vinylic intermediates.

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The easily available 1-bromo-1-seleno alkene, by treatment with sodium phenyl tellurolate under bis(bipyridine)nickel (II) bromide catalysis, furnishes the corresponding telluro(seleno)ketene acetal.64 Ph H

SeMe PhTeNa, EtOH (bipy)2NiBr Br reflux

Ph

SeMe TePh (80%)

The same ketene acetal, as well as telluro(thio)ketene acetals has been obtained with retention of configuration by subsequent treatment of (E)-1-bromo-1-seleno(or thio) alkenes with n-BuLi and diphenyl ditelluride, resulting from the preferential Br/Li exchange over chalcogeno exchange.65

R

YR1

H

Br

n -BuLi hexane, 0°C

R

YR1

H

Li

R

SePh

H

TePh (63%)

(YR1 = PhSe)

PhTeTePh

R R = Ph, C5H11

SPh

H TePh (51, 63%) YR1 = MeS

Telluro(seleno or thio)ketene acetals (typical procedure).65 n-Butyllithium (1.6 M hexane solution, 1.5 mmol) was added to a hexane (5.0 mL) solution of -bromo-vinylic chalcogenide (1.0 mmol) at room temperature. After stirring for 5 min, the electrophile (1.5 mmol) was added at 0°C and the mixture was stirred at room temperature for 8 h (for products with R⫽n-C5H11, Ph; YR1⫽SMe), the solution was allowed to react at 65°C) and treated with saturated NH4Cl solution (5 mL) extracted with hexane and the organic layer washed with brine and dried over MgSO4. The solvents were evaporated and the residue was purified by flash silica gel chromatography eluting with hexane to give the product. Telluro(seleno)ketene acetals have been synthesized by the Al/Te exchange reaction, as shown in the following scheme, starting from hydroalumination of acetylenic selenides.66 R

SePh

Ph DIBAL-H toluene /hexane H N2 /reflux

SePh

BuTeBr Al(i -Bu)2 LiCl

Ph

SePh

H TeBu (22-50%)

+

Ph H

SePh H

R = C3H7, C4H9, C6H13, C7H15, C10H21, Ph,

The reaction furnishes low yield (owing to the low Al/Te exchange in the Al vinyl intermediate and the formation of (Z)-vinyl selenides as by-products) leading therefore to introduce a valuable alternative based on a hydrozirconation protocol. The following scheme illustrates the different regio- and stereochemical outcomes of the sequential addition of the Schwartz reagent (Cp2Zr(H)Cl) and butyl tellurenyl bromide to acetylenic selenides.67

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With R⫽H or Ph unique products are formed with opposite regiochemistry, whereas alkylselenoalkynes (R⫽alkyl) give rise to a mixture of regioisomers. All the acetylenic selenides require 2 equiv of the Zr reagent to ensure the total conversion of the starting material. SeR1 n -BuTeBr

H R

SeR1

Cp2Zr(H)Cl R = H

R1 = n -Bu, Ph

ClCp2Zr

R = Ph R1 = n -Bu Ph

ZrCp2Cl

R

SeBu-n

H

ZrCp2Cl

R

+

ClCp2Zr

Ph

SeBu-n

H

TeBu-n (63%)

SeR1 H

n -BuTeBr SeBu-n

R H

A

SeR1

R

+

SeR1

H n -BuTe (81,66%)

SeBu-n n -BuTeBr

H R = alkyl R1 = n -Bu

H

H

H TeBu-n n -BuTe B (63-70%)

Ratio R = C3H7 n -Bu C6H13 CH3OCH2

A:B 70:30 90:10 92:8 64:36

General procedure for the synthesis of telluroseleno ethenes from acetylenic selenides. 67 To a mixture of Cp2Zr(H)Cl (0.51 g, 2.0 mmol) in THF (6.0 mL) under nitrogen, a solution of the corresponding butylselenoacetylene (1.0 mmol) in THF (2.0 mL) was added via syringe. The reaction was stirred at room temperature until the total transformation of the starting material was confirmed following the reaction by TLC on SiO2 using hexane as eluent. Then the resulting clear yellow solution formed was cooled at 0°C and a solution of butyltellurenyl bromide (2.0 mmol) prepared separately adding bromine (0.16 g, 1.0 mmol) in benzene (10 mL) to dibutylditelluride (0.36 g, 1.0 mmol) in THF (10 mL) was transferred via syringe. The stirring was continued for an additional 15 min, the mixture transferred to an Erlenmeyer flask, diluted with ethyl acetate (10 mL), 95% ethanol (5 mL) and water (10 mL). Butylbromide (0.32 mL; 3.0 mmol) and finally NaBH4 (0.09 g, 3.0 mmol) were added to transform the dibutylditelluride (formed from the excess of butylbromide used) to the corresponding telluride which is more easily removed by distillation. After this treatment the product was extracted with ethyl acetate (5⫻20 mL) and washed with water (5⫻20 mL), the organic phase was dried over anhydrous MgSO4 and the solvent evaporated under reduced pressure. The dibutyltelluride was removed by distillation of the crude product using a Kügelrohr apparatus. The residue is constituted by the telluroseleno ethenes which were obtained as yellow liquids after purification by flash chromatography using hexane as eluent in all cases. In a further line of experiments lithium alkynyl selenolate, instead of acetylenic selenides, has been used to generate the zirconated vinylselenides, precursors of (E)telluro(seleno)ketene acetals.68 RC CH

1) n -BuLi 2) Se

n -BuBr

RC CSeLi

R

SeBu-n

H

ZrCp2Cl

R = C3H7, C4H9, C6H13, Ph,

Cp2Zr(H)Cl

n -BuTeBr

R

R

SeLi

H

ZrCp2Cl SeBu-n

H TeBu-n (71-82%)

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The stereochemistry of the obtained acetals was confirmed by nuclear overhauser enhancement spectroscopy (NOESY) measurement in 1H NMR spectra. Synthesis of telluro(seleno) ketene cetals. General procedure for the acetals via alkynylselenolate anions.68 To a solution of the freshly distilled terminal alkyne (1.0 mmol) in THF (5.0 mL) under a nitrogen atmosphere was added n-butyllithium (1.0 mmol, 0.5 mL, 2.0 M in hexane) at 0°C, and the solution was stirred for 15 min. The mixture was allowed to reach room temperature, and elemental selenium (0.079 g, 1.0 mmol) was added. After total dissappearance of selenium, the solid Cp2Zr(H)Cl (0.257 g, 1.0 mmol) was added rapidly. The resulting mixture was stirred, butyl bromide (0.21 mL, 2.0 mmol) was added, and the reaction was stirred at room temperature for an additional 1 h. Then, a solution of butyltellurenyl bromide (1.0 mmol) prepared separately as described previously was transferred with a syringe. The stirring was continued for an additional 30 min, and the mixture was transferred to an Erlenmeyer flask and diluted with ethyl acetate (10 mL), 95% ethanol (15 mL) and water (100 mL). Butyl bromide (1 mL) and finally NaBH4 (until the solution turned pale yellow) were added. After this treatment, the product was extracted with ethyl acetate and washed with water, the organic phase was dried over anhydrous MgSO4 and the solvent was evaporated under vacuum. Dibutyl telluride was removed by distillation of the crude product using a Kügelrohr apparatus. The residue contained the ketene telluro(seleno) acetals, which were obtained as yellow liquids after purification by flash chromatography using hexane as the eluent. The above-described procedure is advantageous towards the precedent method since: (a) the starting lithium alkynyl selenolate is prepared in situ, avoiding the laborious preparation of the acetylenic selenides; and (b) the hydrozirconation step is regio- and stereoselective, in contrast with the previously discussed hydrozirconation of acetylenic selenides resulting in a mixture of the regioisomers, and requires only 1 equiv of the Schwartz reagent instead of 2 equiv of the precedent procedure. The alkyne hydrozirconation protocol was also applied to acetylenic tellurides furnishing the zirconated vinyl tellurides in cis fashion and high regioselectivity. Subsequent treatment with tellurenyl halides affords telluro ketene acetals with total retention of configuration.68 The use of 2 equiv of the reagent is required (like the selenium route) to achieve the complete conversion of the reagents, in contrast with previous reports, where only 1.1 equiv of the reagent was used.69 The reactions of acetylenic tellurides are faster than the reaction of the selenium acetylides as assessed in separate comparable experiments.

R

TeR1

Cp2Zr(H)Cl

R

TeR1

H

ZrCp2Cl

R = H, C3H7, C6H13, CH3OCH2-, Ph, R1 = R2 = n -Bu X = Br

R2TeX

R

TeR1

H

TeR2

70-79% (2 equiv of Cp2Zr(H)Cl)

R1 = n -Bu, BnO(CH2)3; R2 = n -Bu, Ph 57-60% (1 equiv of Cp Zr(H)Cl) 2 X = Br, I

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General procedure for the synthesis of bis(butyltelluro)ketene acetals.67 To a mixture of Cp2Zr(H)Cl (0.51 g, 2.0 mmol) in THF (6.0 mL) under nitrogen, a solution of the corresponding butyltelluroacetylene (1.0 mmol) in THF (2.0 mL) was added via syringe. The reaction was stirred at room temperature. The total transformation of the starting material was confirmed following the reaction by TLC on SiO2 using hexane as eluent. Then the resulting dark red solution formed was cooled at 0°C and a solution of butyltellurenyl bromide (2.0 mmol) prepared separately as described previously was transferred via syringe. The stirring was continued for an additional 15 min, the mixture transferred to an Erlenmeyer flask, diluted with ethyl acetate (10 mL), 95% ethanol (5 mL) and water (10 mL). Butylbromide (0.32 mL, 3.0 mmol) and finally NaBH4 (0.09 g, 3.0 mmol) were added. After this treatment, the product was extracted with ethyl acetate (5⫻20 mL) and washed with water (5⫻20 mL), the organic phase was dried over anhydrous MgSO4 and the solvent evaporated under reduced pressure. The dibutyltelluride was removed by distillation of the crude product using a Kügelrohr apparatus (100°C/0.6 mmHg). The residue is constituted by the bis(telluroketene) acetals which were obtained as yellow liquids after purification by flash chromatography using hexane as eluent in all cases. General procedure with 1.1 equiv of the reagent. A slurry was prepared from Cp2Zr(H)Cl69 (1.1 mmol) and 5 mL of THF at room temperature under a nitrogen atmosphere, and the acetylenic telluride (1 mmol) in 2 mL of THF was added via a syringe. The mixture was stirred at room temperature for 10–25 min, until hydrozirconation was complete, as evidenced by the disappearance of the insoluble hydride and the formation of a clear solution. To this solution was added 1.1 equiv of alkyltellurenyl iodide (prepared in situ by the addition of iodine solution to a stirred solution of dialkyl ditelluride), at room temperature. After stirring for 1 h, normal work-up was performed. Ketene telluroacetal was isolated and purified by column chromatography using hexane as eluent. The -zirconated vinyl tellurides are valuable intermediates for several synthetic manipulations.67 n -BuSeBr R

TeBu-n

H

SeBu-n

b

R

SeBu-n

H

TeBu-n R = C3H7, C4H9, C6H13, MeOCH2, Ph,

(61-69%)

R

TeBu-n

H

ZrCp2Cl

c I2 c

R

TeBu-n

H

I

d

+

R

I

H

TeBu-n

(73-81%)

NBS

a H O 2

R

TeBu-n

H

Br

R1COCl R

+

TeBu-n

H H (80-92%) R = C3H7, C4H9, C6H13, MeOCH2, Ph,

+

R

H (80, 77%)

R

TeBu-n

H

R1 O (51-71%)

Br

R = C3H7, C6H13

TeBu-n

R1 = Et, pr, Ph, OPh

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Treatment with water gives the corresponding Z tellurides. Reaction with BuSeBr, iodine and N-bromosuccinimide (NBS) gives respectively Te–Se ketene acetals, -iodo- and -bromo vinyl tellurides as mixtures of Z and E stereoisomers (in contrast with the total retention of configuration of the above-discussed Zr/Te exchange reactions). The acylation was effected with acylchlorides in the presence of CuI.70 General procedure for the synthesis of (Z)-vinyltellurides from acetylenic tellurides (a).67 To a mixture of Cp2Zr(H)Cl (0.51 g, 2.0 mmol) in THF (6.0 mL) under nitrogen, a solution of the corresponding acetylenic telluride (1.0 mmol) in THF (2.0 mL) was added via syringe. The reaction was stirred at room temperature. The total transformation of the starting material was confirmed following the reaction by TLC on SiO2 using hexane as eluent. Then, the reaction mixture was treated with water (2.0 mL), diluted with ethyl acetate (150 mL) and washed with a saturated solution of ammonium chloride (3⫻50 mL). The organic phase was dried over anhydrous MgSO4 and the solvent evaporated under reduced pressure. After purification by flash chromatography using hexane as eluent, the products were obtained as yellow oils in 80–92% yield. General procedure for the synthesis of telluroseleno ketene acetals from acetylenic telurides (b).67 To a mixture of Cp2Zr(H)Cl (0.51 g, 2.0 mmol) in THF (6.0 mL) under nitrogen, a solution of the corresponding butyltelluroacetylene (1.0 mmol) in THF (2.0 mL) was added via syringe. The reaction was stirred at room temperature. The total transformation of the starting material was confirmed following the reaction by TLC on SiO2 using hexane as eluent. Then the resulting dark red solution formed was cooled at 0°C and a solution of butylselenenyl bromide (2.0 mmol) prepared separately was transferred via syringe. The stirring was continued for an additional 15 min, the mixture transferred to an Erlenmeyer flask, diluted with ethyl acetate (10 mL), 95% ethanol (5 mL) and water (10 mL). Butylbromide (0.32 mL, 3.0 mmol) and finally NaBH4 (0.09 g, 3.0 mmol) were added to transform the dibutyldiselenide to the corresponding selenide which is more easily removed by distillation. After this treatment the product was extracted with ethyl acetate (5⫻20 mL) and washed with water (5⫻20 mL), the organic phase was dried over anhydrous MgSO4 and the solvent evaporated under vacuum. The dibutylselenide was removed by distillation of the crude product using a Kügelrohr apparatus (70°C/0.6 mmHg). The residue is constituted by the ketene telluro(seleno) acetals which were obtained as yellow liquids after purification by flash chromatography using hexane as eluent. General procedure for the synthesis of 1-halo-1-telluro ethenes from acetylenic telurides (c).67 To a mixture of Cp2Zr(H)Cl (0.51 g, 2.0 mmol) in THF (6.0 mL) under nitrogen, a solution of the corresponding butyltelluroacetylene (1.0 mmol) in THF (3 mL) was added via syringe. The reaction was stirred at room temperature (10–30 min). The total transformation of the starting material was confirmed following the reaction by TLC on SiO2 using hexane as eluent. Then, the resulting dark red mixture formed was treated at room temperature with a solution of iodine or NBS (3.0 mmol) in THF (5.0 mL), transferred via syringe. The stirring was continued for an additional 30 min, the mixture transferred to an Erlenmeyer flask, diluted with ethyl acetate (10 mL), 95% ethanol (10 mL) and water (5 mL) and finally NaBH4 (0.09 g, 3.0 mmol) was added to remove the electrophile excess

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and to perform the dehalogenation of the tellurium atom. After this treatment the product was extracted with ethyl acetate (5⫻30 mL) and washed with water (5⫻50 mL), the organic phase was dried over anhydrous MgSO4 and the solvent evaporated under reduced pressure. The residue is constituted by the 1-butyltelluro-1-halo ethenes which were obtained as yellow liquids after purification by flash chromatography using hexane as eluent. Synthesis of (Z)--organotelluro-,-unsaturated carbonyl compounds from acetylenic tellurides (d).70 A typical procedure for the preparation of the title compounds is as follows. A suspension of zirconocene hydrochloride (1.2 mmol) in THF (2 mL) was stirred at room temperature under nitrogen. A solution of acetylenic telluride (1.0 mmol) in THF (2 mL) was added. After being stirred for 15 min, this reaction mixture was transferred to the solution of acyl halide (1.5 mmol) and CuI (1.0 mmol) in THF (3 mL). After being stirred for 8 h at room temperature, the mixture was quenched by pouring it into saturated aqueous NH4Cl and was extracted with Et2O. Normal handling and chromatography afforded (Z)--organotelluro-,-unsaturated carbonyl compound. Following the precedent methodologies, telluro(stannyl) ketene acetals are achieved by the hydrozirconation of stannyl acetylenes and successive reactions with butyl tellurenyl bromide and NaBH4.71

R

SnBu3-n

Cp2Zr(H)Cl (1.4 equiv) R THF, r.t. H

R = H, n -Bu, C6H13, Ph,

SnBu3-n 1) n -BuTeBr (2 equiv) R SnBu3-n THF, 0°C ZrCp2Cl H TeBu-n 2) NaBH4 (60-81%)

The treatment with NaBH4 is required to reduce the corresponding tellurodibromide formed at the expense of the excess (2 equiv) of n-BuTeBr used, which behaves as brominating agent. General procedure for the synthesis of ketene stannyl(telluro) acetals from stannylacetylenes.71 To a mixture of Cp2Zr(H)Cl (0.721, 2.8 mmol) in THF (10 mL) under nitrogen was added via syringe a solution of the corresponding stannylacetylene (2.0 mmol) in THF (5 mL). The reaction was stirred at room temperature for 15–30 min. The disappearance of the starting material was confirmed by TLC using hexane as the eluent. The resulting solution was cooled to 0°C, and a solution of butyltellurenyl bromide (4.0 mmol) was transferred via syringe. The stirring was continued for an additional 30 min at 0°C and then the mixture was transferred to an Erlenmeyer flask (1 L) and diluted with ethyl acetate (30 mL), water (100 mL) and 95% ethanol (50 mL). Butyl bromide (1.0 mL) and finally NaBH4 (until the mixture turned pale yellow) were added to transform dibutyl ditelluride to the corresponding telluride which is more easily removed by distillation. After this treatment the product was extracted with ethyl acetate (3⫻) and washed with water (3⫻). The organic phase was dried over anhydrous MgSO4 and the solvent partially evaporated. After filtration through Celite® using hexane as the eluent, the product was concentrated under vacuum. Dibutyl telluride was removed by distillation from the crude product using a

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Kügelrohr apparatus. Flash column chromatography (hexane) of the residue afforded ketene stannyl(telluro) acetals as yellow liquids. The E configuration of the products, assessed by NOE experiments, is a consequence of the known 100% regio- and E-stereoselectivity of the hydrozirconation of acetylenic stannanes, and the retained configuration in the Zr/Te exchange. The treatment of the obtained ketene acetals with iodine or NBS in excess promotes exclusively Sn/halogen exchange giving the -halogen vinyl dihalogeno tellurides which are dehalogenated to the corresponding telluride by treatment with NaHSO3 or NaBH4. This process occurs with total retention of the configuration as confirmed by NOE/1H NMR measurements.71 NaBH4 EtOH, -40°C R

SnBu3-n I2 / 2.15 equiv R

H

TeBu-n 0°C, r.t. -6h H

I TeBu-n I I

R

I

H

TeBu-n

+ n -Bu3SnI

1)NaHSO3 2) KF -n -Bu3SnF

R

I

H TeBu-n (56-92%)

O NBS (2.5 equiv), CH2Cl2 R -78°C, 6-12h R = H, C4H9, C6H13, Ph,

H

Br TeBu-n Br Br

+ n -Bu3Sn-N

NaBH4 EtOH, -40°C

NaHSO3

O R

Br

H

TeBu-n

R

Br

H TeBu-n (36-60%)

General procedure for the synthesis of -iodovinyl tellurides from ketene stannyl(telluro) acetals.71 To a solution of the appropriate ketene stannyl(telluro) acetal (1.0 mmol) in THF (5 mL) under nitrogen, and cooled to 0°C was added dropwise a solution of iodine (0.54, 2.15 mmol) in THF (5 mL). The reaction mixture was stirred for 3 h and for an additional 3 h at room temperature. Then it was transferred to an Erlenmeyer flask and treated with a solution of sodium thiosulphate (3 g/80 mL of H2O) under stirring for 30 min. The dark brown colour turned clear yellow, and the organic phase was separated, subsequently treated with a solution of KF·2H2O (2.5 g) in H2O (25 mL), and stirred for an additional 30 min. The mixture was extracted with ethyl acetate and washed with a saturated NH4Cl aqueous solution (3⫻) and water (3⫻). The organic phase was dried over anhydrous MgSO4, and after filtration, the solvent was evaporated under reduced pressure, resulting in a mixture of a white precipitate and a yellow residue. The product was immediately purified by column chromatography, using hexane as the eluent. General procedure for the synthesis of -bromovinyl tellurides from ketene stannyl (telluro) acetals.71 To a solution of the appropriate ketene stannyl(telluro) acetal (1.0 mmol) in CH2Cl2 (10 mL) cooled at ⫺78°C (at 0°C for 5 h) was added dropwise a solution of NBS (0.445 g, 2.5 mmol) in CH2Cl2 (25 mL). Th reaction mixture was stirred for 6 and 12 h (R⫽H). It was transferred to an Erlenmeyer flask and treated with a solution

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of sodium thiosulphate (3 g/80 mL of H2O) under stirring for 30 min. The solution turned clear yellow, and the mixture was extracted with ethyl acetate and washed with a saturated NH4Cl aqueous solution (3⫻) and water (3⫻). The organic phase was dried over anhydrous MgSO4, and after filtration, the solvent was evaporated under reduced pressure. The residue was immediately purified by column chromatography, using hexane as the eluent. Telluro ketene acetals are also accessible by the insertion of unsaturated carbenes into Te–Te bond. By the treatment of solutions of diphenyl ditelluride and an unsaturated triflate in DMF with t-BuOK, the corresponding telluro ketene acetals are formed.72 R

OTf

R1

H

t -BuOk DME, 50°C

R

:

R1

PhTeTePh

R

TePh

R1

TePh

(26-30%)

R, R1 = Me, (CH2)4

General procedure.72 To a flame-dried, three-necked, 250 mL, round-bottomed flask equipped with argon inlet, addition funnel and gas bubbler was added 100 mL of dry 1,2dimethoxyethane (DME) with 0.20 g (0.98 mmol) of triflate [R, R1⫽Me] and 0.50 g (1.2 mmol) of diphenyl ditelluride. The reaction mixture was cooled to ⫺50°C and a solution of potassium t-butoxide (0.15 g, 1.3 mmol) in 50 mL of dry DME was added dropwise. After the addition, stirring was continued for 15 min at ⫺50°C, then the reaction was allowed to warm to room temperature. DME was then removed on a rotary evaporator and the residue taken up in hexane and filtered. The resulting coloured solution was chromatographed on activated silica (hexane as eluent). Isolated yield was 0.12 g (26%), as an orange oil. Further efforts have been devoted to the synthesis of telluro(thio)ketene acetals with high regio- and stereochemistry. The E and Z isomers were obtained by two different routes.73 The E isomer resulted by the syn hydroalumination of thioacetylenes with DIBAL-H followed by the addition of n-BuLi and capture of the obtained intermediate with BuTeBr. In contrast, the Z isomer was attained as the major isomer (80–93%) by the anti addition of the Zweifel reagent [lithium di(isobutyl)-n-butyl aluminate hydride] to the thioacetylene followed by trapping of the intermediate with BuTeBr. R

SPh

DIBAL-H hexane, reflux

R n -BuTeBr benzene / THF, 0°C H

R

SPh

H

Al(i-Bu)2

R

SPh

H

Al-(i -Bu)2 Bu

SPh

TeBu-n E (50-80%)

THF DIBAL-H + n -B uLi 0°C

n -BuLi hexane, 0°C

[HAl-(i-Bu)

R = C3H7, C4H9, Ph,

+ 2]Li

R

R H

Bu TeBu-n R n -BuTeBr / LiCl R + THF-benzene H SPh H 0°C major product (57-65%)

SPh

SPh TeBu-n

Bu Al-(i -Bu)2 Li+ SPh

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Synthesis of (E)-1-butyltelluro-1-phenylthio-1-alkenes (typical procedure).73 To DIBAL-H (2.0 mL, 1.0 mmol, 1 M in hexane) in hexane (5.0 mL) under N2 a solution of 1-phenylthio-1-hexyne (0.2 g, 1.0 mmol) in hexane (1.0 mL) was added via syringe at room temperature. The reaction was refluxed for 60 min, then n-BuLi (1.53 mL, 2.0 mmol, 1.3 M in hexane) was added dropwise at 0°C and stirring was continued for 30 min. A solution of n-BuTeBr/LiCl (4.0 mmol) was prepared by the addition of Br2 (0.32 g, 1.0 mmol) in benzene or CCl4 (10 mL) to a solution of n-(BuTe)2 (0.73 g, 2.0 mmol) in THF (10 mL) at 0°C under stirring for 10 min followed by the addition of LiCl (1.3 g). After additional 30 min stirring, the mixture was transferred to an Erlenmeyer flask and diluted with ethyl acetate (10 mL), 95% ethanol (10 mL) and water (10 mL). Butylbromide (0.64 mL, 6.0 mmol) and finally NaBH4 (0.18 g, 6.0 mmol) were added to transform the (C4H9)2Te into the corresponding telluride, which is more easily removed by distillation. After the usual work-up the product was dried (MgSO4) and the solvent evaporated under vacuum. The (C4H9)2Te was removed by distillation from the crude product using a Kügelrohr oven. The residue was purified by PTLC, using hexane as mobile phase. (E)-1-butyltelluro-1phenylthio-1-hexene – yield: 0.25 g (80%).

3.16.8 The behaviour of vinylic tellurides towards several reagents and reaction conditions used in organic synthesis A systematic study was undertaken to ascertain the behaviour of functionalized vinylic tellurides, such as those depicted in the following figure towards several reagents and reaction conditions.74

HO

TeBu-n

TeBu-n

HO

HO EtO

n -BuTe OH

• •

•

•

TeBu-n OEt

Protection of hydroxy groups as the THP and tert-butyldimethyl silyl ethers and, conversely, deprotection of these derivatives to the original alcohols. Acetylation of the hydroxy group and hydrolysis of the acetoxy group; oxidation of the hydroxy group by MnO2 in ether or alternatively by Dess-Martin periodinane, and the reduction of the obtained aldehyde by NaBH4. Hydrolysis of the diethylacetal function employing p-toluenesulphonic acid in acetone, pyridinium p-toluene-sulphonate in EtOH, and a suspension of SiO2 in hexane. In all cases the corresponding aldehyde is obtained in high yield as a Z/E isomeric mixture. Transmetallation of acetal with Me2Cu(CN)Li2 followed by treatment with c-hexenones giving the 1,4-addition product. Alternatively, transmetallation with n-BuLi and reaction with benzaldehyde giving the expected alcohol.

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The reaction of 3- and 2-butyltelluro aldehydes with n-BuLi gives the corresponding allylic alcohols showing the preferential attack to the carbonyl group.74,75 n -BuTe

CHO

R

1) n -BuLi THF, -78°C 2) H2O

n -BuTe

1) n -BuLi THF, -78°C 2) H2O

TeBu-n CHO

OH 53% R

TeBu-n OH (42-47%)

R = Ph, C5H11

In contrast, when the n-butyltellurium moiety and the carbonyl group are attached to different substrates, the Te atom exhibits high selectivity for the n-BuLi attack, as observed when a 1:1 mixture of a vinyl telluride and a carbonyl compound is reacted with 1 equiv of n-BuLi. O

Ph

+ R TeBu-n

n -BuLi R1 THF, -78°C

R, R1 = (CH2)5 OH R = H; R1 = Ph, ( + Ph O TeBu-n + R

Ph

36%)

n -BuLi R1 THF, -78°C

O

+ TeBu-n R

TeBu-n

O + R

R = R1 = (CH2)5, ( +n -C5H11 OH R = H; R1 = Ph, ( + Ph

R1 OH

Ph OH R R1 69%

n -BuLi R1 THF, -78°C

TeBu-n

R Ph

41%)

R, R1 = (CH2)5 n -C5H11

R R1 OH 61, 64%

70, 59%

OH R, R1 = (CH2)5 R = H; R1 = Ph, ( + Ph Ph

Ph

TeBu-n n -BuLi n -C5H11 OH R1 THF, -78°C R (34, 47%) R1 50%) TeBu-n

Bu-n trace)

Non-conjugated substrates, such as 5-oxocarbonyl vinyl tellurides undergo an intramolecular version of the above-described protocol, achieving the synthesis of cycloalkenols. R OH

O R R = Me, Et

TeBu-n

n -BuLi THF, -78°C 53, 56%

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REFERENCES 1. For preceding review articles see: (a) Comasseto, J. V.; Ling, L. W.; Petragnani, N.; Stefani, H. A. Synthesis 1997, 373. (b) Petragnani, N.; Stefani, H. A. Tetrahedron 2005, 61, 1613, Section 7. 2. Trofimov, B. A.; Gusarova, N. K.; Amasova, S. V. J. Org. Chem. USSR 1977, 13, 413. 3. Trofimov, B. A.; Amasova, S. V.; Gusarova, N. K.; Musorin, G. K. Tetrahedron 1982, 38, 713. 4. Trofimov, B. A.; Amosova, S. V.; Gusarova, N. K.; Potapov, V. A.; Tatarinova, A. A. Sulfur Lett. 1983, 1, 151. 5. Potapov, V. A.; Amosova, S. V.; Kashik, A. S. Tetrahedron Lett. 1989, 30, 613. 6. Potapov, V. A.; Kashik, A. S.; Amosova, S. V. J. Org. Chem. USSR 1988, 1807. 7. Trofimov, B. A.; Gusarova, N. K.; Tatarinova, A. A.; Amosova, S. V.; Sinegovskaia, L. M.; Keiko, V. V.; Potapov, V. A. J. Org. Chem. USSR 1984, 24, 1642. 8. (a) Gusarova, N. K.; Trofimov, B. A.; Tatarinova, A. A.; Potapov, V. A.; Sinegoskaia, L. M.; Amosova, S. V.; Voronkov, M. G. J. Org. Chem. USSR 1988, 24,1686. (b) Gusarova, N. K.; Trofimov, B. A.; Tatarinova, A. A.; Potapov, V. A.; Sinegoskaia, L. M.; Amosova, S. V.; Voronkov, M. G. Tetrahedron 1988, 44, 6739. 9. Barros, S. M.; Dabdoub, M. J.; Dabdoub, V. B.; Comasseto, J. V. Organometallics 1989, 8, 1661. 10. Trofimov, B. A.; Potapov, V. A.; Gusarova, N. K.; Tatarinova, A. A.; Amosova, S. V. J. Org. Chem. USSR 1985, 21, 1029. 11. Uemura, S.; Fukuzawa, S. I.; Patil , S. R. J. Organomet. Chem. 1983, 243, 9. 12. Dabdoub, M. J.; Dabdoub, V. M.; Comasseto, J. V.; Petragnani, N. J. Organomet. Chem. 1986, 308, 211. 13. Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. J. Org. Chem. 1987, 52, 4859. 14. Potapov, V. A.; Kashik, A. S.; Gusarova, N. K.; Mink, V. I.; Sadekov, I. D.; Trofimov, B. A.; Amosova, S. V. J. Org. Chem. USSR 1987, 23, 596. 15. Potapov, V. A.; Amosova, S. V. Phosphorus Sulfur Silicon Relat. Elem. 1993, 79, 277. 16. Zeni, G.; Formiga, H. B.; Comasseto, J. V. Tetrahedron Lett. 2000, 41, 1311. 17. Dabdoub, M. J.; Dabdoub, V. M.; Guerrero Jr., P. G.; Silveira, C. C. Tetrahedron 1997, 53, 4199. 18. Dabdoub, M. J.; Dabdoub, V. M.; Comasseto, J. V. Tetrahedron Lett. 1992, 33, 2261. 19. Dabdoub, M. J.; Dabdoub, V. M. Tetrahedron 1995, 51, 9839. 20. Stefani, H. A.; Cardoso, L. D. G.; Valduga, C. J.; Zeni, G. Phosphorus Sulfur Silicon Relat. Elem. 2001, 172, 167. 21. Dabdoub, M. J.; Dabdoub, V. M.; Pereira, M. A. Tetrahedron Lett. 2001, 42, 1595. 22. Marino, J. P.; Nguyen, H. N. J. Org. Chem. 2002, 67, 6841. 23. Dabdoub, M. J.; Comasseto, J. V. J. Organomet. Chem. 1988, 344, 167. 24. Zhao, C.-Q.; Huang, X. Synth. Commun. 1997, 27, 249. 25. Zhao, C.-Q.; Li, J.-L.; Meng, J. B.; Wang, Y.-M. J. Org. Chem. 1998, 63, 4170. 26. Dabdoub, M. J.; Cassol, T. M. Tetrahedron 1995, 51, 12971. 27. Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. J. Organomet. Chem. 1987, 326, 35. 28. Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. Nippon Kagaku Kaishi 1987, 1469. 29. Wudl, F.; Nalewajek, D. J. Organomet. Chem. 1981, 217, 329. 30. Minkin, V. I.; Sadekov, I. D.; Rivkin, B. R.; Zakharov, A. V.; Nivorozhkin, V. L.; Kompan, O. E.; Struchkov, Y. T. J. Organomet. Chem. 1997, 536–537, 233. 31. Barrientos-Astigarraga, R. E.; Castelani, P.; Sumida, C. Y.; Zukerman-Schpector, J.; Comasseto, J. V. Tetrahedron 2002, 58, 1051. 32. Huang, X.; Liang, C. G.; Xu, Q.; He, Q. W. J. Org. Chem. 2001, 66, 74. 33. Proiadakov, A. G.; Kalabin, G. A.; Buzilova, S. R.; Vereshchagin, L. I.; Panov, A. M.; Ratovski, G. V. J. Org. Chem. USSR 1981, 17, 2157.

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34. Buzilova, S. R.; Sadekov, J. D.; Lipovich, T. L.; Filippova, T. M.; Vereshchagin, L. I. J. Gen. Chem. USSR 1977, 47, 1828. 35. Takahashi, H.; Ohe, K.; Uemura, S.; Sugita, N. Nippon Kagaku Kaishi 1987, 1508. 36. Mo, X. S.; Huang, Y. Z. Tetrahedron Lett. 1995, 36, 3539. 37. Braga, A. L.; Alves, E. F.; Silveira, C. C.; Andrade, L. H. Tetrahedron Lett. 2000, 41, 161. 38. Yang, W. B.; Oh, D. Y.; Lee, C. W. Tetrahedron Lett. 2000, 41, 5103. 39. Marino, J. P.; Nguyen, H. N. J. Org. Chem. 2002, 67, 6291. 40. Zhao, C. Q.; Li, J. L.; Meng, J. B. ; Wang, Y. M. J. Org. Chem. 1998, 63, 4170. 41. Moura Campos, M.; Petragnani, N. Tetrahedron 1962, 18, 527. 42. Uemura, S.; Miyoshi, H.; Okano, M. Chem. Lett. 1979, 1357. 43. Comasseto, J. V.; Stefani, H. A.; Chieffi, A. Organometallics 1999, 10, 845. 44. Zukerman-Schepctor, J.; Castellano, E. E.; Oliva, G. Acta Cryst. 1991, C47, 960. 45. Stefani, H. A.; Petragnani, N.; Zukerman-Schpector, J.; Dornelles, L.; Silva, D. O.; Braga, A. L. J. Organomet. Chem. 1998, 562, 127. 46. Huang, X.; Wang, Y. P. Tetrahedron Lett. 1996, 37, 7417. 47. Zeni, G.; Chieffi, A.; Cunha, R. L. O. R.; Zukerman-Schpector, J.; Stefani, H. A.; Comasseto, J. V. Organometallics 1999, 18, 803. 48. Kauffman, T.; Ahlers, H. Chem. Ber. 1983, 116, 1001. 49. (a) Han, L. B.; Ishihara, K. I.; Kambe, N.; Ogawa, A.; Ryu, I.; Sonoda, N. Phosphorus Sulfur Silicon 1992, 67, 243. (b) Han, L. B.; Ishihara, K. I.; Kambe, N.; Ogawa, A.; Ryu, I.; Sonoda, N. J. Am. Chem. Soc. 1992, 114, 7591. 50. Chen, C.; Crich, D. Tetrahedron Lett. 1993, 34, 1545. 51. Crich, D.; Chen, C.; Hwang, J. T.; Yuan, H.; Papadatos, A. Walter, R. I. J. Am. Chem. Soc. 1994, 116, 8937. 52. Engman, L.; Gupta, V. J. Chem. Soc. Chem. Commun. 1995, 2515. 53. Ogawa, A.; Yokoyama, K.; Yokoyama, H.; Obayashi, R.; Kambe, N.; Sonoda, N. J. Chem. Soc. Chem. Commun. 1991, 1748. 54. Dabdoub, M. J.; Comasseto, J. V. Organometallics 1988, 7, 84. 55. Lee, C. W.; Koh, Y. J.; Oh, D. Y. J. Chem. Soc. Perkin Trans. 1 1994, 717. 56. Silveira, C. C.; Perin, G.; Braga, A. L. Tetrahedron Lett. 1995, 36, 7361. 57. Silveira, C. C.; Perin, G.; Braga, A. L.; Dabdoub, M. J.; Jacob, R. G. Tetrahedron 1999, 55, 7421. 58. Silveira, C. C.; Perin, G.; Braga, A. L.; Dabdoub, M. J.; Jacob, R. G. Tetrahedron 2001, 57, 5953. 59. Silveira, C. C.; Perin, G.; Braga, A. L.; Petragnani, N. Synlett 1995, 58. 60. Silveira, C. C.; Perin, G.; Boeck, P.; Braga, A. L.; Petragnani, N. J. Organomet. Chem. 1999, 584, 44. 61. See ref. 10 in the precedent ref. 60. 62. Gerard, J.; Bietlot, E.; Hevesi, J. Tetrahedron Lett. 1998, 39, 8735. 63. Huang, X.; Liang, C. G. J. Chem. Soc. Perkin Trans. 1 1999, 2625. 64. Stefani, H. A.; Comasseto, J. V.; Petragnani, N.; Braga, A. L.; Menezes, P. H.; Gusevskaya, E. V. Phosphorus Sulfur Silicon 1997, 126, 211 and references therein. 65. Braga, A. L.; Zeni, G.; Andrade, L. H.; Silveira, C. C. Synlett 1997, 595. 66. Dabdoub, M. J.; Cassol, T. M.; Barbosa, S. L. Tetrahedron Lett. 1996, 37, 831. 67. Dabdoub, M. J.; Begnini, M. L.; Guerrero Jr., P. G. Tetrahedron 1998, 54, 2371. 68. Dabdoub, M. J.; Begnini, M. L.; Guerrero, P. G.; Baroni, A. C. M. J. Org. Chem. 2000, 65, 61. 69. Sung, J.-W.; Park, C. P.; Gil, J. M.; Oh, D. Y. J. Chem. Soc. Perkin Trans. 1 1997, 591. 70. Sung, J. W.; Yang, W. B.; Oh, D. Y. Tetrahedron Lett. 1996, 37, 7537. 71. Dabdoub, M. J.; Baroni, A. C. M. J. Org. Chem. 2000, 65, 54. 72. Stang, P. J.; Roberts, K. A.; Lynch, L. E. J. Org. Chem. 1984, 49, 1653. 73. Dabdoub, M. J.; Guerrero, P. G. Tetrahedron Lett. 2001, 42, 7167.

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74. Rahmeier, L. H. S.; Comasseto, J. V. Organometallics 1997, 16, 651. 75. Dabdoub, M. J.; Jacob, R. G.; Ferreira, J. T. B.; Dabdoub, V. M.; Marques, F. A. Tetrahedron Lett. 1999, 40, 7159.

3.17

ACETYLENIC TELLURIDES

Like vinylic tellurides, acetylenic tellurides can be prepared by two general approaches, starting respectively from nucleophilic or electrophilic tellurium species. 3.17.1 3.17.1.1

From nucleophilic tellurium reagents Sodium ethynyl tellurolates

R

H

1) Na / NH3 2) Te

R

TeNa

R1X / NH3 liq. (21-65%)

TeR1

R

(ref. 1-5)

R = H, Ph, Me, t -Bu, S-Pr, vinyl, -CH=CH-CH=CH2; R1= Me R = Me; R1 = Et R = vinyl; R1 = n-Bu

Methyl phenylethynyl telluride (typical procedure).3 To NaNH2 (from 6.0 g, 0.26 mol Na) in liquid NH3 (250 mL) is added phenylacetylene (25 g, 0.25 mol) dropwise, and then Te powder (30 g, 0.24 mol) in small portions, stirring well for 30 min. Methyl iodide (36 g, 0.25 mol) is added over 20 min to the tellurolate solution. The NH3 is then evaporated, the residue extracted with ether and the ether solution washed with H2O and dried (MgSO4). The residue is distilled under vacuum, giving the product (28 g (46%); b.p. 122–124°C/2 torr). 3.17.1.2

Lithium alkyl and ethynyl tellurolates

Lithium butyl tellurolate reacts with phenylbromoethyne to give butyl phenylethynyl telluride: n -BuTeLi + PhC CBr

n -BuTe-C CPh (58%)

(ref. 6)

Otherwise a variety of acetylene tellurides have been prepared by the reaction of lithium ethynyl tellurolates with alkyl halides.

R

H

1) n -BuLi / THF / 0°C R 2) Te, THF, reflux

TeLi

R1X (73-97%)

R

TeR1

R = Me3Si, Me, n -Bu, Ph; R1= CH2Cl; X = I (ref. 5) R = Ph; R1 =Et, n -Bu, n -dodecyl, CH2CH2Ph, i -Pr, CH(Me)C3H7; X = Br (ref. 7)

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Bis-ethynyl tellurides are accessible from bromoacetylenes. TeLi + Ph

R

Br

THF 0°C to r.t.

R

Te

Ph

(ref. 6)

(58,87%)

R = n -Bu, Ph

Phenyl(alkyltelluro)acetylenes (general procedure).7 n-BuLi (1.35 M in hexane, 22.2 mL, 30 mmol) is added dropwise to phenylacetylene (3.10 g, 30.0 mmol) in THF (15 mL) at 0°C under N2. After stirring for 5 min at 0°C, elemental Te (3.90 g, 30.0 mmol) is added and the mixture refluxed until the Te disappears (~30 min). The heat source is removed and the alkyl halide (30 mmol) is added. The mixture is stirred for 40 min at room temperature, then diluted with ether (60 mL), washed with brine and the layers separated. The organic phase is dried (MgSO4), evaporated and the residue purified by SiO2 flash chromatography (elution with hexane). 3.17.2 3.17.2.1

From electrophilic tellurium reagents From alkynyl Grignard and lithium compounds and organyl tellurenyl halides M + ArTeBr

Ph

M = MgBr M = Li

THF 0°C to r.t. (73 %)

Ph

TeAr

Ar = Ph, p -MeOC6H4, p -EtOC6H4 (90-95%) (ref. 8) Ar = p -MeOC6H4 (73%) (ref. 7)

Phenyl(p-methoxyphenyltelluro)acetylene (typical procedure).7 To a solution of lithium phenylacetylide (20.0 mmol), prepared as described above, is added dropwise a solution of p-methoxyphenyltellurenyl bromide (prepared by adding bromine (1.60 g, 10.0 mmol)) in benzene (5 mL) to a solution of the corresponding ditelluride (4.69 g, 10 mmol) in THF (10 mL) at 0°C under N2. The reaction mixture is stirred for 1 h at room temperature and then treated as described above to give the product (4.90 g (73%)), which is recrystallized from EtOH (m.p. 71–72°C). Some related procedures have been reported: •

A one-pot reaction of phenylacetylene with a dialkyl ditelluride and an alkyl iodide under phase transfer catalysis (method a). The same product can be obtained by using the tellurenyl iodide prepared in situ (method b).9 a) RTeTeR + PhC CH + MeI

KOH / benzene dibenzo-18-crown-6

PhC TeR + MeTeR

R = Me, i -pr, n -Bu, t -Bu b) RTeTeR R = i -pr

I2 RTeI benzene

PhC CH KOH / benzene dibenzo-18-crown-6

PhC CTeR + KI + H2O

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The reaction of dialkylditellurides with pressurized acetylenes in the presence of an electrophilic reagent (alkyl halide, Lewis acid), in the system KOH/crown ether/benzene, gives rise to alkylethynyl tellurides and 1,2-bis-alkyltelluroacetylenes in high yields.10

The preferential formation of mono- or bis-substituted product is determined by a minor or major molar ratio KOH/RTeTeR, respectively. The reaction was rationalized assuming the intermediacy of the high electrophilic species [RTeTe(R1)R]X⫺ (R1⫽Alkyl and X⫽Br, I).

RTeTeR + HC CH

KOH / dibenzo-18-crown-6 / benzene

RTeC CH + RTe C C TeR

E

R = Me, Et, i-pr E = MeI, EtI, EtBr, SnCl4, BF3.Et2O

Alkyl ethynyltellurides from dialkylditellurides and pressurized acetylene in the presence of electrophilic reagents (typical procedure).10 To a mixture of di-isopropyl ditellurides (4.5 g, 13 mmol), powdered KOH (20 g, 303 mmol), methyl iodide (2 g, 14 mmol), dibenzo-18-crown-6 (0.2 g) and benzene (50 mL) was heated (40–50°C) in a 1 L rotating autoclave under acetylene pressure (14 atm) for 5 h. After addition of water the mixture was extracted with benzene, the organic layer was separated and dried over K2CO3. After evaporation of benzene the residue was diluted with ether and precipitated dibenzo18-crown-6 was filtered off. After evaporation of ether the residue was distilled in vacuo giving the product (2.3 g, 96% yield). •

The reaction of lithium or sodium aryltellurolate with alkynyl phenyl iodonium triflates11 or tosylates.12 RC

CI+Ph-OTf

PhTeLi (PhLi + Te / ether)

RC CTePh (54-84%)

R = Me3Si, CN, CO2Me, COPh, COt -Bu RC

CI+Ph-OTs

ArTeNa (ArTeTeAr / NaBH4) DMF

RC CTeAr (48-70%)

R = Ph, t -Bu Ar = Ph, naphthyl, p -MeC6H4, p -BrC6H4

Alkynylphenyl tellurides from aryltellurolates and alkynyliodonium salts (general procedure).11 To a stirred solution of PhLi in Et2O/cyclohexane at room temperature under N2, tellurium powder was added and the solution was stirred for 1 h. The appropriate iodonium triflate was added and the solution was stirred for 2 h. The product was eluted through a small portion of silica gel with CH3CN and the volume was reduced in vacuo followed by purification via radial chromatography.

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A mixture of the appropriate diaryl ditelluride (1 mmol) and sodium borohydride (2.2 mmol) in DMF (10 mL) was stirred under an N2 atmosphere at 50–60°C until the solution was clear. The mixture was allowed to cool to room temperature, an appropriate alkynylphenyliodonium tosylate (2 mmol and DMF (10 mL)) was added and stirring was continued at 70–80°C under N2. The course of the reaction was followed by TLC. When the reaction was completed, the mixture was cooled, diluted with water (15 mL) and the product was extracted with petroleum ether (3⫻10 mL). The combined extracts were washed with water, and dried with MgSO4. After removal of the solvent, the residue was chromatographed on a column of silica gel using CH2Cl2 as eluent to give the acetylenic telluride. 3.17.2.2

From tellurium tetrachloride and alkynyllithium compounds

In strict correlation with the early method for the preparation of symmetrical diaryl tellurides by treatment of tellurium(IV) halides with 4 mol equiv of aryl Grignard reagent13 (see Section 3.1.2.4), dialkynyl tellurides are prepared starting from bromomagnesium14 and lithium acetylenic derivatives.15 TeCl4 + 4 R M = MgBr M = Li

M

R

Te

R + R

R

R = Ph (solvent benzene / ether) (ref. 14)* R = Me, Et, n -Pr, Ph, Me3Si (42-62 %, solvent THF) (ref. 15)

*Owing to the difficulty in purification the product was isolated as the corresponding diiodide (Ph C C)2Tel2 in 33% yield.

Dialkynyl tellurides (general procedure).15 To a solution of TeCl4 (11 mmol) in THF (15 mL), cooled at ⫺78°C, is added dropwise with vigorous stirring the 1-alkynyllithium reagent (46 mmol) in THF (25 mL). The solution is warmed slowly to room temperature, heated at 55–60°C for 1 h in the dark and then evaporated under vacuum. The residue is extracted with pentane, the pentane solution evaporated under vacuum and the obtained crude products are purified by vacuum distillation for R⫽Et (b.p. 45–46°C/0.075 torr; 56% based on TeCl4), R⫽n-Pr (b.p. 67–68°C/0.075 torr; 69% based on TeCl4), R⫽Me3Si (b.p. 135°C/15 torr; 60% based on TeCl4), and by crystallization (pentane, m.p. 50–52°C) or distillation (110°C/15 torr) for R⫽Me (42% based on TeCl4). In the case of R⫽Ph, no pure (Ph–h )2Te is obtained, and the by-product (Ph– h –)2 is sublimed from the crude product under a dynamic vacuum at 100°C (30%). In a related procedure, tellurium tetrachloride is treated with lithium amides giving tellurium(II) amides via the successive reduction and amination of TeCl4. These species, which provide Te2+ electrophiles, react with alkynyllithium reagents, giving dialkynyl tellurides in moderate yields.16 TeCl4

LiNR2

[Te(NR2)2]

R1

H / LiNR2 (24-76%)

R1

R1 = Ph, p -Me, p -MeOC6H4, n -Bu, Cl(CH2)3, Me3Si, HC

C(CH2)4,

R = i -Pr, Me3Si

Te

R1

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111

The diynes (R⬘–– h –– h –R⬘) are formed as by-products, therefore suggesting that the previously described reaction of TeCl4 with 4 mol equiv of the lithium alkynyls is partially operative. Bis-phenylethynyl telluride (typical procedure).16 To LDA (8 mmol) in THF is added a solution of TeCl4 (0.28 g, 2 mmol) in THF at ⫺78°C, and the mixture stirred at that temperature for 2 h. Phenylacetylene is injected into the resulting orange solution, the mixture is stirred at ⫺78°C for 2 h and then allowed to rise to 0–18°C for an additional 16 h. The mixture is poured into brine, and the organic layer extracted with ether and dried (NaSO4). The crude material is purified by SiO2 column chromatography (elution with n-hexane/CH2Cl2), giving bis-phenylethynyl telluride and 1,4-diphenylbutadiyne in yields of 40 and 4%, respectively. 3.17.3

Synthesis of internal acetylenes from vinylic tellurides

Treatment of vinylic tellurides prepared by photopromoted carbotelluration of terminal acetylenes (see Section 3.16.3) with aqueous NaOCl, followed by pyrolysis, affords internal acetylenes in good yields.17 The combination of the photopromoted carbotelluration with the elimination reaction, provides a useful method for the introduction of alkyl groups to terminal acetylenes. RTeR1 + R2

H

R 1) NaOCl aq. 2) (62-92%) R2

hν or AIBN

R2 R (46-92%)

R1Te

R = t -Bu; R1 = Me, t -BuCH2, n -C12H25 R2 = Ph, Me3Si, p -MeC6H4 R = i -pr; R1 = R2 = Ph

Synthesis of internal acetylenes.17 Into a CHCl3 solution (5 mL) of vinylic telluride (2 mmol) was added an aqueous sodium hypochlorite solution (5–8 equiv) at 25°C, and the mixture was stirred for 30 min. The products were extracted with CHCl3, and the extract was dried over MgSO4, concentrated and heated gradually in a Kügelrohr distillation apparatus up to 250°C in vacuo, giving rise to the corresponding internal acetylenes.

3.18

ALLENIC AND PROPARGYLIC TELLURIDES

Allenic butyl tellurides have been prepared by two methods: (a) Reaction of butyltellurenyl bromide with the allenyl rnagnesium bromide generated from propargyl bromide and Mg in the presence of HgCl2.18 H

Mg / HgCl2 Br ether, 0°C

n -BuTeBr

. MgBr

. TeBu-n

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(b) SN2-type reaction of propargylic bromides or tosylates with butyltellurolate anions.18,19 R H Br(OTs) R = H, Me

n -BuTeNa

R

[(n -BuTe)2 / NaBH4] EtOH

H

n -BuTeLi

R Br

THF, 0°C

.

.

H TeBu-n

(57-62%)

TeBu-n

(100%)

R=H

Synthesis of allenic tellurides.18 To a solution of allenyl bromomagnesium (0.01 mol) in ether (~15 mL) under N2 at 0°C, a solution of butyltellurenyl bromide (0.01 mol) in THF/benzene obtained as previously described was added at once. The orange-red solution was stirred for 30 min at 0°C, diluted with AcOEt (60 mL) and washed with a saturated NH4Cl solution. After drying over MgSO4 and solvent removal, the residue was distilled in a Kügelrohr apparatus (80–90°C/0.2 mmHg) to give the pure telluro allene. Yield: 1.207 g (54%). To a clear yellow solution of the sodium butyltellurolate (5.0 mmol) in 95% ethanol (50 mL), obtained at room temperature as previously described, 3-bromo-1-butyne (0.659 g, 5.0 mmol) was added. The reaction is complete after stirring for 5 min (for 3-bromo-1propyne reflux for 2 h was necessary). The reaction was then diluted with AcOEt (50 mL) and washed with a saturated NH4Cl solution. After drying over MgSO4 and solvent removal, the residue was distilled in a Kügelrohr apparatus (110–120°C/0.2 mmHg) to give pure telluro allene as a orange yellow oil. Yield: 0.731 g (62%). Propargyl bromide (2 mmol) was added at 0°C into a THF solution (5 mL) of n-BuTeLi (2 mmol), prepared from n-butyllithium (1.63 M, 1.23 mL, 2 mmol) and tellurium powder (256 mg, 2 mmol) at 0°C, and the reaction mixture was stirred for 15 min at the same temperature. At this stage, allenic telluride is formed and the resulting solution can be used for subsequent Li–Te exchange in the one-pot method. Allenic telluride was isolated quantitatively by removal of the solvent in vacuo, filtration using hexane (5 mL), followed by evaporation. The obtained telluride was essentially pure to show reasonable NMR spectra, but elemental analyses have not been successful because of the instability towards light and/or air.19 In disagreement with the above results, the propargyl-bromide of non-terminal acetylene gives only the corresponding propargyl telluride.18

Me

n -BuTeNa Br [(n-BuTe)2 / NaBH4] EtOH

Me (82%)

TeBu-n

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113

Allenyl tellurides can be submitted to Te/Li exchange and subsequent reaction with aldehydes, giving homopropargylic alcohols.18 R H

.

H

1) n -BuLi R1R2CO

TeBu-n 2) THF, -78°C

R = H, Me R1 = Ph, 2-furyl, styryl

H

H

OH R2

R1

(66-74%)

Synthesis of homopropargylic alcohols.18 To a solution of telluroallene (1.0 mmol) in THF (12 mL) under N2 at ⫺78°C, n-BuLi (1.1 mmol) was added at once, followed by the immediate addition of benzaldehyde (1.0 mmol). The reaction was stirred for 1.5 h (monitored by TLC using hexane/AcOEt – 8:2). A saturated solution of NH4Cl (3 mL) was added still at ⫺78°C and then the reaction mixture was allowed to reach room temperature. After normal work-up the product was purified by flash chromatography using a mixture of hexane/ethyl acetate (8:2) as eluent.

REFERENCES 1. Brandsma, L.; Wijers, H. E.; Arens, J. F. Recl. Trav. Chim. Pays-Bas 1962, 81, 583. 2. Petrov, A. A.; Radchenko, S. I.; Mingaleva, K. S.; Savich, I. G.; Lebedev, U. B. J. Gen. Chem. USSR 1964, 34, 1911. 3. Boiko, Y. A.; Kupin, B. S.; Petrov, A. A. J. Org. Chem. USSR 1968, 4, 1307; 1969, 5, 1516. 4. Radschenko, K. S. M.; Mingaleva, K. S. J. Org. Chem. USSR 1977, 13, 2303. 5. Bender, S. L.; Detty, M. R.; Haley, N. F. Tetrahedron Lett. 1982, 23, 1531. 6. Dabdoub, M. J.; Comasseto, J. V.; Braga, A. L. Synth. Commun. 1988, 18, 1979. 7. Dabdoub, M. J.; Comasseto, J. V. Organometallics 1988, 7, 84. 8. Petragnani, N.; Torres, L.; Wynne, K. J. J. Organomet. Chem. 1975, 92, 185. 9. Potapov, V. A.; Amosova, S. V.; Khangurov, A. V.; Petrov, P. A. Phosphorus Sulfur Silicon 1993, 79, 273. 10. Potapov, V. A.; Amosova, S. V.; Shestakova, V. Y.; Zhnikin, A. R.; Petrov, B. V. Recl. Trav. Chim. Pays-Bas 1996, 115, 441. 11. Stang, P. Y.; Murch, P. Synthesis 1997, 1378. 12. Zhang, J. L.; Chen, Z. C. Synth. Commun. 1997, 27, 3881. 13. Rheinboldt, H. R.; Petragnani, N. Chem. Ber. 1956, 89, 1270. 14. Moura Campos, M.; Petragnani, N. Tetrahedron 1962, 18, 527. 15. Gedridge, R. W.; Brandsma, L.; Nissan, R. A.; Verkruijsse, H. D.; Harder, S.; Jong, R. L. P.; O’Connor, C. J. Organometallics 1992, 11, 418. 16. Murai, T.; Imaeda, K.; Kajita, S.; Ishihara, H.; Kato, S. J. Chem. Soc. Chem. Commun. 1991, 832. 17. Terao, J.; Kambe, N.; Sonoda, N. Tetrahedron Lett. 1998, 39, 5511. 18. Dabdoub, M. J.; Rotta, J. C. G. Synlett 1996, 526. 19. Kanda, T.; Ando, Y.; Kambe, N.; Sonoda, N. Synlett 1995, 745.

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–4– Tellurium in Organic Synthesis

4.1

REDUCTIONS

This section is devoted to the application of tellurium compounds as reducing reagents of organic substrates. Inorganic species such as hydrogen telluride, sodium hydrogen telluride, sodium telluride and organic derivatives such as aryltellurols (and tellurolates) and diorganyl tellurides are widely employed in several reduction processes such as reduction of carbonyl compounds, nitro derivatives, epoxides, α,β-unsaturated carbonyl compounds, aryl alkenes, arylalkynes, as well as in the reductive cleavage of several types of carbon–heteroatom bonds. In several cases the use of tellurium reagents is clearly advantageous compared to the usual previously described methods because of the mildness of the experimental conditions and the great selectivity. Additional advantages are the possibility of generating the reducing agents in situ as well in catalytic amounts in the presence of an inexpensive co-reductant, and recovery of the tellurium material (for example, elemental tellurium from the above inorganic reagents and ditellurides from tellurols). 4.1.1 4.1.1.1

Reduction of carbonyl compounds With hydrogen telluride

Hydrogen telluride, H2Te, generated in situ by the hydrolysis of aluminium telluride, reduces aldehydes and ketones to the corresponding alcohols.1,2 In the presence of deuterium oxide, deuterated derivatives are formed. OH

O R

R1

+ 1/ 2 Al2Te3 + 2 H2O

(50-100%)

R

R1 + Te + 2 / 3 Al(OH)3

R = H; R1 = Ph, o -MeC6H4, n -heptyl R, R1 = (CH2)5

Reduction of benzaldehyde with Al2Te/D2O (typical procedure).1 D2O (0.22 mL, 12 mmol) is added at −78°C to a suspension of Al2Te3 (0.876 g, 2 mmol) in THF (10 mL) containing benzaldehyde (0.106 g, 1 mmol) with vigorous stirring. The temperature is raised to 0°C 115

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in 30 min and maintained there for another 2 h. Filtration and evaporation of the solvent leaves pure PhCHDOD (0.11 g (100%)). 4.1.1.2

With phenyltellurol

The reagent is prepared by protonation of lithium phenyl tellurolate with trifluoroacetic acid (TFA)3 or by methanolysis of phenyltellurotrimethylsilane.4 Aldehydes and alkyl aryl ketones or diaryl ketones are reduced to the corresponding alcohols in good yields. O

OH 1 TFA R R PhTeH R R1 (+ 1/2 PhTeTePh) PhTeLi method A MeOH (method B)

PhLi + Te

PhTeTePh

Na

Me3SiCl

PhTeSiMe3

PhTeNa

Method A: R = H; R1 = Ph, p -MeOC6H4 (31,98%) Method B: R = H, Ph, p -MeOC6H4, o -ClC6H4, n -C12H25 (73-98%) R = Ph; R1 = Me, n -C3H7, Ph (80-95%)

Reduction of p-methoxybenzaldehyde, typical procedure; method A.3 To lithium phenyltellurolate prepared from PhLi (2.1 M, 4:8 mL, 10 mmol) and Te (1.27 g, 10 mmol) in THF (30 mL) under N2 is added p-methoxybenzaldehyde (0.272 g, 2 mmol) at −78°C followed by TFA (0.77 mL, 10 mmol). The solution is stirred for a further 2 h, and allowed to warm at room temperature. H2O (200 mL) is added and the product is extracted with ether. The extract is dried (Na2SO4), evaporated and the residue separated by SiO2 chromatography from the formed diphenyl ditelluride, giving p-methoxybenzyl alcohol (0.23 g (93%)). Reduction of o-chlorobenzaldehyde, typical procedure; method B.4 Diphenyl ditelluride (20.5 g, 50 mmol) in THF (50 mL) is treated with Na metal (3.5 g, 0.15 mol) for 18 h under ultrasound. After cooling to room temperature, Me3SiCl (14 mL, 0.11 mol) is added and the mixture stirred for 1 h. The solvent is evaporated and the residue is distilled under vacuum, giving phenyltelluroltrimethylsilane (20.5 g (74%); b.p. 77–79°C/2 torr). o-Chlorobenzaldehyde (0.140 g, 1 mmol) in benzene (5 mL) and MeOH (0.5 mL) is treated dropwise with phenyltellurotrimethylsilane (0.610 g, 2.2 mmol) for 30 min. After evaporation of the solvent the residue is chromatographed on an SiO2 column (eluting with CHCl3), giving o-chlorobenzyl alcohol (0.13 g (92%); m.p. 72.8–73.8°C). Under the above-described conditions, esters are unaffected and aliphatic ketones are barely reduced. In the presence of zinc iodide, however, aliphatic ketones, as well as aldehydes and aromatic ketones, are converted into methyl ethers in high yields.5 If alcohols other than methanol are used, the corresponding alkyl ethers are obtained. O PhTeSiMe3 + ROH

PhTeH + ROSiMe3

R = Me, PhCH2; R1 = H; R2 = Ph, n -C11H23 R1 = Ph; R2 = n -C3H7 R1 = Me; R2 = n -C9H19 R1, R2 = (CH2)5

R1

R2

ZnI2 (5-50% mol) (76-96%)

OR R1

R2

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Benzyl dodecyl ether (typical procedure).5 Phenyltellurotrimethylsilane (0.610 g, 2.2 mmol) is added to a solution of benzyl alcohol (0.238 g, 2.2 mmol) in dry benzene (10 mL) under N2. After stirring for 30 min at room temperature, ZnI2 (0.016 g, 0.05 mmol) in benzene (3 mL) is added, followed by dodecanal (0.184 g, 1.0 mmol). Stirring is maintained for 3 h and then H2O is added, the mixture is extracted with benzene and the benzene extract is dried (MgSO4) and evaporated under vacuum. The residue is chromatographed on SiO2 (elution with benzene/hexane), giving the product (0.265 g (96%)). 4.1.1.3

With diisobutyl telluride/titanium(IV) chloride

The known property of diorganyl tellurides of reducing metallic salts,6 applied to titanium(IV) chloride, generates a titanium(III) species which is a useful reagent for some selective reductions. By this method benzaldehyde is reduced to dihydrobenzoin, and benzyl to benzoin, and successively to desoxybenzoin.7,8 O Ph

H

DME, r.t., 2 h (99%)

O Ph

Ph

Ph

Ph

OH OH

i -Bu2Te/TiCl4 DME, r.t., 1 h (99%)

O

4.1.1.4

OH

i -Bu2Te/TiCl4

Ph

Ph i -Bu2Te / TiCl4

Ph

Ph O

O

With sodium telluride in 1-methyl-2-pyrrolidinone

Aromatic aldehydes are reduced to the corresponding alcohols by the title system.9 R

R R1

CHO

Na2Te NMP, 80°C 0.5-1 h

R, R1 = H R = H; R1 = Me, i -C3H7 R = Me; R1 = H

R1

CH2OH (34-59%)

Benzophenone gives benzhydrol in 89% yield. Typical procedure.9 To a solution of Na2Te (prepared from Te (170 mg, 1.3 mmol), NaH (70 mg, 2.9 mmol) in NMP (2.5 mL), was added acetic acid (1.2 mg, 0.02 mmol) followed by aldehyde (2.1 mmol) in dry benzene (10 mL) at 80°C under argon. The resulting mixture was kept at this temperature for 0.5–1.0 h. During this period, the colour of the solution gradually changed from deep purple to blue and finally to black. The progress of the reaction was monitored by TLC. After disappearance of the starting material (0.5–1.0 h), the reaction was quenched by the addition of 10% hydrochloric acid (0.1 mL), and the mixture was briefly aerated with vigorous stirring. Deposited tellurium was filtered off, and the filtrate was partitioned twice between ethyl acetate (20 mL) and saturated brine (10 mL). The organic phase was separated and dried over Na2SO4. The solvent was removed under reduced pressure, and the residue was chromatographed over silica gel using hexane/ethyl acetate (6:1) as the eluent to give the crude product, benzyl alcohol, which was purified either by Kügelrohr distillation (liquid) or by recrystallization (solid).

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Selective reduction of α,β-unsaturated carbonyl compounds

The regioselective reduction of the C⫽C bond of α,β-unsaturated carbonyl compounds, a very important organic reaction, is achieved by means of hydrogen telluride1 and phenyl tellurol,3 under appropriate experimental conditions. H2Te (Al2Te3 / H2O) method A

O Ph

R

O Ph

PhTeH (PhTeLi / TFA) method B

R

Method A: R = H, Me, Ph (54-84%) Method B: R = Ph (80%)

When the phenyl tellurol is prepared by methanolysis of phenyltellurotrimethylsilane, both the C⫽C bond and the carbonyl group can be reduced.4 Sodium hydrogen telluride was first prepared several years ago,10 and is recognized as a selective and versatile reducing agent. It is inert toward non-conjugated carbonyl, carboxyl, amide, ester, nitrile, haloaryl, non-activated haloalkyl and sulphone groups, but exhibits great selectivity for several functional groups. Sodium hydrogen telluride is a very convenient reagent for the reduction of the C⫽C bond of α,β-unsaturated carbonyl compounds.3,11–13 Representative examples are given below. O Ph

O NaHTe R (50-99%)

Ph

R

R = H, Me, OMe, OCH2Ph, NH2, OH

O O Ar

Ph SO2Ph

O

NaHTe (99%)

O

O O

NaHTe (65-70%)

Ar

Ph SO2Ph

Ar = Ph and haloderivatives

3-Phenylpropanal (typical procedure) NaHTe.10 A stirred mixture of Te powder (0.65 g, 5 mmol) and NaBH4 (0.45 g, 11.8 mmol) in EtOH (20 mL) is heated under argon. After 15 min most of the Te had dissolved. The mixture is then cooled at −20°C and treated with a deoxygenated solution of HOAc (0.6 mL) in EtOH (2.5 mL). Reduction.12 To the above-prepared solution of NaHTe is added 3-phenylpropenal (0.264 g, 2 mmoI) in EtOH (2 mL). The mixture is stirred at room temperature for 4–5 h, then filtered from the formed black Te through Celite® and the filtrate evaporated. The residue is purified by distillation, giving the pure product (0.265 g (99%)).

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The synthetically important isopropylidene malonates (Meldrum acids) can be prepared in a “one-pot” procedure employing this method.14 O R

R1

O

+ O

O

NaHTe O AcOH/ piperidine (69-85%)

O

O

R

R1

O

R, R1 = Me, (CH2)5 R = H; R1 = i -Pr, p -MeO, p -HOC6H4

O

Monosubstituted isopropylidene malonates (typical procedure).14 To a solution of NaHTe (10 mmol) prepared as above is added a solution of isopropylidene malonate (0.72 g, 5 mmol), benzaldehyde (0.83 g, 7.5 mmol), AcOH (1.5 mL) and piperidine (four drops) at −30°C under N2. After 10 min the mixture is warmed to room temperature, stirred for 20 min, filtered and evaporated under vacuum. Aqueous K2CO3 (5 mL) is added to the residue. After extraction with ether (3×3 mL), the solution of K2CO3 is acidified with 6 N HCl (5 mL) and then filtered, giving the pure product (0.99 g (81%); m.p. 80–81°C). 4.1.3

Reduction of conjugated arylalkenes and arylalkynes

In addition to the described reduction of double bonds conjugated to a carbonyl group, sodium hydrogen telluride and phenyltellurol reduce double (and triple) bonds conjugated to aromatic systems.3,4,11 NaHTe (method A) (ref. 3,11) Ar

R PhTeH (PhTeLi / TFA) method B

Ar

R

PhTeH (PhTeSiMe3 / MeOH) (method C) (ref. 4)

High yields are obtained with terminal alkenes (styrene and α-methylstyrene) and with the strained acenaphthylene, whereas stilbene and β-methylstyrene are barely reduced. Acenaphthene (typical procedure).3 Method A. To a solution of NaHTe (10 mmol, prepared as previously described) is added acenaphthylene (0.304 g, 2 mmol) in EtOAc (20 mL), and the mixture refluxed for 9 h. During this time a black Te precipitate is formed. After dilution with H2O (500 mL) the product is extracted with ether, the ethereal solution is dried (Na2SO4) and evaporated and the residue chromatographed on SiO2, giving acenaphthene (0.303 g (99%)). Method B.3 A mixture of diphenyl ditelluride (2.05 g, 5 mmol), NaBH4 (0.397 g, 10.5 mmol) and dry EtOH (20 mL) is heated with stirring under N2 atmosphere for 15 min, when most of the ditelluride had produced an almost colourless solution. The solution is then cooled to 20°C and TFA (0.81 g, 10.5 mmol) in EtOH (5 mL) is added, followed by a solution of acenaphthylene (0.305 g, 2 mmol) in EtOAc (20 mL). The mixture is refluxed for 4 h, then diluted with cold H2O (500 mL) and worked up in the usual manner, including SiO2 chromatography, to give diphenyl ditelluride (1.93 g (94%)) and acenaphthene (0.300 g (98%)).

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Phenylacetylene is reduced by phenyltellurol to ethylbenzene4 in a yield of 42%. Ph

4.1.4

H

PhTeH (PhTeSiMe3 / MeOH)

Ph

Reduction of imines and enamines

Hydrogen telluride,2,15 sodium hydrogen telluride3 as well as phenyltellurol3 afford the reduction of imines to secondary amines. H2Te (Al2Te3 / H2O) / THF, 0°C (method A)

R

N

NaHTe / AcOEt, reflux (method B)

R1

R

H N

R1

PhTeH (PhTeLi / TFA) THF/ -78°C to r.t. (method C)

Method A: R, R1 = Ph (71%) Method B: R = Ph; R1 = Et (65%) Method C: R, R1 = Ph (93%)

Enamines are susceptible to a similar reduction.15 N

(method A) (85%)

N

In the reduction of imines and enamines with hydrogen telluride and sodium hydrogen telluride, hydrolysis leading to primary amines and carbonyl compounds is frequently competitive with the reduction. This undesired side reaction is minimized by the addition of triethylamine, in the case of hydrogen telluride.15 Control of the pH to within the 6–7 range has been shown to be important in the reduction of imines with sodium hydrogen telluride. At a higher pH (10–11) no reduction occurs.16 A valuable application of the above-described reduction is the one-step reductive alkylation of amines with carbonyl compounds.15,17 O R

H2Te (method A) + R2NH2 R1 NaHTe (method B)

R

NHR2

R1

Good yields of secondary amines are achieved using both the methods in the reactions of aromatic and aliphatic aldehydes as well as of dialkyl ketones and cycloalkanones with aliphatic and alicyclic amines (and ammonia). Anilines give low yields, but when 2 equiv is used in the sodium hydrogen telluride method, the yields are improved. In the reaction of ammonia with aldehydes, symmetrical secondary amines are obtained, whereas glutaraldehyde and amines17 lead to N-substituted piperidines.

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Reductive amination of carbonyl compounds. Method A. N-octylbenzylamine (typical procedure).15 A THF (30 mL) suspension of benzylamine (1.67 g, 15 mmol), octanal (1.55 g, 15 mmol), Et3N (7.0 mL, 50 mmol) and Al2Te3 (6.1 g, 14 mmol) is refluxed. The reaction is monitorized by GLC and H2O (3 mL, 168 mmol) is added at 15°C after 30 min. The heterogeneous mixture is refluxed for another 30 min. The black solid is removed by filtration and the filtrate dried (CaCO3) and evaporated. The residue is distilled, giving the product (2.7 g (80%); b.p. 108–112°C/1 torr). Method B (general procedure).17 To a solution of NaHTe (5 mmol) prepared as described previously10 the carbonyl compounds (2 mmol) and the amine (2 mmol) are added at −20°C. The mixture is stirred for 20 h at room temperature. The precipitated black Te is filtered and the filtrate evaporated under vacuum. The residue is dissolved in EtOAc (20 mL) and acidified with dilute HCl. The combined aqueous layers are made basic with NaOH and extracted with EtOAc (2×30 mL). The extract is dried (Na2SO4), concentrated and purified by TLC (elution with hexane/EtOAc). 4.1.5

Reductive desulphuration of aromatic thioketones

Under aprotic conditions, aromatic thioketones are reduced by sodium telluride to the corresponding hydrocarbons.18 S Ar

Na2Te Ar1 DMF/ (60-82%)

Ar

Ar1

Ar = Ar1 = Ph,

O Ar = Ph; Ar1 = p -BrC6H4

Reductive desulphuration of aromatic thioketones: xanthene (typical procedure).18 To a suspension of Na2Te (prepared by heating powdered Te (0.256 g, 2.0 mmol) and NaH (0.10 g, 4.2 mmol) in DMF (3 mL) at 140°C for 1 h under N2) is added xanthene-9-thione (0.210 g, 1.0 mmol) in THF (5 mL), and the mixture is heated under gentle reflux. The colour changes from green to orange to blue and finally to black. After 2 h, excess Na2Te is destroyed by exposure to air, and H2O (20 mL) and CH2Cl2 (20 mL) was added. After filtration, the organic layer is washed with 10% HCl and evaporated. The residue is purified by SiO2 chromatography, giving xanthene (0.161 g (82%); m.p. 96–99°C). Under aqueous conditions (Na2Te/Rongalite method) no reduction occurs, the thioketone being converted into the ketone.18 4.1.6

Reduction of nitro compounds

Hydrogen telluride2,19 and sodium telluride in a protic solvent20 easily reduce nitroarenes to anilines. In contrast, sodium telluride in an aprotic solvent exhibits a milder reducing capacity and reduces the nitro compounds only to azo compounds.18

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The reducing power of sodium hydrogen telluride towards nitro compounds is dependent on their structure. Unhindered nitrobenzenes are reduced to azoxybenzenes whereas sterically hindered nitrobenzenes are reduced to anilines21,22 by simple stirring with 5 mol equiv of the reagent at room temperature for 1 h. Under alkaline conditions, sodium hydrogen telluride reduces nitrobenzene to N-phenylhydroxylamine.23 A more practical procedure for the reduction of nitroarenes to N-arylhydroxyl amines is the use of NaBH4 in the presence of a catalytic amount of tellurium.23 No over-reduction products are observed. The reduction of nitro- and dinitroalkanes with sodium hydrogen telluride gives, respectively, diazenes (dimers of nitrosoalkenes) and olefins.21

ArNO2

H2Te (Al2Te3 / H2O) THF/ (60-94%) Na2Te (Rongalite method) H2O / dioxane, 50°C (55-96%)

Na2Te / DMF

Ar = Ph and alkyl derivatives, o -Ph, p -Ph, p -PhCO, p -PhCH=CHC6H4, 1-naphthyl, p -NO2Me4C6, m -NO2Me4C6 (giving diamines) (ref. 20) ArN=NAr Ar = Ph, m -Cl, p -Cl, m -Me, o -Ph, p -PhC6H4, 1-naphthyl (ref. 18)

(67-97%)

NaHTe (5 equiv)

ArNH2 Ar = Ph, o -Me, p -MeO, p -Cl, p -BrC6H4 (ref. 19) ArNH2

(63-92%)

ArN=N(O)Ar Ar = Ph, o -Me, p -Me, p -MeO, o -Cl, m -Cl, p -Cl p -Br, p -AcC6H4 (ref. 21) ArNH2

(89-92%) NaHTe / OH(60%) NaBH4 / Te cat. EtOH / r.t. (63-92%)

Ar = o, o, p -Me3C6H2, o, o -Me2C6H3, p -t -BuC6H4 ArNHOH (ref. 23) Ar = Ph

ArNHOH Ar = Ph, p -Cl, p -Me, p -HO2C, p -NC, p -NO2C6H4, p -EtO2CCH = CHC6H4 (ref. 23) NaHTe / EtOH RN(O)=N(O)R RNO2 (80-66%) R = cyclohexyl, n -C7H15 (ref. 21) O2N NO2

NaHTe EtOH / r.t. (86%)

Reduction of nitrobenzene with H2Te: aniline (typical procedure).19 Powdered Al2Te (674 mg, 2 mmol), stored under N2 in the dark, is added to a THF (10 mL) solution of nitrobenzene (123 mg, 1 mmol) under N2 at room temperature. The temperature is then raised to 66°C. To the vigorously stirred reaction mixture is slowly added an excess of H2O

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(0.43 mL, 24 mmol), dropwise by a syringe over a period of 40 min. The reaction mixture is refluxed for another 20 min and worked up, giving the product (0.837 g (90%)). Reduction of aromatic nitro compounds with Na2Te (Rongalite method): anilines (general procedure).20 To tellurium powder (0.013 g, 0.10 mmol) and Rongalite (0.77 g, 5 mmol) thoroughly flushed with N2, is added 1 M NaOH (15 mL) followed by the nitro compound (1.0 mmol) in dioxane (3 mL), and the heterogeneous mixture stirred at 50°C for 1–3 h. The progress of the reaction is monitored by TLC. After an appropriate time, when the nitro compound has almost completely disappeared, the reaction is quenched by exposing the mixture to air. NaCl (3–4 g) and CHCl3 (15–20 mL) are added with stirring and the mixture is freed from insoluble inorganic materials by filtration over a thin layer of Celite®. The organic phase is separated and the aqueous solution is extracted with CHCl3. The CHCl3 solution is washed with brine, dried (Na2SO4) and evaporated to give the product as an oil or a solid residue. Reduction of aromatic nitro compounds with Na2Te/DMF: azo compounds (general procedure).18 To a stirred suspension of Na2Te (2.0 mmol) in DMF (3 mL) is added by a cannula a solution of nitroarene (1.0 mmol) in THF (2 mL) at room temperature, and under N2. The mixture is gently heated to 70°C and kept at this temperature for 3–5 h. The progress of the reaction is quenched by bubbling air into the mixture, to which brine (20 mL) followed by ether (20 mL) is then added. The insoluble materials are filtered through a thin layer of Celite® and the organic phase is separated from the filtrate, washed with 10% HCl to remove any DMF, and evaporated to leave the crude product, which is purified by chromatography on SiO2 or recrystallization from an appropriate solvent. Reduction of nitrobenzene with NaBH4 /Te catalyst: N-phenylhydroxylamine (typical procedure).23 To a stirred mixture of powdered Te (51 mg, 0.4 mmol) and NaBH4 (1.52 g, 40 mmol) in EtOH (25 mL) under argon is added nitrobenzene (492 mg, 4 mmol) as a single portion, and the mixture is stirred at room temperature (22°C) for 1.5 h. Then the mixture is poured into ice-H2O, while bubbling argon through it, and acidified to pH 6 with 10% HCl, and immediately extracted with ether. The extract is washed with brine and dried (MgSO4). After evaporation of the solvent under reduced pressure at room temperature, the crude product is purified by short SiO2 column chromatography (eluting with CH2Cl2) and subsequent recrystallization (benzene/petroleum ether) to give the product (0.27 g (63%); m.p. 80–82°C). Dinitro compounds can be similarly reduced to diamines by using twice the amount of reducing agent, increasing the proportion of dioxane in the solvent system, and extracting the product from the aqueous solution with ether. A much longer reaction time (5–7 h) is needed for complete reduction. Phenyltellurol is another valuable reagent for the reduction of nitroarenes (and nitrocycloalkanes) to the corresponding amines. The reagent is easily generated in situ by the previously described methods (methods A3 and B4,24), or by the reduction of diphenyl ditelluride with NaBH424 (method C). In the last

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method, the procedure can be performed with a catalytic amount of ditelluride, since the ditelluride is continuously reformed during the reaction. PhTeLi

F3CCO2H (method A)

PhTeSiMe3 MeOH (method B) PhTeTePh

NaBH4 (method C) benzene / EtOH/ H2O

PhTeH + ArNO2

THF, r.t. (98%)

PhTeH + ArNO2

benzene, r.t. (62-99%)

PhTeH + ArNO2

ArNH2 (+ PhTeTePh)

80°C (65-95%)

Method A: Ar=Ph Method B and C: Ar = Ph, o -Me, m -Me, p -Me, o -MeO, p -MeO o -Cl, o -Br, o -l, m -NO2C6H4, o, p -Me2C6H3, 1-naphthyl, 1-(5-NO2)naphthyl (giving diamine, o -NC-C6H4 (giving anthranilamide)* * Nitrocyclohexane is also reduced to cyclohexylamine with a yield of 62%.

Reduction of aromatic nitro compounds with phenyltellurol (NaBH4 /(PhTe)2 catalyst): aniline (typical procedure).24 Nitrobenzene (0.246 g, 2.00 mmol) and PhTeTePh (0.164 g, 0.4 mmol) are dissolved in benzene (10 mL). To this mixture is added NaBH4 (0.378 g, 10.0 mmol) dissolved in EtOH/H2O (v/v 2:1, 10 mL) dropwise under N2 atmosphere. The reaction mixture is heated to 80°C for 15 h with stirring. After cooling, the mixture is washed with saturated brine and dried (Na2SO4). The crude product is purified by SiO2 column chromatography (solvent: benzene/ether) and then by gel permeation liquid chromatography. Aniline is isolated (134 mg (72%)). An additional method for the reduction of nitrobenzene to aniline involves the previously described i-Bu2Te/TiCl4 system (see Section 4.1.1.3).7,8 ArNO2

i -Bu2Te TiCl4, CH2Cl2 (42-98%)

ArNH2

Ar = Ph, m -CN, p -Cl, p -Br, p -Ph p -PhCOC6H4, m, p -Me2C6H3

Reduction of aromatic nitro compounds with i-Bu2Te/ TiCl4: aniline (typical procedure).8 To a mixture of i-Bu2Te (0.36 g, 1.5 mmol) and a 1 M solution of TiCl4 in CH2Cl2 (4 mL) is added nitrobenzene (0.06 g, 0.5 mmol) in CH2Cl2 (2 mL) at room temperature and under N2. An instantaneous reaction occurs and the colour of the solution turns dark brown. After stirring for 0.5 h, NaCl (3–4 g) followed by 1 M aqueous NaOH is added until the aqueous phase is alkaline. The organic phase is extracted with Et2O and the extract is washed with brine, dried (Na2SO4) and evaporated. The residue is purified by SiO2 chromatography (elution with CH2Cl2/Et2O, 5:1), giving the almost pure amine (74% yield). Sodium phenyl tellurolate is a milder reducing agent than phenyltellurol. The reduction of nitroarenes is therefore limited at the intermediate azoxy or azo compound stage, depending on the reaction temperature.25,26 The reagent is prepared in situ by treatment of diphenyl ditelluride with NaBH4 in ethanol in the presence of sodium hydroxide. Since the

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diphenyl ditelluride is continuously regenerated from the formed phenyl tellurenate (PhTeONa) by reduction with the excess of NaBH4, it is used only in catalytic amounts.

ArNO2

(PhTe)2 cat.

NaBH4 / EtOH / H2O / NaOH r.t. (60-90%) NaBH4 / EtOH / H2O / NaOH reflux (52-91%)

ArN(O)=NAr

ArN=NAr

Ar = Ph, o -MeC6H4, m -MeC6H4, p -MeC6H4, p -MeOC6H4, p -ClC6H4, p -BrC6H4, p -AcC6H4, o, p -Me2C6H3

In the reflux procedure, minor amounts of the corresponding arylhydrazines are formed as by-products. Reduction of nitrobenzene with NaBH4 /(PhTe)2 catalyst/EtOH/aqueous NaOH at room temperature: azoxybenzene (typical procedure).26 To a mixture of diphenyl ditelluride (0.021 g, 0.05 mmol) and NaBH4 (0.378 g, 10 mmol) are added EtOH (10 mL) and 5 M aqueous NaOH (2.5 mL) successively at room temperature under an N2 atmosphere. An EtOH (5 mL) solution of nitrobenzene (0.246 g, 2.0 mmol) is then added to the colourless homogeneous solution and the mixture is stirred at room temperature for 10 h, during which time the yellow colour turns to orange. The mixture is poured into brine, then extracted with CHCl3 (3×50 mL), and the extract is dried (MgSO4). Evaporation of the solvent leaves a yellow oil, which is subjected to column chromatography on SiO2 using hexane/EtOAc (10:1) as eluent, to give the pure product as a yellow solid (0.181 g (91.3%)). Reduction of p-bromonitrobenzene with NaBH4 /(PhTe)2 catalyst/EtOH/aqueous NaOH under reflux: bromoazobenzene (typical procedure).26 To a mixture containing diphenyl ditelluride (0.021 g, 0.05 mmol) and NaBH4 (0.378 g, 10 mmol) is added EtOH (10 mL) followed by 5 M aqueous NaOH (2.5 mL) at reflux. A solution of p-bromonitrobenzene (0.404 g, 2.0 mmol) in EtOH (5 mL) is then added to the colourless homogeneous solution, and the mixture is stirred under reflux for 15 h, during which time the yellow colour turns to red. After cooling, the mixture is poured into H2O and extracted with CHCl3 (3×50 mL), and the extract is then dried (MgSO4). The products, bromoazobenzene (0.224 g, 66%; m.p. 206–207°C) and p,p′-dibromo-N,N′-diphenylhydrazine (0.065 g, 19%), are isolated by column SiO2 chromatography, eluting with hexane/CHCl3 (3:1 and 1:1, respectively). 4.1.7

Reduction of other nitrogenated compounds

Hydrogen telluride reduces hydroxylamines to anilines, and nitroso, azo and azoxy compounds to the corresponding hydrazo compounds.1,2 The reduction of azo compounds to hydrazo compounds is also achieved by means of aryltellurols or sodium hydrogen telluride.3 The last reagent (generated from NaBH4 and

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Te catalyst) reduces tertiary amine N-oxides to the corresponding amines.27 Azides are reduced to primary amines by sodium hydrogen telluride.28 PhNHOH

PhNO

H2Te (Al2Te3 / H2O)

PhNH2

(98%)

H2Te (Al2Te3 / H2O) (78%)

PhNH-NHPh + PhNH2 (4:1)

H2Te (Al2Te3 / H2O) (96%) ArTeH (ArTeLi) / H+ / THF PhN=NPh PhNH-NHPh (66-99%) NaHTe / EtOH / CH2Cl2 (89%) RR1R2NO

RN3

H2Te (Al2Te3 / H2O) (86%)

NaHTe (NaBH4 / Te cat)

RR1R2N

EtOH (70-38%) NaHTe EtOH / ether (69-100%)

PhN(O)=NPh

RNH2

α,β-Unsaturated nitriles and α-halonitriles are reduced to the corresponding nitriles.29 R

R2

R1

CN a) r.t., 24 h, 85-90% b) reflux, 24 h, 75, 67% c) r.t., 24 h, 40%

NaHTe

R

R2

R1

CN

a) R1 = R2 = H R = Ph, p -CNC6H4 ,MeO2C(CH2)6, C8H17, CH2=CH(CH2)7, O b) R1 = Me; R2 = H; R = Ph R2 = H; R, R1 = (CH2)5 c) R, R1 = H; R2 = Ph

R R1

X

O

NaHTe H R 1 CN a) -20°C, 20 min, 90-98% CN R b) r.t., 5h, 94-97% c) -20°C, 20-30 min, 55, 86%

a) X = Cl R = H; R1 = Ph, p -CNC6H4, O O R = Me; R1 = Ph, p -ClC6H4 b) R = H; R1 = C9H19, MeO2C(CH2)7, CH2=CH(CH2)8, Cl(CH2)8 c) X = Br R = H; R1 = C9H19, O O

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Typical procedure.29 Te powder (140 mg, 1.09 mmol) and NaBH4 (95 mg, 2.5 mmol) were refluxed in EtOH (4 mL) for 45–60 min. The resulting deep purple solution was cooled at −20°C; then 0.12 mL of AcOH in 0.5 mL of EtOH was added followed by the substrate (0.5 mmol) in 1 mL of benzene. The reaction mixture was stirred (−20°C, room temperature) at the indicated temperature for the indicated time. Then, the reaction flask was opened to air and a small amount of silica gel added. After 1 h, the mixture was filtered through Celite® and chromatographed on silica gel. Allylamines are important synthetic intermediates.30 The treatment of aziridine sulphonates bearing a trityl or benzydryl group at the nitrogen atom, with Na2Te promotes the opening of the aziridine ring giving allyamines31. CPh3(HPh2) N OTs

4.1.8

CPh3(HPh2) N

Te2Te/NaBH4 DMF

Ph3CN

H+ Te

Ph3CHN

+ Te°

Te-

Deselenylation of α-seleno carboxylic compounds

By treatment with NaBH4 in aqueous NaOH in the presence of dithienyl ditelluride, α-phenylseleno carboxylic esters and malonates, as well as α-phenylseleno carboxylic acids, are deselenylated to the corresponding seleno-free acids in good yields.32 The selective removal of the seleno group without dealkylation of the ester moiety is achieved by the methods depicted in the scheme. SePh R1

R

a) (2-ThTe)2 cat., EtOH, NaBH4 5%

CO2R

R1 = Ar, CO2Et R = Et, H

aq. NaOH 5%, r.t., 5 min b) Te / NaH / DMF c) Te / NaBH4 / DMF

R R

1

CO2H

H R1

CO2R

Method A. Typical procedure.32 To a solution of the ethyl α-phenylseleno phenyl acetate (0.319 g, 1 mmol) and dithienylditelluride (0.01 g, 0.024mmol) in ethanol (8 mL) under nitrogen at room temperature was added a solution of 5% NaBH4 (in aqueous 5% NaOH) until the red colour of the solution was faded. After 5 min stirring at room temperature the reaction was diluted with H2O, acidified with 3 N HCl, extracted with ethyl acetate and dried over MgSO4. After the solvent evaporation the residue was purified by column chromatography on silica gel eluting with hexane/ethyl acetate (9:1). Yield of phenyl acetic acid, 0.11 g (80%), m.p. 74–76°C (lit. 77–78.5°C). Method B. Typical procedure. A mixture of elemental tellurium (0.127 g, 1 mmol) and sodium hydride (0.0528 g; 2.2 mmol) in dimethylformamide (3 mL) was heated at 140°C under nitrogen and magnetic stirring for 1 h. The resulting violet solution was cooled to 0°C in an ice bath and treated dropwise with a solution of ethyl α-(phenylseleno) mesityl acetate (0.361 g, 1 mmol) in DMF (2 mL). A vigorous reaction occurred and the colour of the solution changed from violet to dark brown. After the addition the mixture was diluted

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with diethyl ether and filtered on Celite®, the filtrate was washed three times with water and dried with MgSO4. The solvent was evaporated and the residue was purified as above. Yield of ethyl mesithyl acetate was 0.12 g (60%). Method C. Typical procedure. A mixture of elemental tellurium (0.0127 g, 0.1 mmol) and NaBH4 (1.12 g, 3 mmol) in DMF (3 mL) under nitrogen and magnetic stirring was heated at 70°C for 0.5 h. Then the violet solution was cooled to 0°C and a solution of the α-(phenylseleno)t-butyl acetoacetate (0.313 g, 1 mmol) in ethanol (1 mL) was added dropwise. A vigorous reaction occurred, the colour of the reaction mixture changing from violet to bright brown. Then water (3 mL) was added and the mixture was acidified with 3 N hydrochloric acid (10 mL) and extracted with diethyl ether (3×25 mL). The organic layer was dried with MgSO4 and the solvent was evaporated. The residue was purified as above. Yield of t-butyl acetoacetate, 0.10 g (63%). Similar results are obtained using NaBH4 and catalytic amounts of diphenyldiselenide. α-Phenylseleno ketones and β-ketoesters are also deselenylated by similar methods. 4.1.9 Deoxygenation of oxiranes with alkali O,O-dialkyl phosphorotellurolates A useful complement to the known methods for the deoxygenation of epoxides to alkenes33 is the reaction of epoxides with alkali O,O-diethyl phosphorotellurolates.34,35 R O R2

(EtO)2P(O)TeMet, EtOH, r.t.

R

R2

R1

-Te (Met=Na, Li) (70-90%)

R1

R3

R3

The reagent can be used in stoichiometric amounts or generated more conveniently in situ, employing only catalytic amounts of tellurium, since the metal is continuously regenerated during the reaction. Important points to note are • • •

The tellurium reagents are more effective than their selenium analogues. The lithium salts seem to be the most reactive. Terminal oxiranes are more reactive than other types, Z isomers more than E isomers and cyclohexene oxides more than cyclopentene oxides. The deoxygenation is stereospecific, the (Z)-oxiranes giving the (Z)-olefins. An epitelluride has been postulated as the intermediate.

Deoxygenation of 1,2-epoxyoctane with (EtO)2P(O)TeNa (typical procedure).35 To Te powder (214.9 mg, 1.684 mmol) and 1,2-epoxyoctane (12.180 g, 95.01 mmol) is added from a syringe, under N2 with magnetic stirring, a stock EtOH solution of (EtO)2P(O)Na (107.4 mmol) over 11 h. After stirring overnight, the mixture is filtered through cotton wool, diluted with H2O (1 L) and extracted with isopentane (3×300 mL). The organic extract is dried (Na2SO4), concentrated and distilled in a spinning band apparatus to give 1-octene as a colourless liquid (7.76 g (72%); b.p. 112°C); purity 99% by GLC (silver ionimpregnated columns).

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4.1.10 Reductive opening of oxiranes with sodium hydrogen telluride and sodium telluride Sodium hydrogen telluride reacts with epoxides, in accordance with an SN2 displacement, giving rise to telluro-alcohols. These products are useful intermediates since they are easily converted into the corresponding alcohols and ketones by treatment with nickel boride followed by oxidation (reaction (a)) or to alkenes via the corresponding tosylates (reaction b)).36 R O R2 NaHTe R1 R3 EtOH/

1 1 R R2 R R OH b)TsCl R R OTs R pyridine 3 3 (92%) -Te 1 3 2 2 R R R1 R Te R H Te R H Te R

a)

R3

Ni3B2, r.t. (74-98%)

1 R R OH

H

R2

R R1 H

CrO3, pyridine

2 R3

R

R2 or R3 = H

O R3(2)

a) R1, R2, R3 = H;R = n -C16H33, n -C6H13, PhOCH2 R, R2 = H; R1, R3 = (CH2)4, R2, R3 = H; R, R1= b) R1, R2, R3 = H R = n -C16H33

α,β-Epoxyketones are reduced by sodium hydrogen telluride to the corresponding βhydroxy ketones.37 O

R1 NaHTe / EtOH, 0°C (72-91%) O

R

O

OH R1

R

R = Me, Ph; R1 = Ph O R R1

O

R, R1 = H O R = i -Pr; R1 = H R = H; R1 = CO2Et

O

O

O

O

The following mechanism has been proposed for this reaction: O

O

O

O-

R

R :TeH

Te-

OR

O

OH H O+ 3

OH

R

:TeH

Reduction of α,β-epoxy ketones with NaHTe (typical procedure): β-Epoxy ketones.37 To a solution of NaHTe (prepared from Te (1.30 g, 10 mmol) and NaBH4 (0.90 g, 0.24 mmol)) in EtOH is added isophorone oxide (0.616 g, 4 mmol) in EtOH (4 mL). An instantaneous

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reaction occurs, the colour of the mixture changing to deep black. After 15 min H2O (50 mL) is added and the mixture exposed to air at 0°C. The solution becomes clear after 1 h with deposition of black Te, and is filtered through Celite®. The filtrate is extracted with CH2Cl2, and the extracts washed with H2O, dried (Na2SO4) and evaporated, leaving a crystalline solid which is recrystallized from hexane to give 3-hydroxy-3,5,5-trimethylcyclohexanone (0.578 g (83%); m.p. 79–79.5°C). Epoxides bearing a leaving group in a suitable position, such as chloromethyl epoxides, react with sodium telluride (prepared by the Rongalite method), giving allylic alcohols.38 An unstable epitelluride has been assumed once more as an intermediate. Na2Te

O R

R1

O

O

Cl

Te: R

R1

R

Cl

R1 Te-

OH R

R1

OH

Te

-Te (40-90%)

R1 R

R = H; R1 = Ph, Et, p -MeOC6H4, R = Ph; R1 = H

Bischloromethylcarbinols are converted directly into allylic alcohols.38 Cl R R = Ph,

Cl OH

Na2Te (85, 90%)

OH R

Ph

Reduction of chloromethyl epoxides with Na2Te: allylic alcohols (general procedure.38 To an Na2Te solution (prepared from Te (4.1 mmol), Rongalite (6.2 mmol) and aqueous NaOH (25 mL) by heating at 55°C for 30 min) is added 4.0 mL of the chloromethyl epoxide (or the dichloropropanol) in dioxane (20 mL). After refluxing for 4 h the mixture is filtered through Celite®, extracted with CHCl3 and the solvent evaporated. The residue is purified by flash chromatography or TLC on SiO2 (elution with Et2O/hexane, 3:7).

4.1.11 Correlate reaction: tellurium-mediated resolution of racemic allyl alcohols The combination of the preceding method of obtaining allyl alcohols with the Sharpless kinetic resolution (SKR) of secondary allyl alcohols allows conversion of the original racemic allyl alcohol into a pure enantiomer with a 100% theoretical yield.39 By this procedure, the glycidol obtained by the SKR epoxidation of the secondary allyl alcohol is converted into the corresponding mesylate and then treated with the Te2− ion, furnishing the allylic alcohol with the same configuration of the enantiomer in the SKR which

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remains unreacted. An illustrative scheme is provided. OH

OH O

a, b

R

R R1

OH +

H+ -Te (75-89%)

R R1

R1

a) Ti(O-i Pr)4,TBHP (+)- or (-)-i-Pr2 tartarate, CH2Cl2 b) chromatographic separation(SiO2)

(MeSO2)2O pyridine, DMAP CH2Cl2 OMs O R

OMs O

Na2Te (Rongalite)

Te-

R

Te2-

R1

R1

O-

O

R

R1

R R1

Te

R = Et, n -Bu, Me2CH=CHCH2; R1 = Me R = CH2= CH(CH2)3; R1 = Et R = Ph (CH2)2; R1 = H

The SKR of cis secondary allyl alcohols is often unsatisfactory since low enantioselectivity is observed. R

R

SKR R (10% ee)

R

OH

OH

A valuable alternative for enhancing the selectivity is based on telluride-mediated transposition applied to the trans-allylic alcohols.40 1) t-BuOOH / Ti(O-Pr-i)4/ R (+)-i-Pr2 tartarate

O

OH

2) chromatographic separation

OH

OH

R1

R R1

R

R1 >90% ee

R1

1) (MeSO2)2O, pyridine DMAP, CH2Cl2 2) Na2Te(NaBH4 / Te)/DMF

R1 OH

The transposition of the C⫽C bond and the alcohol functionality is highly stereospecific, erythro-glycidol sulphonates giving cis-allylic alcohols and the threo-isomer the transallylic alcohol. Y X O (+-) 1 R

H R2

Na2Te

H

OH

1 (Te / NaBH4) R R H DMF H trans (83-91%) X = OMS; Y = H = threo

X = H; Y = OMS = erythro 1 Threo R Me c-C6H11 n -pr

CH2

R2 n -C5H11 Me n -pr Me

cis (80-88%) 1 Erithro R Me c-C6H11 n -pr n -C6H13

CH2

R2 n -C5H11 Me n -pr Et Me

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Resolution of racemic allyl alcohols (general procedure).40 Elemental powdered Te (2.0–2.5 equiv) and NaBH4 (4.0–5.0 equiv) in DMF (0.5–1.0 M in Te) are mixed under an inert atmosphere and heated to 70–80°C for 0.5 h. The grey suspension turns a deep purple colour. The reaction is cooled to room temperature and the glycidyl sulphonate (1.0 equiv based on Te) in DMF is added. The reaction is monitored by 1H NMR analysis of reaction aliquots and is generally completed in 3–4 h. Heating to 40°C is sometimes required for complete reaction. At completion, H2O (about 0.5× solution volume) is slowly added. Gas is evolved, and in some instances the solution becomes clear and colourless. When the reaction mixture is exposed to air, elemental tellurium precipitates as a greyish-black solid. H2O2 (five drops, 30% v/v) is cautiously added to complete the oxidation of the telluride ion into tellurium. The mixture is filtered through a bed of SiO2 or Celite®, and the Te is removed. The mixture is extracted with ether (3×20 mL) and the combined organic phases are washed with saturated aqueous Na2S2O3, dried (MgSO4), filtered, and the solvents removed by a rotatory evaporator. Residual DMF is easily removed by flash chromatography. 4.1.12

1,2-Elimination in vicinal disubstituted substrates

4.1.12.1

Debromination of vic-dibromides with tellurium reagents

Several methods achieving the debromination of vic-dibromides by means of tellurium reagents are well established. These methods are particularly advantageous compared to the conventional ones41 in terms of the mildness of the experimental conditions, good yields, lack of important side reactions and inertness of several functionalities to the employed reagents. A relevant characteristic of these reactions is the high E2-type stereospecificity demonstrated by the formation of olefins with Z and E geometry from threoand erythro-dibromides, respectively. (method A) Ar2Te / -Ar2TeBr2 K2S2O5 (method B)

Ar2Te

Ar2TeBr2

benzene / H2O (method C) ArTeTeAr / toluene /

(tellurium reagent) Te:

-Ar2TeBr2, -Te Br R2 3 R

R R1 Br

(method D) NaHTe / EtOH -Te (method E) Na2Te / H2O / dioxane

Representative examples: (Rongalite / Te cat.) RCHBrCHBrR1 1 R = R = H, Ph, CO2Et, CO2H (method F) (EtO)2P(O)TeNa / EtOH, r.t. R = Ph; R1 = 4 - pyridyl, CO2H [(EtO)2P(O)Na/ Te cat.] CO2Et, CHO, COPh R = H; R1 = substituted 3-quinolyl (method G) 2-thienylTeNa / EtOH / EtOH, r.t. R, R1 = (CH2)4, (CH2)10 and (2-thienyl)2Te2 cat./NaBH4 dibromocholesterol (method H) Ph3SnTeSnPh3 / KF.H2O / MeCN, r.t.

R

R2

R1

R3

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The following remarks should be noted: Method A.42 Diaryltellurium dibromides are formed together with the olefinic product. The starting tellurides can easily be recovered by reduction of the dibromides (see Section 3.1.4). Method B. This is an improved modification of method A, effecting the debromination in a catalytic cycle by means of potassium bisulphite and catalytic amounts of diaryl telluride. Di-(p-methoxyphenyl telluride) had at first been used43 but better results have been achieved with electron-rich diorganyl tellurides, such as (p-Me2NC6H4)2Te, (C6H13)2Te, 2(PhTe)C6H4CH2NMe2 associated to different reducing agents, reduced glutathione (GSH) or sodium ascorbate.44 It was suggested that halogenation–dehalogenation with diaryltellurium derivatives is an equilibrium process45 since, at identical concentrations, debromination of erythro-1, 2-dibromo-1,2-diphenylethane with diaryltellurides and bromination of trans-stilbene with dibromodiaryltellurides gives identical product mixtures. Ph

Br

Ph + Ar2Te

Ph

Br

+ Ar2TeBr2 Ph

Method C.46 This method is attractive owing to the easy one-step preparation of the starting ditelluride. The reaction can be rationalized via an aryltellurenyl bromide as an intermediate. Ar2TeBr2 + Te

2 [ArTeBr]

Method D.11,47 Sodium hydrogen telluride furnishes higher yields under milder experimental conditions compared to the previously employed thio and selenium analogues. The tellurium metal can be recovered almost quantitatively and re-employed. Methods E48 and F.49 Since elemental tellurium is separated during the reaction, the debromination can be performed using only catalytic amounts of tellurium, the reagent being continuously regenerated at the expense of excess Rongalite (method E) and (EtO)2P(O)Na (method F). Method G.50 A catalytic cycle is also involved in this method. Sodium thienyl tellurolate, generated from the ditelluride and NaBH4, attacks a bromine atom and the formed tellurenyl bromide reacts with another tellurolate molecule regenerating the starting ditelluride.

ArTeTeAr

NaBH4

Br 2 ArTeNa

ArTeBr + Br ArTeNa

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Method H.51 The reagent is prepared by reacting NaHTe with chlorotriphenylstannane.52 The reaction can be rationalized as shown in the following scheme:

Br Ph3Sn-TeSnPh3 F:

Ph3SnF + BrTe-SnPh + Br

:FBr

+ Ph3SnF + Br2Te Br

Debromination of vic-dibromides. Method B (general procedure).43 To a solution of vicinal dibromide (1 mmol) and bis(4-methoxyphenyl) telluride (0.017 g, 0.05 mmol) in benzene (30 mL) is added K2S2O5 (2.22 g, 10 mmol) in H2O (20 mL). The mixture is refluxed with vigorous stirring, monitoring the progress of the reaction by GLC. After completion of the reaction, the mixture is cooled to room temperature and the organic layer is separated, washed with H2O, dried (Na2S2O4) and evaporated. The crude product is purified by chromatography on SiO2. Method C (general procedure).46 The vicinal dibromide (1–2 mmol) and the diaryl ditelluride (1–2 mmol) are refluxed in toluene or HOAc for 1–4 h (1,2-dibromoethane is used neat). After cooling, the mixture is filtered to remove the Te (quantitative yield) and the diaryltellurium dibromide is precipitated by addition of hexane. The alkene is obtained by evaporation of the filtrate (cinnamic and fumaric acids, both insoluble in petroleum ether, are separated from the tellurium dibromide with EtOH). Method D (general procedure).47 To a solution of NaHTe (10 mmol), prepared in situ from Te (1.30 g) and NaBH4 (0.90 g) in EtOH, is added the dibromide (10 mmol). An instantaneous reaction occurs and the colour of the reaction mixture changes to deep black. The mixture is heated under reflux for 2 h and then poured into ice-cold H2O. The product is extracted with CHCl3, dried and evaporated. The residue is purified by distillation or by recrystallization as appropriate. Method E (typical procedure).48 A solution of 1,2-dibromohexadecane (1.152 g, 0.3 mmol) in EtOH (5 mL) is added to a mixture of Te powder (0.382 g, 3 mmol), Rongalite (0.70 g, 6.0 mmol) and 1 M NaOH (15 mL), under N2 and at room temperature. After stirring at 70°C until the colour of the solution turns deep red, excess reagent is destroyed by addition of 2 M HCl (10 mL). The resulting black suspension is filtered through a thin bed of Celite® and the filtrate extracted with ether. The extracts are washed with H2O, dried (Na2SO4) and evaporated, leaving a colourless oil which is distilled using a Kügelrohr apparatus under vacuum to give the pure product (0.551 g (82%)). Method F (typical procedure).49 (EtO)2P(O)Na (1.10 g, 6.87 mmol) in EtOH (10 mL) is added to Te powder (0.025 g, 0.2 mmol) under N2. The mixture is stirred, and when all the black

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powder disappears meso-stilbene dibromide (0.68 g, 2 mmol) is added. The reaction mixture immediately becomes orange, and then black. The reaction is monitored by SiO2 TLC (eluting with petroleum ether/ether, 9:1) until the reaction is complete (∼6 h). H2O (20 mL) is added and the reaction system is opened to the atmosphere to precipitate tellurium. Half an hour later, the solid is filtered with the aid of ethyl ether (20 mL). The filtrate is extracted with ether (3×20 mL). The organic layer is washed with brine and dried (MgSO4). The solvent is removed and the residue is purified by SiO2 column chromatography (eluent: petroleum ether/ethyl ether, 9:1) to give a white crystalline product (0.32 g (88.9%); m.p. 123.5–125°C). Method G (typical procedure).50 NaBH4, (5%) in 5% aqueous NaOH, is added dropwise under N2 to a solution of 5α, 6β-dibromocholestan-3β-ol (1.50 g, 2.73 mmol) and bis(2thienyl) ditelluride (0.10 g, 0.24 mmol) in EtOH (20 mL) at room temperature until the red colour of the ditelluride disappears. Air is then introduced to the system to oxidize the catalyst to the ditelluride. The mixture is diluted with ether and H2O, and the organic phase is washed with H2O, dried and evaporated. Chromatography of the residue on SiO2 (eluting with CH2Cl2/MeOH, 95:5) furnishes cholesterol (0.95 g (90%); m.p. 149–150°C). The tellurolate-promoted anti-debromination by method G is extended to the preparation of conjugated dienes, starting from 1,2,3,4-tetrabromoalkanes and cycloalkanes, 1, 4-dibromo-2-alkenes and allylic dibromides.53 Br

Br

R Br

Br

Br R1

R

Br

R1 II

Br

Br

I

Br

Br

III

Br II

1 III R = H; R = Et, n -Bu, n -hexyl, (CH2)nOAc(n = 7,8)

R = H; R1 = Me, Ph

Br

Br

R1

R1

R Representative examples: Br Br Br Br I Ph

R

Br

Conjugated dienes (general procedure).53 NaBH4 (10% in 0.1% aqueous NaOH) is added dropwise at room temperature under N2 to a solution of the allylic dibromide in MeOH or THF (1.5–2 mmol, 15 mL solvent) containing bis(2-thienyl) ditelluride (5–10 moI%) until disappearance of the orange-red colour of the ditelluride. Two and up to three equivalents of NaBH4 are required in the MeOH and THF runs, respectively. The mixture is then poured into H2O and extracted with pentane. During this process the ditelluride is usually reformed and extracted into the organic layer. In some cases (synthesis of terminal dienes) the ditelluride is not reformed, but the formation of a white precipitate is observed. The combined organic extracts are washed several times with H2O, dried and evaporated. The diene products are purified by distillation or column chromatography. Another related reaction is the debromination of α,α-dibromo-o-xylenes by sodium aryl tellurolates. The formed o-quinodimethane is allowed to react in situ with dienophiles to

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give the Diels–Alder adducts.54 X THF/ Y 10 equiv (20-53%)

ArTeNa (2 equiv) Br Br (ArTeTeAr / NaBH4)

X Y

Ar = Ph, p -MeC6H4, m -MeOC6H4, m -ClC6H4; X = H; Y = CO2Et, COMe, CN X = Y = CO2Et

The debromination step has been rationalized (in view of the 1:2 ratio of the reagents) as involving an attack by the tellurolate ion at the benzylic carbon, followed by an attack by a second tellurolate at the tellurium of the intermediate product.

Br Br

TeAr Br

:TeAr

:TeAr -ArTeTeAr

The reactivity of the tellurolate ion is peculiar, since the corresponding aryl selenolate gives only the disubstituted o-selenoxylenes, and sodium hydrogen telluride gives only poor yields. Ethyl-2-tetralin carboxylate (general procedure).54 To a solution of sodium phenyl tellurolate (from diphenyl ditelluride (2.05 g, 5 mmol) and NaBH4 (0.400 g, 10.5 mmol) in EtOH (30 mL) at 25°C) is added a solution of ethyl acrylate (5.0 g, 50 mmol) and α, α′-dibromo-o-xylene (1.32 g, 5 mmol) in THF (15 mL). The mixture is refluxed for 2 h with stirring, and then stirred overnight in contact with air at room temperature. The red colour fades and a white precipitate is formed. The precipitate is filtered, the product extracted with ether and the extracts dried (MgSO4) and evaporated. The residue is chromatographed on SiO2 (eluting with benzene), to give the product (0.46 g (45%)). 4.1.12.2

Desulphonation of vic-dimesylates and vic-ditosylates

The title reaction has been achieved by treatment with NaHTe in DMF as depicted in the following scheme.55 R

X

X

R1

NaHTe DMF, r.t.

R R1 (85-95%)

R = H; R1 = C14H29, (CH2)8CO2Me(Bn), (CH2)9CN X = OMs R = H; R1 = C14H29, (CH2)8CO2Me X = OTs R = C8H17; R1 = (CH2)7CO2Me X = OMs R = C8H17; R1 = (CH2)7CON X = OMs R1

R= = (CH2)7CO2Me X = OMs

, (H3C)7CO2C N N

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The reaction is stereospecific, threo- and erythro-isomers giving, respectively, trans-and cis-alkenes only. Typical procedure.55 A solution of tellurium powder (96 mg, 0.75 mmol) and NaBH4 (42 mg, 1.12 mmol) in 1.5 mL DMF-tert-butanol (100:1) was heated at 80°C for 30–45 min under argon. After this time the resulting deep purple solution was cooled at room temperature and the substrate (0.5 mmol), pyridine (43 µL, 0.5 mmol) in 2 mL of benzene–DMF (1:3) was added via syringe. The mixture was stirred at room temperature until completion of the starting material (TLC). Then, the reaction flask was opened to air and water was added. After 1 h, the mixture was filtered trough Celite®, eluted with EtOAc and washed with brine. Chromatography was allowed to obtain the pure products.

4.1.13 4.1.13.1

Reductive fission of carbon–heteroatom bonds Reductive removal of electronegative α-substituents from ketones, acids and derivatives

The removal of functional groups from the α-carbon of carbonyl compounds is an important transformation in organic synthesis.56 Anionic tellurium reagents offer additional useful methods to attain this.

O X

Te-

O H

Sodium hydrogen telluride,57 sodium O,O-diethyl phosphorotellurolate,58 alkali 2thienyl tellurolates59 and bis(triphenylstannyl) telluride60/KF reduce α-haloketones to the corresponding ketones.

(method A) NaHTe/ AcOEt / AcOEt / EtOH / r.t. (78-98%) (method B) (EtO)2P(O)TeNa/ EtOH (67-91%) O R R

X 2 R 1

O

(method C) 2-thyenyl TeLi(Na) C1(2-thienyl Li + Te) (52-94%) C2 (2-thienyl Te)2 + NaBH4 (87-99%) (method D) Ph3SnTeSnPh3 / KF.2H2O (63-100%)

R R1

H R2

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4. TELLURIUM IN ORGANIC SYNTHESIS

α-Haloketones and the methods used for their reduction α-Haloketone PhCOCH2X

α-Haloketone

Method X = Cl X = Br X=I

p -YC6H4COCH2Br Y = Ph, Br, HO Y = Br

Method

O

A, B, C, D A, C, D C

Br

A, B

Cl

C

O A B, C

Me PhNHCOCH2Br PhCH2OCOCH2Cl

C A

O A Br

PhCOCMe2Br

B

CH3COCH2Cl EtCOCH2Br PhCOCH(Me)Br PhCOCMe2Br

D D D D

O

O

A

Br Me C8H17

O X X = Cl X = Br

B, D B

B, C

Me

Br O

O

B

Br

Me O Br

O Cl Me

B

B

MeO

The following mechanisms have been proposed to rationalize the described reactions: Method A O-

O R

X + :TeH

-XTeH

O

H3O +

R

H

R

Method B O R

O X + :TeP(O)(OEt)2

R

O a O OEt TeTe P OEt -EtO) PO R 3 b EtOH a O R

Te-

O-

H3O+ R

b

-Te R

Te O-

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Method C O R

X

ArTe-ArTeX

O R

TeAr

O-

ArTe-ArTeTeAr

R

O

H+ R

Method D Ph3Sn-Te-SnPh3

KF.2H2O

O

Ph3SnF + Ph3SnTe: + X

O Ph3SnTe

R

R + H 2O

:FO R

TeH + Ph3SnOH

-Te

O R

H

Since in method C di(2-thienyl) ditelluride is formed as a by-product and NaBH4 seems to reduce ditellurides preferentially to the carboxyl group, a catalytic procedure can also be employed in which NaBH4 is added to the halo compound in the presence of a catalytic amount (0.1 equiv) of the ditelluride (which is continuously regenerated). 2-Thienyl tellurolates also remove several different substituents such as acetoxy, mesyloxy, phenylthio and 2-thienyltelluro groups, as well as effect the reductive dehalogenation of bromoacetanilide, and of α-haloacids (such as α-bromophenylacetic acid, α-bromo-1-naphthylacetic acid and α-chlorodiphenylacetic acid).59 O X

O

ArTe-

H

Ph

Ph X = OAc, OSO2Me,SPh Ar = 2-thienyl O ArTePh TeAr N H

O Ph

N H

H

Dehalogenation of α-haloketones. Method A (typical procedure).57 A solution of p-bromophenacyl bromide (1.11 g, 4 mmol) in EtOAc (20 mL) is added to a solution of NaHTe, prepared from Te (1.3 g, 10 mmol) and NaBH4 (0.9 g, 22.5 mmol) in EtOH (20 mL). An instantaneous reaction occurs and the colour of the reaction mixture changes to black. After stirring for 1 h at room temperature the mixture is filtered through Celite® 545, the filtrate diluted with ether and the organic layer washed with H2O and dried (Na2SO4). Evaporation of the solvent gives p-bromoacetophenone, which is recrystallized from EtOH (0.737 g (93%); m.p. 50–52°C). Method B (general procedure).58 Finely powdered Te (0.128 g, 1.0 mmol) is treated with an EtOH solution of sodium diethyl phosphite (1.5 mmol) in an apparatus which has been previously evacuated and flushed with argon. After a clear solution has formed, the α-haloketone (1.0 mmol) in dry EtOH or THF (2.0 mL) is injected. The mixture is stirred (25 or 80°C, 1–15 h) and then filtered through Celite® with the aid of ether. The orange filtrate is evaporated at room temperature under vacuum (water pump) and the residue applied to an SiO2 column, proceeding to elution (hexane/EtOAc) only after 30 min, to

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allow decomposition of the residual tellurium species. If necessary the products are further purified by crystallization or distillation. Method C1 (typical procedure).59 n-BuLi solution (1.6 M, 7.5 ml, 12.0 mmol) is added under N2 to a stirred solution of thiophene (1.12 g, 13.3 mmol) in dry THF (50 mL) at 0°C. After 50 min at room temperature, finely powdered tellurium (1.50 g, 11.8 mmol) is rapidly added under N2. After 30 min all the Te will have been consumed, and a solution of p-bromophenacyl bromide (1.6 g, 5.75 mmol) in THF (5 mL) is then added dropwise over 10 min. The solution turns from light yellow to deep red as bis(2-thienyl) ditelluride is formed. After 30 min the solvent is evaporated and the residue partitioned in H2O/ether. The organic phase is dried (CaCl2) and evaporated to give a red solid. Chromatography (SiO2, CH2Cl2/hexane, 1:1) gives p-bromoacetophenone (0.82 g (72%); m.p. 49–50°C). Method C2 (typical procedure)59 A 5% solution of NaBH4 in 5% NaOH is added dropwise to a stirred suspension of bis(2-thienyl) ditelluride (0.75 g, 1.78 mmol) in EtOH under N2, until disappearance of the red colour of the solution. 2-Bromocholestan-3-one (0.80 g, 1.72 mmol) in EtOH (5 mL) is then added dropwise over 10 min, causing an immediate red colouration. After 30 min the mixture is poured into H2O/diethyl ether, and the organic phase is washed several times with H2O and dried (CaCl2). Evaporation of the solvent and chromatography (SiO2/CH2Cl2) gives cholestan-3-one (0.62 g (93%); m.p. 127–128°C). Method C2 (typical catalytic procedure).59 A 5% solution of NaBH4 in 5% aqueous NaOH is added dropwise under N2 to a solution of α-bromophenylacetic acid (1.50 g, 7.0 mmol) and bis(2-thienyl) ditelluride (0.30 g, 0.71 mmol) in EtOH (40 mL) until the red colour of the ditelluride just disappears. At this point air is introduced in the system to oxidize the catalyst back to the ditelluride. The mixture is then partitioned in ether/5% aqueous NaOH. The aqueous phase is separated, acidified with 2 M HCl and extracted with ether. The extract is dried (CaCl2) and evaporated to give phenylacetic acid (0.93 g (98%); m.p. 77°C). Method D (typical procedure).60 A mixture of phenacyl bromide (0.796 g, 0.4 mmol), bis(triphenylstannyl) telluride (3.30 g, 0.4 mmol) and KF⋅2H2O (0.103 g, 1.2 mmol) in MeCN (10 mL) is stirred at room temperature, under N2. Very soon a black solid is deposited. After stirring for 5 h, the reaction mixture is filtered through Celite® and the filtrate evaporated. The product (acetophenone) is isolated by flash chromatography (hexane/EtOAc, 20:1). 4.1.13.2

Dehalogenation of polyhalogenated organic compounds

Some investigations directed to the dehalogenation of polyhalogenated organic derivatives by means of tellurium reagents have been reported. (a)

Na2Te

Cl

Cl

Cl

Cl MeOH / H2O

Cl

H

H

H

Cl

Cl

Cl

Cl

(ref. 61)

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(b)

hexachlorobenzene

(c)

Clnthiophenes

(d) Cl

(e)

(f)

Cl

same procedure

MeOH

Te Ph

Ph Ph

Cl NaHTe Cl EtOH NaHTe EtOH

1,2,4,5-Cl4C6H2

Cln-1thiophenes

Na2Te

Ph

Br Br

same procedure

(ref. 61) (ref. 61)

(ref. 62)* Ph

Te Ph

Ph

Ph

Ph

Ph

+ Ph Ph

H H

H Br

(ref. 63)

(ref. 63)

* Similar reactions performed with Na2S and Na2Se lead to the heterocyclic compounds. Ph Te

Y = S, Se

Y

Ph

4.1.13.3

Reductive removal of tertiary nitro groups

Sodium hydrogen telluride replaces tertiary nitro groups by hydrogen in a very simple procedure which seems to be free from the drawbacks of previously described methods.64 The viability of the reaction is dependent on the presence of a carbonyl or an ester group at the α- or β-position to the nitro group. α-Nitrocumene derivatives undergo a similar reductive fission. A reaction is not observed if the activating group is situated at the γ-position. R1 R R2

NO2

NaHTe EtOH / r.t. (80-100%)

R1 R

H R2

R = Me; R1 = CO2Et; R2 = COMe R1, R2 = Me; R = MeC(CO2Et)2, MeC(CO2Et)(COMe), MeC(CO2Et)CN, p -MeCO, p -NC, p -PhSO2, p -EtO2CC6H4

The reaction has been suggested as involving a one-electron transfer from sodium hydrogen telluride to the nitro compound followed by the detachment of a nitrite anion from the resulting radical anion. Reductive removal of tertiary nitro groups (general procedure).64 A solution of the tertiary nitro compound (4 mmol) in EtOH (4 mL) is added at room temperature under N2 to a solution of NaHTe, prepared in situ by reacting tellurium powder (1.30 g, 10 mmol) and NaBH4 (0.90 g, 25 mmol) for 1 h in refluxing EtOH (40 mL). An instantaneous reaction occurs and the colour of the reaction mixture changes to deep black. After 5 min, H2O (50 mL) is added, and the resulting mixture is exposed to air, with stirring. The solution becomes clear after 1 h, with the deposition of tellurium powder. It is then filtered through Celite® 545 and the filtrate is extracted with three portions of CH2Cl2 (30 mL). The

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combined extracts are washed with H2O, dried (Na2SO4) and evaporated to give the crude product, which is purified by Kügelrohr distillation or recrystallization. 4.1.13.4

Reductive dealkylation of quaternary ammonium salts

Sodium hydrogen telluride efficiently dealkylates quaternary ammonium salts to the corresponding tertiary amines in high yields.36 [PhN+Me3]I[R2N+R1R2]I-

NaHTe / EtOH -MeTeH NaHTe / EtOH

PhNMe2 R2NR1

R1 = Me; R2 = Et; R = O R1, R2 = Me; R = cyclohexyl,

4.1.13.5

, and derivatives

Reductive desulphonation of β-ketosulphones

β-Ketosulphones, inert towards sodium hydrogen telluride in EtOH,28 are easily desulphonylated by the same reagent in the aprotic solvent DMF,13,65 or under phase transfer catalysis.65 SO2Ph R

COAr

(method A) NaHTe / DMF, r.t. (method B) NaHTe / THF / EtOH 18-crown-6-ether, reflux

R

COAr

Ar = Ph; R = Me, PhCH2, p -MeC6H4, p -MeOC6H4, p -ClC6H4, p -BrC6H4CH2 Ar = p -MeC6H4, p -MeOC6H4; R = PhCH2

Desulphonylation of β-ketosulphones (typical procedure)65 Method A. To a violet solution of NaHTe, prepared by heating a mixture of Te powder (0.77 g, 6 mmol) and NaBH4 (0.28 g, 8 mmol) in DMF (10 mL) at 70–80°C for 30 min under N2, is added a solution of the ketosulphone (Ar ⫽p-MeC6H4, R⫽PhCH2) (2 g, 5 mmol) in THF at room temperature. The end of the reaction is detected by TLC (SiO2, benzene). The mixture is then quenched with H2O (20 mL) and filtered. The filtrate is extracted with CHCl3 (3×20 mL). The organic phase is washed with H2O (3×20 mL) and brine (3×20 mL), and dried (MgSO4). After evaporating the solvent, the crude product is obtained and recrystallized from EtOH to give (2-phenyl)ethyl p-tolyl ketone (0.83 g (72.1%); m.p. 66–67°C). Method B. To a solution of NaHTe (5 mmol), prepared by heating a mixture of powdered Te (0.77 g, 5 mmol) and NaBH4 (0.56 g, 12 mmol) in EtOH (15 mL), is added a solution of the same ketosulphone (2 g, 5 mmol) and catalytic amounts of 18-crown-6 (0.02 g) in THF (20 mL) at room temperature under N2. The final work-up is similar to that of method A to give the ketone (1.05 g (90.1%); m.p. 66–67°C). The above-described desulphonylation, combined with the previously described selective reduction of α,β-unsaturated carbonyl compounds, can be applied to α-alkylidene

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β-ketosulphones (easily prepared by a conventional Knovenagel condensation of β-ketosulphones with aldehydes), achieving the direct one-pot synthesis of unsymmetrical ketones.13 SO2Ph NaHTe / DMF /EtOH

Ar

(74-81%)

COPh

COPh

Ar

Ar = Ph, p -Me, p -MeO-, p -Cl, p -Br, m -ClC6H4 H O +

Ar

SO2Ph COPh

Reductive desulphonylation of α-alkylidene β-oxosulphones (typical procedure).13 To a solution of NaTeH, prepared from tellurium (0.65 g, 5 mmol) and NaBH4 (0.45 g, 12 mmol) in EtOH (20 mL) under N2, is added, while stirring, a solution of α-(p-bromobenzylidene) β-ketosulphone (0.73 g, 2 mmol) in DMF (15 mL). The solution, which immediately turns red-black, is stirred at room temperature for 3 h, and after addition of H2O (30 mL) is exposed to air for 30 min, to precipitate tellurium. The mixture is filtered, the filtrate extracted with Et2O (3×40 mL) and the combined ether extracts dried (MgSO4) and concentrated in vacuo to give the crude product, which is purified by SiO2 chromatography (elution with benzene/EtOAc, 10:1), giving pure 2-(p-bromophenyl)ethyl phenyl ketone (0.46 g (80%); m.p. 65–66°C). 4.1.13.6

Desulphonylative condensation of β -cyanosulphones with aldehydes

α-Alkilidene β-cyanosulphones are desulphonylated by sodium hydrogen telluride, giving α,β-unsaturated nitriles with maintenance of the C⫽C bond and retention of its configuration.66 H Ar

SO2Ph NaHTe / EtOH / THF CN

reflux (78-89%)

H

H

Ar CN (method A)

Ar = Ph, p -MeO, p -Cl, o -Cl, m -Cl, p -Br, m -Br, p -Me2NC6H4

Otherwise, the same products can be obtained by a Knovenagel-type reaction between β -cyanosulphones and aromatic aldehydes in the presence of sodium telluride (behaving as a base). This result is consistent with the above-formulated desulphonylation of an intermediate α-alkylidene β-cyanosulphone at the expense of the sodium hydrogen telluride formed during the reaction. PhSO2

CN + ArCHO

Na2Te/THF/H2O -NaTeH

H Ar

SO2Ph CN

NaTeH (75-85%)

H

H

Ar CN (method B)

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α,β-Unsaturated nitriles (typical procedure)66. Method A. To a solution of NaHTe, prepared from tellurium (1.3 g, 10 mmol) and NaBH4 (0.9 g, 24 mmol) in EtOH (20 mL), under N2 atmosphere, is added a solution of (2E)-α-cyano-β-(p-chlorophenyl) ethenyl phenyl sulphone (3.1 g, 10 mmol) in THF (20 mL), and the mixture is stirred under refIux for 30 min. The reaction is quenched with H2O (20 mL), the mixture is filtered and the filtrate is extracted with CHCl3 (3×20 mL). The combined organic extracts are dried (MgSO4) and concentrated to give the crude product, which is purified by SiO2 column chromatography (eluent: benzene), giving pure (Z)-p-chlorocinnamonitrile (1.40 g (85%); m.p. 63–65°C). Method B. To a solution of Na2Te, prepared by heating a mixture of Te (1.30 g, 10 mmol), NaBH4 (0.9 g, 24 mmol) and H2O (10 mL) at 60–70°C under N2 atmosphere, is added a solution of α-cyanomethyl phenyl sulphone (1.81 g, 10 mmol) and p-chlorophenyl aldehyde (1.8 g, 12 mmol) in THF (20 mL). An instantaneous reaction occurs, and the colourless reaction mixture becomes violet. The end of the reaction is detected by TLC (SiO2, benzene as the eluent). The remaining work-up is analogous to method A and gives the pure product (1.3 g (79%); m.p. 63–65°C). 4.1.13.7

Correlate reaction – desulphonylation of α-nitrosulphones

By treating a quaternary α-nitrosulphone with sodium hydrogen telluride, the sulphonyl group is displaced preferentially to the nitro group, as illustrated in the example.64 NO2 NaTeH / EtOH SO2Ph (100%)

4.1.13.8

NO2

Monodesulphuration of diaryl thioketals and bis-sulphenylated β -dicarboxyl compounds, diorganyl trisulphides and disulphides

The title reactions have been performed using sodium telluride in aprotic solvents.67 RS RS

Na2Te R1 RS R2 DMF / HMPA / r.t. R1

R1 R2

R2

(a) R = Et, = Ph; = Ph, p -MeO, p -Cl, p-BrC6H4; R1, R2 = 9-fluorenyl (41-75%) (b) R = Ph; R1, R2 = MeCO, CO2Et, COPh; R1 = MeCO; R2 = PhCO (22-67%)

The method employs mild experimental conditions and is therefore competitive with the existing methods for the preparation of benzhydryl sulphides and sulphenylated β -dicarbonyl compounds.67 Benzhydryl ethyl sulphides (general procedure).67 The diaryl diethylthioacetal derivative (2.0 mmol) in dry DMF (3 mL) is added to a pale yellow suspension of freshly prepared

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Na2Te (1 mmol) in a mixture of DMF (3 mL) and HMPA (3 mL) under N2 at room temperature. The colour gradually changes to deep violet and the mixture is stirred at room temperature for 12 h. The reaction is quenched by the addition of saturated aqueous NH4Cl (10 mL) and the reaction mixture is extracted with benzene (10 mL). The extract is freed from inorganic insolubles by filtration through a thin bed of Celite® and the aqueous layer is extracted with benzene. The combined benzene extracts are washed with H2O, dried (Na2SO4) and evaporated to give the crude product. This is purified by chromatography over SiO2 using hexane as the eluent. Monosulphenylated β-dicarbonyl compounds (general procedure).67 To a suspension of Na2Te (1 mmol) in dry DMF (3 mL) under N2 at room temperature is added a solution of bis-sulphenylated active methylene compound (1 mmol) in dry DMF (3 mL). Immediate separation of tellurium occurs and the reaction goes to completion in 1 h. The reaction is quenched by the addition of 0.5 M H2SO4 saturated with NaCl followed by benzene (10 mL). The reaction mixture is stirred for 30 min and then filtered through a thin bed of Celite® to remove insolubles. The aqueous layer is extracted with benzene and the combined organic extracts are washed with H2O, dried (Na2SO4) and the solvent removed under reduced pressure to leave the crude product. This is purified by chromatography over SiO2 using hexane as the eluent to give diphenyl disulphide. Further elution, with a mixture of hexane and CH2Cl2, gives the monosulphenylated compound. Bis(triphenylstannyl) telluride (see Section 4.1.12.1, method H) has also been employed to afford the monodesulphurization of diorganyl trisulphides68 and for the conversion of diaryldisulphides into arylthiostannanes.69 RS S SR

Ph3SnTeSnPh3 MeCN or toluene, r.t.

RS SR (65-85%)

R = MeC6H4, FC6H4, PhCH2, PhCHCH3, n -C3H7 Ph3SnTeSnPh3

2 ArSSnPh3 + Te° (81-100%) Ar = p -MeC6H4, p -ClC6H4, p -FC6H4, Ph, 2-naphtyl ArS SAr

MeCN, r.t.

Alkyl and benzyl disulphides fail to react. Typical procedure.69 A solution of ditolyl disulphide (25 mg, 0.1 mmol) in 10 mL of dried acetonitrile (CaH2) was purged with a flow of nitrogen for 5 min. To the solution, bis(triphenylstannyl)telluride (83 mg, 0.1 mmol) was added in one portion. A black deposit was formed immediately. The reaction mixture was stirred at room temperature for 3 h. Evaporation of the solvent left a black residue. Extraction of the residue with CCl4 with filtration through Celite® gave a clear solution. Evaporation of the solvent gave a colourless syrup which crystallized upon the addition of ethanol to give a white solid. Recrystallization of the solid from CCl4 and ethanol produced pure triphenyl tolylthiostannane (40 mg, 85%) characterized both by 1H NMR and m.p. 102–104°C (lit. 103–105°C).

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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.

Kambe, N.; Kondo, K.; Morita, S.; Murai, S.; Sonoda. N. Angew. Chem. 1980, 19, 1009. Kambe, N.; Inagaki, T.; Miyoshi, N.; Ogawa, A.; Sonoda, N. Nippon Kagaku Kaishi 1987, 1152. Akiba, M.; Cava, M. P. Synth. Commun. 1984, 14, 1119. Aso, Y.; Nishioka, T.; Osuka, M.; Nagakawa, K.; Sasaki, K.; Otsubo, T.; Ogura, F. Nippon Kagaku Kashi 1987, 1490 . Nagakawa, K.; Osuka, M.; Sasaki, K.; Aso, Y.; Otsubo, T.; Ogura, F. Chem. Lett. 1987, 1331. Moura Campos, M.; Surany, E. L.; Andrade, H.; Petragnani, N. Tetrahedron 1964, 20, 2797. Suzuki, S. H.; Manabe, H.; Enokiya, R.; Hanazaki, Y. Chem. Lett. 1986, 1339. Suzuki, H.; Hanazaki, Y. Chem. Lett. 1986, 549. Suzuki, H.; Nakamura, T. J. Org. Chem. 1993, 58, 241. Barton, D. H. R.; McCombie, S. W. J. Chem. Soc. Perkin Trans. 1 1975, 1574. For mechanistic rationales of reductions with NaHTe see Barton, D. H. R.; Bohe, L.; Lusinchi, X. Tetrahedron Lett. 1987, 28, 6609. Ramasamy, K.; Kalyanasundaram, S. K.; Shanmugam, P. Synthesis 1978, 545. Yamashita, M.; Kato, Y.; Suemitsu, R. Chem. Lett. 1980, 847. Huang, X.; Zhang, H. Synth. Commun. 1989, 19, 97. Huang, X.; Xie, C. Synth. Commun. 1986, 16, 1701. Kambe, N.; Inagaki, T.; Miyoshi, N.; Ogawa, A.; Sonoda, N. Chem. Lett. 1987, 1275. Barton, D. H. R.; Fekih, A.; Lusinchi, X. Tetrahedron Lett. 1985, 26, 3693. Yamashita, M.; Kadokura, M.; Suemitsu, R. Bull. Chem. Soc. Jpn. 1984, 57, 3359. Suzuki, H.; Manabe, H.; Kawaguchi, T.; Inouye, M. Bull. Chem. Soc. Jpn. 1987, 60, 771. Kambe, N.; Kondo, K.; Sonoda, N. Angew. Chem. 1980, 19, 1010. Suzuki, H.; Manabe, H.; Inouye, M. Chem. Lett. 1985, 1671. Osuka, A.; Shimizu, H.; Suzuki, H. Chem. Lett. 1983, 1373. The reduction of nitrobenzene to aniline in 23% yield, by means of NaHTe has also been reported by Akiba, M.; Cava, M. P. Synth. Commun. 1984, 14, 1119. Uchida, S.; Yanada; K.; Yamaguchi, H.; Meguri, H. Chem. Lett. 1986, 1069. Ohira, N.; Aso, Y.; Otsubo, T.; Ogura, F. Chem. Lett. 1964, 853 and personal communication. Ohe, K.; Takayashi, H.; Uemura, S.; Sugita, N. J. Chem. Soc. Chem. Commun. 1988, 591. Ohe, K.; Uemura, S.; Sugita, N.; Masuda, H.; Taga, T. J. Org. Chem. 1989, 54, 4169. Barton, D. H. R.; Fekih, A.; Lusinchi, X. Tetrahedron Lett. 1985, 26, 4603. Suzuki, H.; Takaoka, K. Chem. Lett. 1984, 1733. Blay, G.; Cardona, L.; Garcia, B.; Lahoz, L.; Pedro, J. R. Tetrahedron 1996, 52, 8611. See ref. 8 in the next ref. 31. Pepito, A. S.; Dittmer, D. C. J. Org. Chem. 1997, 62, 7920. Silveira, C. C.; Lenardão, E. J.; Comasseto, J. V. Synth. Commun. 1994, 24, 575. Harrison, I. T.; Harrison, S. Compendium of Organic Synthetic Methods. Vol. 1, Section 204, p. 501, Vol. II, Section 204, p. 200. Wiley, Chichester, 1971. Clive, L. J.; Menchen, S. M. J. Chem. Soc. Chem. Commun. 1977, 658. Clive, C. J.; Menchen, S. M. J. Org. Chem. 1980, 45, 2347. Barton, D. H. R.; Fekih, A.; Lusinchi, X. Tetrahedron Lett. 1985, 26, 6197. Osuka, A.; Taka-Oka, K.; Suzuki, H. Chem. Lett. 1984, 271. Polson, G.; Dittmer, D. C. Tetrahedron Lett. 1986, 27, 5579. Discórdia, R. P.; Dittmer, D. C. J. Org. Chem. 1990, 55, 1414. Discórdia, R. P.; Murphy, C. K.; Dittmer, D. C. Tetrahedrem Lett. 1990. 31, 5603. Schlosser, M. Houben-Weyl, Methoden der Organischen Chemie. (ed. E. Muller). 4th edn, Vol. VIb, p. 180. Georg Thieme, Verlag, Stuttgart, 1972. Young, D. W. Protective Groups in Organic

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42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69.

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Chemistry. p. 309. Plenum Press, London, 1973. Mathai, I. M.; Schug, K.; Miller, S. I. J. Org. Chem. 1970, 35, 1733. Gordon, J. E.; Chang, V. S. K. J. Org. Chem. 1973, 38, 3062 and references therein. Moura Campos, M.; Petragnani, N. Tetrahedron Lett. 1960, 5. Suzuki, H.; Kondo, A.; Osuka, A. Bull. Chem. Soc. Jpn. 1985, 58, 1335. Butcher, T. S.; Detty, M. R. J. Org. Chem. 1998, 63, 177. Leonard, K. A.; Zhou, F.; Detty, M. R. Organometallics 1996, 15, 4285. Petragnani, N.; Moura Campos, M. Chem. Ber. 1961, 94, 1759. Ramasamy, K.; Kalyanasundaram, S. K.; Shanmugam, P. Synthesis 1978, 311. Suzuki, S.; Inouye, M. Chem. Lett. 1985, 225. Huang, K.; Hou, Y. Q. Synth. Commun. 1988, 18, 2201. Engman, L. Tetrahedron Lett. 1982, 23, 3601. Li, C. J.; Harp, D. N. Tetrahedron Lett. 1990, 31, 6291. Einstein, F. B. W.; Jones, C. H. W.; Jones, T.; Sharma, R. D. Can. J. Chem. 1983, 61, 2611. Engman, L.; Bystrom, S. E. J. Org. Chem. 1985, 50, 3170. Kambe, N.; Tsukamoto, T.; Miyoshi, N.; Murai, S.; Sonoda, N. Bull. Chem. Soc. Jpn. 1986, 59, 3013. Bargues, V.; Blay, G.; Fernandez, J.; Pedro, J. R. Synlett 1996, 655. See methods reported in refs. 57–60. Osuka, A.; Suzuki, H. Chem. Lett. 1983, 119. Clive, D. L. J.; Beaulieu, P. L. J. Org. Chem. 1982, 47, 1124. Engman, L.; Cava, M. P. J. Org. Chem. 1982, 47, 3946. Li, C. J.; Harp, D. N. Tetrahedron Lett. 1991, 32, 1545. Mack, W. Angew. Chem. 1967, 6, 1083. Luppold, E.; Winter, W. Chem. Ztg. 1977, 101, 303. Osuka, A.; Takechi, K.; Suzuki, H. Bull. Chem. Soc. Jpn. 1984, 57, 303. Suzuki, H.; Takaoka, K.; Ogura, A. Bull. Chem. Soc. Jpn. 1985, 58, 1067. Huang, X.; Pi, J. H. Synth. Commun. 1990, 20, 2297. Huang, X.; Pi, J. H.; Huang, Z. Phosphorus Sulfur Silicon 1992, 67, 177. Padmanabhan, S.; Ogawa, T.; Suzuki, H. J. Chem. Res. (S) 1989, 266. Li, C. J.; Harpp, D. N. Tetrahedron Lett. 1993, 34, 903. Li, C. J.; Harpp, D. N. Tetrahedron Lett. 1992, 33, 7293.

4.2

4.2.1

TELLURIUM-MEDIATED FORMATION OF ANIONIC SPECIES AND THEIR REACTIONS WITH ELECTROPHILES Reformatsky-type reactions

By treatment with sodium telluride, bromoacetic esters react with aromatic and non-enolizable aldehydes in aprotic solvents, giving α,β-unsaturated esters.1,2 Br

CO2R1 + RCHO

Na2Te / DMF / THF -10°C (37-73%)

R CO2R1

O R1 = Me, Et; R = Ph, o -Me, p -Me2N, p -Cl, p -FC6H4, O PhCH = CH, Me2CPh, Me2C=CH(CH2)2CMe=CH

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In accordance with a Reformatsky reaction mechanism, the telluride ion attacks the bromine atom to generate an ester enolate which reacts in sequence with the aldehyde to afford the α,β-unsaturated ester.

Te2-

CO2R1

Br

O-

O-TeBr-

R1O

O

R R

O

OR1

O

H3O+ R TeBr-

OR1 Te + Br-

The reaction exhibits the following features: chloroacetates give lower yields than bromoacetates; the E configuration is observed for the formed C⫽C bond; α,β-unsaturated carbonyl compounds lead only to 1,2-addition products; ketones are unreactive under the described conditions; a large excess of the bromoacetate and sodium telluride is required to assure high conversions. Reformatsky-type reaction (typical procedure).1 To a solution of Na2Te (prepared from tellurium (0.7 g, 4 mmol), NaH (0.20 g, 8.6 mmol) and DMF (7 mL) by heating at 140°C for 30 min under N2, is added dropwise at −20°C a solution of benzaldehyde (0.10 g, 1 mmol) in THF (1.5 mL) followed by ethyl bromoacetate (0.67 g, 4 mmol) in THF (1.5 mL). Tellurium precipitates instantaneously. After stirring for 1 h, 0.5 M H2SO4 (5 mL) saturated with NaCl is added followed by Et2O (10 mL). Excess Na2Te is destroyed by exposing the mixture to air and stirring for 1 h. The mixture is filtered through Celite®, the organic phase is separated, and the aqueous layer extracted twice with Et2O. The combined extracts are washed with brine, dried (Na2SO4) and evaporated, leaving a yellow oil, which is purified by Kügelrohr distillation under reduced pressure to give ethyl cinnamate (0.10 g (60%); b.p. 95–110°C/1.5 torr). Haloacetonitrile and p-cyanobenzyl bromide react similarly, leading to α,β-unsaturated nitriles2 accompanied by minor amounts of the alcohols. X

CN + RCHO

Na2Te/DMF/THF

CN ( + ROH)

R

0°C (31-70%) X = Cl, Br; R = Ph, p -ClC6H4, PhCH=CH,

O O

Br

C6H4CN-p + RCHO

same conditions

R

C6H4CN-p

O R= O

The previously described diisobutyl telluride/titanium(IV) system (see Section 4.1.1.3) also promotes similar Reformatsky-type reactions.3 In this case, however, the products are

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β-chloroesters.

Br

i -Br2Te / TiCl4

CO2Et + RCHO

CH2Cl2 /

/ (66%)

Cl CO2Et

Ph

The described protocol was later improved by the use of the system PhTeLi/CeCl3. EtO2CCH2Br + RR1CO

PhTeLi (5.0 mmol)-CeCl3 (2.5 mmol) ether, 0°C - r.t., 1 h (47-95%)

R = Ph; R1 = Me, Et R = Me; R1 = C5H11 R, R1 = (CH2)5, O

OH R1RC CH2CO2Et

R = R1 = PhCH2, i -pr R, R1 =

The following scheme rationalizes the reaction.4 CeCl3 H O Br C C OEt H :TePh

PhTeBr

4.2.2

PhTeLi

OCeCl2 H2C C OEt 1

R C R O PhTeTePh

1RRC

O Cl

COEt O

Ce Cl

R1RHC CH2CO2Et OH

Knoevenagel-type reaction

Tellurium tetrachloride is an efficient catalyst in the Knoevenagel reaction of non-enolizable aldehydes with active methylene compounds.5 ArCHO + H2C

CN TeCl4 CN ArCH C R 80°C, 20-75 min R (82-95%)

Ar = Ph and substituted Ph, 2-furyl, E -C6H5CH=CH R = CO2Et, CN, CONH2

Typical procedure.5 A mixture of the carbonyl compound (0.01 mol), the active methylene compound (0.01 mol) and tellurium(IV) tetrachloride (0.001 mol) was thoroughly mixed at room temperature. After being stirred for 5 min, the mixture was heated and continuously stirred at 80°C in an oil bath (20–75 min). The reaction was cooled at room temperature and treated with a solution of 1% aqueous alcohol. The product was extracted with methylene chloride, washed with water. After drying over Na2SO4 the solvent was removed in vaccuo over rotatory evaporator to obtain the product in high purity.

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Pinacol reaction

Tellurium powder/KOH is an efficient system for the pinacolization of aromatic carbonyl compounds.6 The method is advantageous over others since it is a very fast reaction. ArCOR

Te / KOH MeOH, r.t. 10 -15 min

H Ar(R) C CH(R)Ar OH OH (85-95%)

R = H; Ar = Ph and substituted Ph R = Me; Ar = Ph, p-MeC6H4

Typical procedure.6 Benzaldehyde (0.53 g, 5 mmol) was dissolved in methanol (15 mL), tellurium powder (1.276 g, 10 mmol) and KOH (1.40 g, 25 mmol) were added and the reaction mixture was stirred. The reaction became vigorous immediately after the addition of KOH. The reaction mixture was filtered to remove the tellurium powder and water (50 mL) was added to the filtrate. A solid precipitated out which was filtered off under pressure. Some of the diol was obtained by extracting the filtrate with CH2Cl2 (3×20 mL), drying with anhydrous Na2SO4 and then concentrated using a vacuum rotatory evaporator. 4.2.4 Alkylidenation of aldehydes and cyclopropanation of α,β -unsaturated carbonyl compounds with dibromomalonic esters Dibromomalonates react with aldehydes under the assistance of dibutyl telluride (2 equiv) to afford alkylidene malonates.7 H R

Br + O Br

CO2R1 R1

CO2

Bu2Te (2 equiv)

H

CO2R1

r.t. (47-87%)

R

CO2R1

R = Ph, n-hexyl, CH2=CH, MeCH=CH, CH2=CMe, trans -PhCH=CH R1 = Me, Et

Otherwise, dibromomalonates (and nitriles) react with α-unsaturated carbonyl compounds in the presence of only 1 equiv of dibutyl telluride, giving cyclopropane derivates.

R

+

Bu2Te

Br

Y

Br

Y r.t. (32-94%)

R CO2R CO2R

R = CO2Me, CO2Et, CO2Bu, COMe, CN; Y = CO2Me, CO2Et, CN

These reactions have been rationalized as involving the addition of an intermediate tellurium enolate to the carbonyl group or to the β-carbon, respectively, followed by a

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telluride-assisted elimination step or by an intramolecular cyclopropanation.

Bu2Te +

Br

CO2R1

Br

R1

CO2

Bu2Te+Br _

R1

CO2 Br CO2R1

RCHO

Br Bu2TeO R

CO2R1 Br

CO2R1 TeBu2

Bu2

Te(Br)O-

+ Bu2

Te+Br

R R _

CO2R1 CO2R1 Br

R CO2R + Bu2TeBr2 CO2R

H2O

Bu2Te

R

CO2R1

H

CO2R1

OH Br

Alkylidenation of aldehydes with dibromomalonates (typical procedure).7 A mixture of diethyl dibromomalonate (0.48 g, 1.5 mmol) and acrylaldehyde (0.087 g, 1.5 mmol) is treated with dibutyl telluride (0.73 g, 3 mmol) under argon and with stirring. After 30 min the mixture is extracted with CHCl3 (30 mL), washed with H2O, dried (MgSO4) and concentrated in vacuo. The residue is diluted in hexane (20 mL), insoluble material is filtered off and the filtrate chromatographed on an SiO2 column (88–125 mesh, elution with CHCl3), giving dibutyltellurium dibromide (0.03 g (5%)) and then diethyl 2-propenylidenemalonate (0.26 g (87%)), which is purified by Kügelrohr distillation (b.p. 65°C/0.12 torr). Cyclopropanation of α,β-unsaturated carbonyl compounds with dibromomalonates (typical procedure).7 To a mixture of diethyl dibromomalonate (0.95 g, 3 mmol) and methyl vinyl ketone (0.27 g, 3.1 mmol) is added dibutyl telluride (0.73 g, 3 mmol) under argon and with stirring. The exothermic reaction is completed within 1 h. The mixture is chromatographed on an Al2O3, column (70–230 mesh, elution with EtOAc), giving dibutyltellurium dibromide (1.01 g, 84%) and then 1-acetyl-2,2-bis(ethoxycarbonyl)cyclopropane, which is purified by Kügelrohr distillation (0.59 g (86%); b.p. 88–90°C/0.08 torr).

4.2.5

Telluride-assisted sulphenylation and sulphonylation reactions

α-Phenylthiocarbonyl compounds and alkyl aryl sulphones are of great synthetic importance.8–10 New methods for their preparation are therefore the object of great interest. These two classes of compounds can be prepared by treating diphenyl disulphide8 and aryl sulphonyl chlorides9 with sodium telluride followed, respectively, by α-halocarbonyl compounds and alkyl halides. These methods involve the polarity reversal of the formerly electrophilic thio reagents, giving the thiolate and sulphinate anions through an electron transfer from the

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telluride ion. O X

R DMF PhSSPh + Na2Te -Te

2 PhSNa

O

R1 (24-85%)

SPh

R

1

R

R = aryl; R1 = H, SO2Ph, COPh, SPh, CO2Et R, R1 = Me, O-CH2CH2CH; R = Me; R1 = CO2Et H2O / THF/ PTC RX 2 ArSO2Na ArSO2R (Method A) -Te (41-71%) Ar = p -Me, p -BrC6H4, 1-naphthyl , 2-naphthyl RX = alkyl bromide and iodides, PhCH2Cl 2 ArSO2Cl + Na2Te

O,O-diethyl phosphorotellurolates can be employed instead of sodium telluride, the alkylation occurring even with α-withdrawing group-substituted halides.10 ArSO2Cl + RX

(EtO)2P(O)TeNa / THF / EtOH/ TEBA (77-94%)

ArSO2R (Method B)

Ar = Ph, p-MeC6H4 RX = MeI, ClCH2Ph, BrCH2Y (Y = CN, CO2Et, CONHPh, COPh)

α-Bis-sulphenylated carbonyl compounds can be prepared by the inverse addition of the disulphide to the preformed enolate. O Br

Ph

-Te

OPh O

-

O-

Na2Te Ph

PhSSPh -PhS-

OPh

SPh PhSSPh -PhS-

O Ph

SPh

O Ph

SPh SPh

Ph

α-Sulphenyl carbonyl compounds (general procedure).8 To a suspension of Na2Te (0.346 g, 2.0 mmol), prepared from tellurium and NaH in DME, is added diphenyl disulphide (0.876 g, 4.0 mmol) in DMF (5 mL) followed by the α-halocarbonyl compound (2.0 mmol) is the same solvent (3 mL). The dark reduction mixture is kept at −40°C for 2 h and then at room temperature for 1 h, with stirring. The reaction is quenched with saturated aqueous NH4Cl (10 mL) and the mixture is extracted with benzene. The extract is filtered through Celite®, washed with 0.5 M H2SO4 and H2O, and dried (Na2SO4). The solvent is evaporated and the crude product purified by column chromatography on SiO2 (eluent: hexane), giving the unchanged diphenyl disulphide. Further elution with hexane and CH2Cl2 gives the sulphenylated compound. Alkyl aryl sulphones (typical procedure). Method A.9 A solution of Na2Te (prepared by heating a mixture of Te (0.13 g, 1 mmol), Rongalite (0.79 g, 5 mmol) and aqueous 1 M NaOH (15 mL)) is added dropwise to a stirred solution of TsCl (1 mmol) and BzEt3N⫹Cl⫺ (0.022 g, 0.1 mmol) in THF (10 mL) at room temperature under N2. An instantaneous reaction occurs and the colour of the mixture changes to deep black. After stirring for 5 min,

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153

benzyl chloride (0.632 g, 5 mmol) in THF (3 mL) is added and the mixture kept at 90°C for 5 h. After cooling the solvent is evaporated under reduced pressure and the residue treated with aqueous NH4Cl and benzene. The organic phase is evaporated after filtration through Celite® and worked up as usual to give benzyl p-tolyl sulphone (0.179 g (75%); m.p. 143–145°C). Method B.10 To a solution of (EtO)2P(O)TeNa (5 mmol) in EtOH (10 mL) is added a solution of benzene sulphonyl chloride (0.88 g, 5 mmol) in THF (10 mL) under N2. An instantaneous reaction occurs and the colour of the mixture changes from colourless to deep black. After stirring for 20 min, a solution of benzyl chloride (0.64 g, 5 mmol) and catalytic amounts of BzEt3N⫹Cl⫺ (0.01 g) in THF (10 mL) is added. The mixture is refluxed for 4 h. After evaporating the solvent under reduced pressure, the residue is treated with CHCl3 (50 mL). After filtration, the organic phase is evaporated to dryness. The crude product is recrystallized from EtOH, giving the pure product (1.93 g (82.4%); m.p. 145–147°C). 4.2.6

Telluride-mediated aldehyde methylenation

In the presence of dibutyl telluride, iodomethyl triphenylphosphonium iodide reacts with aldehydes, in accordance with a Wittig-type olefination, giving methylenation products.11 Ph3P+

I I-

n -Bu2Te THF, 80°C

Ph3P+

I

TeBu2-n I-

-n-Bu2TeI2

[Ph3P=CH2]

R RCHO Ph3P+

O-

-Ph3PO (51-87%)

R

R = Ph, p -Cl, p -Br, p-FC6H4, PhCH=CH, 2-naphthyl, n -C9H19

Methylenation reaction (typical procedure).11 A mixture of iodomethyl triphenylphosphonium iodide (0.530 g, 1.0 mmol), dibutyl telluride (0.242 g, 1.0 mmol) and p-bromobenzaldehyde (0.096 g, 0.5 mmol) in THF (5 mL) is heated under reflux for 30 h, then cooled and filtered. The residue is purified by TLC, giving p-bromostyrene (0.080 g (87%)).

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

Suzuki, S.; Inouye, M. Chem. Lett. 1986, 403. Suzuki, H.; Manabe, H.; Inouye, M. Nippon Kagaku Kaishi 1987, 1485. Suzuki, H.; Manabe, H.; Enokiya, R.; Hanazaki, Y. Chem. Lett. 1986,1339. Fukuzawa, S. I.; Irai, K. J. Chem. Soc. Perkin Trans. 1 1993, 1963. Khan, R. H.; Mathur, R. K.; Ghosh, A. C. Synth. Commun. 1996, 26, 683. Khan, R. H.; Mathur, R. K.; Ghosh, A. C. Synth. Commun. 1997, 27, 2193. Matsuki, T.; Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. Bull. Chem. Soc. Jpn. 1989, 62, 2105. Padmanabhan, S.; Ogawa, T.; Suzuki, H. Bull. Chem. Soc. Jpn. 1989, 62, 1358.

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4. TELLURIUM IN ORGANIC SYNTHESIS

9. Suzuki, H.; Nishioka, Y.; Padmanabhan, S.; Ogawa, T. Chem. Lett. 1988, 727. 10. Huang, X.; Pi, J. H. Synth. Commun. 1990, 20, 2291. 11. Li, S. W.; Huang, Y. Z.; Shi, L. L. Chem. Ber. 1990, 123, 1441.

4.2.7

Miscellaneous

A convenient protocol for the synthesis of N-sulphonylimines, which are important synthetic building blocks,1 involves the reaction of N,N′-ditosyltellurodiimide generated in situ from elemental tellurium and chloramine-T, with aldehydes in polar or non-polar solvents.2 The mechanism of the reaction is assumed to involve a cycloaddition to a four-membered ring followed by cycloreversion giving the product. 2 TsN(Cl)Na + Te

O R

-2 NaCl

Ts-N=Te=N-Ts

X O Te N R Ts

X Te + N Ts

2 RCHO

2 RCH=NTs + TeO2

Ts + O Te X N R (90-100%)

selected examples: R = Ph, p -MeOC6H4, 3-py, 2-furyl, t -But,

TMS

Typical procedure.2 A suspension of 0.070 g (0.55 mmol) of tellurium metal and 0.24 g (1.05 mmol) of anhydrous chloramine-T in 2 mL of toluene was heated at reflux for 1 h, at which time the suspension became grey. The aldehyde (1.0 mmol) was added and heating continued (1–4 h). The grey suspension became white. Methylene chloride was added and the reaction filtered through Celite®. Removal of solvent in vacuo gave the N-tosylimine suitable for further use. Analytical samples were purified by recrystallization or flash chromatography. Phenyl tellurium pentafluoride, easily prepared from diphenyl ditelluride and xenon difluoride (XeF2), reacts smoothly with olefins affording the corresponding 1,2-difluorides.3 CH2Cl2 -Xe CH2Cl2

Ph2Te2 + 5 XeF2 PhTeF5 + E -PhCH=CHPh

r.t., 10 h

2 PhTeF5 PhCHFCHFPh 65% (erythro:threo = 2:1)

+ PhCH=CH2

+

CH2Cl2 r.t., 4 h CH2Cl2 r.t., 4 h

FCH2-CHFPh 34% F F 40% (trans:cis = 2:1)

Phenyltetrafluorotelluromethoxyde, prepared by the reaction of trimethylsily methoxyde with phenyltelluropentafluoride behaves similarly with trans-stilbene and

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indene but reacts with styrene giving a dihydronaphthalene derivate as major product.4 PhTeF5

Me3SiOMe

Ph-TeF4(OMe) Ph F ( + FCH2-CHFPh)

(97%)

REFERENCES 1. Boger, D. L.; Corbett, W. L. J. Org. Chem. 1992, 57, 4777. 2. Trost, B. M.; Mars, C. J. Org. Chem. 1991, 56, 6468. 3. Lermontov, S. A.; Zavorin, S. I.; Bathin, I. V.; Zefirov, I. V. Phosphorus Sulfur Silicon 1995, 102, 283. 4. Lermontov, S. A.; Bahtin, I. V. Russ. Chem. Bull. 1998, 47, 72.

4.3

DEPROTECTION OF ORGANIC FUNCTIONALITY BY TELLURIUM REAGENTS

The regeneration of substrates with organic functionalities from certain specific derivatives, which frequently execute a protective function, is an important operation in synthetic methodology.1 Some tellurium anionic reagents offer a great contribution to these deblocking reactions owing to the use of mild experimental conditions and a non-hydrolytic medium. 4.3.1

Regeneration of carboxylic acids: cleavage of carboxylic esters

The methods described in this section are of great significance because of the essentially neutral non-hydrolytic conditions employed. 4.3.1.1

Alkyl carboxylates

Common esters such as alkyl and benzyl carboxylates are easily dealkylated by sodium hydrogen telluride, sodium telluride and sodium ditelluride in DMF.2 In accordance with a typical SN2 displacement at the alkoxy group carbon, methyl, ethyl and benzyl esters react smoothly. The nucleophilicity of the reagents is enhanced by the polar aprotic solvent, and the reactivity decreases with higher alkoxy chains due to steric hindrance (e.g.

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n- and i-Pr esters). O

O R

OR1

O

(method A) NaHTe DMF/

(method B) Na2Te DMF/

ONa + R1TeH

R

OR1

R

ONa +

R1TeNa

DMF/

R

O R

ONa

H3O+

R

1 1 ONa + R TeR

2R

O ONa + R1Te2Na

OH + R1TeR1

O OR1

R

O (method C) Na2Te2

O ONa + R

O

O R

O R

O OR1

2R

1 1 ONa + R TeTeR

O R

OH

R = Ph, m -ClC6H4, 2-naphthyl, PhCH2, 1-naphthyl-CH2, n -C11H23 R1 = Me, Et, PhCH2 (yields 88-98%)

Dealkylation of carboxylic esters (typical procedure).2 Method A – with NaHTE. A mixture of Te (1.27 g, 10 mmol), NaBH4 (0.57 g, 15 mmol) evacuated and purged with N2 three times, DMF (20 mL) and t-BuOH (0.2 mL, used as a H+ donor) is heated at 80–90°C under N2 for 30 min until nearly all the tellurium has disappeared, giving a deep violet solution. Methyl benzoate (2.47 g, 18 mmol) is added and the mixture heated at 80–90°C for 15 min (a copious precipitate often appears). H2O (30 mL) is added and the mixture extracted with Et2O (3×50 mL). The aqueous layer is acidified with HCl, extracted with Et2O (3×20 mL) and dried (MgSO4). The solvent is evaporated, giving pure benzoic acid (2.12 g (95%); m.p. 121–122°C). Method B – with Na2Te. A mixture of Te (0.64 g, 5 mmol) and NaBH4 (0.45 g 12 mmol) in DMF (20 mL) is heated at 80–90°C under N2 for 30 min, giving an almost colourless suspension (which has been proved to be Na2Te by reaction with 1-bromobutane to give dibutyl telluride). Methyl benzoate is added, and after heating at 80–90°C for 20 min the mixture is worked up as for method A, giving pure benzoic acid (0.99 g (90%); m.p. 121–122°C). Method C – with Na2Te2. A mixture of tellurium (1.00 g, 7.5 mmol) and NaBH4 (0.19 g, 5.0 mmol) in DMF (20 mL) is heated at 80–90°C under N2 for 20 min, giving a homogeneous deep purple solution (which has been proved to be Na2Te2 by reaction with 1-bromobutane to give dibutyl ditelluride). Methyl benzoate is added, and after 20 min heating at 80–90°C the mixture is handled as in method A, giving pure benzoic acid (0.94 g (85%); m.p. 121–122°C). NaHTe affords the selective carbalcoxy group dealkylation of C-protected and C- and N-protected α-amino acids.3 R NaHTe H2N C CO2R1 DMF H

R H2N C CO2H + R1TeH H

R = H, Ph, p -MeOC6H4, i -But R1 = Me, Et, CH2C6H4, Bu

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R PhCH2OC(O)NHCH-CO2R1

157

R PhCH2OC(O)NHCH-CO2H + R1TeH

R = H, Me R1 = CH2Ph, Et

4.3.1.2

Phenacyl carboxylates

Phenacyl esters, easily prepared from carboxylic salts and phenacyl bromide under phase transfer catalysis,4 regenerate the original carboxylic acid by treatment with sodium hydrogen telluride in DMF.4 O R

Ph NaHTe / DMF r.t.

O O

K2CO3 (solid) PEG / MeCN O R

O

H3O+ (81-95%)

R

O OH + Ph

+ Te

R = Ph, p -Br, p -MeO, p -H2NC6H4, 2-naphthyl 1-naphthyl-CH2, 3-furyl, n -C11H23

O OH + Ph

Br

The reaction involves the attack by anionic tellurium at the phenacyl methylene group. Cleavage of phenacyl esters (typical procedure).4 To a solution of NaHTe (prepared by heating powdered tellurium (1.28 g, 10 mmol) and NaBH4 (0.75 g. 15 mmol) under N2 in DMF at 80–90°C for 30 min, and then cooling at room temperature) is added a solution of phenacyl benzoate (1.92 g, 8 mmol) in DMF (30 mL). An instantaneous reaction takes place and the mixture is stirred at room temperature for 20 min. The solvent is evaporated and the mixture poured into H2O (100 mL) and filtered to remove the tellurium. The basic filtrate is washed with ether and then the aqueous solution is acidified with 6 N HCl and extracted with ether (3×50 mL). The ether extract is dried (MgSO4) and evaporated to give benzoic acid (1.11 g (91%); m.p. 121–122°C). In general the acids obtained by the above procedure are essentially pure. 4.3.1.3

Allyl carboxylates

The allyl group, a less familiar protective group for carboxylic acids, can be easily removed from esters by treatment with sodium hydrogen telluride.5 In contrast to the preceding methods, ethanol has been employed as the solvent. O R

O

NaHTe / EtOH (95%)

O R

OH +

+ Te

R = Ph, p -MeOC6H4, 1-propenyl

Cleavage of allyl carboxylates (general procedure).5 To a solution of NaHTe (prepared in situ from tellurium powder (1.30 g, 1.0 mmol) and NaBH4 (0.90 g, 2.4 mmol) in EtOH (20 mL) buffered with deoxygenated HOAc (1.2 mL in 5 mL EtOH)) is added the allyl carboxylate (0.005 mol). The mixture is refluxed under N2 for 2 h, then filtered, the filtrate

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is evaporated and the residue is dissolved in H2O. The aqueous layer is made slightly acidic with a few drops of dilute HCl. The aqueous layer is then extracted with CHCl3, and the extract dried and evaporated. The residue is purified by recrystallization or distillation. 4.3.1.4

2-Haloethyl carboxylates

The frequently employed 2-haloethyl protecting group can be removed from the carboxylic esters by means of sodium telluride in DMF6 or sodium hydrogen telluride in ethanol.7 (method A) Na2Te / DMF, r.t. X = Cl, 12 h (92-93%)

O R

O O

X

R (method B) NaHTe / EtOH, r.t.

:Te

H+

X = Cl, 30-60 min

O

(84-92%) X = Br, 2 min

X

O

R

OH

+

H

H

H

H

(method A) R = Ph, p -MeC6H4, PhCH2; X = Cl (method B) R = Ph,o -MeOC6H4, PhCH2; X = Cl R = Ph, n -C11H23; X = Br

The reaction can be rationalized as involving an E2 fragmentative attack by the telluride anion at the halogen atom. Sodium hydrogen telluride (method B) requires a shorter reaction time than sodium telluride (method A), the 2-bromoalkyl esters reacting very rapidly. Sodium hydrogen selenide reacts similarly but more slowly.5 Since in the described reactions tellurium is regenerated and sodium hydrogen telluride is prepared from the element and sodium borohydride, a catalytic procedure is also effective, in which only catalytic amounts of preformed reagent are employed.8 O R

O

X

(method C) NaHTe (10 mmol %)/ NaBH4 (14 mol equiv 0/5-20°C (EtOH, 1.5 h) or (EtOH / DMF 1:5, 2 h) (82-95%)

O R

OH

+

H

H

H

H

X = Br; R = Ph, p -MeC6H4, PhCH2,n -C5H11 X = Cl; R = Ph, p -ClC6H4, n -C11H23

Cleavage of 2-haloethyl carboxylates. Method A (general procedure).6 To Na2Te (0.346 g, 2.0 mmol) in DMF (3 mL) is added a solution of 2-chloroethyl ester (2.0 mmol) in DMF (2 mL) at room temperature, and the resulting thick mixture is stirred for 12 h, also at room temperature. The reaction is quenched by adding 0.5 M H2SO4 (5 mL) and the organic phase is extracted with ether (10 mL). The mixture is filtered through Celite® to remove the inorganic material and the ether phase is separated, washed with 0.5 M H2SO4 (2×5 mL) followed by brine and dried (Na2SO4). The solvent is evaporated under vacuum to give the pure carboxylic acid, which is further purified by crystallization from H2O containing a little EtOH.

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Method B (general procedure).7 To an NaHTe solution, prepared from tellurium powder (1.27 g, 10 mmol) and NaBH4 (0.9 g, 24 mmol) in absolute EtOH (25 mL), is added the 2-haloethyl ester (10 mmol) in EtOH (5 mL). (The reaction takes place instantaneously for 2-bromoethyl esters at room temperature.) The mixture is stirred for ∼1 h, then acidified with dilute HCl, extracted with ether and the organic phase dried and evaporated. The residue is recrystallized from appropriate solvents to give pure carboxylic acids. Method C (typical procedure).8 (i) A violet ethanolic solution of NaHTe is prepared by heating a mixture of powdered tellurium (0.06 g, 0.5 mmol) and NaBH4 (0.08 g, 2 mmol) in EtOH (5 mL) at 70–80°C for 0.5 h under N2. After cooling to 5°C, a solution of NaBH4 (0.57 g, 15 mmol) in EtOH (15 mL) and 2-bromoethyl benzoate (1.15 g, 5 mmol) are injected and the mixture is stirred for 0.5 h. A copious precipitate ensues and the mixture is stirred at room temperature for 1 h to ensure completion. The remaining work-up is analogous to that given for method A and affords pure benzoic acid (0.580 g (95%); m.p. 120–122°C). (ii) A violet solution of NaHTe is prepared by heating a mixture of powdered tellurium (0.06 g, 0.5 mmol) and NaBH4 (0.57 g, 15 mmol) in DMF (15 mL) at 70–80°C for 0.5 h. After the mixture is cooled to room temperature a solution of 2-chloroethyl benzoate (0.92 g, 5 mmol) in EtOH (3 mL) is injected and the mixture is stirred for 2 h at room temperature. The work-up is analogous to the above method and gives pure benzoic acid (0.567 g (63%); m.p. 119–121°C).

4.3.2 4.3.2.1

Regeneration of phenols Cleavage of aryl carboxylates and carbonates

Phenols can be regenerated from the corresponding carboxylates9 or carbonates10 by reaction with sodium hydrogen telluride. O a)

O

R

Ar NaHTe / EtOH (90-100%)

O b)

EtO

O

ArOH

Ar

a) R = Me; Ar = Ph, 2-naphthyl, 3-, 6-, 7-flavonyl, 7-coumaryl R = Ph; Ar = 2-naphthyl, o -NO2C6H4, 2-NO2, 4-5(MeO)2C6H3 R = Ar= Ph b) R = Ph, m -Me, o -AcC6H4, 2-naphthyl, 3-flavonyl, 4-coumaryl, 4-(2-hydroxy)quinolyl

Cleavage of aryl carboxylates (general procedure).9 To a solution of NaHTe (prepared from Te (1.30 g, 10 mmol) and NaBH4 (0.90 g, 24 mmol) in EtOH (20 mL)) buffered with AcOH (1.2 ml) in EtOH (5 mL) is added the aryl carboxylate (5 mmol). The mixture is refluxed for 30 min, poured into ice-H2O (200 mL), extracted with CHCl3 and the

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combined extracts dried and evaporated. The residue is purified by recrystallization or distillation. Cleavage of aryl carbonates (general procedure).10 To a solution of NaHTe prepared in situ from tellurium powder (1.3 g) and NaBH4 (0.9 g) in EtOH (20 mL) buffered with deoxygenated HOAc (1.2 mL in 5 mL EtOH) is added the aryl ethyl carbonate (5 mmol) and the mixture refluxed under N2 for 30 min. The reaction mixture is filtered, the filtrate evaporated and the residue dissolved in H2O. The aqueous layer is slightly acidified with a few drops of HOAc. The respective phenols are obtained in quantitative yields. 4.3.2.2

Cleavage of aryl haloacetates

The haloacetate group, also used as a protecting group for the phenolic function, can be removed by treatment with sodium telluride in DMF, giving the parent phenol.6 O Ar O Ar

O

X

X

O

Na2Te / DMF r.t. O

ArOH

(70-91%) -O C CH 2

O Ar

H3O+

Te2ArO-

:

RX (42-70%)

ArOR

By quenching the reaction with an alkyl halide, alkyl aryl ethers are formed: X⫽Br; Ar⫽1-naphthyl X⫽Cl; Ar⫽1-naphthyl, 2-naphthyl, p-Ph, p-N02, p-C3H7CO2C6H4 RX⫽MeI, EtBr, EtI The reaction involves an attack by the tellurium anion at the halogen atom followed by a rapid elimination of ketene from the resulting enolate. Functional groups such as nitro and ester moieties are unaffected. Cleavage of aryl haloacetates (general procedure).6 To a suspension of Na2Te (prepared from tellurium and NaH in DMF (5 mL) under N2) is added a solution of the aryl haloacetate (2.0 mmol) in DMF (3 mL). Immediate separation of Te is observed. After stirring at room temperature for 1 h, the dark mixture was quenched with saturated aqueous NH4Cl (5 mL). The organic phase was extracted with benzene (10 mL). After stirring the mixture exposed to the atmosphere for some time, it is filtered through Celite®, and the organic phase separated, washed with brine, dried and evaporated in vacuo. The crude product is purified by SiO2 chromatography (elution with CH2Cl2/hexane). 4.3.2.3

Cleavage of phenyl allyl ethers

Allyl ethers are cleaved with sodium hydrogen telluride with regeneration of the parent phenols.5 Ar

O

NaHTe / EtOH (85-99%)

ArOH

Ar = Ph, 4-coumaryl, 4-(2-hydroxy)quinolyl

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161

The experimental procedure is identical to that described for the cleavage of allylcarboxylates (see Section 4.3.1.3).

4.3.3

Regeneration of amines by cleavage of trichloro-t-butylcarbamates

Amines are regenerated from the protecting derivatives trichloro-t-butylcarbamates by treatment with sodium 2-thienyl tellurolate.11 The reagent is generated in a catalytic cycle by reduction of the corresponding ditelluride with sodium borohydride. O R1 R1 R2N

O Cl

ArTeNa / THF / Cl Cl (NaBH4 / Ar2Te2)

:TeAr

O R 1 R1 R2N

O Cl

Cl Cl

R2NH + CO2 + R21C=CCl2

R1=

Ar = 2-thienyl; Me R2N(representative examples): 1-indolyl, 2,3-dihydro-1-indolyl, o, p -(MeO)2C6H3NH, p -AcC6H4NH

Cleavage of trichloro-t-butylcarbamates (general procedure).11 A 1% solution of the trichloro-t-butylcarbamate derivative and 24 mol% of bis(2-thienyl) ditelluride in THF is heated to 60°C under N2. A 1% aqueous solution of NaBH4 (stabilized with three drops of a 10% NaOH solution) is added dropwise until the red colour of the catalyst disappears. This usually requires 4 or 5 equiv of NaBH4 and is carried out over a period of 30 min. The volatile components are removed in vacuo. The mixture is then poured into H2O and extracted with ether. The amines are isolated by flash chromatography or by acid extraction.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Greene, T. W. Protective Groups in Organic Synthesis. Wiley, New York, 1981. Chen, J.; Zhou, X. Y. Synthesis 1987, 586. Li, W.; Zhou, X. J.; Ma, Q. Synth. Commun. 1995, 25, 553, Huang, Z.; Xie, L.; Huang, X. Synth. Commun. 1988, 18, 1167. Shobana, N.; Shanmugam, P. Ind. J. Chem. 1986, 25B, 658. Suzuki, H.; Padmanabhan, S.; Ogawa, T. Chem. Lett. 1989, 1017. Chen, J.; Zhou, X. J. Synth. Commun. 1987, 17, 161. Huang, Z.; Zhou, X. J. Synthesis 1990, 633. Shobana, N.; Shanmugam, P. Ind. J. Chem. 1985, 248, 690. Shobana, N.; Amirthavalli, M.; Deepa, V.; Shanmugam, P. Ind. J. Chem. 1988, 27B, 965. Lakshimikantham, M. V.; Jackson, Y. A.; Jones, R. J.; O’Malley, G. J.; Ravichandran, K.; Cava, M. P. Tetrahedron Lett. 1986, 27, 4687.

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OXIDATION OF ORGANIC SUBSTANCES BY MEANS OF TELLURIUM REAGENTS

4.4.1 Bis(p-methoxyphenyl) telluroxides as a mild and selective oxidizing reagent Bis(p-methoxyphenyl) telluroxide (An2TeO), an easily accessible (see Section 3.10), stable crystalline compound, soluble in most organic solvents, finds wide use as a mild and selective oxidizing reagent for several functionalities. The reagent exhibits the advantage of being easily regenerated from the parent telluride produced during oxidation reactions. The oxidation reactions discussed below are well established. 4.4.1.1

Conversion of thio- and selenocarbonyl compounds into their oxo analogues

Y

An2TeO

Y = S, Se

O + An2Te

S

Se

S

R OR thioesters (Ia, b, c)

[1.2]

R OR selenoesters (III)

R SR dithioesters (II)

S

S

S

MeS OR xanthates (IVa, b, c)

RO OR thionocarbamates (V)

RS SR trithiocarbonates (VI)

S

S

S

R NHR thioamides (VIII)

R R thioketones (VIIa, b)

RHN NHR thioureas (IX)

Conversion of thio- and selenocarbonyl compounds into their corresponding oxo derivatives (reaction time, percentage yield) S PhC-OC8H17

(4 h, 91)

Ia C8H17

S O-

Ib R = Ph

(1.5 h, 53)

S O- (0.25 h, 84)

Ic R = Me S R

II R =

Ph

S-

(27 h, 52)

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Se C8H17

III R =

Ph

O-

(0.3 h, 80)

O-

(24 h, 72)

S IVa R =

R

Ph

H O

IVb (1 h, 98)

O S

O

O O O O O S O SMe

S O

SMe S

S t -C4H9

S S VI (20 h) (low yield)

O O V (0.5 h, 96)

IVc (0.75 h, 87)

VIIa (42 h, 23)

S

O

S C4H9-t

VIIb (0.3 h,100)

N S H VIII (20 h, 87)

RHN NHR R = H (63 h,100) IX R = Ph (16 h, 68)

Thiocamphor, under similar conditions, furnishes a vinylic disulphide as the major product (70%), which slowly undergoes a hetero-Cope rearrangement.2

O S

10% SH

S-]2

Cope rearrangement

S ]2

Considering that diaryl tellurides debrominate vic-dibromides, forming diaryltellurium dibromides (see Section 4.1.12.1), which in turn can be easily hydrolysed to the corresponding telluroxides by aqueous bases, a catalytic procedure has been provided for the oxidation of thiocarbonyl compounds. By this procedure excess 1,2-dibromotetrachloroethane acts as a halogen source, converting a small amount of telluride into the corresponding dibromide, which is then

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hydrolysed by the basic aqueous medium to the telluroxide.4 (Cl2CBr)2

R2C=O Ar2Te

Cl2C=CCl2

R2C=S Ar2TeBr2

Ar2TeO HO- / H2O

4.4.1.2

Conversion of tertiary phosphines into tertiary phosphine oxides

R3P

An2TeO

(ref. 2)

R3PO

R = Ph (48 h,72%); t-Bu (0.5 h, 93%)

4.4.1.3

Conversion of phenyl isothiocyanate into diphenylurea

PhN=C=S

4.4.1.4

O

An2TeO

PhHN

0.25 h (52%)

NHPh

(ref. 2)

Conversion of thiourea into ureas S RHN

O

An2TeO NHR

MeOH

RHN

NHR

(ref. 2)

R = H,16 h (73%) R = Ph, 16 h (68%)

4.4.1.5

Conversion of thiols into disulphides

2 RSH

An2TeO

RSSR

R = p -MeC6H4CH2 (0.5 h; 96%), p -MeC6H4 (0.1 h; 98%),

(ref. 1)

(1.5 h; 79%)

HO2C NH2.HCl

The inertness of functional groups (e.g. NH2) to the reagent is noteworthy.

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4.4.1.6

165

Conversion of o- and p-diphenols into quinones OH

OH

O ;

(65%) OH

;

(76%) OH

O

O

O

OH An TeO 2 OH

O An2TeO

An2TeO

(80%)

(ref. 2)

O

Simple phenols are unreactive. 4.4.1.7

Conversion of acylhydrazines into acylhydrazides O Ar

NHNH2

An2TeO

O Ar

NH]2

(ref. 2)

Ar = Ph(1 h, 58%), p -MeOC6H4 (24 h; 86%), p -NO2C6H4 (67%), p -PhOCH2 (24 h, 67%)

The oxidation of arylhydrazines with An2TeO is unsuitable for synthetic purposes, since it furnishes a complex mixture of products. 4.4.1.8

Conversion of N-phenylhydroxylamine into nitrosobenzene

PhNHOH

4.4.1.9

PhNO

(ref. 2)

Conversion of benzophenone hydrazone into diphenyldiazomethane

Ph Ph

An2TeO 0.1 h (90%)

N-NH2

An2TeO

Ph

22 h

Ph

N2

O

p -MeOC6H4CO2H (77%)

p -MeOC6H4

Ph O

Ph

(ref. 2)

The primary reaction product, diphenyldiazomethane, is trapped as diphenyl methyl ester. Oxidation with bis(p-methoxyphenyl) telluroxide (general procedure).2 All the reactions are performed at room temperature under N2 in CHCl3 or CH2Cl2. Approximately 10 mL of solvent is used for every 100 mg of substrate. For thiocarbonyl derivatives, 1.1 equiv of the reagent is used, for thiols 0.55 equiv. The mixtures are concentrated by evaporation and submitted to thin layer or column chromatography to isolate the products. An2Te is always recovered in a yield of 64–96%, while sulphur or selenium is always recovered in near quantitative yields from reaction with thio- or selenocarbonyl derivatives.

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The following compounds are unaffected by bis(p-methoxyphenyl) telluroxide: dithiolanes, enamines, aldehydes, ketones, alcohols, pyrroles, indoles, amino acids, aromatic amines, monohydroxyarenes, esters, hindered thiocarbonates, isonitriles, oximes, arylhydrazones, sulphides, and selenides.2

4.4.2

Polymer-supported bis(p-methoxyphenyl) telluroxide

A useful modification of the bis(p-methoxyphenyl) telluroxide reagent is its immobilization on a polymeric resin.5 This polymer-supported reagent, prepared from p-methoxyphenyl tellurocyanate and poly(p-lithiostyrene), exhibits several advantages over the monomeric reagent, such as easier product work-up and multiple recycle from the spent reagent. P 1)Br2 2) HO-/ H2O

Li + MeO

P

TeCN

Te(O)

P

OMe

OMe

Compared to the analogous supported selenoxide, the tellurium reagent requires shorter reaction times and milder experimental conditions. Prepararion of the polymeric telluride.5 One per cent divinylbenzene styrene co-polymer Bio-Beads S–X (Bio-Rad laboratories), is converted into poly(p-lithiostyrene) according to the described procedure.6 Poly(p-lithiostyrene) prepared in situ from 8.0 g of brominated polystyrene (containing 3.0 mol equiv Br2 g−1) is swollen in dry THF (50 mL) in an N2 atmosphere. A solution of p-methoxyphenyl tellurocyanate (8.0 g, 31 mmol; see Section 3.8) in dry THF (10 mL) is added, and the slurry is stirred at room temperature for 30 min and then overnight at 60°C. The resin is collected by filtration and washed in sequence with THF, THF/H2O, H2O, THF and finally with MeOH. After evaporating to dryness under reduced pressure at 60°C, a reddish-yellow resin of telluride (8.05 g) is obtained. A solution of Br2 (1.92 g, 12 mmol) in CCl4 (10 mL) is added dropwise into a suspension of the telluride resin (8.0 g) swollen in CCl4 (30 mL). The slurry is stirred at room temperature for 4.5 h. The resulting tellurium dibromide is removed by filtration and washed successively with CCl4, and THF. It is then mixed with THF (50 mL) and aqueous 15% NaOH (10 mL), and refluxed overnight. The resin is collected by filtration, and washed once with THF, repeatedly with THF/H2O (1:10) until the washings turn neutral, and finally with MeOH. After evaporating to dryness under vacuum at 60°C a brown resin of telluroxide is obtained (8.1 g). The spent reagent can be recovered as the reduced species by filtration, and reused after oxidation. The activity is not decreased even after 10 recycles. Like its monomeric counterpart, the polymeric reagent is inert to simple amines, amides, alcohols and phenols, but easily oxidizes thiols to disulphides, phosphines to phosphine oxides, hydroquinone and catechol to quinones, and thioketones, thioesters and trithiocarbonates to the corresponding oxo derivatives, in dichloromethane, chloroform or acetic

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acid at room temperature in high yields. The reaction times are 0.5–3.5 h for thiols, thioketones and phosphines, and higher for diphenols (4.5–12 h), thioesters (5 h) and trithiocarbonates (6–36 h). Thioamides are dehydrosulphurated to nitriles, or converted into 1,2,4-thiadiazoles, depending on whether the reactions are performed in non-acidic solvents such as dichloromethane, chloroform and methanol or in acetic acid. S Ar(R)

P An2TeO NH2 CH Cl , CHCl , MeOH 2 2 3 r.t., 1 h (90-98%)

+ P

Ar(R)C N

An2TeO + S

Ar(R) = Ph, p -MeO, p -Cl, o -Cl, p -NO2C6H4, PhCH2, n -C17H35, Ph2N* * Reaction time 24 h. S Ar

P

An2TeO

NH2 HOAc (12.5 h) (53, 31%)

Ar

N Ar

S

N

Ar = Ph, p -ClC6H4

N,N′–diphenylthiourea, structurally unable to form a nitrile, is converted into the corresponding urea by the first procedure; N-phenylthiourea, in acetic acid, giving 5-imino-4, 5-dihydro-1,2,4-thiadiazole. S PhHN

P

NHPh CH2Cl2, r.t., 3-5 h (94%) S

PhHN

P NH2

O

An2TeO

An2TeO

HOAc, r.t., 12 h (97%)

PhHN

Ph HN

N S

NHP

NHPh N

Oxidation with polymeric telluroxide (general procedure).5 The substrate (0.5 mmol) is mixed with the polymeric telluroxide reagent (20% excess) in 10 mL of the solvent. After completion of the reaction, the spent reagent is removed by filtration and washed thoroughly with the same solvent as used in the reaction. The combined filtrate and washings are evaporated under vacuum. The residue is purified by short SiO2 column chromatography to give the pure product. 4.4.3 Bis(p-methoxyphenyl) telluride as a mediator in an electrolytic process Bis(p-methoxyphenyl) tellurium diacetate or ditosylate is generated by the electrolysis of the parent telluride in the presence of Bu4N+ acetate or tosylate. Like the telluroxide, they accomplish the conversion of thioamides into nitriles and 1,2,4-thiadiazoles. Taking into consideration of these facts, the above conversion can be performed in an electrochemical

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catalytic cycle by submitting to eIectrolysis a mixture of a thioamide, Bu4N+ acetate or tosylate and 5% of the telluride.7 S Ar

An2TeX2 NH2

N

(-e, An2Te / (R4N+]X-) Ar (A) X = AcO, MeCN (B) X = TsO, EtOH

S

Ar N

Percentage yield 1,2,4-Thiadiazole

Nitrile Ar = p -MeOC6H4

(A) (B) (A) (B) (A) (B)

p-ClC6H4 PhNH

0 73 18 71 6 77

71 0 63 0 80 4

Electro-oxidation of thioamides to nitriles and thiadiazoles (typical procedure).7 A mixture of thiobenzamide (0.0686 g, 0.5 mmol) and An2Te (0.0086 g, 0.25 mmol) in dry MeCN (16 mL) containing Bu4N+OAc (0.602 g) is electrolysed under a constant current density of 4 mA cm−2. The reaction is monitored by high-performance liquid chromatography (HPLC) (Microsorb C18 standard column, MeOH as the eluent). After passage of 3.0 F mol−1 of electricity (10 h), the starting material is consumed, forming benzonitrile 0.0462 g (89%) and a trace of 3,5-diphenyl-1,2,4-thiadiazole. Both products are isolated as colourless oils and as colourless crystals from column chromatography on SiO2 (eluent: benzene/hexane, 1:1) (m.p. 89.5–90.5 and 89–90°C, respectively). 4.4.4

Bis(p-methoxyphenyl) tellurone

The title compound, prepared by the oxidation of the corresponding telluroxide with sodium periodate, exhibits some peculiarities compared to the described reactivity of the telluroxide.8 It oxidizes thiophenol to diphenyl disulphide, hydroquinone to p-benzoquinone, benzoin to benzyl, benzylic alcohols to the corresponding carbonyl compounds and cleaves hydrobenzoin to benzaldehyde. O

O

An2Te

Ar

O OH toluene/ (72, 79%) O

Ar = O

Ar

MeO , MeO

H

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OH Ph O OH Ph

4.4.5

169

O Ph same conditions (89%)

Ph

Ph

Ph same conditions (79%) OH

O 2 PhCHO

Sodium tellurite as oxidizing agent for thiols

Sodium tellurite exhibits selective and mild oxidizing properties for thiols under phase transfer conditions, at room temperature.9 Aromatic and benzylic thiols are oxidized to the corresponding disulphides very quickly, the ease of oxidation decreasing progressively for primary, secondary and tertiary thiols (PhSH ∼PhCH2SH >RCH2SH >RR1CHSH >RR1R2CSH). Short-chain thiols are more reactive than long-chain ones, probably because of the difference in the solubility in water. Tetrabutylammonium hydroxide is generally used as the phase transfer catalyst (PTC), but in the case of less reactive thiols better results are obtained by employing cetyltrimethylammonium bromide.

RSH

Na2TeO3, [R4N+]X- / H2O, benxene, r.t. -Te

RSSR

R (reaction time, h /% yield) N+]X

[R4

=

n -Bu4N+]OH-

[R4N+]X = CetMe3N+]Br-

Ph (1/ 87), o -H2NC6H4 (4 / 88), p -HOC6H4 (2 / 75) PhCH2 (1/ 88), n -Bu (4/ 56), n -C8H17 (48 / 77), i -Pr (48 / 48) Me3CCH2CMe2 (24 / 0) n -C8H17 (4 / 86), t -Bu (24 / 0)

The reagent is highly selective, as demonstrated by the inertness of several sensitive functionalities such as amino, hydroxyl, azo, hydrazo, phenol, sulphide, disulphide, sulphoxide, aldehyde moieties, and olefinic and acetylenic carbon–carbon bonds. The ability of the reagent to differentiate thiols may be used to prepare unsymmetrical disulphides. RSH + R1SH

same conditions

RSSR1

RSSR1 (reaction time, h / % yield) N+]OH-

[n -Bu4

[cetyl Me3N+]Br-

PhSSBu-t (24 / 52), PhCH2SSBu-t (36 / 74), PhSSPr-i (24 / 21), PhCH2SSPr- i (24 / 56) n -C8H17SSBu-t (24 / 81)

The cross-coupling reaction proceeds through the initial oxidation of the more reactive thiol, followed by the gradual thiolysis of the formed disulphide by the less reactive thiol. The method is therefore useful for the cross-coupling of the reactive aryl and n-alkyl thiols with the less reactive t-alkylthiols.

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Oxidative coupling of thiols to disulphides (typical procedure).9 Symmetrical disulphide. To a stirred solution of phenylmethanethiol (0.629 g, 5.06 mmol) in benzene (20 mL) is added dropwise a soIution of Na2TeO3⋅5H2O (0.471 g, 1.51 mmol) and 10% aqueous [Bu4N+]OH− (0.28 mL, 0.10 mmol) in H2O (20 mL) with a syringe, at room temperature (25–30°C). The reaction mixture quickly turns black, and free tellurium begins to precipitate. Monitoring by GLC indicates that the reaction is almost complete within 1 h. The mixture is filtered through a thin Celite® bed and the organic layer is separated. The aqueous layer is extracted with benzene (2×20 mL) and the combined organic phase is washed with H2O (20 mL), dried (Na2SO4) and evaporated. An oily residue is purified by chromatography on SiO2 (using hexane as the eluent) to give dibenzyl disulphide (0.55 g (88%); m.p. 71–72°C). Unsymmetrical disulphide. A solution of Na2TeO3×5H2O (0.465 g. 1.49 mmol) and 10% aqueous [Bu4N+]OH- (0.28 moI, 0.10 mmol) in H2O (20 mL) is added dropwise to a vigorously stirred solution of phenylmethanethiol (0.307 g, 2.47 mmol) and 2-methyl-2propanethiol (0.251 g, 2.79 mmol) in benzene (20 mL). The resulting mixture is stirred for 36 h at room temperature and then, worked up as described above to give pure benzyl t-butyl disulphide (0.39 g (74% based on phenylmethanethiol)) as an oil (b.p. 97°C/0.9 torr). Sodium tellurate (Na2TeO4) exhibits similar oxidizing properties towards thiols.

4.4.6

TeCl4-promoted oxidation of trialkylphosphites

Di- and trialkyl phosphites react with TeCl4 under a typical Arbuzov reaction to give dialkylchlorophosphates in high yields.10

RO)2POR1 + TeCl4

CH2Cl2 r.t., 30-60 min

RO)2P+Cl + Cl- + TeCl2 O R1

1/2 Te + 1/2 TeCl4

R1

= H; R = Me, Et, i -pr, n -Bu, PhCH2 R1 = R = Me, Et, i -pr, n -Bu

RO)2P(O)Cl (50-90%)

Typical procedure.10 To a solution of TeCl4 (148 mg, 0.55 mmol) in 5 mL of CH2Cl2 was added dropwise a solution of triethyl phosphite (166 mg, 1.0 mmol) in 5 mL of CH2Cl2 under nitrogen atmosphere at room temperature. As reaction proceeded, the mixture became black due to precipitation of tellurium metal. After stirring for about 30 min, the solution was filtered through Celite® and the filtrate was concentrated to give crude diethyl chlorophosphate. The crude product was purified by silica gel column chromatography using EtOAc/n-hexane (1:2) as eluent in 83% yield.

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In the presence of an alcohol and tert-amines the corresponding trialkylphosphates are formed in high yields. Elemental tellurium is formed as by-product.11 RO)3P

R1OH-(lutidine), CH2Cl2, r.t. TeCl4

TeCl2

O (RO)2POR1 + RCl (68-96%)

Te° R = Me, n -Bu, PhCH2, p -ClPhCH2 R1OH = 3-Phenyl propanol, 1-octanol,( +- )-2-octabol, cis-3-hexenol, 3-β-cholestanol

The reaction was considered as an oxidation–reduction process, where the phosphite and TeCl4 are converted into phosphorochloridate and tellurium dichloride, respectively. TeCl2 suffers a disproportionation into Te and TeCl4 which can participate again in the reaction. Typical procedure.11 A solution of trimethyl phosphite (0.85 mmol), 3-phenylpropyl alcohol (0.72 mmol) and lutidine (0.99 mmol) in CH2Cl2 (4 mL) was treated with TeCl4 (0.57 mmol) for 1 h at room temperature. Filtration of the black precipitate and chromatography on silica gel afforded dimethy 3-phenylpropyl phosphate (96% yield). Following the same procedure, phosphoric thiol esters are prepared by the treatment of trialkylphosphites with thiols in the presence of TeCl4 and lutidine.12 (RO)3P + R1SH

TeCl4 lutidine, CH2Cl2

O (RO)2PSR1 (+ R1SSR1) (83-97%)

R = Me, n -Bu, PhCH2 R1 = C12H25, PhCH2, C18H37, CH3CHCO2Et, p-ClC6H4, Ph

High yields are achieved with alkane thiols, but to ensure good results with arene thiols, CaCO3 is employed as acid captor instead of lutidine, avoiding the side reaction forming disulphides. Typical procedure.12 A solution of (dodecane thiol, R1 = Me(CH2)11, 1 mL, 4.17 mmol), trimethyl phosphite (591 µL, 5.01 mmol) and tert-amine (lutidine: 681 µL, 5.84 mmol) or calcium carbonate (1.4 mol equiv based on the thiol) in CH2Cl2 (20 mL) was cooled to −42°C in an MeCN-solid CO2 bath, and TeCl4 (0.8 equiv) was added. The mixture was stirred at the same temperature for 5–10 min and then, after removal of the bath, stirring was continued at room temperature (1.5–3 h). Precipitates were filtered off and the filtrate was washed with water and dried (MgSO4). The solvent was removed under reduced pressure and the phosphorothioate (1.24 g, 96% yield) were isolated by column chromatography on silica gel. 4.4.7

Arenetellurinic anhydrides

The readily accessible arenetellurinic anhydrides (see Section 3.6) exhibit oxidizing properties similar, in many aspects, to those of the diaryl telluroxides.13–15

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From competitive experiments performed to establish, the relative oxidizing capacity of some anhydrides and telluroxides, the following order was determined: (2-naphthylTeO)2O >(p-MeOC6H4)2TeO >(p-MeOC6H4TeO)2O >(p-PhOC6H4Te)2O. As the reactions proceed, the tellurinic anhydrides are reduced to the corresponding ditellurides. Pyrrolidine, indoline, phthaloylhydrazide and 4-phenylurazole are unaffected by tellurinic anhydrides. Like p-methoxyphenyl telluroxide, tellurinic anhydrides do not affect phenols but oxidize hydroquinones to the corresponding quinones, and oxidize thiols, thioesters, thioamides and phosphines to disulphides, esters, nitriles and phosphine oxides. Oxidations with Aryltellurinic Anhydrides

O

OH ( p -PhOC6H4TeO)2O OH OH

HOAc, 80°C, 1 h (95%)

O O

( p -PhOC6H4TeO)2O HOAc, 80°C, 1 h (95%) O OH ( p -PhOC6H4TeO)2O p -H2NC6H4SSC6H4NH2 -p p -H2NC6H4SH CH2Cl2, r.t., 10 min (95%)

ArSH

( p -PhOC6H4TeO)2O HOAc, r.t., 1 h (86, 94%) S

RO

ArSSAr Ar = Ph,

O

( p -PhOC6H4TeO)2O SMe

HOAc, 110°C, 24 h (54%)

RO

SMe

R = β-cholestanyl

Ph3P

( p -MeOC6H4TeO)2O CH2Cl2, r.t., 24 h (92%)

Ph3PO

S p -ClC6H4

NH2 S

Ph2N

CH2Cl2, r.t., 0.5 h (91%) ( p -MeOC6H4TeO)2O

NH2

S Ph

( p -MeOC6H4TeO)2O

HOAc, r.t., 24 h (63%)

p -ClC6H4C N

Ph2NC N O

( p -MeOC6H4TeO)2O OMe

CHCl3, reflux, 24 h (60%)

Ph

OMe

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In contrast to the telluroxide, which oxidizes N,N′-diphenylthiourea to N,N′-diphenylurea, tellurinic anhydrides give N,N′-diphenylcarbodiimide. S PhHN

NHPh

(An2TeO)2O CH2Cl2, r.t., 1 h (89%)

PhN=C=NPh

Like tellurones, and in contrast to telluroxides, tellurinic anhydrides oxidize benzylic alcohol to carbonyl compounds. O

( p -MeOC6H4TeO)2O R

OH

toluene, reflux, 24 h (91, 94%)

R

H

R = p -NO2, p -MeOC6H4

OH Ph

Ph O OH

OMe

Me O

O

( p -MeOC6H4TeO)2O toluene, reflux, 10 min (95%) ( p -MeOC6H4TeO)2O neat, 130°C, 1 h (95%)

Ph

Ph O O

OMe

Me O

Oxidation with p-phenoxyphenyltellurinic anhydride: diphenyl disulphide (typical procedure).13 Thiophenol (0.215 g, 2 mmol) is added to a stirred solution of the anhydride (0.210 g, 0.33 mmol) in HOAc (2 mL) under argon. The mixture is stirred for 1 h at room temperature. After evaporation under vacuum, the residue is extracted with CH2Cl2. The organic phase is washed with H2O, 5% aqueous Na2CO3 and H2O. SiO2 column chromatography yields diphenyl disulphide (0.154 g (86%)) as colourless crystals (from CH2Cl2/EtOH) (m.p. 58–60°C) and the ditelluride (0.167 g (83%)) as purple crystals (from CH2Cl2/EtOH) (m.p. 93–95°C). Oxidation with p-methoxyphenyltellurinic anhydride: benzonitrile (typical procedure).14 Thiobenzamide (0.15 g, 1.0 mmol) is mixed with the anhydride (0.189 g, 0.356 mmol) in dry CH2Cl2 (4 mL). The suspension is stirred at room temperature under N2 and becomes clear in 0.5 h. After evaporation of the solvent under vacuum, benzonitrile is separated from the residue by Kügelrohr distillation (b.p. 122–123°C/100 torr; 0.107 g (95%)). SiO2 column chromatography of the residue (eluent: benzene) gives red crystals of An2Te2 (0.152 g). Mixed anhydrides such as benzenetellurinyl acetate (PhTe(O)OAc), benzenetellurinyl trifluoroacetate (PhTe(O)OCOCF3) and benzenetellurinyl trifluoromethanesulphonate (PhTe(O)OSO2CF3), prepared by reacting benzenetellurinic anhydride with acetic anhydride, trifluoroacetic anhydride and trifluoromethane-sulphonic anhydride, respectively, have been recognized as valuable oxidizing reagents, in some cases superior to the parent benzenetellurinic anhydrides.16

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4.4.8 4.4.8.1

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4. TELLURIUM IN ORGANIC SYNTHESIS

⫽C bond Reaction of oxidizing tellurium reagents with the C⫽ Epoxidation of olefins catalysed by polystyrene-supported tellurinic acid

Anchored aryltellurinic acid has a catalytic activity on the epoxidation of olefins.17 The reagent is prepared by reaction of tellurium tetrachloride with divinylbenzene–styrene copolymer, followed by hydrolysis of the formed trichloride.

TeCl4

P

TeCl3

P

P + H2O2

1) HO2) HCl TeO2H

Te

P

O OH

O

dioxane or t -BuOH

Olefin (relative rates): 1-methyl cyclohexene (54), cyclohexene (21), 3-methyl cyclohexene (21), trans-2-butene (15), styrene (10), cis-2-octene (43), trans-2-octene (4.2), 1-octene (1.0), allylchoride (0.66), allyl alcohol (0.13)

The catalytic activity is dependent on the polymeric structure of the reagent, no epoxidation being observed with free tellurinic acids. The epoxidation is accelerated by increasing alkyl substitution and is stereospecific as demonstrated by the retained configuration of the olefinic bond. Catalytic epoxidation of olefins (typical procedure).17 Solid catalyst (1 g) prepared from XAD-2 resin is stirred with 20 mL of 1.0 M cyclohexane and 1.0 M H2O2 in t-BuOH or dioxane at 60°C for 24 h. Cyclohexenoxide is obtained in a quantitative yield. 4.4.8.2

Diacetoxylation of olefins

Olefins are diacetoxylated by benzenetellurinic anhydride in boiling acetic acid.18 R

R2 (PhTeO)2 / HOAc,

R1

R3

-(PhTe)2

OAc R3 R2 R R1

OAc

Olefins (% yield): styrene (82), 1-octene (71), cis-2-octene (38), cyclohexene (38)

The reaction, which is accelerated by catalytic amounts of sulphuric acid, exhibits a predominantly syn-stereochemistry. The actual reactive species which adds to the C⫽C bond is a reduction product of the tellurinic anhydride, formed at the expense of the ditelluride produced as the reaction proceeds. Two successive anti-acetolyses: (a) of the formed telluronium intermediate and (b) of the resuIting acetoxytelluride (which is acid catalysed), give the final diacetate with an

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overall syn-stereochemistry. (PhTeO)2O

PhTeOAc

(PhTe)2

H+ -HOAc

PhTeOTe(O)Ph or PhTeOTePh

PhTe+

R

R2

R1

R3

R R1 +

Te

HOAc

R2 a) HOAc R3 -H+

PhTeOAc

OAc R2 R3 R R1

TePh

Ph OAc R2 R3

b) HOAc -PhTeH

R R1

OAc

Diacetoxylation of olefins with benzenetellurinic anhydride (typical procedure).18 To a solution of (PhTeO)2O (0.95 g, 2.1 mmol) in dry HOAc (15 mL) is added styrene (0.208 g, 2.0 mmol) in HOAc (4 mL) and 98% H2SO4 (0.020 g, 0.2 mmol) in HOAc (1 mL). The mixture is gently refluxed for 24 h, which turns red with deposition of a small amount of elemental tellurium. After removal of tellurium (0.071 g, 0.56 mmol) by filtration, the solvent is evaporated, the residue extracted with ether and the ether extract dried (MgSO4). Chromatography on SiO2 gives the vic-diacetate (0.363 g (82%)) and 1-phenylethyl acetate (0.029 g (9%)). Other methods achieving the syn-diacetoxylation of olefins use the TeO2/LiBr/HOAc19 (i) or the TeCl4/LiOAc/HOAc20 (ii) systems. (i)

R

R2 TeO2 / LiBr / HOAc

R1

R3

Ac2O reflux, 20 h

reflux, 20h, -Te

OAc R3 R2 R R1

OAc

Olefin (% yield, % erythro / threo or meso / dl): cyclopentene (68), cyclohexene (41), cycloheptene (59), styrene (42), 1-octene (80), cis-2-octene (80.3, 87/13), trans-2-octene (91, 42 / 58), cis-4-octene (44, 89 / 11), trans-4-octene (32.5, 44 / 56), 1-decene (52), allyl acetate (58.5), allylbenzene (61.2), 1,4-cyclohexadiene (51.5)

The diacetates are accompanied by minor amounts of hydroxyacetates, which are converted into the diacetates by treatment with Ac2O/pyridine. As shown for the preceding method employing phenyltellurinic anhydride, the diacetoxylation prefers a syn-stereochemistry, especially for cyclic alkenes and cis-linear alkenes, whereas for trans-alkenes the preference for the syn-stereochemistry is decreased. In accordance with the mechanism proposed in the case of the tellurinic anhydride, the reaction can be rationalized as involving the intermediacy of a trans-adduct followed by an SN2-type detellurative acetolysis. Te x

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(ii)

R

R1 TeCl4 / LiOAc-HOAc

H

H

R1

R

120°C, 20 h

AcO

OAc

Olefin (5 yield, % meso / dl): cyclopentene (42), cyclohexene (50), 1-hexene (49),* styrene (54), cis-2-butene (34, 91/8), trans-2-butene (24, 16/84) * TeBr4 used instead of TeCl4.

The stereochemistry of the reaction is predominantly syn, in accordance with that described for the TeO2/LiBr/HOAc system, with exception of the reaction with cyclopentene, which gives mainly the trans-adduct. Diacetoxylation of olefins with TeO2 /LiBr/HOAc (typical procedure).19 To a mixture of TeO2 (1.60 g, 10 mmol), LiBr (0.87 g, 10 mmol) and HOAc (30 mL) is added cyclohexene (1.64 g, 20 mmol) at room temperature. The resulting yellow-orange heterogeneous mixture is stirred and heated under reflux for 20 h, during which time the mixture gradually becomes a black suspension. The mixture is cooled and the solid filtered off. The filtrate is treated with brine, and then extracted with CHCl3 (3×50 mL). The extract is washed successively with aqueous NaHCO3, brine and then dried (MgSO4). Evaporation of the solvent leaves an oily residue which is treated with (Ac)2O (3 mL) in pyridine (7 mL) at 120°C for 1 h. The solution is made slightly acidic using dilute HCl and then extracted with CHCl3 (3×30 mL). The extract is washed with brine and dried (MgSO4), and Kügelrohr distillation affords almost pure cis-1,2-diacetoxycyclohexane (1.50 g (37.5%); b.p. 160–165°C/3 torr). Diacetoxylation of olefins with TeCl4 /LiOAc/HOAc (typical procedure).20 A mixture of TeCl4 (1.35 g, 5 mmol), LiOAc (1.32 g, 20 mmol) and HOAc (15 mL) is stirred at 80–90°C for 1 h, during which time the solution becomes homogeneous. To the resulting homogeneous solution is added cyclohexene (0.82 g, 10 mmol), and the solution is heated at 120°C for 20 h. The solution is cooled, and the precipitated black tellurium is filtered off. The filtrate is extracted with ether (50 mL) and the ether layer washed with H2O (2×50 mL). The aqueous layer is extracted again with ether (25 mL) and the combined ether extracts washed with aqueous NaHCO3, dried (MgSO4) and evaporated to leave a yellow oil. The oil is treated with Ac2O (1 mL) in pyridine (3 mL) to acylate any free hydroxy groups. The pyridine solution is diluted with ether (25 mL) and acidified with dilute HCl. The ethereal solution is dried (MgSO4) and evaporated giving a yellow oil. The oil is chromatographed on a short SiO2 column (eluting with petroleum ether/ether, 1:1), giving cis-1,2-diacetoxycyclohexane (0.70 g (70%)). If the reaction with TeCl4/LiOAc/HOAc is performed at 80°C in the presence of BF3⋅Et2O, and the mixture treated with sodium thiosulphate after 3 h, acetoxyalkyl ditellurides are obtained, which are converted into the corresponding tribromides by addition of bromine.20 The stereochemistry of the addition is anti, as expected by an attack of the acetate ion at an intermediate telluronium ion. R1 R

1) TeCl4 /LiOAc-HOAc, BF3.Et2O, 80°C, 3 h 2) Na2S2O3

OAc R

Br2

R1 Te

CHCl3 2

OAc R

R1 TeBr3

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The acetoxytellurium tribromides are converted into the diacetates by the same treatment employed for the diacetoxylation reaction (HOAc, 120°C). The formation of an overall syn- or anti-adduct depends on the competition between a rearward attack by the acetate ion at the tellurium atom (as in the case of cyclohexene and 2-butenes), and a front attack by the neighbouring acetate moiety (as in cyclopentenes, where the almost the planarity of the five-membered ring makes the conformation of the acetoxytellurium tribromide susceptible to frontal attack). TeBr3 O

(a)

:OAc

O

overall syn OAc OAc

(b)

OAc

:OAc

overall anti

O +O

OAc

By submitting conjugated dienes to a treatment with the TeO2/LiBr/HOAc system, a mixture of 1,2- and cis- and trans-1,4-diacetoxylated adducts is formed.21 R

Te2O / LiBr / HOAc

OAc + OAc 1 R

R1

AcO

R1

R

R

+ OAc

AcO

R1 OAc

R, R1 = H, Me; R = H; R1 = Me

The yield and the 1,2/1,4 isomeric ratio is highly dependent on the LiBr/TeO2 ratio, the 1,4-isomer being favoured when the ratio is 5. The trans-isomer is the major product in the 1,4-addition to 1,3-butadiene. The reaction can also be performed catalytically relative to TeO2 when a reoxidant such as H2O2 or t-BuOOH is used. Unsatisfactory results are obtained with higher and cyclic dienes. On the basis of the mechanism proposed for the addition of the same TeO2/LiBr/HOAc system to single olefins, the addition to conjugated dienes probably involves the acetolysis of a homoallylic and an allylic telluroacetate (or tellurohalide) generated by the initial addition of a Te(IV) species and an acetate (or halide) anion to the conjugated system. +Te

Y+

Te

Te

+ Y

Te

Y HOAc

OAc

OAc + AcO

OAc + AcO

OAc

Diacetoxylation of 1,3-butadiene (typical procedure).21 To a mixture of TeO2 (0.80 g, 5 mmol), LiBr (2.17 g, 25 mmol), AcOH (18 mL) and Ac2O (2 mL) in a glass pressure bottle, chilled at −20°C, is added chilled 1,3-butadiene (1.35 g, 25 mmol). The resulting suspension is heated (oil bath) at 120–130°C for 20 h under magnetic stirring, producing a

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4. TELLURIUM IN ORGANIC SYNTHESIS

yellow solution which gradually turns to a black suspension. After cooling, the mixture is filtered, and the filtrate is treated with brine, and then extracted with CHCl3 (3×50 mL). The extracts are washed successively with aqueous NaHCO3 and brine, and then dried (MgSO4). GC analysis reveals the presence of the three isomeric diacetates. Evaporation of the solvent gives an oil (IR spectroscopy reveals strong OH and AcO absorption bands). After treatment with Ac2O (3 mL) in pyridine (7 mL) at 80°C for 1 h, GC of the pyridine solution (using ethyl cinnamate as the internal standard) reveals the presence of the 1,2 adduct (0.61 mmol), cis-1,4 adduct (0.71 mmol) and trans-1,4 adduct (4.78 mmol) (total 1.05 g, 6.10 mmol). The diacetoxylation of isoprene and 2,3-dimethyl-1,3-butadiene is performed similarly.

Isoprene 2,3-Dimethyl-1,3-butadiene

4.4.8.3

Reaction time (h)

Temperature (°C)

Total yield (%)

1,2/1,4 ratio

48 72

95 70

15.4 11.5

1/4 1/9

Methoxytellurenylation and dimethoxylation of olefins

The observation that the previously described diacetoxylation of olefins by means of tellurinic anhydrides produces quantitative yields of the corresponding ditellurides suggested the oxidative functionalization of olefins employing diphenyl ditelluride combined with an oxidizing agent instead of the tellurinic anhydride.22 Indeed, aliphatic olefins treated with equivalent amounts of diphenyl ditelluride and t-BuOOH in methanol in the presence of sulphuric acid give methoxytellurenylated adducts.∗ R

(PhTe)2 (1mmol), t-BuOOH (1 mmol) R1 H2SO4 (2 mmol), MeOH (10 mL), 24 h

R MeO

TePh R1

Olefin ( mmol, % yield): R1 = H; R = n-C6H13 (1, 59), R = PhCH2 (1, 73); R, R1 = (CH2)4 (3, 32)

The reaction is regioselective and sterospecific: the tellurium moiety is added exclusively to the terminal carbon of terminal alkenes, and only trans-adducts are obtained with cyclohexene. The reaction may proceed through the tellurenylation of the olefin (in a rapid equilibrium as demonstrated in separate experiments), forming a telluronium intermediate which is then methanolysed. PhTeTePh

R

t -BuOOH

R1

[PhTeTe(O)Ph

+ [PhTe+]

PhTeOTePh TePh

R

+

R1

MeOH

H+ R MeO

[PhTe+] + PhTeOH TePh R1

Methoxytellurenylation of olefins (typical procedure).22 1-Octene (0.112 g, 1 mmol), t-BuOOH (0.090 g, 1 mmol), (PhTe)2 (0.409 g, 1 mmol) and H2SO4 (0.196 g, 2 mmol) are heated under reflux in MeOH (10 mL) for 24 h. The phenyl 1-(2-methoxy)octyl telluride *For alkoxydihalotelluration of olefins by means of the (ArTe)2/Br2/ROH system, see Section 3.9.3.2.

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is obtained in the pure form by diluting with ether (50 mL), neutralizing with K2CO3 and filtration followed by radial layer chromatography (SiO2, pentane/ether) (0.201 g (59%)). Similar reactions performed using ethanol instead of methanol give the corresponding ethoxy derivatives. Under identical conditions the aromatic olefins styrene and p-methylstyrene give the vic-dimethoxy adducts as the sole products. Methoxytellurenylated adducts are formed, however, as minor by-products from different substituted olefins (p-chlorostyrene) or exclusively (from styrene) when the amount of H2SO4 is reduced. OMe

OMe

(PhTe)2, t-BuOOH H2SO4, MeOH

X

X

OMe

+ X

TePh

X = H, Me, Cl

The dimethoxylation of aromatic olefins can also be performed by employing a catalytic amount of ditelluride, e.g. styrene (5 mmol), (PhTe)2 (0.5 mmol), t-BuOOH (5.5 mmol added in 10 portions) and H2SO4 (2 mmol). MeOH (10 mL, reflux for 3.8 days) give 1,2dimethoxy-1-phenylethane in a yield of 88%.22,23 A proposed mechanism (on the basis of the identification of the intermediates) for the dimethoxylation involves an equilibrium between the starting olefin and a regioisomeric methoxytellurenylated adduct, the precursor of the dimethoxylated derivatives. The stereochemistry of the reaction, as demonstrated in the case of indene, involves an anti-methoxytellurenylation followed by methanolysis (with inversion) resulting in a net syn-dimethoxylation. PhTeTePh (1 mmol) t -ButOOH (1 mmol) 1 mmol

H2SO4 (2 mmol), MeOH 10 mL reflux, 24 h

PhTe+ Te+Ph

OMe 37% cis/ trans 79-21% MeOH -PhTeH TePh

MeOH -H+

4.4.8.4

OMe

OMe

Aminotellurinylation of olefins and related reactions

Phenyltellurinyl acetate and trifluoroacetate, prepared in situ from phenyltellurinic anhydride and the corresponding carboxylic acids (or anhydrides),24,25 add to olefins in the presence of excess ethyl carbamate and BF3⋅Et2O to give ethyl(2-phenyltellurinyl)alkyl carbamates.15,24,26 Because of difficulties in their purification, the products are reduced with hydrazine hydrate and isolated as tellurides. The foIlowing features must be pointed out: reactions with the tellurinyl acetates are performed in refluxing chloroform, while the more effective trifluoromethanesulphonate reacts in refluxing dichloromethane, even without the aid of the Lewis acid. Markovnikov-type regioselectivity is observed for terminal olefins, and the electrophilic tellurium moiety adds to the terminal carbon, unless electronic factors are operative (as in allyl phenyl ethers) inverting the isomeric ratio. The

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trans-stereochemistry of the addition to cyclohexene is clearly consistent with an antiaddition of ethyl carbamate on an epitelluronium intermediate. (PhTeO)2O + 2 RCO2H

2 PhTe(O)OCOR + H2O

R = Me, CF3 Ph PhTe(O)OCOR

Te

O H2NCO2Et

+

Ph

Te

O

CHCl3 or CH2Cl2, , 8-20 h

BF3.Et2O

NHCO2Et

H2N-NH2.H2O (75-97%) Olefin: styrene, 2-methylstyrene, allylbenzene 1-hexene, 1-hexadecene, allyl phenyl ether, cyclopentene, cyclohexene, cycloheptene, indene TePh NHCO2Et

Aminotellurination of olefins (typical procedure).25 To benzenetellurinyl trifluoroacetate (1.1 mmol) (generated in situ by treatment of benzenetellurinic anhydride (0.252 g, 0.55 mmol) with (F3CCO)2O (0.116 g, 0.55 mmol) in CH2Cl2 (6 mL) at room temperature, for 10 min) are added successively cyclohexene (0.082 g, 1 mmol), ethyl carbamate (0.446, 5 mmol) and BF3⋅Et2O (0.156 g, 1.1 mmol), and the resulting mixture stirred under reflux for 12 h. The mixture is cooled to room temperature, concentrated under vacuum and the residue reduced with H2NNH2⋅H2O (0.1 g, 2 mmol) in EtOH (6 mL) at 60°C for 10 min. After evaporation of the solvent under vacuum, the mixture is dissolved in CH2Cl2 (6 mL), treated with saturated aqueous NaHCO3 (25 mL) and extracted with CH2Cl2 (2×20 mL). The extract is washed with brine (20 mL) and dried (MgSO4). Evaporation of the solvent under vacuum gives a yellow residue, which is submitted to SiO2 column chromatography (elution with hexane/ethyl acetate, 5:1), giving first a small amount of diphenyl ditelluride and then trans-ethyl(2-phenyltelluro)cyclohexyl carbamate. The carbamate is recrystallized from hexane/CHCl3 as colourless needles (0.36 g (96%); m.p. 72–73°C). The aminotellurinylation of olefins is also achieved by the treatment of olefins with phenyltellurinyl trifluoroacetate, in the presence of BF3⋅Et2O, in acetonitrile, which acts both as a solvent and a nucleophile.27,28 Ph + PhTe(O)OCCF3

MeCN BF3.Et2O

Te +

F3CCO2

O

O

TePh HO

N Me

TePh Me O

TePh

F3C MeC N

H2N-NH2.H2O

HN

O r.t.

-

H2O*

O

(76-92%)

O O

N Me

TePh HN

Me O

Olefin: cyclopentene, cyclohexene, cycloheptene, 1-hexene *Water for hydrolysis is furnished by the reaction forming benzenetellurinyl trifluoroacetate formation25 (see above).

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Aminotellurination of olefins (typical procedure).28 To benzenetellurinyl trifluoroacetate (generated in situ from benzenetellurinic anhydride (0.27 g, 0.59 mmol) and TFA (0.159 g, 1.4 mmol) in MeCN (6 mL) at room temperature for 10 min) are added successively cyclohexene (0.082 g, 1.0 mmol) and BF3⋅Et2O (0.2 g, 1.4 mmol), and the resulting mixture is stirred at room temperature for 12 h. Evaporation of the solvent under vacuum gives a yellow oil, which is reduced with H2NNH2/H20 (0.1 g, 2 mmol) in EtOH (6 mL) at room temperature for 15 min. The mixture is poured into H2O and extracted with CH2Cl2 (2×20 mL). The organic phase is washed with brine, dried (MgSO4) and evaporated. The residue is chromatographed on SiO2 (eluting with hexane/ethyl acetate, 2:3) giving trans-2acetamidocyclohexyl phenyl telluride (0.26 g (76%)). The product is recrystallysed from hexane/CHCl3 as colourless needles (m.p. 146–146.5°C). If the two described aminotellurinylation procedures are performed at a higher temperature, tellurium-free products are obtained in high yields: 2-oxazolidinones25,26 and 2-oxazolines,27,28 respectively. Diphenyl ditelluride is formed as by-product. These heterocycles constitute an important class of compounds with wide applications. These detellurative cyclizations proceed through a rearward attack of the carbonyl oxygen (a) and of the iminol hydroxyl group (b), respectively, at the carbon bearing the tellurinyl moiety. Taking into consideration the regio- and stereochemistry of the amidotellurinylation step, the entire process is highly regio- and syn-stereoselective.

PhTe(O)OCCF3 BF3.Et2O

O

(a) H2NCO2Et

TePh

Cl

Cl + NH

O O

RC N

NH ..

O Et

OEt

(b) H2O

.. PhTeO

(53-92%)

O TePh .. HO

(53-97%)

O

N R

O

NH O

R = Me, Et, Ph, NHCO2Et Olefin: styrene, α-methylstyrene, trans-β-methylstyrene, 1-hexene, cis and trans-4-octene, c-pentene, c-hexene, c-heptene, indene, 1,2-dihydronaphthalene

2-Oxazolidinones (typical procedure).25 To a 1,2-dichloroethane solution (6 mL) of phenyltellurinyl trifluoroacetate (1.1 mmol), generated in situ from benzenetellurinic anhydride (0.252 g, 0.55 mmol) and TFA (0.126 g, 1.1 mmol), are added successively cyclohexene (0.082 g, 1 mmol), ethyl carbamate (0.446 g, 5 mmol) and BF3⋅Et2O (0.156 g, 1.1 mmol), and the resulting solution is stirred under reflux for 20 h. After cooling, the solution is treated with saturated aqueous NaHCO3, (30 mL) and extracted with CHCl3 (2×20 mL). The extracts are dried (MgSO4) and evaporated in vacuo. The remaining ethyl carbamate is removed by sublimation (50°C/1 torr) and the residue chromatographed on SiO2 (elution with EtOAc/hexane, 1:1), giving diphenyl ditelluride (0.14 g, 62%) as red needles and then cis-4,5-tetramethylene-2-oxazolidinone (0.122 g (86%)) as a white solid. Recrystallization from hexane/acetone gives colourless crystals (m.p. 55–56°C).

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4. TELLURIUM IN ORGANIC SYNTHESIS

2-Oxazolines (typical procedure).27 To benzenetellurinyl trifluoroacetate (generated in situ by treatment of benzenetellurinic anhydride (0.27 g, 0.59 mmol) with TFA (0.159 g, 1.4 mmol) in MeCN (6 mL) at room temperature for 10 min) are added successively cycloheptene (0.096 g, 1 mmol) and BF3⋅Et2O (0.27 g, 1.4 mmol). The resulting mixture is heated at 75°C for 3 h, and gradually turns black-red. It is cooled to room temperature, poured into CHCl3 (25 mL) and then extracted with 0.5 M HCl (2×25 mL). The aqueous extract is made alkaline by addition of NaOH pellets with ice cooling, and again extracted with ether (2×25 mL). The combined organic extracts are dried (K2CO3) and evaporated to give pure 2-methyl-4,5-pentamethylene-2-oxazoline (0.149 g (97%)) as a colourless oil (b.p. 130°C/42 torr). A further demonstration of the synthetic utility of the aminotellurinylation reaction is illustrated by the elimination of phenyltellurinic acid from the tellurinyl carbamates, giving amido olefins as shown in the accompanying example. Te(O)Ph NHAc

NaOH H2O / THF, r.t. (76%)

NHAc

Amido olefins (typical procedure).28 To trans-2-acetamidocycloheptyl phenyl telluroxide in THF (5 mL) (generated from cycloheptene (0.105 g, 1.09 mmol) and benzenetellurinyl trifluoroacetate (1.2 mmol) as previously described) is added aqueous NaOH (0.5 M, 5 mL). The mixture is stirred at room temperature for 3 h, poured into H2O and extracted with CHCl3 (2×20 mL), and the organic phase is dried (MgSO4) and evaporated under vacuum. The residue is purified by SiO2 column chromatography (eluting with hexane/EtOAc, 1:1), giving 3-acetamidocycloheptene (0.127 g (7%)) as colourless crystals (m.p. 74–75°C).

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Barton, D. H. R.; Ley, S. V.; Meerholz, C. A. J. Chem. Soc. Chem. Commun. 1979, 755. Ley, S. V.; Merholz, C. A.; Barton, D. H. R. Tetrahedron 1981, 37, 213. For other reagents effecting similar transformations see ref. 8 in the preceding ref. 2. Ley, S. V.; Merholz, C. A.; Barton, D. H. R. Tetrahedron Lett. 1980, 21, 1785. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. Bull. Chem. Soc. Jpn. 1986, 59, 879. Farrall, M. J.; Frechet, J. M. J. J. Org. Chem. 1976, 41, 3877. Matsuki, T.; Hu, N. K.; Aso, Y.; Otsubo, T.; Ogura, F. Bull. Chem. Soc. Jpn. 1988, 61, 2117. Engman, L.; Cava, M. C. J. Chem. Soc. Chem. Commun. 1982, 164. Suzuki, H.; Kawato, S.; Nasu, A. Bull. Chem. Soc. Jpn. 1992, 65, 626. Koh, Y. J.; Oh, D. Y. Synth. Commun. 1993, 23, 1771. Watanabe, Y.; Yamamoto, T.; Iwasaki, T.; Ozaki, S. Chem. Lett. 1994, 1881. Watanabe, Y.; Ioune, S.; Yamamoto, T.; Osaki, S. Synthesis 1995, 1243. Barton, D.H. R.; Finet, J. P.; Thomas, M. Tetrahedron 1986, 42, 2319. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. Tetrahedron Lett. 1986, 27, 6099. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. Phosphorus Sulfur 1988, 38, 177. Fukumoto, T.; Matsuki, T.; Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. Chem. Lett. 1990, 2269. Brill, W. F. J. Org. Chem. 1986, 51, 1149. Kambe, N.; Tsukamoto, T.; Miyoshi, N.; Murai, S.; Sonoda, N. Chem. Lett. 1987, 269.

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19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

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183

Uemura, S.; Ohe, K.; Fukuzawa, S. I.; Patil, S. R.; Sugita, N. J. Organomet. Chem. 1986, 316, 67. Fukuzawa, S. I.; Irgolic, K. J.; O′Brien, D. H. Organometallics 1990, 9, 3073. Uemura, S.; Fukuzawa, S.; Patil, S. R.; Okano, M. J. Chem. Soc. Perkin Trans. 1 1985, 499. Kambe, N.; Fujioka, T.; Ogawa, A.; Miyoshi, N.; Sonoda, N. Phosphorus Sulfur 1988, 38, 167. Kambe, N.; Fujioka, T.; Ogawa, A.; Miyoshi, N.; Sonoda, N. Chem. Lett. 1987, 2077. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. Chem. Lett. 1987, 1327. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. Tetrahedron Lett. 1987, 28, 1281. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. J. Org. Chem. 1989, 54, 4398. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. J. Chem. Soc. Chem. Commun. 1987, 1447. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. Tetrahedron Lett. 1988, 29, 1049. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. J. Chem. Soc. Perkin Trans. 1. 1989, 1775.

4.5

ORGANOTELLURIUM-BASED RING CLOSURE REACTIONS

The addition of an electrophilic tellurium reagent to olefins bearing an effective nucleophilic group at a suitable position, followed by the intramolecular capture of the generated telluronium species, results in the formation of a cyclic structure containing a tellurium moiety linked to the ring. Similar ring closure reactions are well known with selenium electrophilic reagents.1 + Te

Te

Te + Nu

NuH

NuH

These ring closure reactions, combined with the removal of tellurium by well-established methods, constitute useful procedures for the synthesis of cyclic structures, often present in natural products. 4.5.1

Tellurolactonization of unsaturated carboxylic acids

4.5.1.1

With aryltellurium trichlorides

The reaction of aryltellurium trichlorides with γ,δ-unsaturated carboxylic acids gives rise to aryldichlorotellurium butyrolactones in high yields.2,3 ArTeCl3

R2 R3 ArTeCl3 + R

R1

Ar = p -MeOC6H4

OH CHCl3/ -HCl O (77-87%) R1

R2,

+

R HO

ArTeCl2 R1 R2 R3 O

R1 R2 R3

R O O

R3

R, = H; = H, Ph, Me R, R1 = H; R2 = Me; R3 = Ph R, R1 = (CH2)2, (CH2)3*; R2, R3 = H R, R1 = (CH2)3; R2 = H; R3 = Me R, R1 = (CH2)3; R2, R3 = Me

*As established by X-ray4 analysis, this dichlorotellurolactone exhibits a cis -ring fusion in accordance with an anti -addition mechanism.

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Aryldichlorotellurolactones (general procedure).3 A solution of the γ,δ-unsaturated acid (5 mmol) and p-methoxyphenyltellurium trichloride (2.0 g, 5.8 mmol) in CHCl3 (80 mL) is heated under reflux (acyclic substrates, 1 h; cyclic substrates, 2.5–7 h). The solution is evaporated and the residue filtered through SiO2 with the aid of CHCl3. The solution is dried (MgSO4) and evaporated. The residue is recrystallized from CHCl3/petroleum ether at 30–60°C, giving the pure product. Analogous arylselenolactonizations are achieved employing arylselenyl bromides.1,2 The influence of the substrate structure in the tellurofunctionalization reaction of γ, δ-unsaturated carboxylic acids and corresponding benzyl esters has been described.5 γ,δ-Unsaturated carboxylic acids with a monosubstituted carbon–carbon double bond react with aryltellurium trichlorides to give the expected tellurolactone, while the corresponding benzyl esters give the addition products of the aryltellurium trichlorides to the double bond. 1,1-Disubstituted double bonds give a mixture of tellurolactanones with the HCl adducts to the double bond, whereas the corresponding benzyl ester give only the tellurolactones. Cl Cl

R1 R2

R1 O

ArTe

CO2R + ArTeCl3

R2

R1 = H; R2 = Me, Ph 73% 46% R = H R1, R2 = Me 52% R1 = Me; R2 = Ph R = PhCH2

O

R1 = R2 = Me 62% R1 = Me; R2 = Ph 76%

Ar = p-MeOC6H4, p -C6H5OC6H4 R1 R2 CO2CH2Ph

R1 R2

ArTeCl3

CO2CH2Ph ArTe Cl Cl

4.5.1.2

(72%)

With benzenetellurenyl nitrobenzenesulphonate

Diphenyl ditelluride reacts with p-nitrobenzenesulphonyl peroxide (NBSP), generating the title compound. This efficient electrophilic reagent reacts in situ with 4-pentenoic acid, allowing the formation of the corresponding tellurobutyrolactone.6 PhTeTePh + ArSO2-OO-SO2Ar

2 PhTeOSO2Ar

Ar = p -NO2C6H4 PhTeOSO2Ar

HO O

MeCN, 0°C, 1 h (52%)

PhTe O O

Tellurolactonization of 4-pentenoic acid (typical procedure).6 To a solution of diphenyl ditelluride (0.102 g, 0.5 mmol) in freshly distilled MeCN (20 mL) is added solid NBSP (0.170 g, 0.5 mmol), in small portions at 0°C, and the solution is stirred for 10 min. 4-Pentenoic acid (0.130 g, 1.1 mmol) is added, the solution is stirred for 2 h at 0°C, then

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185

poured into H2O (10 mL) and extracted with ether (3×10 mL). The organic layer is washed with 5% aqueous NaHCO3 (10 mL) and brine (10 mL), and then dried (MgSO4). The solvent is evaporated, and the residue purified by SiO2 column chromatography (eluting with hexane/CHCl3, 3:1). 5-Hexenoic and 6-heptenoic acids are ineffective for similar lactonizations. 4.5.1.3

With diaryl tellurium dihalides

Diaryl tellurium diiodides afford the iodocyclization of 4-pentenoic acid after 5 and 4 days reflux in CHCl3 in the presence of pyridine.7 CO2H Ar = p -ClC6H4

ArTeI2 py / CHCl3, reflux 5 days

O

O

+ Ar2Te

I (66%)

Typical procedure.7 A solution of 4-pentenoic acid (0.0500 g, 0.500 mmol), bis(pchlorophenyl)tellurium diiodide (0.302 g, 0.500 mmol) and pyridine (0.040 g, 0.500 mmol) was heated at reflux in 20 mL of chloroform for 5 days. The reaction mixture was concentrated in vacuo, and the residue was purified via chromatography on silica gel eluted with 1:1 hexane/ethyl acetate to give 0.073 g (66%) of the iodolactone and 0.140 mg (80%) of diaryl telluride. Diaryl tellurium dibromides are unreactive. 4.5.1.4

Tellurolactonization of α-allenic acids with phenyltellurenyl chloride

β-Organotellurobutenolides are obtained by the aryltellurenyl halide-induced lactonization of α-allenic acids under mild conditions.8 R H

.

R1

PhTeCl (1.1 equiv) [(PhTe) CO2H 2 + SO2Cl2] MeCN, r.t.

PhTe

R1

R O H O (79-94%)

R = H, alkyl, aryl; R1 = H, benzyl, alkyl

Typical procedure.8 To a solution of diaryl ditelluride (0.50 mmol) in dry MeCN (2 mL) under N2 was added dropwise SO2Cl2 (0.081 g, 0.60 mmol) and the mixture was stirred at room temperature for 1 h. α-Allenoic acid (0.50 mmol) in dry MeCN (2 mL) was then added to above ArTeCl solution with stirring. After the reaction was complete, the mixture was concentrated and the residue purified by flash chromatography or preparative TLC to afford β-organotellurobutenolides. 4.5.1.5

Reductive detelluration of tellurolactones

The reductive removal of the tellurium moiety from aryldichlorotellurolactones is achieved by treatment with tributyltin hydride (TBTH) in refluxing toluene.9 High yields are conditional on a previous careful deoxygenation of the reaction mixture. If the dichlorotellurolactones are reduced previously to the parent tellurolactones by treatment with NaBH4,3 the successive detelluration saves 2 equiv of TBTH, and the

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4. TELLURIUM IN ORGANIC SYNTHESIS

work-up is easier. R

R1 R2 R3

ArTe Cl Cl O O

TBTH (4 equiv)

O

toluene / (87-92%)

O

R NaBH4 (1 equiv)

R1 R2 R3

R

ArTe

O Ar = p -MeOC6H4 O R, R1 = H; R2, R3 = Me, Ph 1 2 3 R, R , R = H; R = Me R, R1 = (CH2)2, (CH2)3; R2, R3 = H

TBHTH (2 equiv) toluene /

R1 R2 R3

Reductive detelluration of aryldichlorotellurolactones (general procedure).9 To a solution of the dichlorotellurolactone (1 mmol) in deoxygenated toluene, (10 mL), under reflux and N2, is added dropwise Bu3SnH (4 mmol). The reaction is monitored by TLC. After 4 h the solvent is evaporated and the residue chromatographed on SiO2 (eluted first with petroleum ether to remove Te and Sn by-products, and then with petroleum ether/EtOAc, 1:5), giving the tellurium-free lactone. Reduction of the dichlorotellurolactones to the corresponding tellurolactones (general procedure).3 A solution of the dichlorotellurolactone (1 mmol) in THF (10 mL) is treated dropwise with NaBH4 (0.04 g, 1.05 mmol) in H2O (5 mL). An immediate reaction takes place with gas evolution and the reaction mixture turns pale yellow. After stirring for 10 min at room temperature, the reaction mixture is diluted with ether (30 mL) and washed in turn with H2O, saturated aqueous NH4Cl and brine. The organic layer is dried (MgSO4) and evaporated, furnishing the tellurolactone as a yellow oil that turns red on standing. By treatment of the dichlorotellurolactone with excess NaBH4, the fission of the Te–C bond takes place with recovery of the starting γ,δ-unsaturated carboxylic acid.3 R

R1 NaBH (6 equiv) 4 R2 R3

ArTe Cl Cl O O O2

R

R1 R2 R3

ArTe H:

O

R1 O H2O

R

R2 R3

OH + [ArTeH]

O

1/2(ArTe)2

The overall process (lactonization/NaBH4 reduction) therefore constitutes a formal protective method for γ,δ-unsaturated acids. Reaction of dichlorotellurolactones with excess NaBH4: regeneration of γ,δ-unsaturated acids (general procedure).3 The dichlorotellurolactone (1 mmol) in THF (10 mL) at room temperature is treated with NaBH4 (0.1 g, 2.6 mmol) in H2O (5 mL). An immediate reaction takes place and the solution turns dark red. After stirring for 10 min at room temperature, the reaction mixture is diluted with ether (30 mL) and extracted with 2 N NaOH (2×5 mL). The organic phase is washed with H2O, dried (MgSO4) and evaporated, furnishing bis(p-methoxyphenyl) ditelluride in a quantitative yield. The cooled aqueous

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phase is acidified with concentrated HCl, and extracted with ether (3×10 mL). The organic extracts are dried (MgSO4), evaporated and the residue Kügelrohr distilled, furnishing the γ,δ-unsaturated carboxylic acid in a yield of ∼90%. 4.5.2 4.5.2.1

Cyclotelluroetherification of unsaturated alcohols and allylphenols With aryltellurinyl acetates

Aryltellurinyl acetates, generated as previously described from the corresponding tellurinic anhydrides and acetic acid, add to hydroxy olefins, giving tellurinylated cyclic ethers.10–12 The reaction is performed in refluxing acetic acid or in chloroform in the presence of BF3⋅Et2O. Owing to their hygroscopicity and intractable nature, the products are reduced with hydrazine hydrate, and isolated as tellurides (as for the aminotellurinylation reactions).

ArTe(O)2O

HO

HOAc

( )n

ArTe(O)OAc

a)HOAc/ , 15 h

n()

b) BF3.Et2O CHCl3, r.t., 30 min

n()

O

O

OAc Te Ar OAc

ArTe O ()n +

HO

H2N-NH2.H2O

O.BF3 TeAr

n()

TeAr

O

n = 1, 2

Ar = Ph, p -MeOC6H4, 2-naphthyl

A variety of unsaturated alcohols with a cyclic structure and allyl phenols lead to bicyclic telluroethers. TeAr OH ( )n

O OH

O

( )m

( )m

( )nTeAr

m = 1; n = 1, 2 m = 2; n = 1

H

HO O TeAr

O

OH

TeAr

TeAr O

OH R

R = H, Me

R

In accordance with the Baldwin rules, formation of five-membered rings is favoured over the four- or six-membered rings, and the six-membered rings over the seven-membered ones. Cyclotelluroetherification of allylphenol (typical procedure).11 To a solution of phenyltellurinyl acetate (1.1 mmol) (prepared in situ by treatment of benzenetellurinic anhydride (0.252 g, 0.55 mmol) with Ac2O (0.056 g, 0.55 mmol) in CHCl3 (6 mL) under reflux for 30 min) is added, after cooling, 2-allylphenol (0.134 g, 1.0 mmol) in CHCl3 (2 mL) and

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4. TELLURIUM IN ORGANIC SYNTHESIS

BF3⋅Et2O (0.170 g, 1.2 mmol). The resulting mixture is stirred at room temperature for 0.5 h and then concentrated in vacuo. The residue is reduced with hydrazine hydrate (0.1 g, 2 mmol) in EtOH (5 mL) at 60°C for 15 min. After cooling to room temperature the mixture is poured into H2O and extracted with CH2Cl2 (2×25 mL). The organic layer is washed with brine, dried (MgSO4) and evaporated in vacuo. The residue is chromatographed on SiO2 (eluting with CH2Cl2), giving 2,3-dihydro-2-[(phenyltelluro)methyl]benzofuran as white needles (0.311 g, (92%); m.p. 46.5–47°C (from MeOH)). The same reaction carried out in HOAc requires heating under reflux for 15 h. 4.5.2.2

With aryltellurium trichlorides

Aryltellurium trichlorides are suitable reagents for the telluroetherification of olefinic alcohols and allylphenols,13–14 exhibiting the advantages of stability and easy separability of the formed crystalline dichlorotelluro derivatives compared to the preceding tellurinyl acetate method. HO

R

( )n

CHCl3,

CHCl3,

R1 R2 ( )n

R

R

O

R1

R2

(85-98%)

TeCl2Ar

n=1

R = i-pr; R1, R2 = Me R = n-Bu; R1 = H; R2 = n -pr R = Ph; R1 = H; R2 = Me

n=2

R = CH2OH; R1, R2 = H R

ArTeCl3 +

TeCl2Ar

O

R Ar = p -MeOC6H4, p -PhC6H4 p -MeC6H4

n = 1; R = H, Me n = 2; R = H OH

n( )

(85-96%)

HO

( )n

HO

CHCl3,

n(

(85-90%)

Ar = p -MeOC6H4 )

TeCl2Ar

O R

(90%)

O

n = 1; R = H, Me n = 2; R = H

Ar = p -MeOC6H4, p -PhOC6H4, p -MeC6H4 ArTeCl2 TeCl2Ar

OH

OH

(78%)

O

TeCl2Ar

(90%)

O

TeCl2Ar OH R

(88, 71%)

O R

R = H, Me

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Cyclotelluroetherification (typical procedure).13 The unsaturated alcohol and a slight excess of the aryltellurium trichloride are refluxed in CHCl3 for 15–40 min. The o-allylphenols require a longer reaction time (12–24 h). The products are purified by filtration through an SiO2 column, eluting with CHCl3 to remove the excess of tellurium trichlorides. The obtained dichlorotelluroethers are recrystallized from CHCl3/hexane. Aryltellurium tribromides (Ar⫽p-MeOC6H4) are also effective in these cycloetherification reactions. The dichlorotelluroethers are easily reduced to the parent telluroethers by treatment with TUDO.9 n( )

TeCl2Ar

O

TUDO PTC / NaOH / THF, r.t.

n( )

O

TeAr

Olefinic benzyl ethers can also be submitted to ethercyclization with aryltellurium trichlorides. The yields and the reaction times are close to those observed for the cyclization of the corresponding alcohols. The stereochemistry of the reaction is low.14 OR3 R

ArTeCl3 CHCl3, reflux

R1 R2

R R1

O

TeCl2Ar

R2

(65-97%)

a) Ar = p-MeOC6H4, p-C6H5C6H4 b) R = i-pr; R1, R2 = Me; R3 = Bz c) R = n-Bu; R1 = H; R2 = pr; R3 = Bz d) R = Ph; R1 = H; R2 = Me; R3 = Bz e) R = CH2OH, CH2OBz; R1, R2 = H; R3 = Bz OR3

O

f)

TeCl2Ar (85%)

R = Bz

Typical procedure.14 A mixture of the benzyl ether (0.246 g, 1 mmol) and p-phenoxyphenyltellurium trichloride (0.403 g, 1 mmol) in anhydrous chloroform (15 ml) was stirred for 30 min at room temperature, then the solvent was evaporated and the residue was filtered through a column of silica gel eluting with chloroform. Evaporation of the solvent in a rotatory evaporator followed by recrystallization of the residue from chloroform/petroleum ether and washing several times with petroleum ether gave 0.50 g (97%) of the product, m.p. 108–110°C. 4.5.2.3

With benzenetellurenyl nitrobenzenesulphonate

The title reagent, employed for tellurolactonizations (see Section 4.5.1.2), is also effective for the tellurocyclization of unsaturated alcohols.6 HO

( )n

PhTeOSO2Ar MeCN, 0°C, 1h (71-81%)

Ar = p -NO2NC6H4

n(

)

O

TePh

n = 1, 2 Ar = p -NO2C6H4

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4. TELLURIUM IN ORGANIC SYNTHESIS

OH

PhTeOSO2Ar MeCN, 0°C, 1h (81%)

TePh O

Ar = p -O2NC6H4

Cyclotelluroetherification (typical procedure).6 The procedure, starting from (PhTe)2 (0.5 mmol), NBSP (0.5 mmol), 4-penten-1-ol or hexen-1-ol (1.1 mmol), is similar to that described for tellurolactonization (see Section 4.5.1.2). 4.5.2.4

With TeO2/HOAc/LiCl or TeO2/HCl

Tellurium dioxide in acetic acid/lithium chloride reacts with 2 equiv of γ- or δ -hydroxyolefins, giving tetrahydrofuran or tetrahydropyran dichlorotelluro derivatives.15

2

( )n

OH + TeO2 HOAc / LiCl (50-58%)

n(

)

Cl Cl Te

O

O

( )n

n = 1, 2 R 2

+ TeO2 OH

Cl Cl Te

HOAc / LiCl (70-78%)

O

O

R = H, Me, Cl

The reaction can be performed in an equimolar ratio, employing tellurium dioxide in alcoholic aqueous hydrochloric acid. The formed trichlorotelluro derivative is reduced without isolation to the corresponding ditelluride (see Section 3.2.3.2).16 OH + TeO conc. HCl 2 MeOH

+ TeO2 OH

conc. HCl MeOH

Na2S2O5 (62%)

Te Te

O

Na2S2O5 (62%)

O

Te Te

O

O

Bis[2-(2,3-dihydrobenzofuranyl)methyl]tellurium dichloride (typical procedure).15 TeO2 (2.4 g, 15.0 mmol), 2-allylphenol (4.0 g, 30.0 mmol) and LiCl (3.0 g, 70.8 mmol) are refluxed in HOAc (50 mL) for 1.5 h. The clear solution is then filtered hot from a small amount of elemental tellurium and, upon cooling (freezing), the product crystallizes as a white solid [5.4 g (78%)]. The product is recrystallized from EtOH/CH2Cl2 (1:1) (m.p. 180–184°C). Bis[2-(2,3-dihydrobenzofuranyl)methyl] ditelluride (typical procedure).16 TeO2 (2.0 g, 12.5 mmol) is dissolved in concentrated HCl (10 ml) and the yellow solution diluted with MeOH (40 mL). 2-Allylphenol (1.98 g, 14.8 mmol) is added and the mixture is heated

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under reflux for 24 h and then allowed to cool to ambient temperature. The yellowish solution (containing a trace of elemental Te) is then poured into a separating funnel containing Na2S2O5 (5 g in 100 mL H2O) and CH2Cl2 (100 mL). After shaking, the red CH2Cl2 phase is separated, dried and evaporated, giving a red oil which is purified by SiO2/flash chromatography (eluting with CH2Cl2 (3.23 g (83%); m.p. 59–63°C). 4.5.2.5

With diaryl tellurium dihalides

Diaryltellurium diiodides afford the iodocyclization of 4-pentenol. Diaryltellurium dibromides give only traces of bromotetrahydrofurans with 3-butenol and 4-pentenol.7 OH

Ar2TeI2 py / CHCl3, reflux 4 days

( )n OH n=1 n=2

O I (57%)

+ Ar2Te

Br

Ar2TeBr2 CHCl3, reflux, 48 h n=1

O traces

Ar2TeBr2 CHCl3, reflux 48 h

O Br traces

Typical procedure.7 A solution of 4-pentenol (0.0860 g, 1.00 mmol), bis(p-chlorophenyl) tellurium diiodide (0.605 g, 1.00 mmol), and pyridine (0.080 g, 1.00 mmol) was heated at reflux in 10 mL of chloroform for 4 days. The reaction mixture was concentrated in vacuo, and the residue was purified via chromatography on silica gel eluted with 1:1 hexane/ethyl acetate to give 0.121 g (57%) of the iodotetrahydrofuran and 0.30 g (85%) of the diaryltelluride. 4.5.2.6

Synthetic utility of the telluroetherification reactions

The synthetic utility of the described reactions is clearly illustrated by the reductive detelluration of telluroethers with Bu3SnH, the entire telluration/detelluration process providing a cyclization under mild conditions for unsaturated alcohols.

O

TeCl2Ar

Bu3SnH (4 equiv)

R Ar = p -MeOC6H4; R = H, Me TeAr O R Ar = Ph

(ref. 9)

toluene, reflux, 6h

O R

Bu3SnH (2.5 equiv) toluene, reflux, 1 h

O R

(ref.11)

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4. TELLURIUM IN ORGANIC SYNTHESIS

Additional synthetically useful transformations are given in sequence. OH

NaOH O

O PhTeO NH2-NH2-H2O

chloramine-T

(ref. 11)

O TePh Br2 NaBr O

O

Br

TePh Br Br

Reductive detelluration of cyclic dichlorotelluroethers (general procedure).9 This procedure is identical to that described for dichlorotellurolactones (see Section 4.5.1.5). Reductive detelluration of cyclic telluroethers (typical procedure).11 To a solution of 2,3dihydro-2-[(phenyltelluro)methyl]benzofuran (0.338 g, 1 mmol) in toluene (6 mL) is injected TBTH (0.67 ml, 2.5 mmol) at room temperature, and the resulting solution is stirred under reflux for 1 h. The solution is evaporated under vacuum and the residual yellowish oil subjected to SiO2 column chromatography (eluting with benzene/hexane, 3:1) to give pure 2,3-dihydro-2-methylbenzofuran as a colourless oil (0.128 g (95%); b.p. 93–94°C/23 torr). Telluroxide elimination in cyclic tellurinyl ethers (typical procedure).11 The cyclofunctionalization of 2-cyclopent-2-enylethanol (0.152 g, 1.35 mmol) with benzenetellurinyl acetate (1.48 mmol) in CHCl3 (6 mL) is performed by the general method described in Section 4.5.2.1. The reaction solution containing the crude telluroxide is diluted with CHCl3 (25 mL), washed with brine and evaporated under reduced pressure. To the residue are added successively THF (5 mL) and NaOH (0.5 M, 10 mL). The resulting mixture is stirred under reflux for 5 h, poured into H2O and extracted with ether (2×20 mL). The extract is dried (MgSO4) and concentrated under reduced pressure. The residue is chromatographed on SiO2, eluting with CH2Cl2/hexene (5:2), to give 8-oxabicyclo[3.3.0]oct-2-ene as a colourless oil (0.079 g (53%)). 4.5.3 Tellurocyclofunctionalization of alkenyl-substituted β -dicarbonyl compounds

α-Alkenyl-substituted β-dicarbonyl compounds, by treatment with aryltellurium trichloride, undergo tellurocyclofunctionalization via an exo-mode process of their enolic form.17,18 TeCl2Ar n(

O

n(

) O

n = 1, 2

O

n(

) OH ArTeCl3

O

)

O

94, 90%

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O

193

O

O R

R

O 80, 93%

R = Me, OEt

TeCl2Ar

O CO2Et

CO2Et O ArTeCl2 65%

Tellurocyclization (typical procedure).18 A mixture of β-keto ester (2 mmol) and pmethoxyphenyltellurium trichloride (2.2 mmol) in 30 mL of recently distilled chloroform was heated under reflux. The solvent was evaporated and the residue filtered through silica gel using chloroform as eluent. γ-Alkenyl-substituted β-dicarbonyl compounds, upon the same conditions, give rise to 2,5-disubstituted tetrahydrofurans bearing exocyclic double bonds. The products, upon treatment with NaBH4, are reduced to the corresponding tellurides which in turn are converted into tellurium-free methyl derivatives by treatment with TBTH.18 O Cl Cl ArTe

CO2Et ArTeCl3 R

reflux

R O

CO2Et

(84, 81%) R = H, Me NaBH4 R=H THF/H2O R O

CO2Et

n -Bu3SnH toluene, reflux

O

ArTe

(77%)

CO2Et

(84%)

α,γ-Diallyl-β-ketoesters, by treatment with aryltellurium trichlorides, furnish polysubstituted furan derivatives as exemplified in the accompanying scheme.19 O

Cl

O

ArTeCl3

Cl

ArTe

O

OEt

Ar = p -MeOC6H4

4.5.4

Tellurocyclization of olefinic carbamates

When olefins bearing a carbamate group at a suitable position are submitted to treatment with phenyltellurinyl acetate in the presence of BF3⋅Et2O (previously described for

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4. TELLURIUM IN ORGANIC SYNTHESIS

amidotellurinylation of olefins, see Section 4.4.8.4), an intramolecular cyclization takes place to give pyrrolidine and piperidine derivatives.20

( ) NHCO2Et n

n = 1, 2 n=1 n=1

PhTe(O)COCF3 BF3.Et2O, CHCl3, , 30 min (73-96%)

R R1 n( )

N CO2Et

TePh

R = R1 = H R = H; R1 = Me R = Me; R1 = allyl

NHCO2Et

same procedure (87%)

TePh N CO2Et

The cyclization is much faster (30 min) than the intermolecular addition (8–20 h).

REFERENCES 1. (a) Paulmier, C. Selenium Reagents and Intermediates in Organic Synthesis. Chapter VIII. Pergamon Press, Oxford, 1986. (b) Nicolaou, K. C.; Petasis, N. A.; Claremon, D. A. in OrganoSelenium Chemistry (ed. D. Liotta). Chapter 2. Wiley, Chichester, 1987. 2. Moura Campos, M.; Petragnani, N. Chem. Ber. 1960, 93, 317. 3. Comasseto, J. V.; Petragnani, N. Synth. Commun. 1983, 13, 889. 4. Husebye, S.; Meyers, E. A.; Zingaro, R. A.; Comasseto, J. V.; Petragnani, N. Acta Cryst. 1987, C43, 1147. 5. Moraes, D. N.; Santos, R. A.; Comasseto, J. V. J. Braz. Chem. Soc. 1998, 9, 397. 6. Yoshida, M.; Suzuki, T.; Kamigata, N. J. Org. Chem. 1992, 57, 383. 7. Leonard, K. A.; Zhou, F.; Detty, M. R. Organometallics 1996, 15, 4285. 8. Xu, Q.; Huang, X.; Yuan, J. J. Org. Chem. 2005, 70, 6948. 9. Comasseto, J. V.; Ferraz, H. M. C.; Brandt, C. A.; Gaeta, K. K. Tetrahedron Lett. 1989, 30, 1209. 10. Hu, N. X.; Aso, Y.; Otsuba, T.; Ogura, F. Tetrahedron Lett. 1987, 28, 1281. 11. Hu, N. X.; Aso, Y.; Otsuba, T.; Ogura, F. J. Org. Chem. 1989, 54, 4391. 12. Hu, N. X.; Aso, Y.; Otsuba, T.; Ogura, F. Phosphorus Sulfur 1988, 38, 177. 13. Comasseto, J. V.; Ferraz, H. M. C.; Petragnani, N.; Brandt, C. A. Tetrahedron Lett. 1987, 28, 5611. 14. Comasseto, J. V. Grazini, M. V. A. Synth. Commun. 1992, 22, 949. 15. Bergman, J.; Engman, L. J. Am. Chem. Soc. 1981, 103, 5196. 16. Engman, L. Organometallics 1989, 8, 1997. 17. Ferraz, M. H. C.; Comasseto, J. V.; Borba, E. B. Quim. Nova 1992, 15, 298. 18. Ferraz, M. H. C.; Sano, M. K.; Scalfo, A. C. Synlett 1999, 5, 567. 19. Stefani, H. A.; Petragnani, N.; Brandt, C. A.; Rando, D. G.; Valduga, C. J. Synth. Commun. 1999, 29, 3517. 20. Hu, N. X.; Aso, Y.; Otsuba, T.; Ogura, F. Chem. Lett. 1987, 1327.

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CONVERSION OF ORGANOTELLURIUM COMPOUNDS INTO TELLURIUM-FREE ORGANIC COMPOUNDS

4.6.1 Detelluration of organotellurium compounds with the formation of new C–C bonds (carbodetelluration) 4.6.1.1

Synthesis of biaryls by Raney Ni-catalysed homocoupling of diaryltellurium dichlorides and aryltellurium trichlorides

Diaryltellurium dichlorides are detellurated by heating with Raney nickel in appropriate solvents, giving the corresponding biaryls.1 ArTe(Cl)2Ar

Raney nickel diglyme, , 8 h (60-91%)

Ar-Ar

Ar = Ph, p -MeO, p -EtO, p -Me, p -Br, p -Me2NC6H4, p -MeO-m -MeC6H3, p,m -(MeO)2C6H3, o,p -(MeO)2C6H3, 2-naphthyl

Synthesis of biaryls (general procedure).1 The diaryltellurium dichloride (50 mmol) is heated with degassed Raney nickel2 (60 g) in diglyme (500 mL) for 8 h. The mixture is filtered while still hot, and the solvent evaporated (water pump). The residue is recrystallized from ethanol or toluene. 2,2′-Binaphthyl is obtained by submitting 2-naphthyltellurium trichloride to a similar treatment.3 TeCl3

4.6.1.2

Raney nickel diglyme, , 2 h (98%)

Pd(0)-catalysed homocoupling of diorganyl tellurides (and ditellurides)

Palladium(0) converts symmetrical and unsymmetrical tellurides into coupled compounds under mild conditions.4 RTeR1

Pd(0) MeCN, (76-100%)

R-R1 + Te

R, R1 = p -MeOC6H4, PhCH2CH2 R = PhCH2CH2; R1 = PhCH2 R = p -MeOC6H4; R1= n -C15H31, t-adamantyl

The reaction is performed by treating the telluride (1 mmol) with the Pd(0) catalyst, prepared in situ from Pd(OAc)2 (1 mmol) and Et3N (2 mmol) in MeCN at 65°C, for 5–10 h under argon atmosphere.

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Diaryl ditellurides are converted into biaryls (Ar⫽p-MeOC6H4) by treatment with an additional equivalent of Pd(0) (HMPA as solvent). The above-described Pd-promoted detelluration is experimentally objectionable since it requires one or more equivalent of the catalyst. Further reports describe the use of the Ni(PEt3)4 system in the presence of phosphines as a successful catalyst for the detelluration of organotellurides and ditellurides.5 Ar2Te(or ArTeTeAr) 2.0 mmol

Ni(PEt3)4 10 mmol%

Ar-Ar +

N P

N

P=Te 3

3

MeCN, 80°C overnight 2,4 mmol for tellurides 5,0 mmol for ditellurides Ar2Te Ar = Ph, 3,4-(MeO)2C6H3, p -MeOC6H4, p -Me2NC6H4 ArTe)2 Ar = Ph, p -MeOC6H4, p -Tol, p -ClC6H4 79-90%

83-96%

Typical procedure.5 A mixture of di(p-methoxy)phenyltelluride (684 mg, 2 mmol) P(pyrro)3 (579 mg, 2.4 mmol) and Ni(PEt3)4 (106 mg, 0.2 mmol) in acetonitrile (5 mL) was heated at 80°C overnight (20 h). The reaction mixture was poured into 10 mL of 1 M HCl to liberate metallic tellurium instantly. Extraction using CH2Cl2, drying over MgSO4 and concentration afforded the crude product, which was subsequently passed through a short silica gel column (ethyl acetate/chloroform/hexane⫽0.5:1:8) to give pure 4,4methoxybiphenyl as a white solid (394 mg, 1.84 mmol, 92%). 4.6.1.3 Correlate reactions Some particular carbocyclic systems are obtained by pyrolitic tellurium extrusion.6,7 Te

175°C toluene (100%) Te

500°C / 0-5 torr

Te

4.6.1.4

500°C / 0-5 torr

Olefin arylation by Pd(II)-catalysed carbodetelluration of aryltellurium compounds

The Pd(II)-catalysed arylation of olefins by means of different aryltellurium(IV) chlorides has been more extensively investigated.8 The reactions are performed treating diaryltellurium dichlorides or aryltellurium trichlorides with excess olefin (5–10 mol equiv) in the presence of Pd(II) chloride–sodium acetate in refluxing acetic acid or acetonitrile. The yields are moderate to good with the dichlorides and poor with the trichlorides. Minor amounts of biaryls are the usual by-products beyond acetoxylated arylalkenes (formed by the addition of acetic acid to the olefins).

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197

The reactions can also be performed catalytically using Pd(II) chloride if a suitable oxidant such as t-butyl hydroperoxide or Cu(II) chloride is added to the system. PdCl2 / NaOAc

Ar2TeCl2 + R

Ar

R

HOAc or MeCN/ (41-98%)

Ar = Ph, p -MeC6H4, p -MeOC6H4; R = Ph, MeO2C, EtO2C, CN, CHO, Ac CN CH2OAc, or R = Me same conditions (3-38%)

ArTeCl3 + R

Ph

Ar

Ar = Ph, p -Me, p -MeOC6H4; R = Ph

The products exhibit the E configuration, with the exception of the acrylonitrile and methacrylonitrile derivatives. In the absence of the olefin, the corresponding biaryls are formed in moderate yields (like the Raney nickel-promoted detellurations (see Section 4.6.1.1)). Ar2TeCl2

PdCl2 /NaOAc

Ar-Ar

HOAc, (10-51%) Ar = Ph, p -Me, p -MeO, p -BrC6H4 ArTeCl3

same conditions (3%)

Ar-Ar

Ar = p -MeOC6H4

Arylation of styrene (catalytic in Pd(II); typical procedure).8 A mixture of Ph2TeCl2 (0.35 g, 1 mmol), styrene (1.04 g, 10 mmol), PdCl2 (0.03 g, 0.17 mmol) and CuCl2 (0.23 g, 1.7 mmol) in HOAc (20 mL) is refluxed for 2.5 h with stirring. After cooling to room temperature the black precipitate is filtered off, and the pale green filtrate is treated with brine (200 mL) and extracted with benzene. The extract is washed with aqueous NaHCO3 and then with brine, and dried (Na2SO4). GLC analysis using ethyl cinnamate as the internal standard shows the presence of trans-stilbene (35.1% based on Ph2TeCl2; 41.3% based on PdCl2) and a trace of biphenyl and unidentified compounds. 4.6.1.5

Pd(II)-catalysed cross-coupling reactions of aryl tellurides with alkenes

The cross-coupling reaction of diaryl tellurides with alkenes in MeOH in the presence of Pd2+ catalyst, Et3N and AgOAc as oxidant, gives the corresponding aryl-substituted (Z)alkenes in good yields.9 Ar2Te +

H H

PdCl2 / AgOAc R Et3N, MeOH, r.t.

H Ar

R

(40-99%) Ar = Ph, p -MeC6H4, p -MeOC6H4, p -BrC6H4 R = Ph, CO2Me, CO2Et, CN, CHO, CH2OAc, C(O)Me

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Typical procedure.9 Into a two-necked, 20 mL, round-bottomed flask containing PdCl2 (8.8 mg, 0.05 mmol), AgOAc (388 mg, 2.00 mmol) and di(p-methoxyphenyl) telluride (0.171 g, 0.50 mmol) were added dry methanol (10 mL), Et3N (0.202 g, 2.00 mmol) and styrene (0.104 g, 1.00 mmol). After the heterogeneous reaction mixture had been stirred at 25°C for 20 h, the solid part was filtered. The filtrate was poured into brine (200 mL) and extracted with diethyl ether (3×50 mL). GLC determination of the ether extract with diphenylmethane as an internal standard showed the presence of 0.99 mmol (99%) of (E)p-methoxystilbene. 4.6.1.6

Ni(II)- or Co(II)-catalysed cross-coupling of Grignard reagents with organic tellurides

Some organic tellurides react with Grignard reagents in the presence of Ni(II) or Co(II). Phosphine complexes give mixtures of cross-coupling products together with the homocoupling products of the telluride in good to moderate yields.10 Ar2Te (ArTeR) + RMgX

Ni(II) or Co(II) THF

ArR + ArAr + RR 44-97% 18-36% 24-54%

Ar = Ph, p -MeOC6H4 R = Ph, p -MeOC6H4, hexyl

4.6.1.7

Palladium- and copper-catalysed cross-coupling of organotellurium dichlorides with organostannanes and organoboronic acids

Diaryl and divinyl tellurium dichlorides undergo cross-coupling reactions with organostannanes11 and organoboronic acids,12 catalysed, respectively, by PdCl2 or Cul in the presence of Cs2CO3, and PdCl2(PPh3). The reaction was extended to carbonylative cross-coupling. R2TeCl2 + R1SnBu3-n

R2TeCl2 + R1B(OH)2

a) PdCl2(10%), MeCN, Cs2CO3 (2 equiv) r.t., 3 h b) CuI (10%), MeCN, Cs2CO3 (2 equiv), 70°C, 7 h c) PdCl2(PPh3)3 (10%), NaOMe (2 equiv)

(56-89%) R-R1 (38-81%)

DME / H2O, 50°C, 5 h

R = Ph, p -MeOC6H4, (Z )-PhCH=CH R1

a), b) = 2-furyl, 2-thienyl, (E )-PhCH=CH c) = Ph, m -NO2C6H4, o,p -Cl2C6H3, p -ClC6H4, p -MeOC6H4 R2TeCl2 + CO ( 1 atm) + R1SnBu3-n

a) or b)

RCOR1 (52-90%)

R = Ph, p -MeOC6H4, (Z )-PhCH=CH R1 = 2-furyl, 2-thyenyl, (E )-PhCH=CH

Typical procedure.12 To a mixture of diphenyltellurium dichloride (0.211 g, 0.60 mmol), PdCl2(PPh3)2 (42 mg, 10 mol%) and NaOMe (65 mg, 1.2 mmol) in DME/H2O (4:1) (3 mL) at 50°C was added m-nitro-phenylboronic acid (0.100 g, 0.60 mmol). The reaction

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mixture was stirred for 5 h at 50°C, and then extracted. The reaction mixture was extracted with diethyl ether (3×20 mL). The organic layer was dried over anhydrous sodium sulphate and evaporated in vacuo. The crude product was separated by SiO2 column chromatography (EtOAc/hexanes, 1:30, Rf ⫽0.42) to afford 3-nitrobiphenyl (91 mg, 76%). 4.6.1.8

Palladium-catalysed cross-coupling of organotellurium compounds with hypervalent iodonium salts

Diaryltellurium dichlorides can be readily coupled with iodonium salts in the presence of palladium catalyst (PdCl2, 10 mol%) and MeONa in MeCN/MeOH.13 Ar2TeCl2 + RIPh+]X-

PdCl2 (10 mol%), MeOH (3 equiv) MeCN / MeOH (1:1), r.t., 7 h

Ar-R (70-88%)

Ar = Ph, p -MeOC6H4 R = p -MeOC6H4, 2-thyenyl, (E )-PhCH=CH X = -OTs, -OTf, -BF4

Typical procedure.13 To a stirred solution of Ph2TeCl2 (716 mg, 1.40 mmol), PdCl2 (25 mg, 0.14 mmol) and NaOMe (220 mg, 4.20 mmol) in CH3CN/MeOH (20 mL) under nitrogen atmosphere was added p-methoxyphenyl(phenyl)iodonium tetrafluoroborate (556 mg, 1.40 mmol) at room temperature. The reaction mixture was stirred at room temperature for 7 h. The reaction mixture was extracted with diethyl ether (3×20 mL). The organic layer was dried over anhydrous sodium sulphate and evaporated in vacuo. The crude product was separated by SiO2 column chromatography (hexane, Rf ⫽0.17) to afford the coupled product (219 mg, 85%). 4.6.1.9

Detellurative carbonylation of organotellurium compounds: preparation of carboxylic acids

Aryltellurium trichlorides and diaryltellurium dichlorides react with nickel tetracarbonyl in DMF, giving benzoic acids. Small amounts of diaryl tellurides and diaryl ketones are by-products of these reactions.14 ArTeCl3 + 2 Ni(CO)4

1) DMF, 70°C, 24 h 2) H2O (52, 35%)

ArCO2H + NiCl2 + NiTe + 7 CO + HCl

Ar = p -MeOC6H4, 2-naphthyl Ar2TeCl2 + Ni(CO)4

same conditions (71, 58%)

2 ArCO2H + NiTe + 2 CO + 2 HCl

Ar = p-MeOC6H4, Ph

Detellurative carbonylation of aryltellurium trichloride: Ni(CO)4 method (typical procedure).14 Ni(CO)4 (2.6 mL, 20 mmol) is added to p-methoxyphenyltellurium trichloride (2.0 g, 5.9 mmol) in dry DMF (50 mL) under N2. After 24 h at 70°C the excess Ni(CO)4 is evaporated off in a stream of N2. The mixture is then poured into H2O (200 mL) containing 48% HBr (10 mL). The product is extracted with ether (3×75 mL), and the ether solution extracted with 2 M NaOH. The ether solution containing non-acid products is dried (CaCl2) and evaporated, giving (p-MeOC6H4)2Te (0.15 g, (15%)) and a trace amount of

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4. TELLURIUM IN ORGANIC SYNTHESIS

(p-MeOC6H4)2CO. The alkaline extract is acidified and extracted with ether. Evaporation of this ether extract gives p-MeOC6H4CO2H (0.45 g (52%)). Detellurative carbonylation of diaryltellurium dichlorides (typical procedure).14 Following the above procedure, Ni(CO)4 (1.5 mL, 11.5 mmol) and di-p-methoxyphenyltellurium dichloride (2.0 g, 4.8 mmol) furnish p-MeOC6H4CO2H (1.05 g (71%)), (p-MeOC6H4)2Te (0.25 g (15%)) and a trace amount of (p-MeOC6H4)2CO. Carboxylic acids are also produced through the detellurative carbonylation of several types of telluride by treatment with carbon monoxide at atmospheric pressure and room temperature in the presence of Pd(II) salts, in various solvents.∗,15 These reactions are of particular interest since the corresponding sulphur and selenium compounds give unsatisfactory results. Among the systems examined, that of PdCl2/Et3N/MeOH provides the best results, furnishing the methyl esters directly. A black precipitate is formed during the reaction. RTeAr

CO(1 atm) / PdCl2

Ar Ph Ph p -MeOC6H4

Et3N / MeOH

RCO2Me + ArCO2Me

R n -C12H25 Ph p -MeOC6H4

A

B Percentage yield A B 22

98 92

99

These reactions can be performed catalytically in PdCl2 if a suitable reoxidant such as CuCl2 is present. Detellurative methoxycarbonylation of diorganyl tellurides. (a) PdCl2 stoichiometric method (general procedure).15 PdCl2 (0.177 g, 1.0 mmol) and the telluride (1.0 mmol) are flushed with CO from a CO balloon connected to the reaction flask at 25°C, to which dry MeOH (10 mL) and Et3N (0.202 g, 2.0 mmol) are added by a syringe. After stirring for 5 h at 25°C the black precipitate formed is filtered off, and the filtrate is poured into aqueous NH4Cl and extracted with ether (3×30 mL). The products are determined by GLC (using an EGSS-X 3% (1 m) column). (b) PdCl2 catalytic procedure. PdCl2 (0.017 g, 0.1 mmol), CuCl2 (0.270 g, 2.0 mmol) and the telluride (1.0 mmol) are flushed with CO from a CO balloon connected to the reaction flask at 25°C, to which dry MeOH (10 mL) and Et3N (0.303 g, 3.0 mmol) are added by a syringe. After stirring for 75 h at 25°C, during which time the green colour turns dark brown, the brown solid is filtered off. The filtrate is poured into aqueous NH4Cl and extracted with ether (3×30 mL). The products are isolated by preparative TLC or Kügelrohr distillation, or determined by GLC (using an EGSS-X 3% (1 m) column with an appropriate inner standard). *Similar reactions with vinyl tellurides will be discussed in Section 4.9.3.5.

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201

Synthesis of enones and cyclopropanes from bis(oxoalkyl)tellurium dichlorides

The sequential treatment of bis(2-oxomethyl)tellurium dichlorides with 2 equiv of LDA in THF at −78°C and 2 equiv of an aldehyde, followed by heating at 25°C, affords the enones with E geometry in good yields. The same reaction performed with methyl vinyl ketone gives rise to the cyclopropane.16 These reactions involve the intermediacy of a bis-ylide which undergoes a Wittig-type reaction with aldehydes or a Michael 1,4-addition to the enone.

R

O Cl Cl O Te

O

LDA 2 equiv R

O O

R = Me, i -pr, i -Bu, t -Bu, Ph, p -MeOC6H4 R1 = Ph, n-C8H17 O

2.5 equiv + R Te O

R R = t -Bu

O

R1CHO 2 equiv THF, -78 -25°C 3h

R

Te

R

THF, -78°C 30 min

R R1 (42-89%)

R O

O 69%

O-

Moreover, if R is an aryl or t-butyl group, by warming at room temperature after the treatment with LDA at −78°C, triacyl-substituted cyclopropanes are formed via a Michael addition involving the intermediates bis-ylide and bis-acylethylene.16

R

O Cl Cl O Te

O

LDA 2 equiv R

THF, -78°C 30 min

R

O

R

Te

R

R

O

O

R = t -Bu, Ph, p -MeOC6H4, 1-adamanthyl, 1-MeC-prop

R

O O

O R R (46-89%)

4.6.1.11

Conversion of telluroesters into ketones

By treatment with dialkylcuprates, telluroesters are easily converted into ketones in high yields.17 RCOTePh

R2CuLi / ether, -78°C

ArCOR

R = Me (1.1 equiv; 15 min, 87%) R = n -Bu (1.2 equiv; 30 min, 97%)

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4. TELLURIUM IN ORGANIC SYNTHESIS

Replacement of the tellurium moiety by other functionalities By amino group – allylic amine by imination of allylic tellurides

Allylic phenyl tellurides are converted into the corresponding allylic amines by imination with [N-(p-toluene-sulphonyl)imino] phenyliodinane. The reaction proceeds via [2,3]-sigmatropic rearrangement of a tellurimide intermediate. Similar results are obtained with chloramine-T (TsNClNa).18 R1 R3 R R2

PhI=NTs TePh EtOH, 25°C 20 h

R

TePh

[2,3]-sigmatropic

Ts

NTs

NH N

TePh

EtOH

R2, R3 = H; R = Me CH2 R1 =

R = Ph; R1, R2, R3 = H R, R1 = Me; R2, R3 = H R2, R3 = H; R1 = Me CH2 R=

R

R NHTs

R, R2 = H R1, R3 = (CH2)3

(59-85%)

Typical procedure.18 Phenylcinnamyl telluride (prepared from sodium phenyl tellurolate, 1 mmol, and cinnamyl bromide, 1 mmol in EtOH, 10 mL) was treated with Phl=NTS (1.5 mmol) and the mixture was stirred at 25°C for 20 h. After removal of the solvent, the product was purified by column chromatography on SiO2 (eluent: hexane/AcOEt). Chiral allylic amines are isolated with high enantiomeric excess (ee), by submitting chiral ferrocenyltellurides to the above-described protocol. Br + (Fc*Te)2

Ph

Fc* =

NaBH4 EtOH r.t., 1-2 h

Ph

Fe

TeFc*

PhI=NTs 0°C, 24 h

Ph NHTs 45% (93% ee)

Me Me2N H (S, R )

This chirality transference parallels that observed in the oxidation of chiral allylic ferrocenyl tellurides (see Section 4.7.1). 4.6.2.2

By hydroxy group – hydrolysis of telluroesters to carboxylic acids and esters

Telluroesters can be easily hydrolysed or converted into the corresponding oxygenated esters by treatment respectively with CuCl2 dihydrate in acetone or anhydrous CuCl2 in the appropriate alcohols.19 O R

O CuCl2.2H2O TeBu-n acetone, r.t.

R OH (65-96%)

R = Ph, p -ClC6H4, p -MeOC6H4, C6H5CH2, Me, t-Bu

CuCl2 R1OH O R OR1 (68-88%)

R = Ph, p -MeOC6H4, p -ClC6H4, C6H5CH2 R1 = Et, Me, n -Bu

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Typical procedure.19 To a solution of the telluroester (R⫽p-MeOC6H4) (1.6 g, 5 mmol) in dry acetone (40 mL) under stirring at room temperature was added CuCl2⋅H2O (1.11 g, 6.5 mmol). A sudden formation or a yellow-orange precipitate was observed and the reaction stirred for 5 min. The mixture was filtered, diluted with ethyl ether and washed with a 10% sodium hydroxide solution (2×50 mL). The aqueous phase was acidified with concentrated HCl and extracted with ethyl ether. The organic phase was dried over MgSO4 and evaporated. The residue was identified by 1H NMR, IR and gas chromatography-mass spectrum (GC-MS) as the anysic acid. Yield: 1.23 g (77%), m.p.184°C. 4.6.2.3

By halogens

The transformation of a carbon–tellurium bond into a carbon–halogen bond has been achieved in several types of organotellurium compound. (a) Halogenodetelluration of vinylic tellurium trichlorides By treatment of (Z)-2-chlorovinyltellurium trichlorides, easily obtained by addition of tellurium tetrachloride to phenylacetylenes (see Section 3.16.2.1), with 1–2 mol equiv of iodine or N-bromosuccinimide-aluminium trichloride, a halogenodetelluration occurs, generating the corresponding (Z)-iodo- or (Z)-bromochloroalkenes.20 I2(1-2 equiv) MeCN, reflux, 2 h (60-94%) Ph

R + TeCl4

CCl4 70°C, 1 h

Ph

R

Cl

TeCl3

Ph

R

Cl

I

R = H, Me, Et, Ph AlCl3 / NBS CCl4, reflux, 2 h (60-70%)

Ph

R

Cl

Br

R = H, Me

The chlorotelluration–iododetelluration of propargyl alcohol proceeds analogously. H

H OH

I

OH Cl

This two-step procedure therefore offers a useful method for the syn-iodo and synbromochlorination of acetylenes. (b) Halogenodetelluration of aryltellurium(IV) halides Aryltellurium trichlorides bearing a para-electron-releasing group undergo substitution of the trichlorotelluro group by iodine, furnishing the corresponding iodoarenes.21 The reaction is greatly enhanced by the addition of potassium, cesium or ammonium fluoride. Diaryltellurium dichlorides (and dicarboxylates) react similarly but with lower yields. -

R

TeCl3

R = MeO, Me

I2 / F / MeCN, reflux (85, 46%)

R

I

I2 / F-/ MeCN, reflux (23%)

TeCl2

R

2

R = MeO

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Similar bromodetellurations afford o,p-dibromo derivatives as the main product (formed by a further bromination of the primary p-bromo derivative under Te(IV) catalysis). The addition of F− ions is unnecessary. Br R

TeCl3

Br2 / MeCN / 35-45°C (85%)

Br + R

R

Br trace

R = MeO

(76%) Br2 / MeCN 40-45°C R

TeCl2 2

An electrophilic mechanism is plausible for these reactions, on the basis of the dependence on para-electron-releasing groups and the enhancement by F− ions (forming ArTeCl3F22− or Ar2TeCl2F22− species which make the electrophilic attack by halogens easier). Iododetelluration of aryltellurium trichlorides (typical procedure).21 A mixture of pmethoxyphenyltellurium trichloride (0.34 g, 1 mmol), iodine (1.01 g, 4 mmol) and NH4F (0.15 g, 4 mmol) in CH3CN (20 mL) is stirred under reflux for 5 h. After cooling to room temperature the mixture is treated with brine (200 mL) and extracted with benzene (3×50 mL). The extracts are washed with 10% aqueous Na2S2O3 and then with brine, and dried (Na2SO4). GLC analysis, using p-iodotoluene as the internal standard, shows the presence of p-methoxyiodobenzene (85%). Bromodetelluration of aryltellurium trichlorides (typical procedure).21 A mixture of p-methoxyphenyltellurium trichloride (0.68 g, 2 mmol) and bromine (0.64 g, 4 mmol) in MeCN (20 mL) is stirred at 35–45°C for 20 h. After work-up as described above, GLC analysis using ethyl cinnamate as the internal standard shows the presence of o,p-dibromop-methoxybenzene (69.5%) and a trace of p-methoxybromobenzene. (c) α-Elimination of organic halides from organotellurium(IV) halides Organotellurium tri- and dihalides undergo α-elimination by oxidative, photolytic or thermal routes, giving the corresponding halides with a selective transference of the halogen at the position where the tellurium moiety was originally attached (ipso-substitution). The oxidative procedure A22,23 is based on the treatment with oxidants, preferably t-butyl hydroperoxide in refluxing dioxane, acetic acid or acetonitrile. The photolytic procedure B23,24 is performed by irradiation in benzene with a highpressure mercury lamp, at room temperature, in the presence of atmospheric oxygen. The reaction is accompanied by deposition of elementary tellurium. In the thermal procedure C the substrate is heated at 230–250°C under reduced pressure (in a Kügelrohr distillation apparatus) (method C1)20 or heated in DMF at 70–100°C in the presence of an additional alkali metal halide (method C2).25 The tellurium moiety is isolated as diphenyl ditelluride, despite the formal formation of phenyltellurenyl halide.

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α-Elimination of organic halides from organotellurium(IV) halides Cl TeCl3

R

Cl

(A)

Cl

(40%, 70%) R

(1)

R = n -C8H17, PhCH2 Cl

Cl

(A) (92%)

(2)

(B) (29%)

TeCl3

Cl

KI Cl

Cl

(A)

(3)

(64%)

Tel3

I

(A) Ph R TeCl3 (A) R = Ph, H (20%, 30%) Cl Cl (B) R = H (38%) (A) TeCl3 R (B) (A) R = MeO, Me (53%, 25%) (B) R = MeO, Me (81%, 36%)

Ph

R

Cl R

TeCl3

(4)

(5)

Cl

Cl TeCl3

(A) (42%) (A)

R

R

TeCl2

Cl

(A) (60%)

(6)

(7)

Cl

2

(B)

R = MeO, H, Br

(4-37%) Cl

(A) Te Cl Cl

OMe

R

Cl (55%)

(C1)

TeBr2Ph

PhTeX2CHRR1

(58, 70%)

(C1) (83-98%)

R

+ MeO

Cl

(8)

(22%)

R = MeO, H

(9)

Br

RR1CHX + [PhTeX] (10)

(C2) (59-95%) C1

C2

X = Br R = H; R1 = C11H23, PhCH2 R = Me; R1 = C6H13, C12H25 X = Cl, Br, I

R = H; R1 = C10H21, C11H23, C13H27, C15H31, PhCH2, PhCH2CH2 R = Me; R1 = C11H23

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The following remarks are pertinent: • • • • •

Retention of the configuration is observed in all the reactions involving the cyclohexyl and alkenyl derivatives. By comparison of the yields of the reactions shown in the scheme, the following order for the trichlorides has been assessed for method A: alkyl >aryl >alkenyl. Diaryltellurium dichlorides are unreactive by method A, and by method B furnish lower yields than aryltellurium trichlorides (eq. (7)). Eq. (8) shows that the cycloalkyl group is more reactive towards the oxidative α-elimination than the aryl group. By method B, the reactivity order of the trichlorides is aryl>alkenyl>alkyl (eqs. (5), (2) and (4)).

The described oxidative α-elimination reactions have been rationalized as proceeding through a 1,2-tellurium–halogen shift. X Te X X

O

X

Te X X

O-

X -Te(O)X2 Te X + X

X

Organic halides from organotellurium(IV) halides (typical procedures). Method A.23 To 1-chlorocyclohexyltellurium trichloride (0.35 g, 1.0 mmol) in dioxane (5 mL) is added t-BuOOH (70%, 0.26 g, 2.0 mmol) and the mixture stirred under reflux for 30 min, during which period a pale yellow solid precipitates. After cooling at room temperature, the precipitate is filtered off, and the filtrate poured into brine and then extracted with ether (3×30 mL). The ether extract is dried (MgSO4), and GLC analysis reveals the presence of 1,2-dichlorocyclohexane (0.14 g (92%), cis/trans = 7:93) (using phenyl acetate as the internal standard). Method B.23 A solution of p-methoxyphenyltellurium trichloride (0.34 g, 1.0 mmol) in benzene (200 mL) is irradiated with a high-pressure mercury lamp in the presence of atmospheric oxygen for 1 h (formation of a black precipitate). The mixture is filtered and the filtrate is washed with brine and then dried (MgSO4). GLC analysis shows the presence of p-chloromethoxybenzene [0.14 g (70%)] (and minor amounts of o,p-dichloro-methoxybenzene and p-methoxybiphenyl) using p-chlorotoluene, 2,4-dichlorotoluene and p-methylbiphenyl as the internal standard, respectively. Method C1.23 Neat n-dodecylphenyltellurium dibromide (0.53 g, 1.0 mmol) is heated at 230–250°C at 3 torr using a Kügelrohr distillation apparatus. A colourless liquid distills which is identical to n-dodecyl bromide (1H NMR and GLC analysis) [0.25 g, (90%)]. (A red-brown residue seems to be mainly PhTeBr and on standing exposed to the ambient atmosphere solidifies to a grey solid PhTe(O)Br and the starting dibromide by IR and combustion analytical data.) Method C2.25 Hexadecylphenyltellurium dichloride (0.199 g, 0.40 mmol) and NaCl (0.030 g, 0.51 mmol) are heated in DMF (2 mL) under nitrogen atmosphere, with stirring at

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100°C for 1 h. The mixture is quenched with H2O and extracted with hexane. The extract is dried (MgSO4) and filtered through a short column of SiO2 to give 1-chlorohexadecane as a colourless oil (0.096 g (92%)). Since alkyl phenyl tellurides are easily prepared from alkyl halides and phenyltelluromethyllithium, procedure C constitutes a homologation of alkyl halides. PhTeCH2Li

RX

PhTeCH2R

X2

RCH2X

Similarly, 1,1-dibromoalkanes can be prepared from lithium diphenyltelluromethane, whereas homologated aldehydes are obtained via iodination (probably through the hydrolysis of an intermediate diiodide).25 RX LiCH(TePh)2

(PhTe)2CHR

Br2/NaBr

RCHBr2

(76-85%)

I2 / NaI

RCHO

(77-93%)

R = n-C11H23, n-C12H25, n-C14H29, n-C16H33, Ph(CH2)3

An alternative iodination method is provided by treatment of the telluride with methyl iodide and sodium iodide in DMF.25 PhTeR + MeI

Ph + NaI TeMe IDME, 55°C R (75-90%)

RI + PhTeMe

R = n -C11H13, n -C12H25, n -C14H29, n -C16H33, Ph(CH2)2, Ph(CH2)3, C12H25CHCH3

Iodination of alkyl phenyl telluride with MeI/NaI (typical procedure).25 MeI (0.20 mL, 3.21 mmol) and NaI (75 mg, 0.50 mmol) are successively added to a solution of 1-phenyltellurododecane (149 mg, 0.40 mmol) in DMF (2 mL) under N2. The mixture is stirred for 10 min at room temperature and then for 2 h at 55°C. It is quenched with H2O and extracted with hexane. The extract is dried (MgSO4) and filtered through a short column of SiO2 to give 1-iodododecane as a colourless oil (103 mg (87%)). (d) Halogenodetelluration of α-dichloroaryltelluroketones Reaction of α-dichloroaryltelluroketones (see Section 3.9.3.1) with equimolar amounts of chlorine in dichloromethane at room temperature leads to the corresponding α-chloroketones in good yields, accompanied by the corresponding aryltellurium trichlorides.26 O TeCl2Ar

R R

1

O

Cl2 / CH2Cl2, r.t., 2-5 h (68-100%)

Cl + ArTeCl3

R R

1

1

R = H; R = Ph, t -Bu R, R1 = (CH2)3, (CH2)4, (CH2)5 Ar = p -MeOC6H4

Similar reactions with bromine produce a mixture of α-chloro- and α-bromoketones.

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4. TELLURIUM IN ORGANIC SYNTHESIS

Reaction of α-dichloroaryltelluroketones with chlorine: α-chloroketones (general procedure).26 To a solution of the α-(dichloroaryltelluro)ketone (2 mmol) in CH2Cl2 (10 mL) is added a solution of Cl2 (2 mmol) in the same solvent. The mixture is vigorously stirred until precipitation of the aryltellurium trichloride, which is then filtered off. The filtrate is washed with H2O (25 mL) and then with brine (2×30 mL). The organic phase is dried (MgSO4) and evaporated in a rotatory evaporator. The residue is purified by preparative TLC on SiO2 (eluting with petroleum ether 30–60°C). (e) Halogenodetelluration of acetylenic tellurides Acetylenic tellurides (see Section 3.17), on treatment with 3 mol equiv of bromine or iodine, undergo halogenolysis, accompanied by addition of halogen to the acetylenic carbon–carbon bond, giving trihaloalkenes and organyltellurium trihalides.27 TeR1

R

X2 / CH2Cl2 / benzene r.t., 20-30 min

R

X

X

X

+ R1TeX3

X = Br (51-76%); X = I (73, 74%) X = Br; X = I;

R = n -C5H11, Ph, p -BrC6H4; R1 = n -C4H9 R = Ph; R1 = p -MeOC6H4 R = n -C5H11, Ph; R1 = n -C4H9

Reaction of acetylenic tellurides with iodine (typical procedure).27 To a solution of 1-(butyltelluro)heptyne (1.40 g, 5 mmol) in CH2Cl2 (10 mL) is added I2 (3.81 g, 15 mmol). The pale yellow colour of the solution turns dark red and a precipitate is formed. The mixture is stirred at room temperature for 30 min and then the solvent is evaporated. The residue is filtered through an SiO2 column (eluting with hexane), giving 1,1,2-triiodoheptene (1.76 g (74%)). ( f ) Correlate reactions Dibenzyl ditelluride, benzyl telluride28,29 and benzyl tellurocyanate30 generate benzyl bromide on treatment with excess bromine.

4.6.2.4

Ph

TeTe

Ph

Ph

Te Ph

Ph

TeCN

Br2

Br2 -Te

Ph

2 Br2 Te Ph -TeBr 4 Br Br

2 Ph

Br

Br2 Ph

Br2 TeCN -BrCN Br Br

Ph

TeBr3

Br2 -TeBr4

Ph

Br

By the methoxy group

(a) Detellurative methoxylation Oxidation of alkyl phenyl telluride with excess meta-chloroperbenzoic acid (MCPBA) (3–5 equiv) in methanol affords the replacement of the phenyltellurium moiety by a methoxy group, giving the corresponding methyl ethers31,32 (method A). This reaction,

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which also proceeds by using alkyl phenyl selenides, therefore contrasts sharply with the well-known selenoxide (and with the less familiar telluroxide) elimination leading to olefins. Improved yields are obtained by the similar treatment of telluroxides (prepared by hydrolysis of the parent dibromides)33 and different alcohols such as ethanol, propanol and isopropanol can be employed to give the corresponding ethers (method B). The detellurative methoxylation of the telluroxides also proceeds in high yields by treatment with trifluoroperoxyacetic acid (generated in situ from TFA and H2O2)32 (method C). Finally, alkyl phenyl tellurones (prepared by oxidation of telluroxides with NaIO434 are susceptible to similar conversions32 (method D). (method A) MCPBA (3-5 equiv) / MeOH, r.t. RTePh ROMe ( 45-86%) R = n -C12H25 , PhCH2CH2, n -C12H25CHCH3, OMe cycloheptyl,

(1) Br2/CCl4 -

(2) OH /H2O

RTe(O)Ph.H2O

(method B) MCPBA (2 equiv)/R1OH, r.t. ROR1 (65-95%) R = n-C12H25; R1 = Me, Et, n-Pr, i-Pr R = C8H17CH(OMe)CH2; R1 = Me

NaIO4 MeOH/H2O

(method C) F3CCO2H/H2O2 / MeOH, r.t.

ROMe

(88%) R = n-C12H25 O R TePh O

(method D) MCPBA (2 equiv)/MeOH, r.t.

ROMe

(87%) R = n-C12H25

In the above procedures, the telluroxide syn-elimination (see Section 4.7.1) was observed as a competitive interference only in the case of cycloheptyl phenyl telluride, where cycloheptene is formed in a yield of 45%. When a phenyl group is linked at a vicinal position to tellurium, the replacement of tellurium by the methoxy group is accompanied by phenyl migration.31,32 R Ph 1 R

TePh

MCPBA (1 equiv) MeOH, r.t. (73-90%)

R MeO 1 R

Ph

H3O+ R1 = MeO

O R

Ph

R = H; R1 = Me, MeO R = Me; R1 = MeO (1) Br2 /CCl4 (2) HO- /H2O

R Ph 1 R

Te(O)Ph

MCPBA (1 equiv)/ MeOH,r.t. (73-84%)

R = H, Me; R1 = OMe

(2-Methoxy)cyclohexyl phenyl telluride and cycloheptyl phenyl telluride (easily accessible by methoxy telluration of the corresponding cycloalkenes, see Section 4.4.8.3) and

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(2-hydroxy)cyclohexyl phenyl telluride (prepared by the opening of cyclohexenoxide with the phenyltellurolate ion, see Section 3.1.3.2) are converted by the above-described procedure into the dimethyl acetals of the ring-contracted aldehydes, in contrast, therefore, to the detellurative methoxylation of the tetrahydronaphthalene derivative (see above). The difference in stability between the cyclic six- and seven-membered methoxy telluroxides is noteworthy. Indeed, while the cyclohexane derivative is stable and isolable, giving ring contraction on treatment with 1 equiv of MCPBA, the cycloheptane derivative is unstable, suffering telluroxide elimination (like cycloheptene formation from cycloheptyl phenyl telluride), as will be shown in Section 4.7. MCPBA (2-5 equiv) MeOH, r.t. (77, 90%) TePh n=1 R = Me, H

OR ( )n

OMe

( )n CHO

MCPBA (1equiv)/ MeOH,r.t. n = 1 (84%) Te(O)Ph

( )n

MCPBA (1 equiv)/MeOH, r.t. n=2 R = Me

( )n

H3O+

OR

(1) Br2 /CCl4 (2) HO- /H2O

OMe

OR

OR

TePh -PhTeOH (40%) O

Detellurative methoxylation of alkyl phenyl tellurides (typical procedure).32 2-Methoxy 3-phenyltelluro-1,2,3,4-tetrahydronaphthalene (1.85 g, 5 mmol) is treated with MCPBA (3.3 g, 15 mmol) in MeOH (30 mL) at 25°C with stirring. After 1 h the mixture is treated with aqueous Na2S2O3 followed by aqueous NaHCO3 and extracted with ether (3×30 mL). The extract is dried (MgSO4) and evaporated, to leave a residue which is purified by SiO2 TLC (eluting with hexane/EtOAc, 4:1) to afford pure trans-2,3-dimethoxy-1,2,3,4-tetrahydronaphthalene (0.37 g, 1.94 mmol) and the cis-isomer (0.15 g, 1.76 mmol (54%)). Detellurative methoxylation of alkyl phenyl telluroxides (typical procedure).32 To a heterogeneous solution of dodecyl phenyl telluroxide (0.82 g; 2 mmol) in MeOH (10 mL) is added solid MCPBA (80% purity, 0.88 g, 4 mmol) at 25°C with stirring. The resulting solution becomes homogeneous after 1 h, when it is treated with aqueous Na2S2O3 followed by aqueous NaHCO3. The mixture is treated with ether (3×30 mL), and the extract dried (MgSO4) and evaporated to leave a pale yellow oily residue which is purified by SiO2 column chromatography (eluting with hexane/EtOAc, 5:1) to afford pure dodecyl methyl ether (0.38 g (95%)). (b) Correlate reaction – synthesis of α-arylpropanoic acids The title compounds, important anti-inflammatory pharmaceuticals, can be synthesized starting from aromatic ketones, as depicted in the accompanying scheme, involving the above-described detellurative methoxylation in the key step.35,36 In an analogous sequence using the corresponding selenides, the yield of the substitution of the bromine for the phenylseleno group is higher. This result depends on the

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self-oxidation of the telluride to the telluroxide, followed by elimination to the vinylic derivative. O (1) Br2 /HOAc

Ar

OH

(2) HO

O Ar

O Ar

Br

(PhTe)2Na THF/DMF

O Ar

O

MCPBA MeOH, r.t., 1 h

TePh H2O (58-75%)

O

O

Ar HO(81-91%)

O

HO

O

Te(O)2Ph

Ar

CO2H

Ar = Ph, p-Me, p-i-Bu, p-Ph, p-BrC6H4, MeO

4.6.2.5

Reductive detelluration of tellurides by triphenyltin hydride

Phenyl alkyl tellurides are reduced by triphenyltin hydride under mild conditions, giving the corresponding hydrocarbons in high yields.37 An excess of the reagent is necessary (2–2.8 equiv), but a radical initiator need not be employed. RTePh

Ph3SnH (2-2.8 equiv), 25-80°C benzene

RH + Ph3SnTePh

R = C12H25, CH3CHC10H21, C8H17CH(OH)CH2, C8H17CH(OAc)CH2, C3H7CH(OH)CHC3H7, 5-α-cholestanyl

The corresponding tellurium dichlorides can also be employed as starting materials, and although a greater excess of the reagent is needed (3.5–4 equiv) the reactions are faster. These reductive cleavages can be rationalized through a radical mechanism. Ph3SnH

Ph3Sn.

Ph3Sn. + RTePh

RSnTePh + R.

R. + Ph3SnH

RH + PhSn.

Since the starting tellurides are easily prepared from alkyl halides or epoxides by displacement with tellurolate anions (see Section 3.1.3.2), the overall sequence constitutes a mild reduction of these substrates and is advantageous over the analogous reductions of selenides, which require more severe conditions (a temperature of 120°C is necessary). The example in sequence, where a carbonyl function remains untouched, clearly demonstrates the value of this method. O O

PhTe-

O OH TePh (70%)

Ph3SnH

O OH

(70%)

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4. TELLURIUM IN ORGANIC SYNTHESIS

Reduction of alkyl phenyl tellurides to alkanes (general procedure).37 All the operations are executed under red light using a one-piece apparatus (condenser fused to the flask) to avoid loss of material, closed with a septum through which are inserted inlet and exit needles for N2. The telluride is weighed directly in the flask, benzene is introduced from a syringe, and N2 is swept for ∼5 min through the system, which is then closed (by removing the exit needles) to ensure a slight static pressure. Ph3SnH is injected into the mixture and the reaction mixture maintained at room temperature or heated under reflux. The course of the reaction is monitored by TLC. The product is isolated by chromatography and/or distillation. In a typical experiment, n-dodecyl phenyl telluride (0.160 g, 0.42 mmol) in benzene (1 mL) is treated with Ph3SnH (0.361 g, 1.03 mmol) during 5 h at room temperature, furnishing n-dodecane (0.080 g (87%)). Further examples of reductive detelluration of tellurium compounds have been shown in connection with tellurolactonization and telluroetherification reactions (see Section 4.5).

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

(a) Bergman, J. Tetrahedron 1972, 28, 3323. (b) Bergman, J. Org. Synth. 1977, 57, 18. Petit, G. R.; Van Tamelen, E. E. Org. React. 1962, 12, 409. Bergman, J.; Engman, L. Tetrahedron 1980, 36, 1275. Barton, D. H. R.; Ozbalik, N.; Ramesh, M. Tetrahedron Lett. 1988, 29, 3533. Han, L. B.; Tanaka, M. Chem. Commun. 1998, 47. Cuthbertson, E.; MacNicol, D. D. J. Chem. Soc. Chem. Commun. 1974, 498. Cuthbertson, E.; MacNicol, D. D. Tetrahedron Lett. 1975, 1893. Uemura, S.; Wakasugi, M.; Okano, M. J. Organomet. Chem. 1980, 194, 277. (a) Nishibayashi, Y.; Cho, S. C.; Uemura, S. J. Organomet. Chem. 1996, 507, 197. (b) previous reports: Kawamura, T.; Kikukawa, K.; Takagi, M.; Matsuda, T. BuII. Chem. Soc. Jpn. 1977, 50, 2021. Uemura, S.; Fukuzawa, S. I.; Patil, S. R. J. Organomet. Chem. 1983, 243, 9. Kang, S. K.; Lee, S. W.; Ryu, H. C. Chem. Commun. 1999, 2117. Kang, S. K.; Hong, Y. T.; Kim, D. H.; Lee, S. W. J. Chem. Res. (S) 2001, 283. Kang, S. K.; Lee, S. W.; Kim, M. S.; Kwon, H. S. Synth. Commun. 2001, 31, 1721. Bergman, J.; Engman, L. J. Organomet. Chem. 1979, 175, 233. Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. J. Org. Chem. 1987, 52, 4859. Han, L. B.; Kambe, N.; Ryu, I.; Sonoda, N. Chem. Lett. 1993, 561. Sasaki, K.; Aso, Y.; Otsubo, T.; Ogura, F. Chem. Lett. 1986, 977. Nishibayashi, Y.; Srivastava, S. K.; Ohe, K.; Uemura, S. Tetrahedron Lett. 1995, 36, 6725. Dabdoub, M. J.; Viana, L. H. Synth. Commun. 1992, 22, 1619. Uemura, S.; Miyoshi, H.; Okano, M. Chem. Lett. 1979, 1357. Uemura, S.; Fukuzawa, S. I., Wakasugi, M.; Okano, M. J. Organomet. Chem. 1981, 214, 319. Uemura, S.; Fukuzawa, S. I. J. Chem. Soc. Chem. Commun. 1980, 1033. Uemura, S.; Fukuzawa, S. I. J. Organomet. Chem. 1984, 268, 223. Uemura, S.; Fukuzawa, S. I. Chem. Lett. 1980, 943. Chikamatsu, K.; Otsubo, T.; Ogura, F.; Yamaguchi, H. Chem. Lett. 1982, 1081. Stefani, H. A.; Chieffi, A.; Comasseto, J. V. Organometallics 1991, 10, 1178. Dabdoub, M. J.; Comasseto, J. V.; Barros, S. M.; Moussa, F. Synth. Commun. 1990, 20, 2181.

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SYNTHESIS OF OLEFINS

213

28. 29. 30. 31. 32. 33. 34.

Vicentini, G. Chem. Ber. 1956, 91, 801. Spencer, H. K.; Cava, M. P. J. Org. Chem. 1977, 42, 2937. Spencer, H. K.; Lakshmikantham, M. V.; Cava, M. P. J. Am. Chem. Soc. 1977, 99, 1470. Uemura, S.; Fukuzawa, S. I. Tetrahedron Lett. 1983, 24, 4347. Uemura, S.; Fukuzawa, S. I. J. Chem. Soc. Perkin Trans. 1 1985, 471. Detty, M. R. J. Org. Chem. 1980, 45, 274 (see also Section 3.10.2). (a) Lee, H.; Cava, M. P. J. Chem. Soc. Chem. Commun. 1981, 277. (b) Engman, L.; Cava, M. P. J. Chem. Soc. Chem. Commun. 1982, 164. 35. Uemura, S.; Fukuzawa, S. I.; Yamauchi, T.; Hattori, K.; Mizutaki, S.; Tamaki, K. J. Chem. Soc. Chem. Commun. 1984, 426. 36. Uemura, S.; Fukuzawa, S. I. J. Chem. Soc. Perkin Trans. 1. 1986, 1983. 37. Clive, D. L. J.; Chittattu, G. J.; Farina, V.; Kiel, W. A.; Menchen, S. M.; Russel, C. G.; Singh, A.; Wong, C. K.; Curtis, N. J. J. Am. Chem. Soc. 1980, 102, 4438.

4.7 4.7.1

SYNTHESIS OF OLEFINS

By telluroxide elimination

The telluroxide elimination appeared initially to be of little value compared to the popularity of the analogous selenoxide elimination. However, after the previous results which established the syn stereochemistry of the elimination,1 more detailed investigations clearly demonstrated the synthetic utility of this methodology.2–4 1 2 3 R R R R

PhTeO

H

R R

1

R2 R3

+ PhTeOH

The starting telluroxides (see Section 3.1) are prepared by hydrolysis of the corresponding tellurium dibromides3 (method a) or by direct oxidation of the tellurides (MCPBA is the more appropriate oxidizing agent4) (method b). By the first method the telluroxides are obtained as hydrates and normally higher elimination yields are achieved. s-Alkyl phenyl telluroxides and cycloalkyl phenyl telluroxides, with the exception of cyclohexyl derivatives, are unstable compounds, suffering the elimination reaction at room temperature. In contrast, n-alkyl phenyl telluroxides and cyclohexyl phenyl telluroxide are stable compounds and undergo elimination only by prolonged heating in toluene or THF, or by pyrolysis at 200–240°C. The addition of Et3N and other amines exhibits a remarkable effect on the telluroxide elimination, improving the alkene yields, suppressing the formation of side products and promoting the elimination even from primary alkylphenyl telluroxides.5 General procedure for telluroxide elimination by oxidation of alkyl phenyl tellurides in the presence of base.5 To a two-necked, round-bottomed flask (25 mL) containing alkyl phenyl telluride (1 mmol), triethylamine (1–2 mmol) and diethyl ether (5 mL) was added solid MCPBA (purity 80%) (2 mmol as pure MCPBA) portionwise at 25°C. The mixture was stirred with a magnetic stirrer for 2 h at the same temperature before being poured into

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4. TELLURIUM IN ORGANIC SYNTHESIS

saturated aqueous Na2CO3 (100 mL) containing hydrazine (10 mL). It was then stirred for 0.5 h to remove the resulting meta-chlorobenzoic acid (MCBA) and to reduce the remaining MCPBA and telluroxide to MCBA and telluride, respectively. The solution was extracted with diethyl ether (3×50 mL), the extract dried over MgSO4 and analysed by GLC using a suitable internal standard. In the case of 2-hydroxyalkyl phenyl tellurides, the product was isolated by column chromatography on SiO2 (hexane/ethyl acetate, 9:1). Moreover the 2-pyridyltelluro moiety was shown to be a better leaving group than the usual phenyltelluro moiety.5 The following important features should be noted. Since the starting tellurides are easily prepared (see Section 3.1.3.2) from the corresponding alkyl bromides and tellurolate ions, and β-hydroxyalkyl tellurides by the opening of epoxides with the same reagents, the combined procedures furnish a method for the dehydrobromination of alkyl bromides and for the conversion of epoxides into allylic alcohols. Moreover, combining the telluroxide elimination with the methoxytelluration of olefins (see Sections 3.9.3.2 and 4.4.8.3), allylic and vinylic ethers are easily prepared. The telluroxide elimination shows a preference towards the less substituted carbon (as observed by the 2.48–2.50:1 ratio assessed in the elimination of 2-bromooctyl phenyl telluroxide and 2-bromodecyl phenyl telluroxide) that is more marked than that observed for the selenoxide and sulphoxide eliminations. By procedure b (MCPBA oxidation), the ratio is decreased to 1.5–1.7:1. In all cases, small amounts of the corresponding alcohols and ketones are detected. The by-product of the elimination, phenyltellurenic acid, is detected, at least partly, as diphenyl ditelluride, and is probably formed by disproportionation and oxidation reactions. 2 PhTeOH

PhTe(O)OH + PhTeH [O]

2 PhTeH

(PhTe)2 + H2O

The telluroxide elimination proceeds much slowly than selenoxide elimination. The accompanying scheme illustrates these useful synthetic transformations. a or b

R1 (a) + c (70-80 %)

R Ph

R1 PhTeNa

R Br

Te

O

TePh R

b

Ph

Te

R1

a + d

O

(50 %)

R

R1

R1 = H; R = n-C10H21 Br

R1 + R

R1 = Me; R = n -C5H11, n -C7H15, n -C11H23

R1

R

(b) + c (48-64 %)

R

PhTeNa

n = 2,3 a + c 70% b+c n =1 a + d (72%) b + d (19%)

( )n Br

PhTeNa

a + c (80%)

( )n

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SYNTHESIS OF OLEFINS

O

215

OH

PhTeNa

TePh

OH

a + c (78 %)

OH a or b Ph

Te O

b + c (42 %)

n(

)

O

PhTeNa

n(

)

OMe

PhTeBr3

n = 2, 3 a + c (67 %) n( ) Te(O)Ph n = 2 b + c (46 %) OMe a c (80 %) Te Ph O OH

a or b

MeOH PhTeBr2

)

PhTeBr3 n( )

MeOH

O Te Ph

c (n = 2, 3) n(

(81-99 %)

)

HOOMe O

Te Ph

OMe

d (70 %)

OMe R

OMe

OMe

OMe n(

OH

a

d (56-78 %)

R Ph

Te

OMe R

R = Ph, n -C6H17

O.H2O

a: (1) Br2 /CCl4, (2) OH-/H2O (telluroxide as hydrate) b: MCPBA (1.5 equiv) ether c: 20-25°C, 1-3 h (for a) or 2 h (for b) d: Kugelrohr distillation, 200-240°C/20-760 torr

In contrast to selenides, in the case of tellurides the double bond geometry of the formed alkenes is markedly dependent upon the amount of oxidant employed. Thus, trans-cyclododecene is nearly the sole product of the oxidation of cyclododecyl phenyl telluride with 1 equiv of oxidant, whereas with excess oxidant (even 2 equiv) a mixture of cis and trans-isomers is formed, in some cases the cis-isomer being the major product.6 Olefins and allylic alcohols from alkyl or cycloalkyl bromides and from epoxides (general procedures).3 The s-alkyl or cycloalkyl bromide is treated with sodium phenyl tellurolate (1 equiv from diphenyl ditelluride and NaBH4, see Section 3.1.3.2). The crude telluride is purified by SiO2 chromatography (elution with hexane) and converted into the corresponding dibromide by addition of bromine (1 equiv) in CCl4, at 0°C. Epoxides are converted into β-hydroxylalkyl and β-hydroxycycloalkyl phenyl tellurium dibromide by a similar procedure, except that the intermediate tellurides are chromatographed on SiO2 using hexane/EtOAc (5:1) as the eluent.

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4. TELLURIUM IN ORGANIC SYNTHESIS

Method a + c. The tellurium dibromide (1.0 mmol) in THF (20 mL) is treated with 0.5 N NaOH (10 mL) at room temperature with stirring, and the resulting solution is stirred for 1 h, the colour of the solution turning to orange. The mixture is diluted with brine and extracted with ether. The extracts are dried (MgSO4), evaporated under vacuum and the residue purified from diphenyl ditelluride by SiO2 chromatography (elution with hexane). The pure olefin is isolated by distillation of the residue. Method a + d. The tellurium dibromide is treated with 0.5 N NaOH at room temperature for 1 h. The mixture is diluted with H2O and extracted with ether. The extracts are dried (MgSO4), and evaporated under vacuum, furnishing the telluroxide hydrate in a quantitative yield. Pyrolysis of the telluroxide is performed in a Kügelrohr apparatus at 200–240°C/760 torr, giving the olefin as an oil. Allylic and vinylic ethers via methoxytelluration of olefins (general procedure).3 To a solution of phenyltellurium tribromide (2.22 g, 5 mmol), prepared from diphenyl ditelluride and bromine in methanol (5 mL), is added the olefin (10 mmol) and the mixture refluxed for 1 h. On cooling, the (β-methoxy)alkyl or cycloalkylphenyltellurium dibromide precipitates and is separated by filtration. Method c. The dibromide is converted into the corresponding allylic methyl ethers by treatment with NaOH as described previously. Method d. The dibromide is converted into the stable telluroxide hydrate and then pyrolysed to the allylic and vinylic methyl ether as described previously. The telluroxide elimination in allylic tellurides proceedes via a [2,3]-sigmatropic rearrangement affording allylic alcohols after hydrolysis.7 O R

TePh

R1

R

[O]

TePh

R1

R2

R2

[1,2]

R

OTePh

R1

R1

OH R2

OTePh H2O

R1 2

-PhTeH

O

R

CHO

R1

R2

R

OH

R1 R2

R, R2 = H; R1 = Ph R, R1 = Me; R2 = H

R2 -PhTeH

R

R

R

[O] = H2O2, t -BuOOH, NaIO4

H2O

[2,3]

R = H; R1 = R2 = (CH2)3

1

R R = R2 = H

R1 = H; R, R2 = (CH2)3

The scheme shows that non-rearranged allylic alcohols are formed via a [1,2] tellurium–oxygen shift followed by hydrolysis. Elimination of phenyltellurol from the telluroesters intermediate explains the formation of α,β-unsaturated carbonyl compounds.

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217

The above-described oxidation of allylic tellurides7 was applied on chiral allylic ferrocenyl tellurides, giving evidence of chirality transference to the allylic aIcohol.8 [O]

R

R

TeFc*

R1

R

1

TeFc* O

[2,3]

R

R

H2O

R1 OTeFc*

R1 OH ee 14-22 %

R = Me; R1 = R= ; R1 = Me 1 R = H; R = Ph

4.7.2 Correlate method: reaction of alkyl phenyl tellurides with chloramines-T The reaction of alkyl phenyl tellurides with excess chloramine-T (N-chloro-N-sodium-ptolysulphonamide) in refluxing THF leads to olefins, presumably through a tellurosulphimino intermediate.9 Owing to the high yields obtained, this method seems to be highly competitive with the telluroxide elimination.

TePh

R

NSO2C6H4Me-p

p -MeC6H4SO2NCINa THF (66-89 %)

R

TePh

R + PhTeNHSO2C6H4Me-p

R = n-C8H17, n-C10H21, n-C12H25, n-C13H27 nC12-H25

TePh

same conditions (93 %)

n C11-H23

+ n C12-H25 (2.2:1)

Olefins by reaction of alkyl phenyl tellurides with chloramine-T (typical procedure).9 A solution of 1-dodecyl phenyl telluride (0.198 g, 0.52 mmol) and commercial chloramineT trihydrate (0.300 g, 1.0 mmol) in THF (5 mL) is refluxed for 40 min under N2. After evaporation of the solvent, the residue is treated with hexane (20 mL), and then filtered. The filtrate is passed through a short SiO2 column with hexane, giving, after evaporation, 1-dodecene as a colourless oil (0.068 g (78%)). Vinylsilanes, compounds of great synthetic utility,10 can be synthesized by this procedure, starting from 1-phenyltelluro-1-trimethylsilylalkanes11 (thio and seleno analogues react similarly).

(PhTe)2 R

X

NaBH4 benzene/EtOH PhTe Me3Si

Me3Si

Cl

PhTe

SiMe3

R Chloramine T/DMF or THF, (56-64 %) H

R = n -C9H19, n -C11H23, n -C13H27, n -C15H31

n-BuLi/TMEDA, 0°C or n -BuLi/LDA, -78°C R

H

H

SiMe3

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4. TELLURIUM IN ORGANIC SYNTHESIS

4.7.3

From telluronium ylides

4.7.3.1

Stabilized telluronium ylides

Stabilized telluronium ylides such as dibutyltelluronium carbethoxy,12 phenacyl,13 cyano-13 and carbamoyl14 methylide (easily prepared by the reaction of dibutyl tellurides with the appropriate substituted methyl halides, followed by treatment with a base), undergo Wittig-type olefination reactions with a variety of carbonyl compounds, giving the expected olefins in satisfactory yields (method A). This behaviour is in sharp contrast to that of stabilized sulphonium ylides, which are inert towards carbonyl compounds. +

n -Bu2Te + X

Y

n -Bu2Te

Y]X-

R1

1) Base 2) R

1

O

R2

Y

+ Bu2TeO

(method A)

R2

Y

X

Base/ solvent/temperature

CO2Et

Br

KOt -Bu /THF/-20°C

Percentage Yield

R1R1CO aliphatic and aromatic aldehydes aliphatic, cycloaliphatic and aromatic ketone, α,β-unsaturated aldehydes and ketones

52-90

COPh

Br

KOt -Bu/THF/-20°C

aromatic aldehydes

74-81

CN

Cl

KOt -Bu/THF/-20°C

aromatic aldehydes aliphatic and cycloaliphatic aldehydes

60-83 36-49

aliphatic and cycloaliphatic aldehydes α,β-unsaturated aldehydes

40-71

CONHBu-i Br

NaH/THF/HMPA/-50°C,r.t.

Noteworthy features of these reactions are the high predominance of E stereochemistry in the case of aldehydes and the good results obtained even with highly enolizable ketones (such as cyclopentanone), α,β-epoxy ketones (isophorone oxide) and α,β-unsaturated compounds (benzalacetophenone and cinnamaldehyde). Dibutyltelluronium benzylide, generated by treatment of the telluronium salt with potassium t-butoxide, behaves similarly to the above-stabilized ylides, undergoing Wittigtype olefinations with aromatic aldehydes.15 +

n -Bu2Te

Ph]Br-

1) KOt -Bu/THF 2) ArCHO (63-84 %)

Ar

Ph

Ar = Ph, p -ClC6H4, p -MeC6H4, PhCH=CH, 2-naphthyl

In the case of carbamoyl derivatives (Y⫽CONHBu-i) the reactions can be performed even under phase transfer catalysis, by simply treating the aldehyde with the telluronium

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salt (itself behaving as a PTC) in THF/trace H2O, in the presence of K2CO3 at 50–60°C (method B).16 + n -Bu2Te

CONHR]X- + R1CHO

K2CO3 (solid) (71-82%)

R1

CONHR

R = i -Pr, i -Bu; R1 = Ph, p -ClC6H4, p -BrC6H4, p -MeC6H4, p -MeOC6H4 X = Cl, Br

Simplified procedures The low energy and high polarity of the Te–C bond allows the reactions to be performed starting from the telluronium salts, instead of the ylides, under neutral conditions (method C).13,17 Finally, the preparation of the telluronium salts and the olefination reaction can be combined in a simplified and very practical one-pot procedure (method D).13,17 ArCHO + n-Bu2Te n -Bu2Te + X

Y]X-

THF, reflux

Y + ArCHO

(88-97%) THF, reflux (83-93%)

Ar

Y

Ar

Y

(method C) (method D)

Y = CO2Et, CO2Me, COPh; X = Br Y = CN; X = Cl Ar = p -NO2C6H4, m -NO2C6H4, p -BrC6H4, p -ClC6H4

Catalytic one-pot procedure. Since in the described telluronium ylide olefination telluroxide is formed as a by-product, and the telluroxide is susceptible to reduction by triphenyl phosphite, a catalytic procedure can be employed, providing a practical one-pot synthesis of α,β-unsaturated esters and ketones (method E).18 By this procedure, a catalytic amount of n-dibutyl telluride reacts with the α-bromoester or α-bromoketone, and the formed telluronium salt is converted in situ under phase transfer conditions (solid K2CO3/trace H2O) into the ylide, which reacts in turn with the aldehyde, giving the olefin. Since the reaction is performed in the presence of triphenyl phosphite, the formed dibutyl telluroxide is reduced back to the dibutyl telluride, which is then recycled. n-Bu2Te + Br

Y

+ n-Bu2Te

Y

(method E)

RCHO K2CO3 (72-98 %) n-Bu2TeO + R

Y

(PhO)3P Y = CO2Me, COPh, COPr-i R = Ph, p -ClC6H4, p -MeC6H4, styril, 2-pyridyl, 2-furyl, cyclohexyl, n -Bu, n -C9H10

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Reaction of stabilized telluronium ylide with aldehydes. Method A – using telluronium ylide (typical procedure).12 A solution of carbethoxymethyldibutyltelluronium bromide (1.23 g, 3 mmol) in dry THF (3 mL) is added dropwise to a solution of KOt-Bu (0.337 g, 3 mmol) in THF (4 mL) at −20°C. After a few minutes, trans-cinnamaldehyde (0.264 g, 2 mmol) is added over 1 min. The reaction mixture is then stirred for 1 h at −20°C. After the usual work-up, the product is purified by SiO2 column chromatography, to give ethyl 5-phenylpenta-(2E,4E)-dienoate (0.291 g (72%)). GC and 1H NMR of the product reveals a purity of 98%. Method B – using telluroniun ylide under PTC (general procedure).16 A mixture of dibutyl(alkylcarbamoylmethyl)telluronium halide (2.5 mmol), aldehyde (2.5 mmol) and K2CO3 (2.5 mmol) in THF (25 mL) and a trace of H2O is stirred at 50–60°C for ∼6 h. The end-point of the reaction is monitored by TLC (GF254 petroleum ether/acetone, 4:1). Brine (15 mL) is added, and the reaction mixture is extracted with ether (3×20 mL). The combined ether extracts are dried (MgSO4) and concentrated under vacuum to give the crude α,β-unsaturated amide, which is purified by SiO2 chromatography. Method C – using telluronium salts (typical procedure).13 A mixture of carbethoxymethyldibutyltelluronium bromide (0.99 g, 2.5 mmol) and 4-nitrobenzaldehyde (0.38 g, 2.5 mmol) is refluxed in THF (30 mL). After 6 h, it is quenched with H2O and extracted with ether. The organic extract is purified by recrystallization to afford pure (E)-methyl 3-(4′-nitrophenyl)propenoate (0.491 g (95%)). Method D – using dibutyl telluride (typical procedure).13 A mixture of 3-nitrobenzaldehyde (0.38 g, 2.5 mmol), methyl bromoacetate (0.38 g, 2.5 mmol) and dibutyl telluride (0.61 g, 2.5 mmol) is refluxed in THF. After 6 h, the mixture is worked up as for method C to give pure (E)-methyl 3-(3′nitrophenyl)propenoate (0.460 g (89%)). Method E – catalytic procedure (typical procedure).18 Benzaldehyde (106 mg, 1.0 mmol), methyl bromoacetate (165 mg, 1.1 mmol), triphenyl phosphite (356 mg, 1.2 mmol), dibutyl telluride (48 mg, 0,2 mmol), K2CO3 (179 mg, 1.3 mmol) and THF (4 mL) are mixed and stirred at 50°C for 13 h (monitored by TLC). The reaction mixture is filtered rapidly through a small amount of SiO2 with EtOAc as the eluent to remove inorganic salts and dibutyltellurium oxide. Preparative TLC with EtOAc/petroleum ether at 60–90°C (1:9) as the eluent yields 3-phenylpropenoate (160 mg (98%)).

4.7.3.2

Semi- and non-stabilized telluronium ylides

In sharp contrast to the olefination reaction employing stabilized telluronium ylides, semiand non-stabilized ylides react with carbonyl compounds to give epoxides (like non-stabilized sulphonium and selenonium ylides).

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(a) Dialkyltelluronium allylides Moderate to good yields of α,β-unsaturated epoxides are obtained, allowing aromatic and aliphatic aldehydes to react with dialkyltelluronium allylide (the diisobutyl derivatives are the reagents of choice).19 1) KOt -Bu Br- 2) Ar(R)CHO THF, -78°C

+ n -Bu2Te

O + i -Bu2Te

Ar(R)

Ar = Ph and (methyl, alkoxy, halo, nitro) derivatives, 3-pyridyl, 1-naphthyl, 4-cyclohexenyl, cyclohexyl (55-94 %) R = PhCH2CH2, n -C6H13, n -C9H19 (30-38%)

The moderate cis-selectivity, observed for aromatic and aliphatic aldehydes, decreases if an NO2 group or ortho-substituents are linked to the aromatic ring. Reaction of telluronium allylides with aldehydes (typical procedure).19 Allyldiisobutyltelluronium bromide (0.87 g, 2.4 mmol) and KOt-Bu (0.269 g, 2.4 mmol) are placed in a reaction vessel under N2, and dry THF (4 mL) is added with stirring at –78°C. After a few minutes, a solution of benzaldehyde (0.216 g, 2 mmol) in THF (2 mL) is added dropwise, and the reaction mixture is slowly allowed to warm to room temperature. Normal work-up followed by SiO2 column chromatography gives diisobutyl telluride (0.55 g (95% based on the used salt); eluted by hexane) and 2-phenyl-3-vinyloxirane (0.356 g (82%); cis/trans = 85:15; eluted by 5% ether/hexane). (b) Diphenyltelluronium methylide This was the first-described, non-stabilized ylide, obtained by treatment of the corresponding telluronium tetrafluoroborate with lithium 2,2,6,6-tetramethylpiperidide (LiTMP). Epoxides are obtained by reaction with both aldehydes and ketones.20 +

[Ph2TeMe]BF-4-

1) LiTMP, THF, -78°C 2) RR1CO (55-74 %)

R O R

+ Ph2Te

1

R = H; R1 = Ph, p -ClC6H4, p -BrC6H4, p -FC6H4, p -biphenyl, cyclohexyl, 2- naphthyl R = Ph; R1 = Me, Ph

Reaction of telluronium methylide with carbonyl compounds (general procedure).20 A solution of LiTMP (1.2 mmol) in THF is added to a solution of diphenyltelluronium methyl tetrafluoroborate (1.2 mmol) in THF (8 mL) at −78°C under N2. The mixture is warmed to −70°C and stirred for 30 min. After cooling to −78°C, the carbonyl compound (1.0 mmol) in THF (2 mL) is added. The reaction mixture is then allowed to warm at room temperature. After the reaction is complete (monitored by TLC), the usual work-up and flash chromatography gives the product.

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4. TELLURIUM IN ORGANIC SYNTHESIS

(c) Diisobutyltelluronium trimethylsilylpropynylide As in the preceding case, the ylide is obtained using LiTMP as the base and gives epoxides with aldehydes and ketones.21,22 i-Bu2Te + Br

SiMe3

neat r.t.

+

SiMe3 Br

n -Bu2Te

R = H; R1 = Ph, p -ClC6H4, p -BrC6H4, 2-naphthyl, p -biphenyl, cyclohexyl, n -Bu, n -C9H19

1) LiTMP THF, -78°C 2) RR1CO (76-96 %)

R = Ph; R1 = Me, Ph

cis /trans : 99/1-81/19 i -Bu2Te +

R1 O R

SiMe3

As shown by the obtained products, the reaction is highly cis-stereospecific. The utility of this method is evident in view of the extensive use of unsaturated silicon reagents in organic synthesis.10 Reaction of telluronium trimethylsilylpropynylide with carbonyl compounds (general procedure).21 A solution of LiTMP (1.2 mmol) in THF (2 mL) is added dropwise to a solution of [i-Bu2Te+CH2C ≡ CSiMe3]Br− in THF (8 mL) at −78°C under N2. The mixture is stirred for 30 min and then the carbonyl compound (1.0 mmol) in THF (2 mL) is added. The reaction mixture is allowed to warm at room temperature, and when the reaction is complete (monitored by TLC) the usual work-up and flash chromatography give the pure (trimethylsilyl)ethynyl epoxide. Intensive investigations have been devoted to the synthesis of vinyl cyclopropanes by using allylic telluronium salts.23 Vinyl cyclopropanes are important compounds as versatile synthetic intermediates24 and as participants in the structure of several biologically active natural compounds.25 3-Trimethylsilyl diisobutyltelluronium prop-2-enylide reacts with α,β-unsaturated esters and amides to give trimethylsilylvinyl cyclopropane derivatives.26 The presence of lithium salts plays an important role in the stereochemistry of these reactions.27 Thus, allyl telluronium ylides generated in situ from the corresponding telluronium salts in the presence of Li salts react with α,β-unsaturated esters and amides to afford trans2-vinyl-trans-3-substituted cyclopropyl compounds in high yieds.

+

i -Bu2Te

1) a) LiTMP, THF, -78°C b) LiBr TMS Br- 2) 2 R

; R1 = H; R2 = Ph

CO2Et(Me) R1

R1

R R = CO2Et, CO2Me R1 = H; R2 = Ph, p -MeOPh, Me, CO2Et, O R1 = Me; R2 = H R = CO-N

H R2

TMS (74-97%)

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(a) General procedure.26 A solution of freshly prepared LTMP (1.2 mmol) in THF (2 mL) was added dropwise to a solution of 3-trimethylsilylprop-2-enyldiisobutyltelluronium bromide (1.2 mmol) in THF (8 mL) at −78°C under N2. The mixture was stirred for 30 min, and then the α,β-unsaturated ester (1.0 mmol) in THF (2 mL) was added. The reaction mixture was then allowed to warm to room temperature. After the reaction was completed (monitored by TLC), usual work-up and flash chromatography gave the pure product. (b) A solution of sodium bis(trimethylsilyl)amide.27 (0.75 mmol) in THF (0.75 mL) was added dropwise to a solution of telluronium salt (0.75 mmol) and LiBr (0.75 mmol) in 6.5 mL of solvent at −78°C under N2. The mixture was stirred for 5 min, and then α,β-unsaturated compound (0.5 mmol) in solvent (1 mL) was added. The reaction mixture was then allowed to warm to room temperature after the reaction was completed. Usual work-up and flash chromatography gave the pure product. Employing sodium and potassium salts instead of lithium the corresponding cis–transcompounds are obtained as main product. Temperature and solvent are also influential factors in the stereochemical outcome. A useful and practical version of the above protocol involves a catalytic process in which the enone, a bromoallyl silane, cesium carbonate and diisobutyl telluride react in a one-pot procedure to give the desired cyclopropanes28 with a high cis-stereoselectivity. R

H

H

Bz

Br

Bui2Te

SiMe3

SiMe3 Bui2Te+

Ph

Bz

Bui2Te+

-

SiMe3Br-

Cs2CO3 SiMe3

General procedure – with telluronium salt: method A.28 A mixture of trimethylsilylprop-2enyl(di-isobutyl)telluronium bromide (0.33 g, 0.75 mmol), cesium carbonate (0.25 g, 0.75 mmol), chalcone (0.5 mmol), and DME (5 mL) and water (5 mm3) was heated at 70°C for specific periods of time. When the reaction was complete (monitored by TLC), the resulting mixture was eluted with ethyl acetate through a short column of silica gel. Removal of the solvent and flash chromatography on silica gel gave the desired pure product, of purity >98% (GC). With a catalytic amount of diisobutyl telluride: method B. Chalcone (0.5 mmol), cesium carbonate (1.0 mmol), (E)-3-bromo-1-trimethylsilylprop-1-ene (0.75 mmol), diisobutyl telluride (0.1 mmol), THF (5 cm3) and water (5 mm3) were mixed in a reaction tube and stirred at 50°C for a specific period of time. After 28 h, additional trimethylsilylallyl bromide (0.25 mmol) was added and stirring continued for 20 h. When the reaction was complete (monitored by TLC), work-up as for method A and flash chromatography of the residue on silica gel afforded the product, of purity >98% (GC).

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4. TELLURIUM IN ORGANIC SYNTHESIS

In contrast to the above-described results obtained with α,β-unsaturated esters and amides, α,β-unsaturated ketones submitted to a similar reaction give rise to cis-2-vinyltrans-3-substituted cyclopropyl ketones with high stereoselectivity and high yields.29 The stereochemistry of this reaction is almost independent of choice of the base. The addition of lithium salts, is ineffective. COPh +

i-Bu2Te

SiMe3

1) base, THF 2) Ph

SiMe3

H

COPh Ph H (50-94%)

base = LiBr + NaN(SiMe3)2, KDA, KN(SiMe3)2, KOBu-t

General procedure.29 Condition A: A solution of base (0.75 mmol) in THF (0.75 mL) was added dropwise to a solution of telluronium salt (0.75 mmol) + LiBr (0.75 mmol) in 6.5 mL of solvent at −78°C under N2. The mixture was stirred for 5 min, and then the α, β -unsaturated compound (0.5 mmol) in solvent (1 mL) was added. The reaction mixture was allowed to warm to room temperature after the reaction was completed. Usual workup and flash chromatography gave the pure product. Condition B: Similar to condition A, only there was no lithium salt addition. Similar cyclopropanation reactions have been performed with benzylidene and alkylidene malonic esters.30 +

i -Bu2Te

-

R + R1CH

CO2Et CO2Et

EtO2C CO2Et H H R1

R = H, Me3Si, Me, Ph R1 = Ph, p -MeC6H4, C6H13

(74-92%)

Propargylic silylated telluronium ylides react analogously. General procedure.30 A solution of potassium bis(trimethylsilyl) amide (0.75 mmol) in THF (0.75 mL) was syringed to a solution of telluronium salt (0.75 mmol) in solvent (6.5 mL) at −78°C under N2. The mixture was stirred for 2 min, and a solution of α,β-unsaturated compound (0.5 mmol) in solvent (1 mL) was added. The reaction mixture was then allowed to warm to room temperature. After the reaction was completed, usual work-up and flash chromatography gave the pure product. The reaction of allyl telluronium salts with phenols in the presence of solid NaOH in THF leads to allylic ethers in excellent yields.31 R R1

+ TeBu2]Br- + ArOH

NaOH THF, r.t., 5 h

R ArO

R1

R = R1 = H Ar = Ph, p -ClC6H4, 2 -naphthyl, 1 -naphthyl, o -MeC6H4, p -t - BuC6H4 R = R1 = Me Ar = Ph, p-ClC6H4, 1-naphthyl, 2-naphthyl

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Typical procedure.31 A mixture of p-chlorophenol (128 mg, 1 mmol), NaOH (40 mg, 1 mmol) and THF (10 mL) was stirred for 10 min; allyldiisobutyltelluronium bromide (360 mg, 1 mmol) was then added. The reaction mixture was stirred for another 5 h at room temperature under nitrogen. Aqueous saturated NaHCO3 solution was added and extracted with CH2Cl2. The extract was dried over anhydrous MgSO4 and concentrated in vacuo. The residue was chromatographed on silica gel with 95:5 hexane–ethyl acetate as eluent to give a colourless oil of allyl p-chlorophenyl ether (145 mg, 86%). 4.7.3.3 Correlate reaction: the reaction of telluronium salts with carbonyl compounds mediated by organolithium reagents – formation of secondary alcohols Telluronium salts, precursors of non-stabilized, semi-stabilized and stabilized telluronium ylides, react with carbonyl compounds on treatment with organolithium reagents to give secondary alcohols instead of alkenes.15,22,32,33 This different pathway arises due to the conversion of the telluronium salts into unstable tetraorganyltellurium intermediates (instead of the deprotonarion giving ylides), which, through a cleavage of one of the Te–C bonds, generates a nucleophilic species suitable to be added to the carbonyl group.

+ n-BuLi [R2Te-R1]X-

Bu-n R2Te-R

1

R2 Bu-n R3 δ+ δ− R2Te------R1------Li+

O

R2 R

3

OTe(n-Bu)R2 R

H2O

1

R2

OH

R3

R1

R = Ph; R1 = Me + telluronium hydroxide R = n -Bu; R1 = PhCH2, NC CH2CH2, Me3SiC CCH2 R2 Bu-n R3 δ+ δ− + 1 RR Te------R------Li

O

R2

OTe(n -Bu)RR1

R3

R

H2 O

R = Ph; R1 = Me

Telluronium salt (a1) [Me3Te+]I- [21], (a2) [Ph2Te+Me]BF4- [32] (b) [n -Bu2Te+CH2Ph]Br- [15]

(c) [n -Bu2Te+CH2CN]Cl- [33]

R2

OH

R3

R

+ telluronium hydroxide

Carbonyl compound

Yield (%)

benzaldehyde and derivatives, 2-naphthyl-CHO, 2-pyridyl-CHO

56-85

benzaldehyde and derivatives, cinnamaldehyde, 2-pyridyl-p-CHO, cyclohexanone

58-96

benzaldehyde and derivatives, aliphatic aldehydes, benzophenone, acetophenone

85-98

(d) [i -Bu2+ TeCH2C CSiMe3]Br- [22] benzaldehyde and derivatives, 2-naphthyl-CHO, 2-pyridyl-CHO, cyclohexyl-CHO, acetophenone, cyclohexanone, 2-cyclohexenone

67-93

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In the reactions with trimethyltelluronium salts (a1), alcohols with the structure RCH(–Bu)OH are formed as minor by-products (6–23%), resulting from the transference of the n-butyl group. With the telluronium salts (a2), (b), (c) and (d), phenyl, benzyl, cyanomethyl and (trimethylsilyl)propargyl groups, respectively, are transferred in preference to the other groups. The β-hydroxynitriles obtained from telluronium salt (c) form an important class of compounds since the cyano group can be submitted to several transformations.33 The importance of the propargylic anions in synthetic chemistry34 enhances the utility of the reaction involving telluronium salt (d). Reaction of telluronium salts/n-BuLi with carbonyl compounds: synthesis of alcohols with telluronium salt (a2) (typical procedure).32 A solution of n-BuLi (1.1 mmol) in hexane is added to a solution of the telluronium salt (0.459 g, 1.2 mmol) in THF (6 mL) at −78°C under N2. The mixture is warmed at −60°C and stirred for 10 min. After cooling again at −78°C, p-chlorobenzaldehyde (0.140 g, 1.0 mmol) in THF (2 mL) is added. The mixture is then allowed to warm at room temperature. After the reaction is complete (monitored by TLC), the usual work-up and flash chromatography yield pure p-chlorophenyl(phenyl)carbinol. With telluronium salt (b) (general procedure).15 A solution of n-BuLi (1.2 mmol) in hexane (0.6 mL) is added to a solution of the telluronium salt (0.494 g, 1.2 mmol) in dry THF (4 mL) at −70°C under N2. After 30 min, a solution of a carbonyl compound (1.0 mmol) in THF (2 mL) is added. The mixture is then allowed to warm at room temperature. After the reaction is complete (monitored by TLC), the usual work-up and flash chromatography yield pure benzylcarbinol. With telluronium salt (c) (general procedure).33 A solution of n-BuLi (1.5 mmol) in hexane is added to a solution of the telluronium salt (0.475 g, 1.5 mmol) in dry THF at −70°C under N2. After 1 h a solution of the carbonyl compound (1.0 mmol) in THF (2 mL) is added. After the reaction is complete (monitored by TLC), AcOEt is added. The resulting mixture is filtered through a short SiO2 column. The solvent is evaporated under vacuum, and flash chromatography gives pure cyanomethylcarbinol. With telluronium salt (d) (typical procedure).22 A solution of n-BuLi (1.5 mmol) in hexane is added to a solution of the telluronium salt (0.65 g, 1.5 mmol) in dry THF (10 mL) at − 78°C under N2. After 30 min, a solution of p-chlorobenzaldehyde (0.168 g, 1.2 mmol) in THF (2 mL) is added dropwise at −78°C and the reaction mixture is allowed to warm at room temperature. After the reaction is complete (monitored by TLC), H2O (1 mL) is added and the solution is stirred for another 1 h. The mixture is then extracted with ether (3×5 mL). The combined extracts are washed with brine, dried (Na2SO4), filtered and concentrated under vacuum. Flash chromatography on an SiO2 column gives 1-(pchlorophenyl)-4-(trimethylsilyl)-3-butyn-1-ol (0.265 g (87%) (98% GC purity)). 4.7.4 Tellurium-catalysed decomposition of α-lithiated benzylic sulphones into 1,2-diarylethylenes

α-Lithiated benzyl sulphones slowly decompose to give 1,2-diarylethylenes. In the presence of catalytic amounts of elemental tellurium this reaction acquires synthetic utility

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227

since the decomposition time is decreased to 3 h from 2 to 10 days.35 In contrast to the uncatalysed reaction, which furnishes pure trans-olefin, in the catalytic process, substantial amounts of cis-isomers (15–35%) are formed. However, cis/trans isomerization is promoted by tellurium tetrachloride in chloroform under reflux, and pure trans-stilbenes can, therefore, be obtained. Sulphur and selenium are less effective as catalysts. Since the required starting sulphones are easily prepared by the reaction of sodium benzene sulphinate with benzyl bromides, the described method comprises the formal dimerization of a benzylic fragment. The reaction has been rationalized as involving an attack of lithium tellurolate on a lithiated sulphone, promoting the elimination of lithium phenyl sulphinate and the formation of a labile epitelluride that readily collapses into stilbene and elemental tellurium.

Ar

SO2Ph

1) n-BuLi 2) Te

Ar

SO2Ph

Ar SO2Ph -PhSO2Li

Te -PhSO2Li

Ar

Ar

-Te (70-95 %)

Li Te Ar

TeLi

TeLi

Ar

Ar

SO2Ph

TeCl4 Ar CHCl , reflux 3

Ar Ar

Ar = Ph, o -MeC6H4, p -MeOC6H4, o - ClC6H4, m -ClC6H4, p -BrC6H4, m,m-MeC6H3, 2-naphthyl, o -biphenylyl

Tellurium-catalysed conversion of aryl phenyl sulphones into stilbenes (typical procedure).35 To a solution of benzyl phenyl sulphone (1.0 g, 4.3 mmol) in dry THF (50 mL) is added n-BuLi (1.6 M, 2.7 mL, 4.3 mmol) at 0°C with stirring. Finely ground elemental tellurium (0.10 g, 0.78 mmol) is then added to the orange-yellow solution, and stirring continued at room temperature for 3 h. During this period the colour of the solution slowly fades. Protonation of any resulting anion is effected by the addition of aqueous HOAc (50%, 2 mL). After evaporation of the solvent, the product is extracted with CH2Cl2 and washed once with 5% aqueous Na2CO3. The extract is dried, evaporated and chromatographed on SiO2 (eluting with CH2Cl2/petroleum ether at 40–60°C, 3:1), giving stilbene (0.35 g (90%)) as a 1:4 cis/trans mixture (determined by integration of 1H NMR spectrum). The product is heated under reflux in CHCl3 (15 mL) containing TeCl4 (0.10 g, 0.37 mmol) for 1 h. The cooled solution is then shaken with 5% aqueous Na2CO3 in a separating funnel. The organic layer is dried and evaporated, giving pure trans-stilbene (0.33 g (85%); m.p. 124°C). REFERENCES 1. Sharpless, K. B.; Gordon, K. M.; Lauer, R. F.; Patriel, D. W.; Singer, S. P.; Young, M. W. Chem Scripta 1975, 8A, 9. 2. Lee, H.; Cava, M. P. J. Chem. Soc. Chem. Commun. 1981, 277. 3. Uemura, S.; Fukuzawa, S. I. J. Am. Chem. Soc. 1983, 105, 2748. 4. Uemura, S.; Ohe, K.; Fukuzawa, S. I. Tetrahedron Lett. 1985, 26, 895.

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4. TELLURIUM IN ORGANIC SYNTHESIS

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Nishibayashi, Y.; Komatsu, N.; Ohe, K.; Uemura, S. J. Chem. Soc. Perkin Trans. 1 1993, 1133. Uemura, S.; Hirai, Y.; Ohe, K.; Sugita, N. J. Chem. Soc. Chem. Commun. 1985, 1037. Uemura, S.; Fukuzawa, S. I.; Ohe, K. Tetrahedron Lett. 1985, 26, 921. Chiba, T.; Nishibayashi, Y.; Singh, J. D.; Ohe, K.; Uemura, S. Tetrahedron Lett. 1995, 36, 1519. Otsuba, T.; Ogura, F.; Yamaguchi, H. Chem. Lett. 1981, 447. Colvin, E. W. Silicon Reagents in Organic Synthesis, p. 7. Academic Press, London, 1988. Ogura, F.; Otsubo, T.; Ohira, N. Synthesis 1983, 1006. Osuka, A.; Mori, Y.; Shimizu, H.; Suzuki, H. Tetrahedron Lett. 1983, 24, 2599. Huang, X.; Xie, L.; Wu, H. J. Org. Chem. 1988, 53, 4862. Osuka, A.; Hanasaki, Y.; Suzuki, H. Nippon Kagaku Kaishi 1987, 1505. Li, S. W.; Zhou, Z. L.; Huang, Y. Z.; Shi, L. L. J. Chem. Soc. Perkin Trans. 1 1991, 1099. Huang, Z. Z.; Wen, L. W.; Huang, X. Synth. Commun. 1990, 20, 2579. Huang, X.; Xie, L.; Wu, H. Tetrahedron Lett. 1987, 28, 801. Huang, Y. Z.; Shi, L. L.; Li, S. W.; Wen, X. O. J. Chem. Soc. Perkin Trans. 1 1989, 2397. Osuka, A.; Suzuki, H. Tetrahedron Lett. 1983, 24, 5109. Shi, L. L.; Zhou, Z. L.; Huang, Y. Z. Tetrahedron Lett. 1990, 31, 4173. Zhou, Z. L.; Huang, Y. Z.; Shi, L. L. J. Chem. Soc. Chem. Commun. 1992, 986. Zhou, Z. L.; Huang, Y. Z.; Shi, L. L.; Hu, J. J. Org. Chem. 1992, 57, 6598. See a related review: Huang, Y. Z.; Tang, Y.; Zhou, Z. L. Tetrahedron 1998, 54, 1667. Reed, J. W. Rearragement of Vinylcyclopropanes and Related Systems in Comprehensive Organic Synthesis (eds. B. M. Trost; I. Fleming). Vol. 5, pp. 899–970. Pergamon Press, UK, 1991. See ref. 73 in the precedent ref. 23. Huang, Y. Z.; Tang, Y.; Zhou, Z. L.; Huang, J. L. Chem. Commun. 1993, 7. Tang, Y.; Huang, Y. Z.; Dai, L. X.; Chi, Z. F.; Shi, L. P. J. Org. Chem. 1996, 61, 5762. Huang, Y. Z.; Tang, Y.; Zhou, Z. L.; Xia, W.; Shi, L. P. J. Chem. Soc. Perkin Trans. 1 1994, 983. Tang, Y.; Huang, Y. Z.; Dai, L. X.; Sun, J.; Xia, W. J. Org. Chem. 1997, 62, 954. Tang, Y.; Chi, Z. F.; Huang, Y. Z.; Dai, L. X.; Yu, Y. H. Tetrahedron 1996, 52, 8747. Xu, C.; Lu, S.; Huang, X. Synth. Commun. 1993, 23, 2527. Shi, L. L.; Zhou, Z. L.; Huang, Y. Z. J. Chem. Soc. Perkin Trans. 1 1990, 2847. Zhou, Z. L.; Shi, L. L.; Huang, Y. Z. J. Chem. Soc. Perkin Trans. 1 1991, 1931. (a) Patai, S. (ed.). The Chemistry of the Carbon–Carbon Triple Bond. Wiley, New York, 1978. (b) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes, Allenes and Cumulenes. Elsevier, Amsterdam, 1981. Engman, L. J. Org. Chem. 1984, 49, 3559.

25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

35.

4.8

TRANSMETALLATION REACTIONS

4.8.1 Lithium–tellurium exchange: generation of organolithium reagents By treatment of diorganyl tellurides with alkyllithiums in THF at −78°C, a lithium–tellurium exchange occurs, generating another organolithium if the latter is more stable than the former. Alkyl, aryl, allyl, benzyl, vinyl, ethynyl tellurides, tellurobutadienes, divinyltellurides, tellurobutenines, telluro(thio)- and telluro(seleno)ketene acetals and β-(phosphorovinyl) tellurides are susceptible to such exchange, giving the corresponding lithium compounds trapped in sequence with selected electrophiles.1–3 Depending on whether 1 or 2 equiv of the starting organolithium reagent is used, only one or both organic groups, respectively, are displaced from the telluride (methods A and B).

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Moreover, since organic tellurides are easily accessible from lithium tellurolates and organic halides, a one-pot procedure can be performed, overcoming the isolation of the telluride (method C). RTeR1 + R2Li

THF, -78°C -R1TeR2

E

RLi

RE

(method A)

R2 = n -Bu RTeR + 2 R1Li

THF, -78°C

RX + R1TeLi

THF, -78°C

-R1TeR1 -LiX

2 RLi E RTeR1

2 RE

(method B)

R1Li RLi -R1TeR1

E

RE

(method C)

Transmetallations of vinylic tellurides deserve particular attention. These tellurides (prepared by anti-addition of tellurols to acetylenes, see Section 3.16.1.2) exhibit the Z configuration and therefore generate (Z)-vinyllithiums. These results are in sharp contrast to the earlier tin–lithium exchange performed with vinylstannanes (characterized by the E configuration), giving (E)-vinyllithiums.4 To our knowledge, the first reported example of an Li–Te exchange in a vinylic tellurium system is the reaction of 2,5-diphenyltellurophene with n-BuLi giving the intermediate 1,4-dilithium-1,4-diphenylbuta-1,3-diene, which was trapped in situ with several electrophiles leading to the corresponding disubstituted dienes with retention of configuration.5 Later, 1-phenyltelluro-1-trimethylsilylethene was converted into the 1,1-bissilyl derivative via an intermediate Li/Te exchange.6

Ph

n -BuLi Te Ph TMEDA, r.t.

Ph

Ph Li Li

E

Ph E

E Ph

E = MeI, CO2, PhCOCl, H2O TePh n-BuLi SiMe3 THF, -70°C

Li SiMe3

Me3SiCl

SiMe3 SiMe3

In the last decades, transmetallation reactions of vinylic tellurides have been extensively studied and applied in useful synthetic transformations. In the case of symmetrical divinyl tellurides, the displacement of both vinyl groups is achieved by employing 2 equiv of n-butyllithium. Aryl vinyl tellurides give a mixture of products, since both Ar–Te and vinyl–Te bonds are transmetallated on reaction with n-BuLi, leading to vinyl- and aryllithiums. The butyl vinyl tellurides give only the desired vinyllithiums. The reactions are stereospecific with retention of the C⫽C bond geometry.2,3,7–10 Lithium–tellurium exchange: generation of vinyllithium (typical procedure). Method A.2 To a solution of butyl vinyl telluride (0.211 g, 1 mmol) in THF (4 mL) at −78°C under N2 is added dropwise a solution of n-BuLi (1.5 M in hexane, 0.67 mL, 1 mmol). After stirring for 40 min at −78°C, benzaldehyde (0.016 g, 1 mmol) is added. The mixture is allowed to react at room temperature for 30 min, diluted with EtOAc (40 mL) and washed with brine.

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4. TELLURIUM IN ORGANIC SYNTHESIS

After normal work-up of the organic layer, the crude product is chromatographed on SiO2. Elution with hexane removes dibutyl telluride, and elution with EtOAc removes the product [PhCH(OH)CH=CH2] (0.115 g (85%)). Method C.1 To a solution of lithium phenyl tellurolate (prepared in situ from Te (2 mmol) and PhLi (2.0 M, 1 mL, 2 mmol)) in THF (5 mL)) is added benzyl bromide (0.342 g, 2 mmol) at −78°C. The mixture is warmed at 25°C and stirred for 30 min. At this stage the conversion into phenyl benzyltelluride is complete. The subsequent Li–Te exchange and reaction with an electrophile are the same as described for method A. It has been reported that sodium–, potassium–, calcium– and magnesium–tellurium exchanges are achieved by similar procedures with alkyl, aryl, ethynyl, vinyl, allyl and benzyl tellurides.11 Tables 4.1, 4.2 and 4.3 collect a variety of Li–Te transmetallation reactions in accordance with methods A, B and C. Te–Li exchange in butyltelluro-1,3-butadienes and reaction with electrophiles (typical procedure)14 (run 12). To a solution of (1Z,3Z)-1-butyltelluro-4-methoxy-1,3-butadiene (0.267 g; 1.0 mmol) in THF (8.0 mL), cooled to −78°C under N2, n-BuLi (0.76 mL, 1.1 mmol, 1.43 M) was added in one portion. The reaction mixture was stirred at this temperature and after 10 min the butadienyl lithium formed can be used in situ. The facts observed in reactions of these intermediates (total disappearence of the starting material – followed by TLC – and formation of only one product) indicated that it is formed in high yield. (1Z,3Z)-1-ethylseleno-4-methoxy-1,3-butadiene. To a solution of butadienyl lithium formed as above, elemental selenium (0.079 g, 1.0 mmol) was added, the cooling bath removed and the mixture stirred at room temperature until all selenium disappeared (∼20 min). Then ethyl bromide (0.08 mL, 1.1 mmol) was added, the solution turned yellow and after 10 min of stirring was treated with a saturated solution of NH4Cl, diluted with ether (30 mL) and washed with NH4Cl saturated solution (3×15 mL). After drying the organic phase over anhydrous MgSO4, the solvent was removed under reduced pressure and the residue purified by flash chromatography using hexane as eluent and the product was obtained as a yellow oil. Yield: 0.104 g (54%). (1Z,3Z)-1-methoxy-5-hydroxy-5-phenyl-1,3-pentadiene. To a solution of butadienyl lithium formed as above, freshly distilled benzaldehyde (0.101 mL, 1.0 mmol) was added at −78°C and stirred for 30 min at this temperature, then allowed to reach room temperature and stirred for an additional half hour. Work-up affords the product as a pale yellow liquid. Yield: 0.10 g (53% yield). Te–Li exchange in bis(2-arylethenyl)tellurides15 (run 15). To a solution of bis-[2-(p-tolyl) ethenyl] telluride (0.361 g, 1 mmol) in THF (4 mL) at −78°C under nitrogen, a solution of n-BuLi (0.91 mL, 2.1 mmol, 2.42 M solution in hexane) was added dropwise. After 20 min of stirring at this temperature, DMF (0.2 mL, 2.58 mmol) was added. The temperature was raised to room temperature and stirred for 1 h, then diluted with ethyl acetate (30 mL) and washed with water (3×10 mL). The organic layer was separated, dried with MgSO4 and the solvent removed under vacuum. Elution with hexane removed dibutyl telluride, and elution with ethyl acetate gave a low-m.p. solid. Yield: 0.175 g, 60% (characterized as the semicarbazone: m.p. 108.4–109.4°C).

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231

Table 4.1 Li–Te exchange by method A

RTeR1 + R2Li

THF, -78°C -R1TeR2

E

RLi

RE

R2 Li = n -BuLi

Run

Telluride

1

BuTeBu

E

Yield (%) Ref.

Product OH

PhCHO Bu

86

1

77

1

80

1

74

1

72, 68

12

65, 96

12

C8H7-n

70

2

Ph

89

1

79

2

68-70

13

72

2

89

1

Ph OH

2

PhTepr-n

3

PhTePh

4

Ph

PhCHO

Ph

Ph OH

PhCHO

TeBu

PhCHO

Ph Ph

Ph Ph OH

5

Ar

TeBu

n -prCHO

OH

Ar = p -Me, o -BrC6H4 a 6

Ar

TeBu

pinacolone

Ar OH

Ar = o -I,m -NCC6H4a,b 7

Ph

TeBu

pr-n

Ph

C8H17Br

Ph

PhCHO

Ph

OH O Ph HO Et 8

H

Bu-n TeBu

R1RCHO

Et H

R, R1 = Ph, H, (-CH2)59

THPO

TeBu

PhCHO

THPO

Bu-n R 1 OH R Ph OH

10

Ph

TePh

PhCHO

Ph

OH Ph

Cont.

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4. TELLURIUM IN ORGANIC SYNTHESIS

Cont. Run

Telluride

Product

E

R 11

Yield (%)

Ref.

70-90

3

93

3

63

3

R H+

TeBu

H

R1 R1 R , R = Ph, p -MeC6H4, p -MeOC6H4 1

1

R

Me2SO4

R , R = Ph

Me R1 R

R1, R = Me

PhCHO

Ph OH

R1 12

1) Se TeBu 2) EtBr

MeO

SeEt

MeO

Ph OH R Me

RCOMe R = H, Me

TeBu

14

OH

PhCHO MeO 13

54 53 60, 65

14

85-92

8

60-72

15

40-63

16

OH TeBu

14

15

Ar

Te

Ar

R

SPh

CHO

DMF

Ar only E

Ar = Ph, p-Tol 16

R R1

RCOR1 R = H; R1 = alkyl, aryl R, R1 = (CH2)4 R = Ph; R1 = Me

DMF

R

R = Ph, p -ClC6H4, C3 H7

SPh CHO

TePh H2 O

R

SPh

R

SPh

62

H MeI

86

Me PhCHO

R

SPh OH Ph

85

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233

Cont. Run

Telluride

17

Product

E

Bu

SeBu

H

TeBu

Bu

DMF

Bu

n -BuCHO

17

SeBu

91

H

Bu

CO2

70

CHO

H ClCO2Et

Ref.

SeBu

H H2 O

Yield (%)

SeBu

H

CO2Et

Bu

SeBu

H

CO2H

Bu

SeBu

H

OH

65

70

68

Bu-n O (EtO)2P

18

TeR

O (EtO)2P

H+

Ph R = Ph, n -Bu

R = n -Bu

O (EtO)2P PhCHO

O EtO

P

H +

Ph O

40, 61

18

Ph

Ph

8-30 31

aSolvent bn-BuLi

ether. added to the telluride in the presence of pinacolone at -100°C.

Te–Li exchange in telluro(seleno)ketene acetals (typical procedure)17 (run 17). To a solution of the telluro(seleno)ketene acetal (1.0 mmol) in THF (5.0 mL) under N2 at −78°C was added n-butyllithium (0.47 mL, 1.0 mmol, 2.1 M in hexane), followed by immediate addition of the electrophile (H2O, DMF, ClCO2Et, CO2, t-BuCHO). The reaction was stirred (15 min–1 h). Total transformation of the starting material was confirmed following the reaction by TLC on SiO2 (hexane as eluent). Then the reaction mixture was treated with water, diluted with EtOAc and washed with a saturated solution of NH4Cl and water. The organic phase was dried (MgSO4) and the solvent evaporated under reduced pressure. After purification the products were obtained as yellow oils. Further investigations of the Li–Te exchange discussed above are reported in sequence. The one-pot generation (by method C) of allyl- and benzyllithiuns followed by capture with electrophiles has been investigated in more detail.12 RX

n -BuTeLi THF, 0°C

RTeBu

n -BuLi 0°C

+ [RLi] E RE

X = Cl, Br R = allyl, benzyl E = aliphatic and aromatic aldehydes and ketones, Me3SiCl

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4. TELLURIUM IN ORGANIC SYNTHESIS

Table 4.2 Li–Te exchange by method B RTeR + 2 R1Li

THF, -78°C -LiTeR1

Telluride

R1

E

Ph2Te

n-Bu, t-Bu

PhCHO

E

2 RLi

2 RE

Product

Yield (%)

Ref.

80, 72

1

86

1

70

2

72

2

66

2

82

2

Ph Ph OH

n -Bu2Te

Ph

PhCHO

s-Bu

n -Bu OH

Ph

THPO

)2Te

n -Bu

n -C8H17Br

)2Te

n -Bu

PhCHO

Ph

C8H17 Ph

THPO

OH p -BrPh

)2Te

n -Bu

Ph

)2Te

n -Bu

Ph

p -BrPh

PhCHO

OH cyclohexanone

Ph HO

Table 4.3 Li–Te exchange by method C RX + R 1TeLi

R1

RX Ph

THF, -78°C

Br

Ph

RTeR 1 E

PhCHO

R1 Li

RLi

-R1 TeR1

Product Ph

Ph

E

RE

Yield (%)

Ref.

72

1

96

1

73a

12

84

1

OH

Br

Cl

OH Ph

PhCHO

Bu

PhCHO

Ph g

OH Ph OH

I a1:1

s-Bu

PhCHO

mixture of diastereoisomers.

Ph

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235

It was observed that in the case of bromo- or iodo-substituted benzylic tellurides the lithium–Br exchange and/or the halogen displacement by the n-butyllithium competes with the Te–Li exchange, resulting in lower yields of the desired products. This drawback is avoided by using ether as solvent. The reaction of o-cyanobenzyllithium, generated by Te–Li exchange in the corresponding telluride, with aldehydes or ketones followed by acid, promotes lactonization of the obtained hydroxyderivatives, affording 3,4-dihydroiso-coumarins compounds.19 O CN TeBu n-BuTeLi THF, 0°C

n-BuLi THF, ether -105°C

Li

CN

1

RCOR -105°C

R1

H2SO4, 66°C

OH or p-TsOH, H2O R 85°C

O

R R1

47-100%

1

R = H; R = n-pr, t -Bu, Ph, (E)CH=CHMe R = Me; R1 = t -Bu, Ph R = CH2CH2NMe2; R1 = Ph R, R1 = (CH2)5, 2-norbornyl

CN Br

4.8.2

CN

Acyl- and aroyllithium compounds

Acyl- and aroyllithiums are generated by submitting telluroesters, easily accessible from the corresponding acid chlorides and tellurolate anions (see Section 3.11), to the abovedescribed lithium–tellurium exchange reactions.20 The exchange step and the capture of the formed lithium intermediate with the electrophile take place in a high yield with aromatic telluroesters (employing n-BuLi in THF/ether at –105°C) and with aliphatic telluroesters bearing no α-hydrogen atoms (employing t-BuLi in THF at −78°C). O R

R Ph, p -F3CC6H4 , o,o -F2 C6 H3

R

solvent R1Li(eq) (temperature)

E

n -BuLi (1.2)

THF/ether (-105°C )

O

O

solvent 1 TeBu-n + R Li -105-78°C -R1TeBu-n

E Li

R

E

Product O

pinacolone

Yield % 60-85

R OH O

t -Bu

t -BuLi (1.6)

THF (-78°C)

pinacolone

66

t -Bu OH O

t -BuCHO

25

t -Bu OH O

1-adamantyl

t-Bu

n-BuLi (1.6)

n-BuLi (1.2)

THF (-78°C) THF/ether HMPA (-105°C)

pinacolone

Me3SiCl

1-adamantyl

57 OH

O t-Bu

70 SiMe3

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4. TELLURIUM IN ORGANIC SYNTHESIS

It must be pointed out that the corresponding selenoesters and acylstannanes are inadequate for similar reactions. Telluroesters bearing an α-anion stabilizing group, upon treatment with n-BuLi (2 equiv) in the presence of chlorosilanes, give enolsilyl ethers of the corresponding acyl silanes exhibiting main Z geometry at the double bond.21 The reaction pathway is depicted in the following scheme.

EWG

n-BuLi

TeBu EWG

TeBu OLi

o 1,2-silyl migration

R 3SiCl

OSiR 3

n-BuLi TeBu -Bu2 Te EWG

Li EWG

OSiR 3

R 3SiCl

SiR 3 EWG (54 -100%)

SiR 3 EWG

EWG = Aryl, PhS, PhCH2O

4.8.3

OSiR 3

OLi

O

Heterosubstituted methyllithium compounds

Lithium–tellurium exchange can be put to further use, allowing the generation of a variety of heteroatom-substituted methyllithiums suitable for trapping with electrophiles.22 n-BuTe

Y + n-BuLi

Y

E

Run 1

Ph

THF, -78°C -n-Bu2Te

PhCHO

O

Y

E

Li

OH

O

E

Percentage Yield %

Product Ph

Y

66

Ph OH 2

MeO

3

MeO

4

MeO

5

MeO

O

O

PhCHO

Ph2CO

MeO

MeO

O

O

Ph OH Ph Ph

70

60

O

6

Me2N

O

PhCOCl

O O PhCHO

MeO MeO Me2N

O

Ph

O

Ph OH

40 51 73

Ph 7

Me3Si

PhCHO

Me3Si

OH

89

Ph 8

n -BuTe

PhCHO

n-BuTe

OH

81a-b

Ph 9

n -BuTe

65a-b

n-BuTe O

OH

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Run

Y

237

E

Percentage Yield %

Product

10

n -BuTe

Me3SiCl

n-BuTe

11

Bu3Sn

Me3SiCl

Bu3Sn

12

PhSe

H+

SiMe3 98a (NMR yield) SiMe3 98b,c (NMR yield) 90

PhSeMe

a

One equivalent LiCl is added. reaction at -95°C. cSolvent ether. bExchange

The reaction can also be performed satisfactorily in different solvents such as ether, toluene and hexane (in contrast to the previously reported lithium–tin exchange, which is sluggish in solvents of low polarity23). The following important remarks should be noted: • • •

•

Only 1.2 adducts are obtained with methyl vinyl ketone (runs 5 and 9). Tin and selenium derivatives (runs 11 and 12) undergo lithium–tellurium exchange preferentially to lithium–tin and lithium–selenium exchange.22,24 The formation of BuTeCH2Li from BuTeCH2TeBu is improved markedly by the addition of LiCl (runs 8–10). Therefore, it can be generated in situ from CH2Cl2 and BuTeLi owing to the concomitant formation of LiCl. Two different electrophiles can be introduced successively by a consecutive one-pot lithium–tellurium exchange/trapping sequence.

CH2Cl2 + 2 BuTeLi

-LiCl

BuTe

TeBu

1) n -BuLi

BuTe

2) Me3SiCl (73%) 1) n -BuLi 2) Bu3SnCl

SiMe3

BuTe

SnBu3 1) n-BuLi 2) PhCHO (63%)

Me3Sn

OH Ph

1) n -BuLi 2) Me3GeBr

BuTe

GeMe3 1) n-BuLi 2) Ph2CO (83%)

Me3Ge

Ph Ph OH

Heteroatom-substituted methyllithiums (typical procedure).22 To a solution of MeO(CH2)2OCH2TeBu-n (0.274 g, 1.0 mmol) in THF (5 mL) is added n-BuLi (1.6 M in

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hexane, 0.63 mL, 1.0 mmol) at −78°C. After 15 min, benzaldehyde (0.117 g, 1.0 mmol) is added in one portion. The reaction mixture is warmed at 25°C over a period of 1 h, and then poured into saturated aqueous NH4Cl (30 mL). The product is extracted with ether (3×30 mL) and dried (MgSO4). Radial layer chromatography on SiO2 gives MeO(CH2)2OCH2CH(OH)Ph (0.137 g (70%)) with dibutyl telluride (0.183 g (76%)). Bis(butyl tellurium)methyl sulphide, prepared from bis(bromomethyl)sulphide and n-BuTeLi by treatment with 2 equiv of n-BuLi, gives rise to the corresponding bis(lithium methylsulphide), which can be isolated under vacuum as a colourless powder and stored at −20°C for 6 months. It decomposes under argon at 60°C and ignites explosively upon contact with traces of air. Reaction with Bu3SnCl and Me2PSiCl leads to the corresponding bis-stannylated and bis-silylated derivatives.25 Br

S

Br

2 n -BuTeLi

n -BuTe

S

n -BuTeLi ether, -30°C

TeBu-n

Li

n-Bu3SnCl n -Bu3Sn

S

Li

Me2PhSiCl

SnBu3-n Me2PhSi

4.8.4

S

S

SiPhMe2

Ferrocenyltellurium derivatives

Mono- and bis-tellurenyl ferrocenes are achieved respectively by treatment of lithiated ferrocenes with butyltellurenyl bromide (route a) or with dibutyl ditelluride (route b). Mono tellurenyl ferrocene is also obtained in a two-step procedure by treating lithiated ferrocene with Te to give the ditelluride followed by reductive alkylation (route c).26,27 Te)2

TeBu-n 1)t -BuLi route a

Fe

2) n-BuTeBr THF-hexane 0°C, 67%

Fe

route b 1) n -BuLi / TMEDA

Fe

TeBu-n

Fe 2) (BuTe)2 (56-68%) n-BuTe R

1) t -BuLi

Fe 2) Te THF-hexane 0°C TeR 1) NaBH 4 route C EtOH, 0°C Fe 2) RX (64-100%)

=

n -Bu, C12H25-CHCH3, C8H17CHCH3, C6H13CHCH3, c-C12H25, C14H29, C16H33, C18H37, C20H41, Ph(CH2)n- (n=1-6), PhCH=CHCH2,(CH3)2C=CHCH2-

Butyltelluroferrocene.27 A solution of BuTeBr (freshly prepared from 8 mmol of BuTeTeBu and 8 mmol of Br2) was added to a THF solution of lithiated ferrocene (prepared from 10 mmol of ferrocene) at 0°C. The reaction mixture was stirred at room

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temperature (2 h), quenched with a saturated solution of NH4Cl, and the whole was extracted with EtOAc. The organic layer was washed with brine, dried (MgSO4) and evaporated to dryness in vacuo. Flash column chromatography (hexane as eluent) afforded 2.47 g (67%) of the product. Tellurium–lithium exchanges in the mono- and bis-telluroderivatives are a suitable route for the preparation of mono- and disubstituted ferrocenes.27 TeBu-n E 1) n-BuLi (1.1 equiv), THF, -78°C Fe Fe 2) E+, -78°C

E = MeI, TMSCl, Ph2PCl, Ph2CO, DMF PPh2 Fe TMS

1) n -BuLi (1 equiv), THF, -105°C, r.t. 2) TMSCl, -100°C , r.t., 64% 3) n-BuLi (1.1 equiv), THF, -78°C 4)Ph2PCl, -78°C, 78% n -BuTe

79, 95%

TeBu-n Fe

E

1) n -BuLi (2.5 equiv), THF, -78°C 2) E+, -78°C, r.t. E = Mei, TMSCl, Ph2PCl, DMF

Fe

E (74, 81%)

4.9 REACTIVITY AND SYNTHETIC APPLICATIONS OF VINYLIC TELLURIDES 4.9.1

Vinylcuprates by copper–tellurium exchange

The most promising synthetic applications of vinylic tellurides involve their reactions with higher-order dilithium cyanocuprates. The nature of the counterions in the higher-order cyanocuprates has a dramatic effect in this reaction. Treatment of (Z)-vinylic tellurides with higher-order dilithium cyanocuprates promotes a Cu–Te exchange to give higherorder vinylic cyanocuprates with retention of the configuration, instead of the crosscoupling reaction observed with Li+MgBr+ or (MgBr)22+ analogues. 4.9.1.1

Conjugate addition of enones

The formed vinylic cuprates readily perform conjugate addition to α,β-unsaturated carbonyl compounds giving β-vinyl-substituted ketones. A preliminary investigation28,29 directed to determine the influence of the substituents in both the telluride and the starting cuprate, employing cyclohexenone as substrate, showed that phenylvinyl tellurides are not appropriate as the source of the vinyl copper reagent, since both the vinyl and phenyl groups are transmetallated and add to the enone. This result could be anticipated since both the formed carbanions are sp2 hybridized. Otherwise, butyl vinyl tellurides undergo exclusive Te/vinyl transmetallation followed by the desired vinyl transfer.

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The following scheme summarizes the reported results. O O R2Cu(CN)Li2 +

R3

THF, r.t. TeR1 -RTeR1 R3 R Me Me n -Bu n -Bu

R3

Cu(CN)Li2 -78°C R R2 H Ph H H

R3 Ph H H Ph

Ph

O

R2

R2

R1 n -Bu n -Bu Ph n -Bu

R

+

O + R

R2

2

90% 88% 55% 75%

trace trace -----23%

----------45% ------

TeR (R = n -Bu, Ph, 2-Th

H

Te2Th TeBu , THPO

THPO other vinylic and bis vinylic tellurides

TeBu

Ph(Me) TeBu (Me)Ph R

Te

O ( R R = Aryl,

NCH2)

)2Te ,

THPO

higher order cyanocuprates: Bu(2-Th)Cu(CN)Li2, Me2Cu(CN)Li2, Bu(imid)Cu(CN)Li2 enones : MCK,

O,

O

Bis-vinylic tellurides are very convenient reagents for this transmetallation reaction since only 1 equiv of the telluride is required to generate 2 equiv of the mixed vinylcuprate. Concerning the R group in the cyanocuprate reagent, it was observed that the butyl group is not a convenient residual group, since it is partly transferred to the enone. In contrast, methyl, 2-thienyl and imidazolyl cuprate groups are efficient residual groups, promoting the excIusive 1,4-addition of the vinyl group. An especially efficient organocuprate reagent is the thienyl derivative Bu(2-Th)Cu(CN)Li2, which can be easily prepared from the commercially available 2-ThCu(CN)Li. An extensive investigation was undertaken30 involving several vinylic and bis vinylic tellurides, higher-order mixed cyanocuprates and three representative enones. In all cases the yield of the 1,4-addition products was good. This 1,4addition reaction is also sensitive to steric factors and the yields of the 1,4-adduct obtained with β,β-disubstituted enone, like mesityl oxide, are low. 2-Thienyl vinyl tellurides are valuable reagents for the transfer of the vinyl group. By treatment with Bu2Cu(CN)Li2 they are converted in a single step into the mixed cuprate

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vinyl-(2-Th)Cu(CN)Li2, the residual non-transferable ligand (2-Th) arising from the starting telluride. O O

Bu2Cu(CN)Li2 R

Te(2-Th)

THF, r.t. -BuTeBu

R

R

Cu(CN)Li2 (78-82%) 2-Th

Cu–Te exchange with divinyl telluride and addition to enones (general procedure).29 n-BuLi (1.35 mL, 2.5 mmol of a 1.84 M solution in hexane) is added to thiophene (0.21 g, 2.5 mmol) in THF (2 mL) in a 10 mL, two-necked flask under N2, at −70°C. The temperature is allowed to reach 0°C, while stirring is maintained for 30 min. The yellowish solution is then transferred via a cannula into a 25 mL, two-necked flask containing CuCN (0.18 g, 2.0 mmol of a commercial sample dried (P2O5) under vacuum in an Abderhalden apparatus) and THF (3 mL), previously purged with N2, and cooled to −70°C. Warming to 10°C produces a homogeneous solution which is recooled to −70°C. n-BuLi (1.1 mL, 2.0 mmol of a 1.84 M solution in hexane) is added. Stirring is continued for 15 min and then the mixture is allowed to reach room temperature. The bisvinyl telluride (1.1 mmol) in THF (2 mL) is added, the solution stirred for 1 h and then cooled to −70°C, and the enone (2.0 mmol) added rapidly via a syringe. The cooling bath is removed and the solution is stirred for 20 min at room temperature. The reaction is quenched with a 4:1 mixture of saturated aqueous NH4Cl and NH4OH, and extracted with EtOAc (30 mL). The organic layer is separated, washed with brine (2×50 mL) and dried (MgSO4). The solvent is evaporated and the residue chromatographed on SiO2, eluting first with hexane and then with hexane/EtOAc (5:1), giving the desired 1.4-addition product. Cu–Te exchange with vinyl 2-thienyl telluride and addition to enones (general procedure).29 To a stirred suspension of CuCN [(0.089 g, 1.0 mmol) of a commercial sample dried (P2O5) under vacuum in an Abderhalden apparatus] in THF (3 mL), under N2, cooled to −70°C, is added n-BuLi (1.1 mL, 2.0 mmol of a 1.84 M solution in hexane). The resulting homogeneous solution is stirred for 30 min at −70°C and then allowed to reach room temperature. A solution of vinyl thienyl telluride (1.0 mmol) in THF (2 mL) is added. After 1 h the reaction mixture is cooled to −70°C, the neat enone (1.0 mmol) is added rapidly and the mixture stirred for 20 min at room temperature. The reaction is quenched with a 4:1 mixture of saturated aqueous NH4Cl and NH4OH and extracted with EtOAc (15 mL). The organic layer is separated, washed with brine (2×25 mL) and dried (MgSO4). The solvent is evaporated and the residue purified by SiO2 chromatography, eluting first with hexane and then with hexane/EtOAC (5:1), giving the 1,4-addition product. The conjugate addition of higher-order (Z)-vinylic cyanocuprates, to enones,31 has been submitted to further useful modifications. Thus it was shown that the serious drawback of this methodology, which is the inertness of hindered enones to the mentioned reagents when THF is used as solvent, can be overcome by the addition of BF3⋅etherate to the reaction mixture, or simply by performing the reaction in ether in the presence or absence of

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BF3⋅etherate.32 Ph

Te(2-Th)

n-Bu2Cu(CN)Li2 Ph

Ph

TeBu-n

Cu(2-Th)CNLi2 O

n-Bu2(2-Th)Cu(CN)Li2 conditions THF/ BF3.Et2O Et2O / BF3.Et2O

O

Et2O

Ph (66-72%)

Another highlight of these additions was a systematic investigation33 of the use of alkylseleno- and alkyltelluro groups as non-transferable ligands of alkyl higher-order cyanocuprates [(RY)Cu(CN)R1Li2, Y⫽Se, Te] in addition to enones. The following example is illustrative. n-BuTeCu(CN)BuLi2

Ph TeBu-n -78°C, 3h, r.t.

Ph

Cu(n-BuTe)CNLi2 O

-78°C, r.t.

O Ph (86%)

Addition of alkyltellurocyanocuprates to enones (typical procedure).33 In a two-necked, 50 mL flask under nitrogen and magnetic stirring was placed elemental tellurium (511 mg, 4 mmol) in dry THF (5 mL). To this suspension at room temperature was added n-butyllithium (3.08 mL of a 1.3 M solution in hexane, 4 mmol). A yellow solution was formed. This solution was transferred via cannula to a second two-necked, 50 mL flask under nitrogen and magnetic stirring containing a suspension of CuCN (358 mg, 4 mmol) in THF (5 mL) at −78°C. The mixture was kept at this temperature for 15 min and then t-butyllithium (6.15 mL of a 0.65 M solution in hexane, 4 mmol) was added. The cooling bath was removed and the mixture was stirred until a clear solution formed. Then it was cooled again to −78°C and cyclohexenone (370 mg, 3.8 mmol) was added. The mixture was allowed to reach the room temperature and maintained under stirring for 1 h. A dark precipitate was formed. The organic phase was diluted with NH4Cl/NH4OH (3:1, 5 mL) and then with a 10% solution of sodium hypochlorite (3×10 mL). The organic phase was further washed with NH4Cl/NH4OH solution until the blue colour of the aqueous phase disappeared. The organic phase was dried MgSO4 and the solvent was evaporated. The residue was distilled in a Kügelrohr oven under vacuum. Yield of 3-t-butylcyclohexanone: 490 mg (83%).

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The yields of the 1,4-addition to hindered enone are improved by using BF3⋅Et2O as additive. 4.9.1.2

Conjugate addition of higher-order cyanocuprates to enone, followed by O-functionalization

The enolate intermediate, generated by the addition of higher-order cyanocuprates to enones, has been trapped with several electrophiles. Thus the addition of trimethylsilyl chloride, diethyl or diphenyl phosphorochloridate and N-phenyltrifluoro methanesulphonamide affords the corresponding vinyl silyl ethers, vinyl phosphates and vinyltriflates.34 R

A

or

TeBu-n

Te R B Me2Cu(CN)Li2

R

R

Cu(CN)Li2 Me

O

OR

PhNSO2CF3 Me3SiCl/TMEDA (RO)2P(O)Cl c HMPA, THF TMEDA THF, -78°C a b THF, 75 to 0°C -75°C to r.t. OSiMe3 R

OP(O)(OR)2 R

OSO2CF3 R

General procedure for the 1,4-addition of higher-order mixed cyanocuprates to enones followed by O-functionalization34 (a) By chlorotrimethylsilane Methyllithium (4.0 mmol, 1.0 M in diethyl ether, 4.0 mL) was added to a suspension of CuCN (2.0 mmol, 0.18 g) in THF (10 mL) at −75°C. The reaction mixture was then stirred until a clear solution was obtained and allowed to warm to room temperature. The appropriate (Z)-vinylic telluride A (2.0 mmol) or B (1.0 mmol) was added and stirred for 45 min. The solution was cooled back to −75°C and the corresponding enone (2.2 mmol) was added. After 20 min, chlorotrimethylsilane (2.6 mmol, 0.60 g) diluted in THF (5 mL) was added. The reaction mixture was stirred for 1 h, allowed to warm to room temperature and then treated with 1:1 solution of saturated aqueous NH4Cl and NH4OH (20 mL), extracted with ethyl acetate (3×20 mL), dried, evaporated and the residue was purified by Kügelrohr distillation affording the silyl enol ethers.

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(b) By diethylchlorophosphorochloridrate The (Z)-vinylic cyanocuprate was prepared as described above. After 20 min, N,N,N′,N′-tetramethyl-1,2-ethanediamine (TMEDA) (6.0 mmol, 0.70 mL) and diethylphosphorochloridrate (2.6 mmol, 0.46 g) in THF (5 mL) were added and the solution was allowed to warm to 0°C. The reaction mixture was stirred for 1 h and then treated with 1:1 solution of saturated aqueous NH4Cl/NH4OH (3×20 mL), extracted with ethyl acetate (3×20 mL), dried, evaporated and the residue was purified by flash silica gel chromatography using a 3:1 hexane/ethyl acetate as eluent affording the vinyl phosphates. (c) By N-phenyltrifluoromethanesulphonamide The (Z)-vinylic cyanocuprate was prepared as described above. After 20 min, HMPA (6.0 mmol, 0.6 mL) and N-phenyltrifluoromethanesulphonamide (2.6 mmol, 0.6 g) in THF (5 mL) were added and the solution was allowed to warm to room temperature. The reaction mixture was stirred for 4 h and then treated with 1:1 solution of saturated aqueous NH4Cl/NH4OH (20 mL), extracted with ethyl acetate (3×20 mL), dried, evaporated and the residue was purified by flash silica gel chromatography using hexane as eluent affording the vinyl triflates. Several synthetic transformations have been performed with the O-functionalized systems. Enol silylethers can be submitted to cyclopropanation, followed by F⫺-promoted annulation.35 OSiMe3

CO2R1

O 1) N2CCO2R

1

SO2Ph

R 2) F

R = CH=CHSO2Ph

Vinylphosphates are converted into dienes by reduction36 or organometallic coupling, respectively,37 and into the corresponding iodides by treatment with TMSCl/NaI in MeCN.38 R reduction R1 OP(O)(OEt)2

1

R Cu

R

R I NaI/TMSCl R = Ph

I R +

R

60% (1:1)

As described in Section 3.16.1.6, vinylphosphates and triflates are converted into functionalized tetrasubstituted vinylic tellurides39,40 by coupling with butyllithium tellurolate.

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Finally, triflates react with (Z)-vinyl zinc chloride (prepared by treating (Z)-butylvinyl telluride with n-BuLi and then with ZnCl2) and with terminal alkynes under Pd(PPh3)4 catalysis to afford, respectively, the coupled products.41,34

R

TeBu-n

R

1) n-BuLi

R

2) ZnCl2

ZnCl THF, r.t.

Ph n(

n = 1 (75-80 %)

OSO2CF3

R

Ph Pd(PPh3)4 n(

)

n=1 R H pyrrolidine, r.t.

Ph n(

)

R = Ph, n -Bu, CH2OH, SiMe3

n=0 R = (E) CH=CHCH2OTs

(75-80%)

4.9.1.3

Bu-n, TBS

R = Ph,

)

Reaction with epoxides

Vinylic organocuprates, prepared by transmetallation of vinylic tellurides with Bu (2-Th)Cu(CN)Li2, are highly reactive towards simple epoxides.30 The table, which collects representative examples, shows that monosubstituted epoxides afford homoallylic alcohols resulting from the attack to the less substituted carbon atom (runs 1, 5 and 7). Homoallylic alcohols are useful intermediates in several important total synthesis.42 Disubstituted epoxides fail to react (run 4). Styrene oxide leads to a mixture of homoallylic alcohols (run 2) and allylic epoxides give mixture of 1,2- and 1,4-opening product, with predominance of the 1,4 product (run 3, 6 and 8). Telluride 1) n-Bu(2-Th)CuCNLi2, THF, 1 h, r.t. 2) epoxide

product + (n-Bu)2Te Me

Telluride

Ph

Te

,

Ph , BuTe

Me B

A Run 1

Telluride A

Epoxides

R

TeBu

OTHP Me

Products OH R

O

C Yields %

1

R = Me R = Bu

85 82

+ OH Ph Ph Ph (1.2:1)

91

Bu R OH

2

A

3

A

Ph

( )n

O

Ph

( )n Ph HO

+ ( )n

n = 1 3.4:1 n = 2 3.0:1

Ph OH

74 94 Cont.

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Cont. Run

Telluride

4

A

5

B

6

B

Epoxides

Products no reaction

O n =1,2

( )n

Yields %

OH

7

8

C

C

O

Bu

Bu

O ( )n

+

( )n

OH

n = 1 3.6:1 OTHP n = 2 5.2:1 OH

O

OTHP

67

Bu

O ( )n

( )n HO

78 89

( )n

HO

Bu

61

OTHP

+ ( )n

OH

86 72

n = 1 1.9:1 n = 2 1.2:1

General procedure for the transmetallation of vinylic and bis-vinylic tellurides with (2-Th)Cu(Bu)(CN)Li2.30 n-Butyllithium (1.35 mL of a 1.84 M solution in hexane, 2.5 mmol) was added to a solution of thiophene (0.21 g, 2.5 mmol) in THF (2 mL) previously cooled to −78°C under nitrogen. The temperature was raised to −10°C, and the solution was stirred for 30 min. The yellow solution was transferred via cannula to another flask containing a suspension of CuCN (0.18 g, 2.0 mmol) in THF (3 mL) previously cooled to −78°C. Heating the mixture to room temperature produced a homogeneous solution which was then cooled to −78°C and treated dropwise with n-butyllithium (1.1 mL of a 1.84 M solution in hexane, 2.0 mmol). The stirring was maintained for 15 min at −78°C, and then the mixture was heated to room temperature. A solution of the bis-vinylic telluride (1.1 mmol) or the vinylic telluride (2.1 mmol) in THF (2 mL) was added. After being stirred for 1 h at room temperature, the solution containing the vinylic cuprate was ready to be reacted with the electrophile. General procedure for the epoxide opening with vinylic telluride/(2-Th)Cu(Bu)(CN)Li2 systems. To a vinylic telluride/(2-Th)Cu(Bu)(CN)Li2 system prepared as described above, cooled to −78°C, was added the appropriate epoxide (2.0 mmol) via syringe. In the case of the allylic epoxides, the mixture was stirred for 1 h at −78°C and then worked up. In the other cases the cooling bath was removed and the mixture was stirred for 2 h at room temperature. In both cases the work-up was performed by adding a mixture of saturated solutions of NH4Cl and NH4OH (4:1) and extracting with ethyl acetate (30 mL). The organic phase was separated, washed with saturated solution of NaCl (2×50 mL), and dried with MgSO4. The solvent was evaporated, and the residue was chromatographed on silica gel eluting first with petroleum ether to remove the tellurium-containing by-product and then

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with petroleum ether/ethyl acetate (4:1). BF3⋅Et2O (0.25 mL, 2.0 mmol) was added at −78°C before the addition of the epoxide, and the mixture was stirred at −78°C for 2 h before performing the work-up described above. 4.9.1.4

Reaction with bromoalkynes

Vinylic cyanocuprates, prepared from vinylic tellurides, react with bromoalkynes giving conjugated (Z)-enynes and (Z)-enediynes.43 Me2Cu(CN)Li2 R

R

TeBu

R=O

Cu(CN)Li2 1) ZnCl2/Et2O, -20°C Me 2) R1 C CBr /THF, -20°C (65, 85%)

R1

R1 = n-Pent, Ph

NCH2 R1 R

R

1)Me2Cu(CN)Li2 / THF, r.t. TeBu 2) ZnCl / Et O 2 2 3) R2C CBr (48-74%)

R, R1 = Me, Ph, O

R1 R

R1

R2 = n-Pent, Ph, THPOCH2

NCH2

Reaction with bromoalkynes (typical procedure).43 The butyltelluroenine (R, R1 = n-methylene morpholine) (0.47 g, 1.1 mmol) was added to dilithium dimethylcyanocuprate (1 mmol) in THF (5 mL) at room temperature and the mixture was stirred for 1 h, cooled to −20°C and treated with ZnCl2 (1.0 mL, 1.0 mmol of a 1 M solution in Et2O), and then stirred for 1 h at the same temperature. Bromophenylacetylene (0.199 g, 1.1 mL) was added and the mixture was stirred at −20°C for 1 h. A 4:1 mixture of saturated solution of NH4Cl and NH4OH was added (40 mL) and the organic phase was extracted with EtOAc, dried (MgSO4) and the solvent was evaporated. The product was purified by SiO2 gel column eluting first with hexane and then with hexane/EtOAc (9:1); yield 0.23 g (66 %). 4.9.1.5

−)-macrolactin A Synthesis of (−

One of the subunits of the natural product (−)-macrolactin A, a strong anti-viral agent, has been synthesized as shown in the following scheme, involving the hydrotelluration of a enyne (a), and the opening of an epoxide with a vinylic higher-order cyanocuprate (b).44 OTBS OTBS

a)

(n-BuTe)2 NaBH4 / EtOH

TeBu-n OH OTBS

b)

1) n -BuCu(CN)2-Th)Li2 2) O

OH O S

O HO

pTol

HO

O S

O

HO pTol

HO (-)-Macrolactine A

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Tellurium–zinc and tellurium–aluminium exchange

Alkenylzinc compounds have been prepared by a Te–Zn exchange reaction on vinylic teIlurides under halide-free conditions, with retention of the geometry of the starting telluride.45 Quenching of the reaction mixture with aqueous HCl yields the tellurium-free alkenes with retained geometry of the C–C double bond. R n-BuTe

R

Et2Zn THF, 20°C -n-BuTeEt

telluride R

Ph

n-BuTe

Bu-t , n-BuTe

Bu-t

R

R HCl aq.

R

EtZn

R

H (62-85%)

Ph , PhTe

Pr-i ,

Ph Ph t-Bu

Te

Bu-t

R = Me3Si, CO2Et

Tellurium–Zn exchange (typical procedure).45 Into a THF (5 mL) solution of E-1-(butyltelluro)-1-phenyl-3,3-dimethyl-1-butene was added diethylzinc (1.0 equiv) at 20°C. After stirring for 5 h the reaction mixture was quenched with 1 N HCl at 0°C. Purification of the resulting mixture by HPLC afforded 1-phenyl-3,3-dimethyl-1-butenine in 85% yield (E/Z ratio, 6:94) along with butylethyl telluride in 89% yield. A Pd-catalysed cross-coupling reaction takes place upon treatment of the formed alkenylzinc with p-iodotoluene giving the expected coupled product. Ph EtZn

Bu-t

p -ITol Pd(PPh3)4 cat.

Ph

Bu-t

p -Tol (72%)

It was later observed46 that the above Te–Zn exchange reaction is not a general method to prepare alkenyl Zn reagents, but is restricted to vinylic tellurides bearing Ph, ester, or Me3Si groups at the α-position, able to stabilize the formed vinyl zinc. An additional report describes Te/Zn exchange of diaryl tellurides and diaryl ditelluride by treatment with Zn under Ni catalysis.47 The resulting Zn derivatives submitted to transmetallation with CuCN⋅2LiCl and subsequent allylation with allylic bromide, furnishes the expected product in high yield.

ArTeAr or ArTeTeAr

R Et2Zn Ni(acac)2 5-10 mol% 25°C, 6 h

ArZnEt

CuCN.2LiCl R Br

Ar (60-92%)

R = H, Br, CO2Et Ar = p-MeOC6H4, Ph, p-C6H5C6H4, p-Me2NC6H4, p-BrC6H4, 3-thyenyl, 2-thienyl

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Preparation of 2-bromo-3-(p-tolyl)propene (typical procedure).47 A three-necked, 50 mL flask equipped with an argon inlet, a rubber septum and an internal thermometer was charged with bis(p-bromophenyl)ditelluride (1.7 g, 3.0 mmol, 1 equiv) and Ni(acac)2 (77 mg, 0.3 mmol, 10 mol%). The reaction mixture was cooled to −40°C and THF (6 mL) was added. It was further cooled to −78°C and Et2Zn (1.5 mL, 15 mmol, 5 equiv) was slowly added via syringe. The reaction was allowed to warm to room temperature and was stirred for 6 h. Meanwhile, a mixture of copper cyanide (2.68 g, 23 mmol) and lithium chloride (2.54 g, 60 mmol) was dried under vacuum (130°C, 2 h) and dissolved in THF (10 mL). This solution was added to the reaction mixture at −60°C, followed by 2,3-dibromopropene (6.0 g, 30 mmol, 10 equiv). The reaction mixture was warmed up to room temperature and worked up as usual. The crude oil obtained after evaporation of the solvents was purified by flash chromatography (hexanes), affording the product (1.45 g, 5.2 mmol, 88% yield) as a colourless oil. Similar reactions were also performed with alkylphenyl and dialkyl tellurides. The following scheme illustrates a cyclization process probably proceeding via a radical intermediate. Me BnO

TePh O

1) Et2Zn, THF Ni(acac)2 7 mol% 25°C, 3 h 2) H2O

O OBn (56%) cis:trans = 95:5

Vinylic tellurides undergo Te–Al exchange with triethylaluminium giving the corresponding alkenylaluminium derivative with retention of the original stereochemistry. Successive quenching with aqueous HCl or reaction with allylbromide in the presence of CuI gives, respectively, the corresponding alkenes or a cross-coupling product.48 Ar RTe

R1 AlEt3 3equiv Ar CH3CCl3 Et Al 2 -RTeEt

Ar = Ph, p-ClC6H4 R = n -Bu, s-Bu R1 = n-Bu, s -Bu, t -Bu

R1 HCl aq.

Ar

R1

(67-94%) Br

Ar

R1

CuI (66%) Ar = Ph R = n-Bu; R1 = t-Bu

Tellurium–Al exchange (typical procedure).48 Into a CHCl3 (3 mL) solution of the telluride ((E); Ar⫽Ph; R⫽n-Bu; R1 ⫽t-Bu; 344 mg, 1.0 mmol) was added triethylaluminum (3.0 equiv) at 20°C. After 5 h stirring, the reaction mixture was quenched with 3 N HCl at 0°C. Extraction followed by purification of the resulting mixture with HPLC afforded 3,3-dimethyl-1-phenyl-1-butene (Ar⫽Ph; R1 ⫽t-Bu) in 94% yield (E/Z⫽>99:1). As by-product BuTeEt was formed almost quantitatively.

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REFERENCES 1. Hiiro, T.; Kambe, N.; Ogawa, A. Miyoshi, N.; Murai, S.; Sonoda, N. Angew. Chem. Int. Ed. 1987, 26, 1187. 2. Barros, S. M.; Comasseto, J. V.; Berriel, J. Tetrahedron Lett. 1989, 30, 7353. 3. Dabdoub, M. J.; Dabdoub, V. M.; Comasseto, J. V. Tetrahedron Lett. 1992, 33, 2261. 4. See Pereyre, M.; Quintard, J. P.; Rahm, A. in Tin in Organic Synthesis, p. 153. Butterworths, London, 1987. 5. Luppold, M.; Müller, E.; Winter, W. Z. Naturforsch. 1976, 31b, 1654. 6. Kauffman, T. Angew. Chem. Int. Ed. 1982, 21, 410. 7. Dabdoub, M. J.; Dabdoub, V. M. Tetrahedron Lett. 1995, 51, 9839. 8. Mo, X. S.; Huang, Y. Z. Tetrahedron Lett. 1995, 36, 3539. 9. Dabdoub, M. J.; Begnini, M. J.; Cassol, T. M.; Guerrero, P. G.; Silveira, C. C. Tetrahedron Lett. 1995, 36, 7623. 10. Ogawa, A.; Tsuboi, Y.; Obayashi, R.; Yokoyama, K.; Ryu, I.; Sonoda, N. J. Org. Chem. 1994, 59, 1600. 11. Kanda, T.; Sugiro, T.; Kambe, N.; Sonoda, N. Phosphorus Sulfur Silicon 1992, 67, 103. 12. Kanda, T.; Kato, S.; Sugino, T.; Kambe, N.; Sonoda, N. J. Organomet. Chem. 1994, 473, 71. 13. Gerard, J.; Bietlot, E.; Hevesi, L. Tetrahedron Lett. 1998, 39, 8735. 14. Dabdoub, M. J.; Dabdoub, V. M.; Guerrero, P. G. Tetrahedron 1997, 53, 4199. 15. Silveira, C. C.; Perin, G.; Boeck, P.; Braga, A. L.; Petragnani, N. J. Organomet. Chem. 1999, 584, 44. 16. Silveira, C. C.; Perin, G.; Braga, A. L.; Dabdoub, M. J.; Jacob, R. G. Tetrahedron 1999, 55, 7421. 17. Dabdoub, M. J.; Begnini, M. L.; Guerrero, P. G.; Baroni, A. C. M.; J. Org. Chem. 2000, 65, 61. 18. Jang, W. B.; Oh, D. Y.; Lee, C. W. Tetrahedron Lett. 2000, 41, 5103. 19. Kanda, T.; Kato, S.; Sugino, T.; Kambe, N.; Ogawa, A.; Sonoda, N. Synthesis 1995, 1102. 20. Hiiro, T.; Morita, Y.; Inoue, T.; Kambe, N.; Ogawa, A.; Ryu, I.; Sonoda, N. J. Am. Chem. Soc. 1990, 112, 455. 21. Inoue, T.; Kambe, N.; Ryu, I.; Sonoda, N. J. Org. Chem. 1994, 59, 8209. 22. Hiiro, T.; Atarashi, Y.; Kambe, N.; Fujiwara, S. I.; Ogawa, A.; Ryu, I.; Sonoda, N. Organometallics 1990, 9, 1355. 23. Seyferth, D.; Weiner, M. A. J. Am. Chem. Soc. 1962, 84, 361. 24. Brandt, C. A.; Comasseto, J. V.; Nakamura, W.; Petragnani, N. J. Chem. Res(S). 1983, 156. 25. Strohmann, C. Angew. Chem. Int. Ed. 1996, 35, 528. 26. Nishibayashi, Y.; Chiba, T.; Singh, J. D.; Uemura, S. J. Organomet. Chem. 1994, 473, 205. 27. Chieffi, A.; Comasseto, J. V.; Snieckus, V. Synlett 2000, 269. 28. Comasseto, J. V.; Berriel, G. N. Synth. Commun. 1990, 20, 1681. 29. Tucci, F. C.; Chieffi, A.; Comasseto, J. V. Tetrahedron Lett. 1992, 33, 5721. 30. Tucci, F. C.; Chieffi, A.; Comasseto, J. V.; Marino, J. P. J. Org. Chem. 1996, 61, 4975. 31. See Comasseto, J. V.; Ling, L. W.; Petragnani, N.; Stefani, H. A. Synthesis 1997, 373, Section 3.3. 32. Araujo, M. A.; Barrientos-Astigarraga, R. E.; Ellensohn, R. M.; Comasseto, J. V. Tetrahedron Lett. 1999, 40, 5115. 33. Zinn, F. K.; Ramos, E. C.; Comasseto, J. V. Tetrahedron Lett. 2001, 42, 2415. 34. Moraes, D. N.; Barrientos-Astigarraga, R. E.; Castelani, P.; Comasseto, J. V. Tetrahedron 2000, 56, 3327. 35. Marino, J. P.; Simonelli, F.; Stengel, P. G.; Ferreira, J. T. B. J. Braz. Chem. Soc. 1998, 9, 345 and references therein.

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36. (a) Ireland, R. E.; Pfister, G. Tetrahedron Lett. 1969, 26, 2145. (b) Muchmore, D. C. Org. Synth. 1972, 52, 109. (c) Heathcock, C. H.; Delmar, E. G.; Graham, S. L. J. Am. Chem. Soc. 1982, 104, 1907. 37. Moorhoff, C. M.; Schneider, D. F. Tetrahedron 1998, 54, 3279 and references therein. 38. Lee, K.; Wiemer, D. F. Tetrahedron Lett. 1993, 34, 2433. 39. Barrientos-Astigarraga, R. E.; Castelani, P.; Sumida, C. Y.; Comasseto, J. V. Tetrahedron Lett. 1999, 40, 7717. 40. Barrientos-Astigarraga, R. E.; Castelani, P. Sumida, C. Y.; Zuckerman-Schpector, J.; Comasseto, J. V. Tetrahedron 2002, 58, 1051. 41. Barrientos-Astigarraga, R. E.; Moraes, D. N.; Comasseto, J. V. Tetrahedron Lett. 1999, 40, 265. 42. See Corey, E. J.; Cheng, X. M. in The Logic of Chemical Synthesis, Wiley, New York, 1989. 43. Araujo, M. A.; Comasseto, J. V. Synlett 1995, 1145. 44. Marino, J. P.; McClure, M. S.; Holub, D. P.; Comasseto, J. V.; Tucci, F. C. J. Am. Chem. Soc. 2002, 124, 1664. 45. Terao, Y.; Kambe, N.; Sonoda, N. Tetrahedron Lett. 1996, 37, 4741. 46. Dabdoub, M. J.; Dabdoub, V. M.; Marino, J. P. Tetrahedron Lett. 2000, 41, 433. 47. Stüdemann, T.; Gupta, V.; Engman, L.; Knochel, P. Tetrahedron Lett. 1997, 38, 1005. 48. Terao, J.; Kambe, N.; Sonoda, N. Synlett 1996, 779.

4.9.3 4.9.3.1

Coupling reactions Pd(II)-catalysed homocoupling of vinyl tellurides

Vinyl tellurides bearing a stiryl moiety give the corresponding 1,3-dienes homocoupling products with moderate to good yields, by treatment with a catalytic amount of Pd(OAc)2 in the presence of AgOAc as reoxidant.1 The characteristic feature of this catalytic reaction is the preferential formation of a (Z,Z)-diene, the Z stereochemistry of the starting telluride being largely retained. The formation of the stylbenes, only in low yield, clearly shows that the fission of a vinyl tellurium bond is favourable towards the Ph–Te bond. Each of the above tellurides reacts smoothly to give the corresponding dienes where the selectivity to Z,Z and E,E isomers in the product is high from Z and E isomers. Cross-over experiments suggest that the homocoupling reaction occurs between an alkenyl telluride and an alkenyl Pd species, which is formed via the migration of an alkenyl moiety from Te to Pd.

Ar

TePh Pd(OAc)2 cat. / AgOAc MeCN

Ph

Ar + Ar

Ar Ph

Ph TePh

Ar

Te 2

Te

2

Ar = Ph, p-Tol

Styryl tellurides are also detellurated on treatment with lithium chloropalladate (2 equiv), giving stereoisomeric mixtures of the homocoupled 1,3-butadienes.

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On treatment with Pd(II) acetate, distyryl telluride furnishes styryl acetate.2 Li2PdCl4, MeCN r.t., 20 h Ph (61%) (E,E )/(E,Z )/(Z,Z ) 56/36/8

Te

Ph

Li2PdCl4, MeCN r.t., 20 h

Ph

(41%) (E,E )/(E,Z )/(Z,Z ) 63 /35 /2

Ph

TePh

Ph same conditions Te

Ph

same conditions

Ph (69%) (E,E )/(E,Z )/(Z,Z ) 22/47/31

(73%) (E,E )/(E,Z )/(Z,Z ) 20 /52 /28

TePh

Pd(OAc)2, MeCN, r.t., 2 h

Te Ph

Ph

(70 %) (E )/(Z ) = 100 / 0

Ph

Ph Te

Ph

Ph

OAc

Pd(OAc)2, MeCN, r.t., 2 h (58%) (E )/(Z ) = 18 / 82

4.9.3.2

Pd(II)-catalysed cross-coupling of vinylic tellurides with alkenes

Vinylic and divinylic tellurides react with alkenes in MeOH in the presence of a combination of catalytic amounts of PdCl2 together with AgOAc and Et3N to afford the corresponding vinyl-substituted alkenes.3 The products result respectively from the coupling of the styryl and phenyl moieties of the starting telluride with retention of its double-bond stereochemistry. Ph

TePh

PdCl2 or Pd(OAc)2

+ Tol

Tol

AgOAc/Et3N MeOH, r.t., 20 h Ph TePh

, Ph

Te , 2

Ph

+

(56-92%)

Ph

Tol Ph (28-67%)

Te 2

4.9.3.3 Ni(II)- or Cu(I)-catalysed cross-coupling of vinyl tellurides with Grignard reagents Vinylic tellurides react with Grignard reagents under Ni(II) or Co(II) complex catalysis giving cross-coupling products, with results that depend on the nature of the substrate or the Grignard reagent.4 Ph

TePh

PhMgBr / THF, cat.

Ph

Ph + Ph-Ph

20°C; 60% (Z/ E = 90/10) refl.; 100% (Z/ E = 98 / 2) cat. = NiCl2(PPh3)2, NiCl2[Ph2P(CH2)3PPh2] PhTe

CO2Et

PhMgBr Ni(II) or Co(II) cat.

Ph CO2Et

+ Ph2Te + Ph-Ph

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Typical procedure.4 To a THF solution (10 mL) of phenylstyryl telluride (0.307 g, 1.0 mmol) containing NiCl2(PPh3)2 (0.032 g, 0.05 mmol) was added a THF solution (1 M) of PhMgBr (2.5 mL, 2.5 mmol) under N2 at 20°C. The mixture was stirred for 10 min during which period a black precipitate of elemental tellurium was formed. The mixture was then decomposed with diluted HCl and filtered from the precipitates which were washed to leave black tellurium (0.11 g). The filtrate was treated with aqueous NaCl and extracted with diethyl ether (3×30 mL) and the extract was dried over MgSO4. GLC analysis of the extract revealed the presence of biphenyl (0.142 g, 0.92 mmnl) and cis- and trans-stilbene (0.108 g, 0.60 mmol; cis/trans = 90:10), dibenzyl being used as an internal standard. By submitting tellurophene to a similar treatment 1,4-disubstituted (Z,Z)-(1,3)-butadienes are formed.5

Te

+ RMgX

R

NiCl2-L benzene,

,2h

(66, 88%)

R

R = Ph; L = 2(Ph3P) R = Me, n-Bu; L = Ph2P

PPh2 molar ratio: tellrophene / RMgX / cat. = 1:2.1:0.1

Furans, thiophenes and selenophenes react similarly, but with the reactivity decreasing from tellurophens to furans. (Z)-β-substituted cynnamic esters are formed with retention of the configuration, by the coupling reaction of Z-β-ayltelluro cynnamic esters with Grignard reagent in the presence of CuI.6 Ph ArTe

H CO2R

+ R1MgBr

CuI THF

R = CH3, C2H5 R1 = CH3, C2H5, n-Bu, i-Bu, Ph Ar = Ph, p -MeOC6H4

Ph

H

R1 CO2R (79-94%)

Typical procedure.6 Under N2, CuI (0.19 g, 1 mmol) was stirred in 10 mL anhydrous THF, then several drops of Grignard reagent were added. The cynnamic ester (1 mmol) was added and stirred for 5 min. The Grignard reagent (2 mmol, ∼1 M solution in THF) was added slowly in drops at 10°C. The mixture was stirred for 2 h. Filtered away the solid and washed it with Et2O. The combined organic phase was washed with NH4Cl solution and then with water. Dried with Na2SO4, evaporated the solvent and the residue was purified by prepared TLC on silica gel (AcOEt/n-hexane = 1:8). Disubstituted vinylic tellurides are converted into trisubstituted olefins by a similar reaction catalysed by Ni(0).7 n -C5H11

Et

PhMgBr

PhTe

H

Ni(0)

n -C5H11

Et

Ph H (77%)

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Ethyl 5-telluro-(2E,4Z)pentanedienoate reacts with different copper reagents Bu2Cu(CN)Li2 (a), Bu2(CN)(MgBr)2 (b) and Bu2CuMgBr (c) to give the cross-coupled tellurium-free dienes with high E,E stereoselectivity.8 CO2Et

n -BuTe

"BuCu" THF, -78°C, 2-30 min -n -Bu2Te

CO2Et n -BuTe

(92-96%)

(2E/ 4E : 2E/ 4Z = 81/19(a);99/1(c))

Lower E,E selectivity is observed with the reagent Bu2CuLi. The stereochemistry outcome of the above reaction, proceeding with inversion of the C3–C4 double bond geometry, is anomalous since it differs from the well-established retention of the geometry in the coupling reaction of (Z)-vinylic tellurides with higher- and lower-order cyanocuprates.9 Optimum reaction conditions were achieved with the bromomagnesium cuprate, and explored in several examples. CO2Et

n -BuTe

CO2Et

R2CuMgBr THF, -78°C, 2-30 min R

R = n -C6H13, n -C7H15, n -C10H21, Ph, p -Tol, p -MeOC6H4, (E ) PhCH=CH

(82-98%)

(2E/ 4E : 2E / 4Z = 98 / 2-99 / 1)

Trisubstituted 1,3-butadienes are afforded by Ni-phosphine-catalysed coupling of the corresponding tellurobutadienes with Grignard reagents.10 Ph H Ph

H

TePh H

+ PhMgBr

NiCl2(PPh3)2cat. THF

Ph H Ph

H (75%)

Ph H

Typical procedure.10 NiCl2(PPh3)2 (3%×0.5 mmol) was placed into a dry 50 mL flask equipped with a magnetic stirrer and a solution of the tellurobutadiene (0.5 mmol) in THF was added. A solution of phenylmagnesium bromide (2.0 mmol) in THF (3 mL) was then added dropwise to the stirred suspension in THF at room temperature. After reaction completion (TLC), the mixture was quenched with saturated aqueous NH4Cl and extracted with ether (3×10 mL). The combined extracts were washed with water and dried over Na2SO4. After removal of the solvent by evaporation, the residue was taken up in a minimum of petroleum ether and filtered through a short plug of silica gel to remove the catalyst and concentrated to give a residue. The residue was purified by preparative TLC on silica gel eluting with light petroleum ether to give the product, yield 75%, m.p. 107–109°C.

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255

Pd(II)- and Ni(II)-catalysed Sonogashira-type cross-coupling of vinyl tellurides and vinyl tellurium dichlorides with terminal alkynes

Cross-coupling reactions of alkynes with (Z)-vinylic tellurides under PdCl2/CuI catalysis gives rise to the corresponding enynes and enediynes with retention of the configuration.11 1) PdCl2 / CuI cat. MeOH, r.t. R

TeBu-n 2) R1

H, Et3N

R (62-85 %)

R1

R = Ph, HOCH2, CO2Et, (E)-THPOCH2CH=CH, C5nH11 R1 = n -C5H11, (CH2)3OH, HOC(Me)Et, n -C5H11

Typical procedure for the coupling reaction.11 To a two-necked, 25 mL, round-bottomed flask under N2 atmosphere containing PdCl2 (0.035 g, 20 mmol%), CuI (0.040 g, 20 mmol%) and dry methanol (5 mL) was added the vinylic telluride R⫽Ph (0.143 g, 0.5 mmol). After stirring the mixture for 15 min at room temperature, were added 1-heptyne (0.096 g, 1 mmol) and Et3N (0.3 mL). The reaction was exothermic and the temperature was maintained between 15 and 20°C by using a water bath. The stirred reaction was kept at room temperature for 4 h. Then the solid part was filtered and the filtrate was treated with saturated solution of NaCl (30 mL). The aqueous layer was extracted with ethyl acetate (3×20 mL), the combined organic layers were dried over MgSO4 and concentrated under vacuum. The residue was purified by silica gel column chromatography eluting with hexane. (Z)-1-(1-nonen)-3-ynyl benzene; yield: 0.085 g (85%). The (Z)-enynes are also obtained in good yields by applying the above Pd-catalysed process to (Z)-divinyl tellurides.12 R

Te

R

+

R1

H

PdCl2 / CuI MeOH / Et3N

R R1

Despite the excess of alkyne used, the transfer of only one vinylic group was observed. A more general approach of this method involves the reaction of (Z,Z)- and (E,E)-divinylic tellurides with alkynes under nickel catalysis, to give (Z)- and (E)-enynes with complete retention of configuration.13 R

Te

R

+ 4 R1

H

Ni(dppe)Cl2 (5 mol%) CuI (5 mol%), pirrolidine r.t., 16-24 h

Z, Z; E, E; R = Ph Z, Z; R = CH2OH R1 = Ph, C5H11, CH2OH, (CH2)3OH, CMe2OH

R 1 (54-83%) R

Typical procedure.13 To a two-necked, 25 mL, round-bottomed flask under an argon atmosphere containing Ni(dppe)Cl2 (0.026 g, 5 mol%), Cul (0.01 g, 5 mol%) and dry pyrrolidine (1.5 mL) was added (Z,Z)-distyryl telluride (0.33 g, 1.0 mmol). After the mixture was stirred for 15 min at room temperature, 1-heptyne (0.38 g, 4.0 mmol) was added. An

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exothermic reaction was observed, and the temperature was maintained between 15 and 20°C by using a water bath. The reaction was stirred at room temperature for 20 h. The solids were filtered off over Celite®; the filtrate was treated with a saturated solution of NH4Cl and extracted with ethyl acetate, and the organic layers were dried over MgSO4 and concentrated in vacuo. Column chromatography (hexanes) of the residue gave the expected product as a pale yellow oil (0.308 g, 78%). It must be emphasized that the enyne units are present in several anti-tumour agents and anti-biotics. Similar couplings have been performed with telluroketene acetals yielding geminal enedyines. PdCl2 is used instead of the PdCl2⋅CuI system.14 R1 H

TeBu-n

1) PdCl2, MeOH, r.t.

H

R

TeBu-n

2) R1

R

H, Et3N

1 (75-91%) R

R = Ph, n -C5H11, C3H7

R1 = n -C5H11, (CH2)3OH, SiMe3, CH2OH, HOC(Me)Et

Typical procedure.14 To a two-necked, 25 mL, round-bottomed flask under argon atmosphere containing PdCl2 (0.035 g, 20 mol% and dry methanol, 5 mL) was added the ketene butyltelluroacetal (R = Ph) (0.470 g, 1 mmol). After stirring the mixture for 15 min at room temperature, 1-heptyne (0.192 g, 2 mmol) and Et3N (0.8 mL) were added. The reaction was stirred at room temperature for 4 h. After this time, the solids were filtered under vacuum, to the filtrate was added brine. The organics were extracted with dichloromethane (3×25 mL), dried over MgSO4 and concentrated under vacuum. The residue was purified by flash silica gel chromatography eluting with hexane; yield 0.227 g (75%). A similar Sonogashira cross-coupling reaction has been successfully extended to vinylic and heteroaromatic tellurium dichlorides.15 R

TeCl2Bu-n + R1

Pd cat. / CuI Et3N, solvent, r.t.

R (61-87%)

R = Ph; R1 = HOCH2, HOCMe2, HO(CH2)3, Et2NCH2, O R = C8H17 ,(EtO)2P(O); R1 = HOCH2, HOCMe2 X

TeCl2Bu-n + R1

X = O, S R1 = HOCH2, HOCMe2

H

same conditions

R1 NCH2

X

R1

(77-83%)

Typical procedure.15 To a two-necked, 25 mL, round-bottomed flask under an argon atmosphere containing PdCl2 (0.035 g, 20 mol%), CuI (0.038 g, 20 mol%) and dry methanol (5 mL) was added the vinylic tellurium dichloride (R⫽Ph) (0.358 g, 1 mmol). After stirring the mixture for 5 min at room temperature, propargyl alcohol (0.112 g, 2 mmol) and Et3N (0.8 mL) were added. The reaction was stirred at room temperature for 6 h. After this

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time the solid part was filtered under vacuum. The filtrate was treated with brine and extracted with dichloromethane (3 ×25 mL). The combined organic layers were dried over MgSO4 and concentrated under vacuum. The residue was purified by flash chromatography eluting with hexane/ethyl acetate (80:20, yield 82%). 4.9.3.5

Pd/Cu-catalysed cross-coupling of vinylic tellurides with organyl zinc reagents

The cross-coupling reaction of vinylic tellurides with dialkylzinc or alkynylzinc reagents under Pd(PPh3)4⋅CuI catalyst was established as a useful method to synthesize different types of dienes,16 enynes or enediynes.17 R

TeBu-n

H

H

R3 2 Zn (6 equiv) Pd(PPh 3 )4.CuI THF / DMF, r.t.

R = SPh, 2-naphtyl,

R

R3

H

H

OH

,

R3 = Me, Et

yield 69-92%

TeBu-n

Me2Zn (10 equiv)

Ph

Pd / (PPh3)4.CuI THF / DMF, r.t.

Ph (42%)

Typical procedure.16 To a solution of the vinylic telluride (R⫽2-napht) (1.0 mmol) in THF (5 mL), Pd(PPh3)4 (0.055 g, 0.05 mmol), CuI (0.19 g, 1.0 mmol) and DMF (5 mL) were added at room temperature under N2. Then a commercial dimethylzinc solution (3.0 mL, 2.0 M, in toluene) was transferred dropwise via syringe. The dark brown resulting mixture was stirred for 3 h (followed by TLC) and was carefully treated with water and extracted with diethyl ether. The product was purified by column chromatography using hexane as eluent (yield 92%). R1 H

TeBu-n

R

H E

R or

TeBu-n

H

H Z

R1

"Zn"

Pd(PPh3)4 / CuI THF / DMF, r.t.

Alkynyl "Zn" Ph n-C4H9 n-C4H9

)2Zn )ZnEt )ZnEt2Li

H R

R or

H

Me3Si

) 2Zn

H

H

(79-93%) (R) Telluride Ph (E ), CO2Me (Z ) Ph (Z )

OH ) 2Zn

R1

Ph (Z )

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4. TELLURIUM IN ORGANIC SYNTHESIS

Ph

ZnEt2Li

Me2N

)2Zn

C5H11-n(Z )

THPO ZnEt C5H11

2

Zn

OTHP

Alkynil zinc reagents have also been employed to afford coupling reactions with phenylbutyl telluride and aryl iodides.17 PhTeBu same catalytic ZnEt conditions

Me2N

Me2N (65%)

Ph

PhTeBu OH OH

)2Zn MeOC6H4I

Ph (50%)

OH C6H4OMe (79%)

General procedure.17 To a flask containing the appropriate organotellurium compound (2.0 mmol) in THF (20 mL), Pd(PPh3)4 (0.11 g, 0.2 mmol), CuI (0.38 g, 2.0 mmol) and DMF (15 mL) were added at room temperature under N2. Then, the alkynyl zinc reagent (3.0 mmol) previously prepared in another flask was transferred dropwise via syringe. The dark brown mixture was stirred at room temperature and the reaction time was determined monitoring the reaction by TLC. The mixture was extracted with diethyl ether, washed with water, dried over MgSO4 and the solvent removed under vacuum. The product was purified by column chromatography. Ethylalkynyl reagents of RC⬅CZnEt type were prepared by Te–Zn exchange of Et2Zn (1.5 equiv) with acetylenic telluride, reagents of RC⬅CZnEt2Li and (RC⬅C)2Zn type by the treatment of lithium acetylides respectively with Et2Zn or ZnCl2 (0.5 equiv).

RC CLi RC CLi

4.9.3.6

Et2Zn (1.5 equiv)

RC CZnEt + EtTeBu THF, r.t. Et2Zn RC CZnEt2Li THF, r.t.

RC CTeBu

ZnCl2 (0.5 equiv) THF, r.t.

(RC C)2Zn + LiCl

Detellurative carbonylation of vinylic tellurides

Treatment of phenylvinyl tellurides with CO at atmospheric pressure and room temperature in MeCN in the presence of equimolar amounts of palladium(II) salts leads to the detellurative formation of the corresponding unsaturated benzoic acid.18,19 The stereochemistry of the reaction greatly depends on the CO pressure and the choice of the palladium(II) salt (PdCl2, Li2PdCl4 or Pd(OAc)2). The addition of Et3N to the reaction mixture greatly improves the reaction.

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259

The PdCl2/Et3N/MeOH system was revealed to be the best among the examinated systems.20 With this modification the expected α,β-unsaturated methyl carboxylates are obtained in high yields and with complete retention of the C⫽C bond configuration. In all cases a black precipitate is deposited during the reaction. R

CO (1 atm)

R

PdCl2 / Et3N (1:2) MeOH, r.t.

R1

TePh

R1

R = Ph, CO2Et R = H; R1 = Ph

1

R =H

CO2Me + PhCO2Me (71-96%)

(30-24 %) (ref. 20)

Methoxycarbonylation of organic tellurides by use of stoichiometric Pd(II) salts (typical procedure).20 In a two-necked, 50 mL, round-bottomed flask with a septum inlet and a three-way stopcock were placed PdCl2 (0.177 g, 1.0 mmol) and a telluride (1.0 mmol). The system was then flushed with CO from a CO balloon connected to the flask at 25°C, to which dry methanol (10 mL) and triethylamine (0.202 g, 2.0 mmol) were added by a syringe. After the mixture was stirred with diethyl ether (3×30 mL). The products were determined by GLC using an EGSS-X 3% (1 m) column. Hydroxyvinyl phenyl tellurides submitted to the carbonylation reaction in non-polar solvents (CH2Cl2 or CHCl3) lead to butenolides in moderate yields.20 R R1

OH H

TePh

CO (1 atm)

H

PdCl2/Et3N CH2Cl2

R + PhCO2H R1 O O (22-51%)

R, R1 = H, Me, (CH2)5 R = H, R1 = Me R = Me; R1 = i-Bu

These carbonylation reactions can also take place by using catalytic amounts of Pd(II) salts in the presence of an oxidant such as CuCl2, CuCl/O2, FeCl3, Ce(IV) salts or benzoquinone, the most effective of them being CuCl2.20 CO (1 atm) Ar CO2Me PdCl 2 cat./CuCl2 H H H H Et3N/MeOH 60-80% Ar = p -tolyl, p -ClC6H4, p -MeOC6H4, o -ClC6H4 Ar

TePh

PdCl2-catalysed methoxycarbonylation of organic tellurides with carbon monoxide (typical procedure).20 In a two-necked, 50 mL, round-bottomed flask with a septum inlet and a three-way stopcock were placed PdCl2 (0.017 g, 0.1 mmol), copper(II) chloride (0.270 g, 2.0 mmol) and a telluride (1.0 mmol). The system was then flushed with CO from a CO balloon connected to the flask at 25°C, to which dry methanol (10 mL) and triethylamine (0.303 g, 3.0 mmol) were added by a syringe. After the mixture was stirred for 75 h at 25°C, during which time the green colour turned to dark brown, the brown solid was filtered off. The filtrate was poured into NH4Cl aqueous solution and extracted with diethyl

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ether (3×30 mL). Products were isolated by preparative TLC or Kügelrohr distillation, or determined by GLC using an EGSS-X 3% (1 m) column with an appropriate internal standard. Alkinyl tellurides react similarly. PhC C TePh

similar cond.

PhC C CO2Me

R = Ph, C6H13

(60,37%)

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19. 20.

Nishibayashi, Y.; Cho, C. S.; Ohe, K.; Uemura, S. J. Organomet. Chem. 1996, 526, 335. Uemura, S.; Takahashi, H.; Ohe, K. J. Organomet. Chem. 1992, 423, C9. Nishibayashi, Y.; Cho, C. S.; Uemura, S. J. Organomet. Chem. 1996, 507, 197. Uemura, S.; Fukuzawa, S. I.; Patil, S. R. J. Organomet. Chem. 1983, 243, 9. Wenkert, E.; Leftin, M. H.; Michelotti, E. L. J. Chem. Soc. Chem. Commun. 1984, 617. Huang, X.; Zhao, C. Q. Synth. Commun. 1997, 27, 237. Gerard, G.; Bietlot, E.; Hevesi, L. Tetrahedron Lett. 1998, 31, 8735. Huang, Y. Z.; Mo, X. S.; Wang, L. Tetrahedron Lett. 1998, 39, 419. See Comasseto, J. V.; Ling, L. W.; Petragnani, N.; Stefani, H. A. Synthesis 1997, 373, Section 3.2.2. Wang, Y. P.; Wu, L. L.; Huang, X. Synth. Commun. 2001, 31, 2803. Zeni, G.; Comasseto, J. V. Tetrahedron Lett. 1999, 40, 4619. Zeni, G.; Menezes, P. H.; Moro, A. V.; Braga, A. L.; Silveira, C. C. Synlett 2001, 9, 1473. (a) Silveira, C. C.; Braga, A. L.; Vieira, A. S.; Zeni, G. J. Org. Chem. 2003, 68, 662. (b) Raminelli, C.; Prechtl, M. M. G.; Santos, L. S.; Eberlin, M. N.; Comasseto, J. V. Organometallics 2004, 23, 3990. Zeni, G.; Perin, G.; Cella, R.; Jacob, R. G.; Braga, A. L.; Silveira, C. C. Synlett 2002, 10, 1. Braga, A. L.; Lüdtke, D. S.; Vargas, F.; Donato, R. K.; Silveira, C. C.; Stefani, H. A., Zeni, G. Tetrahedron Lett. 2003, 44, 1779. Dabdoub, M. J.; Dabdoub, V. M.; Marino, J. P. Tetrahedron Lett. 2000, 41, 433. Dabdoub, M. J.; Dabdoub, V. M.; Marino. J. P. J. Org. Chem. 2000, 41, 437. Uemura, S.; Ohe, K.; Kim, J. R.; Kudo, K.; Sugita, N. J. Chem. Soc. Chem. Commun. 1985, 271. Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. J. Organomet. Chem. 1987, 326, 35. Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. J. Org. Chem. 1987, 52, 4859.

4.10

FREE RADICAL CHEMISTRY

Increasing interest has been directed over the recent years in organic radical reactions as a method for organic synthesis.1 Special attention was focused on organotellurium compounds as precursors for carboncentred radicals.

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4.10.1

261

Telluride-ion-promoted coupling of allylic halides

The first reported radical reaction promoted by tellurium reagent was probably the conversion of allylic halides into the coupled 1,5-dienes by treatment with telluride anions.2 The reaction, which gives the best results when employing the reagent prepared in situ from elemental tellurium and lithium triethylborohydride, proceeds through the intermediacy of the thermally unstable bis-allylic telluride followed by extrusion of tellurium and coupling of the formed allylic radicals. X

Te2-(Te+LiHBEt3)

Te -Te dioxane (53-93%) 110°C 2

.

Allylic halide: n(

)

X = Cl; n = 1 X X = Br; n = 1-3; Ph

Cl X (X = Cl, Br); Br ; n-C H 3 7

; Br

Cl ;

; PhCH2Br

Substituted allylic bromides furnish mixtures of the expected and rearranged products. The same mixture of products is formed independently from the geometry of the double bond (e.g. 1-bromo-2-hexenes). Coupling of 3-bromocyclohexene (typical procedure).2 3-Bromocyclohexene (322 mg, 2.0 mmol) in dry dioxane (4 mL) is added by syringe to the tellurium reagent (1.0 mmol, prepared adding super hydride (2.6 equiv) to tellurium powder under argon and stirring at room temperature for 5–7 h). The mixture is immersed in a bath preheated to 110°C and refluxed for 1 h. After cooling, the mixture is filtered through Celite® with the aid of CH2Cl2. The filtrate is concentrated to an oil and partitioned between CH2Cl2 and H2O (100 mL, 1:1). The aqueous phase is extracted with CH2Cl2 (2×25 mL) and the combined organic extracts are dried and concentrated to an oil. Chromatography over SiO2 with hexane, followed by Kügelrohr distillation at 110°C/15 torr gives 2,2′-bicyclohexenyl (140 mg (86%)); purity 96% (by GLC). 4.10.2

Organyl tellurides as exchangers of carbon radicals

It was discovered that the acetyl derivalive of N-hydroxy-2-thiopyridone is especially suitable as a source of methyl radicals, being photolysed by irradiation with a simple tungsten lamp. On this basis, and because of the high radicophilicity of tellurides (especially anisyl tellurides), a radical exchange occurs when the reagent is irradiated in the presence of an

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alkyl anisyl telluride, to afford methyl anisyl telluride and a new alkyl radical. In the presence of an electrophilic olefin, the alkyl radical is trapped, giving a relatively electrophilic radical, which in turn reacts with the thiocarbonyl function of the starting reagent to afford a tandem adduct, and reforming the methyl radical, thus beginning a new cycle.3,4

N O

hυ (W)

S Me

+ CO2 + Me.

S.

N

O AnTeR + Me. R. +

R

AnTeMe + R.

X

.

X

R

.

X X N

R S

N O

S

+ Me. + CO2

Me O

An = p -MeOC6H4

The radical chemistry discussed above is very useful for the manipulation or synthesis of complex products. 4.10.2.1

Tellurium-mediated addition of carbohydrates to olefins

Carbohydrate anisyl tellurides are easily prepared by treatment of the corresponding mesylates or tosylates with the anisyl tellurolate anion. By irradiation of these tellurocarbohydrates in the presence of N-acetoxythiopyridone and the electrophilic olefin, the tandem adduct is formed. The oxidative elimination of the thiopyridine moiety leads to the transolefins.4

AcO AcO AcO

(86%)

AcO AcO AcO

O

TeAn

+

OAc H O

N O Me same procedure O OAc O

N S OAc R CH2Cl2, 5°C, hυ, 10 min (-Me., -CO2) R = SO2Ph, CO2Me, COPh

O

N S

OAc

R (76-82%) MCPBA

AcO H AcO AcO N O Me

H

O

H

OAc R

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Radical addition (general procedure).4 To the tellurocarbohydrate (1 mmol) and the appropriate olefin (5 mmol) in dry CH2Cl2 (4 mL) under argon at 5°C is added acetoxy-2-thiopyridone (0.5 mmol). Photolysis with a 150 W tungsten lamp for 10 min is followed by further addition of the reagent (0.25 mmol). This is repeated until all the carbohydrate disappears (TLC; ∼1.5 mmol of the reagent is usually required). The synthesis of the antibiotic showdomycin, starting from a ribose derivative, is illustrative.4 O O

Ph3CO O

OH O

O H N

O O

HO HO

4.10.2.2

O

Ph3CO

TeAn

O

Ph3CO O

O

H N

O

O

O

OH

Intramolecular radical cyclization

(a) Synthesis of six-membered carbocycles The intramolecular version of the described radical reaction provides an easy approach to the synthesis of six-membered carbocycles.3 In accordance with the following scheme the hydroxyaldehyde A was submitted to a Wittig olefination followed by the mesylation of the hydroxyl group to give the α,β-unsaturated compound B which was in turn converted into the telluride C by treatment with the appropriate aryltellurolate. Irradiation of C in the presence of N-acetoxy-2-thiopyridone gave rise to the cyclic compound D. Compound D was transformed into the exocyclic olefin E by a oxidation–elimination procedure. HO

CHO A

1) Ph3P=CHR toluene, refl., 12 h 2) MsCl, Et3N, CH2Cl2

MsO B Ph

ArTe

C (77-78 %)

R = CO2Me, CN Ar = Ph, An

N S hυ OAc

R N

. . + CO2 + Me S.

R

S

ArTeNa ArTe2 /NaBH4 /EtOH

R

+ ArTeMe + PySMe to l M C ue PB R ne A , re (69-74 %) f l. D

E

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4. TELLURIUM IN ORGANIC SYNTHESIS

Compound B (R⫽SO2Ph, P(O)(OEt)2) has also been prepared by the alternative route depicted in sequence.

HO

CHO A

1) NaIO4, RuO2 CCl4, MeCN, H2O 2) MsCl, Et3N, CH2Cl2

MsO

CO2H

S O

MsO

N

O

MsO

SPy R hυ, CH2Cl2

MsO

R

1) (COCl)2, CH2Cl2, DMF cat. 2) NaO(S py) 1) MCPBA, CH2Cl2 2) Toluene, ref.

R B

Aryltellurium-mediated free radical cyclization (general procedure).3 A mixture of N-acetoxy-2-thiopyridone (0.60 mmol, prepared by the reaction of equimolar amounts of N-hydroxythiopyridone sodium salt with acetyl chloride, with the exclusion of light) and the telluride (0.2 mmol) in CH2Cl2 (2 mL) is stirred vigorously under argon. The mixture is irradiated using a 250 W halogen lamp for 15 min at room temperature. The course of the reaction is monitored by TLC. lf the reaction is not complete after the consumption of the 2-thiopyridone ester, a further quantity of this reagent is added, and the irradiation is continued. This procedure is repeated until all the starting material is converted. The solution is evaporated, and by chromatography of the residue on a short SiO2 column the cyclized product is separated from methyl aryl telluride and the thyopyridyl derivative. (b) Synthesis of cyclonucleosides The following synthesis of cyclo-5,6-dihydrouridine is self-explanatory.5 O

O

NH

NH ON

HO O

O

ON

O AnTe O

O

O

O S

PyS

OAc O CH Cl , hυ 2 2

ON O

NH

NH

O

Raney Ni O EtOH, reflux (60%)

ON O

O

O

Aryltellurium-mediated synthesis of cyclo-5,6-dihydrouridine (typical procedure).5 To a solution of the starting alcohol (see scheme) (6.22 g, 22 mmol) in pyridine (100 mL) is added dropwise mesityl chloride (2.55 mL, 33 mmol). The mixture is stirred overnight at room temperature, then H2O is added and the solution evaporated to dryness. The residue is purified on an SiO2 column (eluting with EtOAc/heptane, 1:1) to yield the mesylate as crystals (6.34 g (80%)). The mesylate (1.81 g, 5 mmol) in dry and degassed THF (50 mL) is added slowly to sodium anisyltellurolate (from (AnTe2) (1.29 g, 0.55 equiv) in degassed THF (2.5 mL) and NaBH4 (0.380 g, 2 equiv) in EtOH (8 mL)) under argon. The reaction mixture is stirred at room temperature for 6 h, and then H2O (40 mL) is added. The solvent is evaporated to dryness. After extraction with EtOAc, the organic layer is dried

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(MgSO4), filtered and evaporated. The residue is purified on an SiO2 column (eluting with EtOAc/heptane, 6:4) to yield the telluride as crystals (2 g (80%)). To a solution of the telluride (0.252 g, 0.5 mmol) in dry and degassed CH2Cl2 (5 mL) is added N-acetoxythiopyridone (0.338 g, 2 mmol) under argon. The reaction mixture is irradiated with a tungsten lamp (150 W) for 3 h and the solution is allowed to warm at 40°C. The reaction is purified on an SiO2 column (eluting with ether/pentane, 8:2) to yield the thiopyridyl cyclonucleotide as a white foam (0.14 g (75%)). This product (0.2 g, 0.53 mmol) is heated overnight under reflux in EtOH in the presence of Raney nickel. The mixture is filtered through Celite® and evaporated. The residue is purified on an SiO2 column (eluting with EtOAc/heptane, 6:4) to yield the final product, cyclo-5.6-dihydrouridine (0.085 g (60%)). 4.10.3

Reactions of tetraorganyl tellurium with acetylenes

Tetralkyltelluriums, prepared in situ by the reaction of tellurium tetrachloride with 4 equiv of alkyllithiums, react with arylacetylenes to afford dialkyl tellurides A, alkylation products B, an alkene C and minor amounts of a vinyl telluride D.6 R4Te

R3Te. + R.

ArC CH

Ar

R

Ar

R3Te

R 1

TeCl4 + 4 RLi

R = R CH-CH2R R2Te + Ar A

R + R1CH=CHR2 C B

R

R2Te 1

H R2

2

Ar +

R3Te D

R

The alkylation is rationalized as involving the radical addition of R4Te to the acetylene and the decomposition of the formed adduct to afford the dialkyltelluride and the olefin, originated from the displaced alkyl moiety via transfer of a hydrogen to vinyl cation. The alkylation proceeds mainly via a trans-mode giving cis-1,2-disubstituted adducts. When an excess of phenylacetylene is used, the yield of the alkylation product is raised (98%, E/Z⫽9:91), indicating that the alkylation competes with the self-degradation of n-Bu4Te (to n-Bu2Te, butane and octane). An electron-donating (MeO) or an electron-withdrawing (F) substituent linked to the benzene ring is without effect in the reaction. Reaction of n-Bu4Te with phenylacetylene (typical procedure for the reaction of R4Te with acetylenes).6 To a stirred suspension of TeCl4 (323 mg, 1.2 mmol) in 6 mL of Et2O at 0°C was added n-BuLi (4.8 mmol, 3.1 mL, 1.58 M in hexane) dropwise. The suspension turned brown initially and then yellowish orange as the addition came to completion. The mixture was stirred for 0.5 h at 0°C and phenylacetylene (123 mg, 1.2 mmol) was added to it. The mixture was then warmed to room temperature and stirred until no more increase of B (1-phenyl hexene) was observed by GC (8 h). GC analysis of the mixture using dodecane as an internal standard showed that n-Bu2Te, B and D were formed in 93%, 62% (E/Z⫽8:92) and 4% (E/Z = 50:50) yields, respectively. The reaction mixture was then poured into water

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(5 mL), and the products were extracted with ether (2×20 mL) and dried over MgSO4. After removal of the solvent under a reduced pressure, a yellow oil was obtained. By separation on a recycling preparative HPLC, n-Bu2Te, B and D were obtained in 89% (258 mg, 1.07 mmol), 60% (115 mg, 0.72 mmol, E/Z⫽8:92), and 2% (8.3 mg, 0.02 mmol, E/Z ⫽51:49) yields, respectively. The E/Z ratio was determined by GC. Alkylacetylenes, such as 1-octyne, are unreactive. The reaction of tetradecyltellurium with 2 equiv of phenylacetylene gives rise to PhCH⫽CHDec-n (86%, E/Z⫽14:86), n-Dec2Te (92%) and 1-decene (91%). The reaction of dibutyldidecyl tellurium with phenylacetylenes yields nearly statistical ratios of products. PhCH⫽CHBu-n (41%, E/Z⫽10:90), PhCH=CHDec (35%, E/Z⫽11:89), Bu2Te (20%), n-BuTeDec (41%) and Dec2Te (18%) are formed as a result of a random transfer of primary alkyl substituents. In contrast, di-n-butyl-di-i-propyl tellurium reacts faster than n-tetrabutyl tellurium, giving only PhCH=CHi-pr; tetraphenyl tellurium does not react at all; and n-Dec2TePh2 decomposes to n-DecTePh (95%), 1-decene (93%) and benzene (89%). 4.10.4

Telluroesters as source of acyl radicals

Telluroesters (acyl tellurides) have been recognized as excellent sources of acyl radicals upon photolysis with a 250 W tungsten lamp, or thermal process (benzene at reflux) in the dark. The formed acyl radicals are reactive towards efficient radical trapping reagents such as 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO), diphenyl diselenide and diphenyl disulphide, and N-t-butyl-α-phenylnitrone giving the respective adducts.7–9 O TeAr

O

TEMPO a) 2.1-5 h Ar = p -FC6H4, p -MeOC6H4; 86, 100% b) 5 h, Ar = p -FC6H4; 100%

O

N

O a) Y = Se Y=S b) Y = Se Y=S

PhYYPh Y = S, Se Ar = p -FC6H4 1.5 h Ar = p -FC6H4 6 h Ar = p -FC6H4 2 h Ar = p -FC6H4 16 h + N O-

Ph

a) tungsten lamp 250 W b) benzene reflux/dark

YPh

100% 85% 80% 16% O

O. N

*

Ph

*Characterized by ESR spectroscopy

Photolytic reaction of telluroesters; typical procedure for photolysis in the presence of TEMPO.9 Photolysis of the telluroester (Ar = 4-FC6H4) (34 mg, 0.1 mmol) and TEMPO

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(16 mg, 0.1 mmol) in benzene (5 mL), under argon in a cold water bath (8°C) with a 250 W GE sunlamp from a distance of 15 cm, gave, after 2.5 h and subsquent chromatography on silica gel (eluent hexane/ethyl acetate, 9:1), the product (86% yield) as a white crystalline solid: m.p. 87–88°C. Se-phenyl selenobenzoate; typical procedure for photolysis in the presence of diphenyl diselenide. A solution of telluroester (Ar = p-FC6H4) (64 mg, 0.2 mmol) and diphenyl diselenide (63 mg, 0.2 mmol) in benzene (5 mL), under nitrogen in a circulating cold water bath (8°C) was irradiated with a 250 W GE sunlamp from a distance of about 15 cm until TLC monitoring indicated loss of the starting compound. The solvent was removed and the residue chromatographed on silica gel (eluent: hexane/ethyl acetate, 9:1) to give the selenoester (52 mg, 93%) whose spectral characteristics were identical to those recorded in the literature. S-phenyl thiobenzoate; typical procedure in the presence of diphenyl disulphide. Photolysis of the telluroester (Ar = 4-FC6H4) in the presence of 6 equiv of diphenyl disulphide according to the standard for the formation of selenoesters gave 71% of the thiobenzoate after chromatography on silica gel (eluent: hexane/ ethyl acetate, 9:1). The product had a melting point and spectral characteristics in accordance with those described in the literature. Further evidence for the generation of acyl radicals is the formation of benzaldehyde on photolysis of benzoyl-1-naphthyl telluride in the presence of thiophenol. O Ph

PhSH Te-1-napht hυ, 8°C

O Ph H (80%)

Photolysis of the telluroester in the presence of thiophenol isolation of benzaldehyde. A solution of the telluroester (Ar = 1-naphthyl) (15 mg, 41.7 µmol) and thiophenol (17 mg, 154 µmol) in CDCl3 (0.6 mL) was photolysed with a 250 W white light source at 8°C for 2 h. Preparative TLC on silica gel of the reaction mixture, eluting with ethyl acetate/hexanes, 1:9, gave benzaldehyde (3.5 mg, 80%). These results, which could be supported in terms of homolysis of the Ac–Te bond with capture of the acyl radical by the trapping reagents, have been rationalized, however, on the basis of further experiments, as a degenerated background reaction in which an acyl radical abstracts an aryltelluro group from an additional molecule of acyl telluride. O Ph

TeAr

O .

Ar

Ph

O . +

O ArTe

Ar

o-Allyloxy-substituted telluroesters are also an efficient source of acyl radicals. They are trapped under photolysis with diphenyldiselenide and TEMPO and undergo radical cyclization with transfer of the aryltellurium group.9 The following scheme, illustrates the formation of 3-(aryltelluro)-methyl or ethyl chromanones via an exo-mode cyclization.

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These compounds are relatively unstable, and treatment with hydrogen peroxide promotes telluroxide elimination to give the corresponding α-methylene ketones. O

hυ

TeAr R1

O

benzene 15-30 min

R

O

R

R1

O

TeAr

H2O2

R1 O R = H; R1 = Me Ar = 4-FC6H4 (94%)

O (80-100%)

Ar = p -FC6H4, p -MeOC6H4, 2, 4-(MeO)C6H3; R, R1=H Ar = p -FC6H4; R = H; R1 = Me R = Me; R1 = H

Typical procedure for rearrangement with aryl tellurium group transfer (formation of 3-[(aryltelluro)methyl]chromanones).9 In a 5 mL flask fitted with a reflux condenser the telluroester (0.1 mmol) was photolysed under argon in benzene (3 mL) with a 250 W GE sunlamp from a distance of 15 cm. Either no cooling bath was used and the heat from the lamp caused the solution to reflux or the reaction was maintained at 8°C by means of a circulating cold water bath. After completion (TLC control) the solvent was removed in vacuo and the residue analysed by 1H NMR spectroscopy. In every case the product mixture was composed of the expected 2-((aryltelluro) methyl)chromanone as the very major product together with the corresponding elimination product and the diaryl ditelluride. Spectral analysis was facilitated by the possession of authentic samples of the ditellurides and, eventually, of the elimination products. The product ratios, and hence yields, were determined by integration of the 1H NMR spectra. Attempted purification by chromatography on silica gel led only to less pure samples contaminated with increased amounts of ditelluride and elimination products. The thio analogue behaves similarly, whereas the propargylic derivative is converted into the corresponding stable vinylic telluride. O

O TeAr

TeAr

S

S

Ar = p -FC6H4, p -MeOC6H4

(69, 90%)

O

O TeAr

O Ar = p -FC6H4

TeAr O (100 %)

Cyclization of substrates containing a cyclopropyl substituent proceeds with concomitant cleavage of the cyclopropyl ring. O

O TeAr

TeAr O Ar = p-FC6H4

O (100 %)

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The α,β-unsaturated acyltelluride also takes part in the photolytic cyclization. O

O

O TeC6H4F-p H2O2

TeC6H4F-p (100%)

(74%)

Each of the above cyclizations proceeds upon a 6-exo-trig mode process promoting the formation of six-membered rings as opposed to the more common five-membered ring formation. The following examples demonstrate, however, the applicability of the above procedures to 5-exo-trig cyclization giving a five-membered ring. O

O

O TeC6H4F-p

TeC6H4F-p

H2O2

(100%)

A 8-endo-trig cyclization leading to a cyclo-octanone, foreseeable in the case of a o-homoallylic benzoyl telluride, did not occur. Such a result was achieved, however, with a more conformationally unstrained substrate A. Treatment of the product with H2O2 furnishes a 3.7:1 mixture of the desired dibenzocyclooctanone B and the methylene cycloheptanone C. O TeC6H4F-p

degradation

O O

O

O TeC6H4F-p O

TeC6H4F-p O

O

O O A

+

O

TeC6H4F-p O O

H2O2

+ O

O

B

(63%) O O

C

As shown, each of the above-described cyclizations involves an acyltelluride group directly conjugated with an aromatic ring or with an alkene. Noteworthy non-conjugated aliphatic acyl tellurides are unable to be cyclized. O TeC6H4F-p

O O

degradation

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A series of experiments were effected performing the photolysis in the presence of thiophenol, TEMPO, PhSeSePh and, PhSSPh where the radical captors compete with the 6-exo-cyclization in trapping the acyl radical.9 Diphenyl disulphide has been shown to be a poor radical captor. O

O TeC6H4F-p PhSH

O

H

R

R

17% 8%

O

R

+

O R=H R = Me

O

O

R TeAr

+ O

O

59% 19%

24% 73% O

TEMPO

TeC6H4OMe-p

O

O

N

O (78%) O PhSeSePh

SePh O (87%) O

O TeC6H4OMe-p

SePh

PhSeSePh

O (73%)

O

4.10.5

Aryl telluroformates as precursors of oxyacyl and alkyl radicals

Photolysis at room temperature of aryl telluroformates gives rise to oxoacyl radicals, which can be trapped with diphenyl diselenides giving the corresponding phenyl selenoformates.10 Each of the reactions was performed in NMR tubes and a half-life of 15, 11 and 14, 10 and 6 h was found for methyl, primary alkyl, c-hexyl and benzyl, and phenyl telluroformate. O RO

TeAr

PhSeSePh 250 W low pressure Hg lamp, benzene, r.t.

O RO

SePh

Ar = Ph, p-FC6H4, p-MeOC6H4; R = Me Ar = Ph, p -FC6H4; R = i -Bu, c-Hex, β-cholestanyl Ar = Ph; R = n -C8H17, C6H5CH2, β-cholesteryl

In contrast, thermolysis of the telluroformates at 160°C in the dark leads to the formation of alkyl aryltellurides in good yields, presumably through the transient oxyacyl

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radicals which undergo thermal decarboxylation to afford alkyl radicals. O RO

TeAr

160°C

Ar

R-TeAr

R (half life)

Ph

Me (33 d), i -Bu (19 d), C6H5CH2 (8 H), c-Hex (6 d)

p -FC6H4 Ph, p -FC6H4

i -Bu (12 d), c-Hex (6 d) β-chlolestanyl (10, 11 d)

Some runs have been performed in preparation scale. Ar⫽Ph; R⫽c-Hex (71%). Ar⫽Ph; R⫽ α- and β-cholestanyl (85%). 4.10.6 Aryltelluroformates as precursors of selenium-containing heterocycles Benzylselenoalkyl-substituted telluroformates, synthesized according to the sequence depicted in the accompanying scheme, give different Se-containing heterocycles (depending from the length of the alkyl chain) by photolytic or thermal processes.11 O O

160°C

TePh

O

OH

Br2

PhTeCH2Ph +

SeCH2Ph

Se

OH

Se Br Br (74%) O

1) COCl2 O TePh ( )nSe Ph 2) (PhTe) / 2 ( )nSe Ph R NaBH4 A R = n-octyl (b) (a) n=2 n = 3-5 hυ (250 W lamp Hg 160°C low pressure) O O O O TePh ( )n-2 ( )n-2 . R Se + R Se O Se O R ( )n + Ph Se Ph - TePh Ph Se R ( )n R PhTeCH2Ph (73%) C B n = 3 (87%), 4 (94%), 5 (43%)

H

X ( )n

RMgX

R

( )nX

(PhCH2Se)2 NaBH4

R

Route (a) can be considered as the first example of intramolecular homolitic substitution at selenium, whereas route (b) must likely involve a nucleophilic attack by the selenium moiety with decarboxilative removal of phenyltellurolate. 4-Octyl-3-oxaselenan-2-one.11 A 0.15 M solution of 1-(benzylseleno)-3-undecyl (phenyltelluro)formate A (n = 2) (103 mg, 180 µmol) in benzene in a water-cooled jacket was irradiated with a 250 W low-pressure mercury lamp (white light) at a distance af 20 cm for 64 h. Removal of the solvent in vacuo and preparative TLC (hexane/ethyl acetate, 85:15) afforded the product B as a pale oil (37 mg, 73%).

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2-Octyltetrahydroselenophene was prepared using 1-(benzylseleno)-4-dodecyl (phenyltelluro)formate A (n = 3) in benzene (0.46 M) with heating at 160°C for 2 days. Preparative TLC (hexane/ethyl acetate, 98:2) afforded the product C as a pale oil (87%). 4.10.7 2-Allyloxy and 2-propargyloxy alkyl tellurides as precursors of tetrahydrofuran derivatives The allyloxy and propargyloxy compounds are easily prepared by the opening of monosubstituted epoxides with sodium aryl tellurolates, followed by allylation and propargylation of the obtained β-hydroxytellurides, submitted to irradiation with a sun lamp in the presence of hexabutylditin, and suffer group-transfer cyclization under 5-exo-mode to give the 2,4-disubstituted tetrahydrofuran derivatives.12 TeAr

(n -Bu3Sn)2 TeAr hν

O

O 1b Ar = 2-Th

1a

Yield %

63

cis / trans

1/4

TeAr

R

TeAr

(n -Bu3Sn)2

O 2a

TeAr

54-65

hν

TeAr

H

(n -Bu3Sn)2

O 3a

1/ 3-1/10

R O 2b Ar = p -CF3C6H4 R = Et, Ph, CH2 = CHOCH2 PhOCH2, PhCH2OCH2

hν

69, 72

1/2

H 3b Ar = p -CF3C6H4, 2-Th TeAr

TeAr (n -Bu3Sn)2 O 4b Ar = p -CF3C6H4

hν

O 4a

46

1/1.5

TeAr

R

TeAr O

(n -Bu3Sn)2 hν

5a

TeAr O 6a

R

46-49

1/1.2-1/1.1

40

1/1.2

O

5b Ar = p -CF3C6H4 R = Et, Ph, CH2=CHOCH2 PhOCH2, PhCH2OCH2 TeAr

H (n -Bu3Sn)2 hν

O

H 6b Ar = p -CF3C6H4

Typical procedure for group-transfer radical cyclization; preparation of compound 3b.12 (Ar ⫽ p-CF3C6H4) To a solution of 2-(allyloxy)cyclohexyl 4-(trifluoromethyl)phenyl

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telluride (0.46 g, 1.1 mmol) in dry benzene (10 mL) under nitrogen, was added hexabutylditin (0.026 g, 0.45 mmol). The resulting reaction mixture was irradiated with a sun lamp. The position of the sun lamp was adjusted so that a vigorous reflux could be maintained. After 1 h, TLC showed complete consumption of the starting material. The flask was cooled and the solvent removed in vacuo. The residue on purification by flash chromatography (4% ether/pentane) afforded 0.31 g (69%) of the title compound as a 2:1 mixture of exo- and endo-isomers. 4.10.8

Telluroglycosides as source of glycosyl radicals

Telluroglycosides generate glycosyl radicals by homolitic C–Te bond cleavage promoted by photolysis with a UV lamp, or by thermolysis at 140°C in the dark.13 A rapid equilibration of the β-isomer into the α-isomer takes place by a unimolecular radical mechanism, giving a 83:17 α,β-mixture. The same mixture is formed by irradiation of the pure α-isomer. These results reveal that the α-isomer is thermodynamically more stable than the β-isomer. The radicals so generated are trapped with TEMPO and diaryl diselenides. The addition of alkynes gives rise to vinylic tellurides. In the last case, the α-isomers are the main products and the reactivity is practically insensitive to the nature of the substituents of the alkynes. PO

O

PO TeTol

PO PO

PO PO

OP β 1-a P = Ac 1-b P = Bz 1-c P = Bn

O

PO

O

. + .TeTol PO PO

OP

OP

TeTol

α

TolSeSeTol P = Ac R PO

PO PO

O OP

H

R

TEMPO

-1/2(TeTol)2

(α-isomer main product)

TeTol

R = Ph p-CO2MeC6H4 70-83% p-MeOC6H4 2-pyridyl 2-furyl (42 %), CO2Me (50 %), n-C6H13 (56%),

PO PO PO

AcO

O

O N

OP

AcO AcO

O OAc SeTol (90 %)

O

P = Ac, Bz, Bn O H H H

(41%)

Reaction of telluroglycosides with alkynes (typical procedure).13 A mixture of the telluroglycoside (R⫽Ac) and phenylacetylene (2.55 g, 25.0 mmol) in a sealed Pyrex tube was irradiated with a UV lamp (Rayonet RMR-600 equipped with RMR-3500 Å lamp (4.5 W × 8)) at 120°C for 20 h. Purification of the crude mixture by silica gel chromatography afforded the addition product (R⫽Ph) in 93% yield (3.02 g, 4.65 mmol) as a 47:29:13:11 mixture of the α-E, α-Z, β-E and β-Z isomers. The reaction in the dark at 120°C also gave the adduct in good yield. By the addition of 10 mol% of AIBN, the reaction proceeded under milder conditions (80°C).

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4. TELLURIUM IN ORGANIC SYNTHESIS

Radical-mediated group-transfer imidoylation with isonitriles

Intensive investigations have been directed recently to group-transfer imidoylation of organotellurium compounds with isonitriles.14

NAr1 R-TeAr + C ..

NAr1 C .. R.

hυ/100°C -ArTe.

TeAr

. R

NAr1

R

NAr1

Ar = Tol Ar1 = o, o -xylyl

It was found that a variety of stabilized carbon-centred radicals such as glycosyl, benzyl, α-amino, α-alcoxy, α-carbalcoxy and aryl derivatives, generated from the corresponding organotellurium compounds, are effectively trapped by the isonitriles. R

;

h; yield %

OAc O

O 40; 80

N

AcO AcO AcO

3; 90a

O

N

3; 78

N

PhCH2

3; 51

OSiMe2 CPh2

12; 92b

N EtO2CCH2

7; 66

R = a i -prop b t -Bu

9; 40 4; 50

R1 = a b c d e

R=

R1CO

7; 78b C-prop i-prop i-Bu t-Bu

19; 73b 12; 73b 12; 83b 12; 73b 12; 61b

f

6; 48-52b

g R

2

R2 = H, Cl, MeO h Ph

6; 52b

a) reaction at 80°C b) reaction in the dark

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The reactions are normally performed at 100°C with 2 equiv of isonitrile (2,6xylylisonitrile) in benzene in a Pyrex vessel with UV irradiation (250 W high-pressure Hg lamp). The reactions also work in the dark, but a higher temperature and Iarger reaction times are required. The described imidoylation of α-acyl radicals deserves great interest since α-acyl compounds are not only versatile building blocks but also exhibit several biological activities.15 In the case of acyltellurides bearing sec- and tert-alkyl substituents, the decarboxylation of CO from the acyl radical competes with the imidoylation. Such a drawback is avoided by conducting the reaction under CO pressure (50 atm). The imidoylation of non-stabilized radicals (R⫽i-prop, t-Bu), also takes place, but the reaction is slow and requires a longer reaction time, the efficiency being increased by the addition of a radical initiator [1,1-azobis(cyclohexane-1-carbonitrile)]. The effect of solvents was also examined. It was observed that the change from nonpolar to polar solvents has only a small effect on the yields and rates of the reactions. Such a result suggests that the imidoylation reactions operate under a radical mechanism. The synthetic potential of the imidoylated product is noteworthy since some manipulations of the C–Te bond are useful for synthetic transformation. O TeTol 1

NAr

O

-e (2.0 F /mL) LiClO4 H2O/MeCN r.t.

NHAr1 O

[14b]

(100%)

(TolSe)2 2.1 equiv, C6H6 R SeTol mV Lamp, 100°C, 43 h NAr1 (100%) AcO AcO AcO

O TeTol OAc NAr1

OAc

1.1 equiv n-Bu3SnH / AIBN, C6H6 R

H H -20°C, 5 days AcO AcO NAr NAr 1 (100%) 1 (78%) [14c]

80°C, 1 h

Ar1 = 2,6-Me2C6H3 -e (2.0 F / mL); MeOH LiClO4 / EtCN, r.t.

R

OMe NAr1

(81%)

HCl 1M R NHAr1 THF, r.t. O 45 h (63%)

4.10.10 Three-component coupling of silyltellurides, carbonyl compounds and isocyanides Three-component coupling proceeds under mild conditions, giving the group-transfer product.16 O Me3SiTePh + R

.. R1 + C NPh

EtCN, 100°C 12-24 h

R R1

OSiMe3 TePh NPh

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4. TELLURIUM IN ORGANIC SYNTHESIS

The competitive formation of the silyloxytelluride detected during the progress of the reaction, and of the silylated TEMPO formed by adding the radical scavenger TEMPO, suggests the following reaction pathway:

O Me3SiTePh + R

[Me3Si.] R1 [-PhTe.]

TEMPO

OSiMe3 . R R1

.. C NPh

OSiMe3 . R

R1

.TePh

-PhTe. + PhTe. OSiMe3

Me3SiO N

NPh

R R1

R TePh R1

OSiMe3 TePh NPh

1

RR (yields %) R = Ph; R1 = Ph (82), p -MeOC6H4 (73), p -Me2NC6H4 (84), p -ClC6H4 (63) p-pyridyl (81), Ph(MeO)CH (73), c-propyl (93) RR1 = (CH2)5 (61) R = H; R1 = C6H13 (55), Ph (46)

The reaction is of general character, applicable to a variety of aldehydes and ketones. Aromatic ketones give the highest yields. The synthetic utility of the reaction is demonstrated by the oxidative hydrolysis of the products, giving silyloxy amides. Me3SiO R

TePh R1 NPh

Me3SiO R

-e (2.0 F mmol) LiClO4, H2O/EtCN r.t.

NHPh

R1 O (92 %)

1

R = R = Ph

Alkynes have also been employed as a third coupling partner to afford silylated allylic derivatives.17 Trimethylsilylphenyl telluride, by treatment with benzophenone in MeCN at room temperature, gives compound A which is treated in sequence with phenylacetylene at 100°C to give compound C in 85% yield (route a). The reaction can also be performed in one step by heating a mixture of the telluride (1.2 equiv), benzophenone (1.0 equiv) and phenylacetylene (1.2 equiv) without solvent at 100°C for 12 h to give C in 93% yield (route b). O a) Me3SiTePh + Ph

MeCN, r.t. Ph [- .PhTeH]

OSiMe3 [+ .PhTe] . Ph Ph [- .PhTe]

OSiMe3 TePh Ph Ph A

PhC CH 100°C

OSiMe3 . Ph + . TePh Ph - . TePh Ph B

Me3SiO Ph

Ph C

Ph TePh (85%)

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Me3SiO

O b)

Me3SiTePh + R

1

R + R

2

100°C net

R2

TePh R R1 C (51-97%)

1

R = Ph; R = Ph, p -MeOC6H4, p -Me2NC6H4 R, R1 = (CH2)3 R = H; R1 = Ph, C-hexyl R2 = Ph, p -EtO2CHC6H4,p-BrC6H4, 3-pyridyl, 1-c-hexenyl, CO2Et

The reaction shows high E selectivity. The coupled product C is a radical precursor of the vinylic radical B which can be generated by treating C with n-butyltin hydride (n-Bu3SnH) in the presence of AIBN and submitted to several manipulations. OSiMe3 R2 OSiMe3 R2 n-Bu3SnH (1.2 equiv) R R H TePh AIBN (0.1 equiv), 80°C R1 R1 C [92% (94% Z)] Sn(n-Bu3) AIBN (0.1 equiv) 80°C, 3 h

CO2Et 3 eq

OSiMe3 Ph CO2Et R R1[78% (96% Z)]

dimethylfumarate (5 equiv) n-Bu3SnH (2.4 equiv) AIBN (0.1 equiv) benzene, 80°C,1 h OSiMe3 Ph R CO2Me R1 CO2Me [58% (97% Z)]

R, R1, R2 = Ph

4.10.11 Synthesis of substituted quinones via organotellurium compounds Carbon-centred radicals, generated under photo-thermal conditions from organotellurium compounds, react with a variety of quinones to afford the mono addition products in good yields.18,19 The reaction in the dark is slow, but eventually gives the same product by heating at 100°C. The effect of the solvent is marginal. O

O R3

RTeTol + R1

R2 O

hυ, heat -1/2(ArTe)2

R

R3

R1

R2 O (53-71%)

R = PhCH2, CH2CH=C(CH3)2, i -prop R1, R2, R3 = H; R1, R2 = Cl; R3 = H; R1 = Me; R2, R3 = OMe R1 = Ph; R2, R3 = H; R1 = t -Bu; R2, R3 = H

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O

O Ph PhCH2-TeTol +

R

R

O (87, 64%)

O

R = OH, Me

O

O N

TeTol

O

+

O

N

O

O

O

O (44%) AcO AcO AcO

O O

TeTol +

OAc O

AcO

O

AcO AcO AcO

O

O (39%)

Reaction of organotellurium compounds with quinones (typicaI procedure, 2-benzyl-1, 4-benzoquinone).19 A solution of benzyl p-methylphenyl telluride (111 mg, 0.36 mmol) and 1,4-benzoquinone (78.0 mg, 0.72 mmol) in benzene (0.6 mL) in a Pyrex tube was irradiated with a 200 W high-pressure mercury lamp at 100°C for 1 h. After the solvent was removed under reduced pressure, the crude mixture was purified by flash chromatography (silica gel 6.4 g; elution with 5% ethyl acetate in hexane) to give the product in 57% yield. The reaction has been applied for the synthesis of polyprenyl quinol natural product ubiquinone and vitamin K.

O OMe OMe TeTol

O

O

OMe (41%) O

ubiquinone

OMe O O

O (43%) vitamin K

O

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The photolytic reaction of acyltellurides with quinones surprisingly takes place at the oxygen moiety. OCOAr

O hυ, 100°C EtCN

ArCOTeTol +

OCOAr

O

(78%)

Ar = Ph, p -ClC6H4

The reductive chemoselective bis-silylation of quinones was afforded by thermal reaction with silyl tellurides.20 O R

OSiMe3 + 2 Me3SiTePh

O

THF, r.t.

R OSiMe3 (66-100%)

2,3-dichloro-4,5-dicyano-1,4-benzoquinone 2,3,5,6-tetrachloro-1, 4-benzoquinone 2,6-dichloro-1,4-benzoquinone 2,3-dimethoxy-5-methyl-1,4-benzoquinone 1,4-naftoquinone, duroquinone, antraquinone

A radical mechanism has been proposed involving an initial electron transfer (ET) from the silyl telluride to the quinone followed by the formation of a phenoxy radical as precursor of the product. Quinones with higher reduction potential react faster that those with lower reduction potential. 1,2-Benzoquinones are also reduced to the corresponding bis-silylated hydroquinones. 4.10.12 Thiotelluration of vinyl cyclopropanes. Thio- and selenotelluration of acetylenes As part of a systematic study of the reaction of chalcogen radicals, generated by irradiation of the corresponding chalcogenides, with carbon–carbon multiple bonds, the highly selective and efficient thiotelluration of vinyl cyclopropanes to give allylic sulphides bearing a phenyl telluroethyl group at the terminal carbon has been investigated. The thio- and selenotelluration of acetylene to give vic-thio- and selenotelluroalkenes have been afforded successfully upon visible light irradiation in the presence of the systems (PhS)2/(PhTe)2 and (PhSe)2/(PhTe)2.21 It must be emphasized that radical addition of seleno and telluro moiety to acetylenes is unprecedented. The following features are noteworthy. The thiotelluration of a variety of olefins by visible light irradiation does not proceed at all (except for the special case of norbonene), probably because of a photoinduced

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regeneration of the starting materials. R

+ (PhS)2 + (PhTe)2 hυ

R

.

SPh

PhTe PhS. -PhS.

S .

SPh (PhTe)2

-PhTe.

hυ

In contrast, the thiotelluration of vinylcyclopropane proceeds efficiently since the intermediate radical (R⫽C-prop) can be converted into the acyclic primary radical avoiding the reverse process. + (PhS)2 + (PhTe)2 Ph

hυ (> 400 nm) CDCl3, 40°C, 7 h

PhTe

SPh Ph (84%) E / Z = 14 / 86

21

Thiotelluration of 1-styrylcyclopropane. In a Pyrex glass tube were placed 1-styrylcyclopropane (0.2 mmol, 28.8 mg, 0.4 M), diphenyl disulphide (0.2 mmol, 43.7 mg, 0.4 M), diphenyl ditelluride (0.2 mmol, 81.9 mg, 0.4 M) and CDCl3 (0.5 mL). The tube was filled with Ar, and the mixture was irradiated at 40°C for 7 h through a filter (hν >400 nm) with a tungsten lamp (500 W). The solvent was evaporated in vacuo. Purification by a recycling preparative HPLC (CDCl3) yielded 77 mg (84%, E/Z= 14:86) of 2-phenyl-1-(phenylthio)5-(phenyltelluro)-2-pentene as a pale yellow oil. The thiotelluration of acetylenes works well owing to the higher kinetical stability of the thiovinyl radical

S .

SPh

, compared to the thioalkyl radical, therefore suppressing the

reverse process. Ph

+ (PhS)2 + (PhTe)2

hυ (> 400 nm) CDCl3, 45°C, 1 h

Ph

SPh

PhTe (80%) 100% E

The thiotelluration of 1-octyne requires prolonged photoirradiation, giving rise to an isomeric mixture of products in moderate yields. This result depends on the different stability of the two vinylic radicals. The selenotelluration of acetylenes works well in the case of aromatic acetylenes, whereas aliphatic acetylenes provide a mixture of the expected selenotelluro adducts and the diselenation product. Ph

+ (PhSe)2 + (PhTe)2

hυ (> 400 nm) CDCl3, 45°C, 2 h

Ph

SePh

PhTe

[95% (E / Z = 90 /10)] n -C6H13

hυ (> 500 nm) n -C6H13 + (PhSe)2 + (PhTe)2 CDCl3, 45°C, 114 h PhTe

SePh + n -C6H13 PhSe

[29% (100% E)]

SePh

(29%)

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Thiotelluration of phenylacetylene.21 In a Pyrex glass tube were placed phenylacetylene (0.25 mmol, 25.5 mg, 1 M), diphenyl disulphide (0.25 mmol, 54.6 mg, 1 M), diphenyl ditelluride (0.25 mmol, 102.4 mg, 1 M) and CDCl3 (0.25 mL). The tube was filled with Ar, and the mixture was irradiated at 45°C for 1 h through a filter (hν >400 nm) with a tungsten lamp (500 W). The solvent was evaporated in vacuo, and purification by preparative TLC on silica gel (hexane/Et2O = 10:1) yielded 86 mg (80%, E/Z = 100:0) of α-(phenyltelluro)-β-(phenylthio)styrene as a pale yellow oil. Selenotelluration of phenylacetylene.21 In a Pyrex glass tube were placed phenylacetylene (0.25 mmol, 25.5 mg, 0.5 M), diphenyl diselenide (0.25 mmol, 78.0 mg, 0.5 M), diphenyl ditelluride (0.25 mmol, 102.4 mg, 0.5 M) and CDCl3 (0.5 mL). The tube was filled with Ar, and the mixture was irradiated at 45°C for 2 h through a filter (hν >400 nm) with a tungsten lamp (500 W). The residual mixture was purified by a recycling preparative HPLC (CDCl3) to provide 110 mg (95%, E/Z = 90:10) of α-(phenyltelluro)-β-(phenylseleno)styrene as a pale yellow oil. 4.10.13

Perfluoroalkyltelluration of terminal olefins and alkynes

The title reaction has been achieved by the treatment of alkenes and alkynes with sodium phenyltellurolate in the presence of perfluoroalkyl halides.22 The reaction proceeds via a single electron transfer (SET) from sodium tellurolate to perfluoroalkyl halides followed by a radical chain reaction of the SRN1 mechanism. R

PhTeNa / R1fX EtOH, 0°C

TePh R1f

R (30-81%)

R = n -C6H13, CH2CN, H, O R1fX = CF3Br, C4F9I, C8F17I I R

PhTeNa / R1fX EtOH / benzene, -40°C

R = n -Bu, MeC(OH)H, H R1fX = CF3Br, C4F9I, C8F17I X=I

TePh R1f (6-81%)

+ RfTePh + R1f

R

R

I + PhTe

R

Perfluoroalkyltelluration of olefins (typical procedure).22a Into a suspension of sodium phenyltellurolate prepared from (PhTe)2 (0.3 mmol) and NaBH4 (1.8 mmol) (dissolved in dry ethanol (1 mL) at 0°C for 15 min, 1-octene (6 mmol) and C4F9I (1.2 mmol) were added under N2 at −40°C. The colour of the solution turned from colourless to deep brown, immediately indicating the formation of (PhTe)2. The solution was stirred for 4 h while allowing the temperature to rise to room temperature, affording the product in 81% yield. Perfluoroalkyltelluration of alkynes (typical procedure).22b Sodium benzenetellurolate was prepared from (PhTe)2 (122.8 mg, 0.3 mmol) and NaBH4 (68.1 mg, 1.8 mmol) dissolved

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in EtOH (0.5 mL) and benzene (0.5 mL) at 0°C for 20 min. After the mixture was cooled at −40°C, 1-hexyne (0.69 mL, 6.0 mmol) and C4F9I (0.21 mL, 1.2 mmol) were added to the tellurolate solution under N2. The temperature of the reaction mixture was allowed to rise to room temperature for 2 h under constant stirring; then the organic product were extracted with ether several times. The combined extracts were washed with brine and dried on Na2SO4. After evaporation of the solvent, the residue was chromatographed through silica gel using hexane to give a mixture of 1-perfluorobutyl-2-tellurophenyl1-hexene (246.6 mg, 81%, E/Z = 75:25) and perfluorobutylphenyl telluride (20 mg, 8%) with 1-perfluorobutyl-2-iodo-1-hexene (13.2 mg, 5%). In the case of alkynes, a photochemical addition of the in situ-formed perfluoalkyl telluride (RfTePh) is also plausible. 4.10.14 Synthesis of indole derivatives via radical cyclization of N-(ortho ethynylbenzene)-phenyltelluro trifluoro acetimidates Trifluoromethylated organic compounds play an important role in medicinal and agricultural chemistry.23 The following scheme illustrates the synthesis of a trifluoromethyl indole derivatives via the title radical reaction. R R PhTeNa (PhTe)2 / NaBH4 EtOH

Cl F3C

N A .TePh

R

H2O

. F3C

TePh

R F3C

N O

PhTe F3C

hυ

N (90-95%) B H O TePh R

N H2O

F3C

N H

R -PhTeH

F3C

N H

X = Cl, I R = n -Bu, Ph, PhOCH2, MEMOCH2

C (54-68%)

Photolysis of phenyltellurotrifluoroacetimidates (typical procedure).23 Into a suspension of sodium phenyltellurolate prepared from (PhTe)2 (0.3 mmol) and NaBH4 (1.2 mmol) dissolved in dry EtOH (0.3 mL) and dry toluene (1.0 mL) at 0°C for 20 min, the chloride A (0.6 mmol) dissolved in dry toluene (0.2 mL) was added quickly and the solution was stirred under N2 at −80°C for 10 min. The usual work-up and chromatography (SiO2, hexane/AcOEt) gave the telluroimidates B (90–95%). Then a solution of B (0.3 mmol) and a drop of water in distillated THF (3.0 mL) was irradiated with a UV lamp (4 W × 2, 250–400 nm) at room temperature for 48–85 h to give 2-trifluoromethyl-3-acylindoles C (54–68%).

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283

4.10.15 Organotellurium compounds as initiators for controlled living radical polymerization The use of several diorganyl tellurides as initiators for controlled “living” radical polymerization of styrenes has been investigated.24 X

R-TeR1

heat

X

n [R. + .TeR]

R

( )n TeR1

R = Me; R1 = PhCHMe, PhCH2, Ph2CCO2Et, PhCHO R = Ph; R1 = PhCHMe, PhCOSiMe3

The efficiency of the polymerization was discussed25 on the basis of the bond dissociation energy (BDEn) and the reactivity of the initiating radical towards styrenes. R1 R-TeMe

R

heat

[R. + .TeMe]

4 R3 R

R1 R2 n

TeMe n m

n

R5 R6

R3

R2

R

1

R R R

2

n

R4

n TeMe R5 R6

4 R3 R

R1 R2 n

m

lTeMe

REFERENCES 1. (a) Curran, D. P. in Trost, B. M.; Fleming, I.; Semmelhack, M. F. (eds.). Comprehensive Organic Synthesis. Vol. 4, pp. 715 and 779. Pergamon, Oxford, 1991. (b) Curran, D. P. Synthesis 1988, 417 and 489. 2. Clive, D. L. J.; Anderson, P. C.; Moss, N.; Singh, A. J. Org. Chem. 1982, 47, 1641. 3. (a) Barton, D. H. R.; Ozbalik, N.; Sarma, J. C. Tetrahedron Lett. 1988, 29, 6581. (b) Barton, D. H. R.; Dalko, P. I.; Gero, S. D. Tetrahedron Lett. 1991, 32, 4713. 4. Barton, D. H. R.; Ramesh, M. J. Am. Chem. Soc. 1990, 112, 891. 5. Barton, D. H. R.; Gero, S. D.; Sire, B. Q.; Samadi, M.; Vincent, C. Tetrahedron 1991, 47, 9383. 6. Han, L. B.; Kambe, N.; Ogawa, A, Ryu, I.; Sonoda, N. Organometallics 1993, 12, 473. 7. Chen, C.; Crich, D.; Papadatos, A. J. Am. Chem. Soc. 1992, 114, 8313. 8. Chen, C.; Crich, D. Tetrahedron Lett. 1993, 34, 1545. 9. Crich, D.; Chen, C.; Hwang, J. T; Yuan, H.; Papadatos, A.; Walter, R. I. J. Am. Chem. Soc. 1994, 116, 8937. 10. Lucas, M. A.; Schiesser, C. H. J. Org. Chem. 1996, 61, 5754. 11. Lucas, M. A.; Schiesser, C. H. J. Org. Chem. 1998, 63, 3032. 12. (a) Engman, L.; Gupta, V. J. Chem. Soc. Chem. Commun. 1995, 2515. (b) Engman, L.; Gupta, V. J. Org. Chem. 1997, 62, 157. 13. Yamago, S.; Miyazoe, H.; Yoshida, J. I. Tetrahedron Lett. 1999, 40, 2339.

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14. (a) Yamago, S.; Miyazoe, H.; Goto, R.; Yoshida, J. I. Tetrahedron Lett. 1990, 40, 2347. (b) Yamago, S.; Miyazoe, H.;Sawazaki, T.; Goto, R.; Yoshida, J. I. Tetrahedron Lett. 2000, 41, 7517. (c) Yamago, S.; Miyazoe, H.; Goto, R.; Hashidume, M.; Sawazaki, T.; Yoshida, J. I. J. Am. Chem. Soc. 2001, 123, 3697. 15. See ref. 23 in the precedent ref. 14c. 16. Miyazoe, H.; Yamago, S.; Yoshida, J. I. Angew. Chem. Int. Ed. 2000, 39, 3699. 17. Yamago, S.; Miyoshi, M.; Miyazoe, H.; Yoshida, J. Angew. Chem. Int. Ed. 2002, 41, 1407. 18. Yamago, S.; Hashidume, M.; Yoshida, J. I. Chem. Lett. 2000, 1234. 19. Yamago, S.; Hashidume, M.; Yoshida, J. I. Tetrahedron 2002, 58, 6805. 20. Yamago, S.; Miyazoe, H.; Lida, K.; Yoshida, J. I. Org. Lett. 2000, 2, 3671. 21. Ogawa, A.; Ogawa, I.; Obayashi, R.; Umezu, K.; Doi, M.; Hirao, T. J. Org. Chem. 1999, 64, 86. 22. (a) Uneyama, K.; Kanai, M. Tetrahedron Lett. 1991, 32, 7425. (b) Ueda, Y.; Kanai, M.; Uneyama, K. Bull. Chem. Soc. Jpn. 1994, 67, 2273. 23. Ueda, Y.; Watanabe, H.; Uemura, J.; Uneyama, K. Tetrahedron Lett. 1993, 34, 7933. 24. (a) Yamago, S.; Lida, K.; Yoshida, J. I. J. Am. Chem. Soc. 2002, 124, 2874. (b) Yamago, S.; Lida, K.; Yoshida, J. I. J. Am. Chem. Soc. 2002, 124, 13666. 25. See ref. 11 in the preceding ref. 24a.

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–5– Telluroheterocycles

5.1

5.1.1

TELLURA-3,5-CYCLOHEXANEDIONE DICHLORIDES (3,5-DIOXOTELLURANE-1,1-DICHLORIDES)

From 2,4-dioxopentanes and tellurium tetrachloride and derivatives1

R3 O

R R2 R1

Cl Cl R3 Te R

TeCl4 CHCl3, reflux

O

O

R2 R1

O

R, R1, R2, R3 = H R, R3= n-alkyl; R1, R2 = H and derivatives with R1 or R1 and R2 = n-alkyl

No reaction with 1,1-dialkyl, 1,1,5,5-tetralkyl and 1,5-diisopropyl dioxopentane. The obtained dichlorides are easily reduced to the corresponding 3,5-dioxotellurane by treatment with aqueous potassium bisulphite. Cl Cl R3 Te R O

5.2 5.2.1

R2 R1

O

K2S2O5 H2O

R3 O

Te R R2 R1

O

OXA AND 1-THIA-4-TELLURANES

From Na2Te and 2-haloethyl ether or sulphide2,3 Cl Y Cl Y=O Y=S

Na2Te rongalite 44.5 % 6.6 % 285

Y

Te

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5.3

TELLUROPHENES

Tellurophene, the most important member of chalcogenophenes, is a light yellow, bad smelling and toxic oil, rather stable in air. Its aromaticity follows the order: benzene>thiophene>selenophene>tellurophene>furan.4 5.3.1

Preparation

5.3.1.1 From alkali metal tellurides (a)

With 1,3-butadieynes A convenient procedure for the synthesis of tellurophene employs the reaction of Na2Te with 1,4-bis(trimethylsilyl)-1,3-butadiene. The crude product is isolated as the corresponding dibromide, which is then reduced to tellurophene.5 Me3Si C C C C SiMe3

Na2Te rongalite method

Br2 Te

Na2S2O5 Te Br Br (84%)

Te (59%)

Tellurophene.5 A mixture of tellurium (4.0 g, 31 mmol), sodium formaldehyde sulphoxylate of 85% (28 g, 200 mmol), sodium hydroxide (17 g, 425 mmol) in 150 mL water is heated at reflux, under N2 atmosphere for 15 min, and then cooled at 20°C. A solution of 1,4bis(trimethylsilyl)-1,3-butadiene (8.2 g, 42 mmol) in 100 mL of ethanol is slowly added to the stirred sodium telluride solution, the mixture is heated at reflux for 15 min, then stirred at 20°C for 3 h and extracted with ether. The extract is dried (Na2SO4), filtered, and 10 mL (200 mmol) of bromine are added dropwise until the bromine colour persists. This solution is concentrated in a water bath under aspiration vacuum to a volume of 50 mL, and the red precipitate of tellurophene dibromide is collected: 8.9 g (84%), m.p. 120°C dec. The crude product, suspended in 100 mL of diethyl ether is treated with a solution of sodium sulphite (21 g, 170 mmol) and potassium carbonate (16 g, 161 mmol) in 250 mL of water in a 500 mL separatory funnel. The mixture is shaken until all the solid has disappeared, the ether layer is separated, the aqueous layer is extracted with ether and the combined organic phase is washed with water, dried (Na2SO4) and evaporated (aspiration vacuum) giving the yellowish oil of pure tellurophene: 3.3 g (59%), b.p. 102°C/350 torr (partial dec.). Sodium telluride reacts with 1-organo- and 1,4-diorgano-1,3-butadyines (prepared in situ by the treatment of the corresponding 1,4-dichloro-but-2-yines with NaOH in methanol) to give tellurophenes.6 Na2Te + R C C C C R1

MeOH

R, R1 = H, Ph, CH2OH, (CH3)2COH, R1 = H; R = Ph, (CH3)2COH

The yields of these reactions do not exceed 50%.

R

Te R1

N-CH2-

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(b)

With b-chloroenals

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The reaction of Na2Te with the title compounds, followed by the treatment of the formed sodium ethenetellurolates with an activated chloromethane derivative, gives 2,4-disubstituted tellurophenes.7,8 R

Cl CHO

R

Na2Te Na / Te/ NH3

TeNa X

Cl (Br) R

CHO

Te X

R = t -Bu, p- MeOC6H4, m -MeOC6H4 X = CHO, CH3CO, EtCO2, NO2

(c)

With 1,4-diiodotetraphenyl-1,3-butadiene The following scheme is self-explicative.8 Ph Ph

Ph I I

Ph

TeLi2

Ph EtOCH2OEt

Ph

Ph

Te Ph (82%)

Tetraphenyltellurophene.8 A solution of 1,4-diiodotetraphenyl-1,3-butadiene (2.0 g, 3.3 mmol), finely powdered lithium telluride (1.5 g, 11 mmol), in 125 mL of 2-ethoxyethyl ether is heated at reflux, under stirring and flushing with N2, for 8 h. The mixture is then poured in 1000 mL of water, the organic phase is separated and submitted to usual workup. After evaporation, the residue is recrystallized from dicloromethane/ethanol, giving the product: 1.3 g (82%), m.p. 239°C. 5.3.1.2

From tellurium tetrachloride

TeCl4 reacts with 1,4-dilithiotetraphenyl-1,3-butadiene to give tetraphenyltellurophene.9 Ph Ph

Ph I I

Ph

TeCl4 EtO2

Ph Ph

Ph

Te Ph (56%)

Tetraphenyltellurophene.9 To a suspension of 1,4-dilithiotetraphenyl-1,3-butadiene in 100 mL of ether, prepared from 10 g (56 mmol) of diphenyl acetylene and excess of lithium, is added over 15 min a solution of TeCl4 (5.3 g, 19.7 mmol). The green mixture is poured into a mixture of CH2Cl2 and water, the organic phase is separated, filtered through anhydrous MgSO4, filtered and evaporated. The residue is recrystallized from dichloromethane/ethanol, giving tetraphenyltellurophene: 5.35 g (56%), m.p. 239°C.

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5.3.1.3

From 1,4-dibutyltellurobutadiene

Bis-tellurobutadiene exhibits an uncommon property, giving tellurophene by treatment with 1 equiv of n-BuLi followed by quenching with water.10

n-BuTe

TeBu-n

n -BuLi THF, -78°C -Bu2Te

n -BuTe

SO2Cl2

H2O Te Li PhCHO

Te

Te

Te Cl Cl (61%)

1) Se 2) EtBr

OH

Ph (60%)

Li

Te SeEt (43%)

The tellurophene is formed via a first Te–C sp2 cleavage followed by an unusual Te–C sp3 cleavage. The treatment of the reaction mixture with benzaldehyde or with elemental selenium followed by ethyl bromide affords the 2-substituted tellurophenes. The selective Te/Li transmetallation and the subsequent functionalization reactions occur with total retention of the double bond configuration. 5.3.1.4 (a)

From butyltellurobutenines

By iodocyclization

Treatment of (Z)-butyltellurobutenines with iodine in petroleum ether produces 3iodotellurophenes.11 This conversion involves the intermediacy of an iodonium ion which suffers the attack of the iodide anion, promoting cyclization through the tellurenyl iodide (pathway a) or by the direct pathway b. The crude products are treated with aqueous NaBH4 to remove excess I2 and reduce the formed diiodide.

R R2Te

I+

2 I2 petroleum ether r.t. R1

R1

R Te Bu I I

a -BuI

R

Te I

R1

I-

IR = R1 = Ph, p-MeC6H4, p-MeOC6H4 Me, H; R2 = n -Bu R = H; R1 = Ph; R2 = n -Bu R R = R1 = Ph; R2 = p-MeOC6H4

I+

b I Te R1 I I

I

I2

Te R1 (40-80%)

NaBH4 R

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(b)

Under Rupe reaction conditions

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289

(Z)-butyltellurobutenines, upon treatment with boiling 85% formic acid (Rupe reaction conditions), do not form the expected ,-unsaturated ketones, but are converted into substituted tellurophenes.12 Ph

Ph

Te

HCO2H

BuTe

R=H

R

Te Ph (78%)

HCO2H O BuTe

Ph

Ph

R = Ph

Te Ph (41%)

The unexpected formation of the tellurophene compounds is probably due to the presence of the phenyl group in the starting tellurobutenines. 5.3.2

Reactions

5.3.2.1

Via 2-lithiotellurophene 2-substituted derivatives

By treatment with equimolar amounts of methyl-13 or n-butyllithium,6 tellurophene is converted into 2-lithiotellurophene, which is the starting material for several synthetic manipulations. MeS)2 (14,15) MeS

(16)

Te

X Te X = Cl (53%) X = Br (44%)

(16) Te Te Te (11%) (16)

Te

5.3.2.2

CO2 / Et2O

Te (50%)

I+ Te

C2X6 ether, -70°C

RLi Et2O

Te Li

Te ether / benzene 20°C Cl2+I H H Cl ether / benzene -70°C

Me Ph N CHO ether, reflux CH3CHO ether, -15°C

Te CO2H + (38%) Te CHO (6) (24%) Te

(6)

(60%) Me2SO4 ether

Te Me (65%)

Formylation17 COCl2 /DMF R

OH

Te

R = H, Me

R

Te CHO

(6)

Te

Te O (6%)

(6)

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5. TELLUROHETEROCYCLES

5.3.2.3

Acetylation18 MeCO)2O R

Te

R

SnCl4

Te O

R = H, CO2CH3

5.3.2.4

Chloromethylation18 ClH2C Ph

5.3.2.5

Te Ph

CH2O / HCl HOAc

Ph

CH2Cl Te Ph

Acetoxymercuriation19 CH3CO2)2Hg Te

5.3.2.6

EtOH, reflux

H3CO2CHg

Te HgO2CCH3

Modifications of the functionalized tellurophenes

The functionalized tellurophenes obtained in accordance with the above scheme can be submitted to further transformations such as (a)

replacement of halogen atom in chloromethylderivatives by hydroxyl, alcoxy groups, bromine, pyridine18, sulphur, or selenium;20 conversion of hydroxymethyl tellurophenes into bromomethyl,18 formyl,18 and acetyloxy derivatives;21 conversion of carboxytellurophenes into the corresponding methylesters,6,22 alcohols,14 amides,23 and into decarboxylated tellurophenes;24 hydrolysis of alcoxycarbonyl tellurophenes to the free acids;24,22 and reactions of 2,5-diphenyl-3-iodo tellurophene with n-butyllithium.25

(b) (c) (d) (e)

The treatment of the title compounds (see Section 5.3.1.4a) with 0.75 equiv of n-BuLi at ⫺78°C leads to the ditelluride B, whereas the telluride C is formed by the slow addition of 2 equiv of n-BuLi at room temperature. The following scheme rationalizes these conversions. I Ph

n- BuLi 0.75 equiv, -78°C Te Ph Ph n- BuI A n- BuLi 2 equiv, r.t. Te

Ph

Ph

Li B

Ph

Ph C

Ph

Ph

Ph Te-

Ph

+ Ph

TeI

Ph

n- BuLi n - BuTe Ph

Ph

Ph

A

Te Ph

Te Te

Ph n - BuLi Ph

B

Ph

-

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The sequence depicted in the following scheme is additional evidence that the iodocyclization of butyltellurobutenine (see preceding Section 5.3.1.4a) occurs by pathway a. Te Te

Ph

I2

ITe Ph

R Ph

Ph

I

I+

I2 Te I

R1

Ph

Te Ph

I-

In additional experiments the iodotellurophene was treated with 1.0 equiv of the nBuTeLi to give the corresponding butyltelluro compound, which is the result of an unexpected nucleophilic substitution. I

Li

TeBu- n

PhTeLi Ph

5.3.2.7

Te Ph

Ph

+ PhTeI

Te Ph

Ph

Te Ph

Formation of complexes

Tellurophene forms a charge-transfer complex with tetracyanoethylene, and a complex with chromium tricarbonyl and with sodium tetrachloro-palladate(II). Tetraphenyltellurophene forms a complex of indefinite structure with triiron dodecarbonyl.26 Tetraphenyltellurophene forms a complex of indefinite structure with triiron dodecarbonyl.26 5.3.2.8

Removal of tellurium from the ring

2,4-Diphenyl tellurophene suffers a Te/Li exchange by treatment with n-BuLi giving the 1,4-dilithio derivative (see Section 4.8.1).

Ph

Te Ph

n- BuLi hexane, Me2NCH2)2

Ph

Li Li

Ph

Grignard reagents in the presence of Ni-phosphine complexes remove tellurium from tellurophene to give 1,4-disubstituted butadienes.27

Te

5.4

+ RMgX

L2.NiCl2 R

HH

R

1-BENZOTELLUROPHENES

Te

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5.4.1 5.4.1.1

5. TELLUROHETEROCYCLES

Preparation From tellurium

Ortho-lithiated phenylethynyllithium reacts with elemental tellurium to produce 1-benzotellurophene.28 Ph C CH

n- BuLi THF, -40°C

Ph C CLi

K

t - ButOK THF, -70°C

LiBr THF, -10°C Li

Li

TeLi

Te THF, 0°C

1)THF, -30°C Li

Li

HMPA 2) 30°C

Te (85%)

In a one-pot procedure ortho-bromoethynylbenzene, prepared by Pd-catalysed coupling of iodobromobenzene with alkynes, is converted into the ortho-telluroderivatives which suffer an intramolecular anti-hydrotelluration giving 1-benzotellurophenes.29 R I

R H Br PdCl2 /(Ph3P)2CuI

Te (47-54%)

R 1) t - BuLi 2) Te 3) EtOH

Br (78-93%)

TeH

NaBH4

R

Te

R = TMS

R = Me, Bu, t-Bu, Ph, TMS

5.4.1.2

From TeO2

Phenylacetylenes (3 mol) react with TeO2 (1 mol) and excess of lithium halide in refluxing HOAc to produce 3-halobenzotellurophenes via the addition of a Te(IV) acetate halide to the triple bond, cyclization (probably by loss of HOAc) and reduction of the Te(IV) cyclic product to 3-halobenzotellurophene by an excess of the phenylacetylene. Owing to isolation facilities, the product is converted into the crystalline dichlorides that is reduced to the benzotellurophenes.30 X X OAc Te R

R1

TeO2 / LiX HOAc

"reductive elimination" LiX

R

X = Cl, Br, I R = H, Me, Bz; R1 = H, Me

R1

R

X -HOAc

X

Te

R1

Cl2 Na2S2O5 R

R

Te X X

R1 Te Cl Cl

R1

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3-Bromobenzotellurophene.30 A mixture of 2.0 g (19.6 mmol) of phenylacetylene, 1.0 g (6.3 mmol) of tellurium dioxide, 2.0 g (23 mmol) of lithium bromide and 50 mL of acetic acid is heated under reflux for 20 h, cooled to 20°C, and poured into 150 mL of diethyl ether. Aqueous sodium hydrogen carbonate solution (5%) is added until all the acid has been neutralized. The organic phase is separated, dried with anhydrous calcium chloride, filtered and evaporated. The brown, oily residue is dissolved in a mixture of 30 mL of carbon tetrachloride and 10 mL of petroleum ether (b.p. 30–40°C). Chlorine is carefully bubbled through this solution until precipitation of the product ceases. The yellow precipitate is filtered and recrystallized from acetonitrile. Yield: 2.2 g (92%); m.p. 263–265°C. The obtained 3-bromobenzotellurophene dichloride is suspended in diethyl ether and an excess of 5% aqueous sodium disulphite solution is added. The mixture is shaken thoroughly until all of the organic material has dissolved. The organic layer is separated, dried with anhydrous calcium chloride, filtered, and the filtrate is evaporated. The oily residue is pure 3-bromobenzotellurophene. Yield: 100%. 5.4.1.3 Via cyclization of ortho-acetyl or formyl-substituted phenyl telluro compounds The intramolecular condensation of o-acetylphenyl tellurenyl bromide in basic medium gives 3-oxo-2,3-dihydro benzotellurophene.31 TeBr Me

Te

KOH / EtOH -HBr

O

O

3-Oxo-2,3-dihydrobenzotellurophene.31 KOH (0.85 g, 15 mmol) in ethanol is added to a solution of o-acetylphenyl tellurenyl bromide (5.0 g, 15 mmol) in 100 mL of ethanol. The solution is stirred and then poured in 200 mL of water. The precipitate is filtered off and recystallized from cyclohexane (3.0 g (80%); m.p. 107°C). By treatment of the same starting compound with aromatic aldehydes in HOAc at reflux in the presence of pyridine, 2-benzylidene derivatives are produced.32 TeBr

a) ArCHO Me HOAc, reflux (-H2O) piperidine O

b) HOAc / piperidine -HBr

TeBr

-HBr

Te CHAr

Ar O Te

O

H

ArCHO

-H2O O Ar = Ph, p-MeC6H4, p-MeOC6H4, p-ClC6H4, p -Me2NC6H4, m-ClC6H4, p-IC6H4, 1-naphthyl

The two formulated pathways can be operative.

(80%)

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o-Formylphenyl carbonyl or carboxylmethyl tellurides, refluxed in a py/Ac2O mixture, give rise to o-carbonyl benzotellurophenes.31,33 TeCH2COR Ac O/py 2 CHO R = OH (80%) R = Me

reflux

Te

COR

2-Carboxybenzotellurophene.33 A mixture of 15.5 g (53 mmol) of 2-formylphenyl carboxymethyl tellurium, 50 mL of pyridine and 50 mL of acetic anhydride is heated under reflux for 2 h. Most of the solvent is then evaporated under vacuum, and the residue is extracted with boiling of 1 M aqueous sodium hydroxide solution. The extract is neutralized, the precipitate is filtered off and the solid is recrystallized from ethanol/benzene. Yield: 11.6 g (80%); m.p. 206–208°C. 5.4.1.4

From o-phenylethenyl tellurium trichloride

3-Chlorobenzotellurophenes were obtained by the thermal cyclization of 2-chloro-2phenylethenyl tellurium trichlorides.30,34

Cl TeCl3 R Cl

1,2,4-Cl3C6H3 reflux 2.5 h R = Ph

Te

Cl R

Na2S.9H2O Te

R

Cl Cl Cl

LiCl/HOAc reflux R=H

Te

By treatment with trifluoroacetic acid at reflux, 3-chlorobenzotellurophene produces the corresponding 3-oxo-2,3-dihydro compound.30 Cl

O

Cl H2O + + Te -H , -HCl

H+ Te

Te

In turn the 3-oxo-2,3-dihydrobenzotellurophene is converted into 3-halobenzotellurophene by treatment with carbon tetrachloride or bromide in the presence of Ph3P.30

O

X CX4 / Ph3P

Te

X = Cl, Br

Te

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5.4.2 Reactions of 1-benzotellurophene and 3-oxo-2,3-dihydrobenzotellurophene 2-Lithiobenzotellurophene reacts with DMF and CO2, giving the expected 2-formyl and 2-carboxy derivative.35 DMF

CHO

Te

n- BuLi Et2O Te

Te

Li CO2 Te

CO2H

2-Formylbenzotellurophene.35 Benzotellurophene (2.3 g, 10 mmol) is dissolved in 15 mL of dry diethyl ether, and a solution of 11 mmol of n-butyllithium in diethyl ether is added dropwise. The mixture is stirred for 0.5 h and then 1.0 g (14 mmol) of dimethylformamide in 10 mL of diethyl ether is added. The yellow product is isolated by normal work-up of the reaction mixture; yield: 1.5 g (60%); m.p. 108°C. 3-Oxo-2,3-dihydrobenzotellurophene has been submitted to several transformations: (a) (b) (c)

reduction to benzotellurophene by NaBH4 in EtOH;31 condensation of the methylene groups with aromatic aldehydes,31 DMF36 or (EtO)2CH-NMe2,31 4-nitroso-N,N-dimethylaniline;31 reaction with hydroxylamine, 2,4-dinitrophenyl hydrazine and semicarbazide31 giving the expected derivatives;

NaBH4 Te (20 %) O ArCHO

CHAr Te Ar = Ph, m - MeC6H4, p - NO2C6H4

O Te

ON

O

NMe2

N Te O DMF or (EtO)2CH-NMe2

Te NY

H2N-Y Te Y = OH, -NHCONH2, O N 2

NH NO2

CH-NMe2

NMe2

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(d)

oxidation to telluroindigo by refluxing in DMF in the presence of air31 or by treatment with K2FeCN6.37 O

O

air Te or K2FeCN6

O

Te Te (73 %)

Benzotellurophenes and 3-oxo-1,3-dihydrobenzotellurophene are converted into the corresponding dihalides by treatment with halogens in inert solvents.30,31,33,35 In turn, benzotellurophene dichlorides are reduced back to benzotellurophenes by sodium disulphite or sodium sulphide nonahydrate.30,34 R

R

X2 R solvent Te

R Te X X

Na2S2O5 or Na2S.9H2O X = Cl, Br, I O X2

O

Te

Te X X

X = Cl, Br, I

5.4.3

Ring cleavage of the tellurophene ring

3-Bromobenzotellurophene is cleaved by n-BuLi forming butyl-2-ethynylphenyl telluride.30 Br Te

H n - BuLi hexane, -50°C -LiBr

TeBu

Sodium hydrogen sulphite,31 hypophosphorous acid38 and concentrated hydrobromic acid32 cleave 3-oxo-2,3-dihydrobenzotellurophenes. O Te]2

NaHSO3 / EtOH or H3PO2

2

Te

Me

(80%) O O

O Te

CHC6H5

HBr HOAc (95%)

CH=CHC6H5 TeBu

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5.5

BENZOTELLUREPINES, TELLUROCHROMENES AND TELLUROCHROMONES O Te Te

Te

R 1-Benzotellurepines

5.5.1

Te R

R 2-Alkylidene-2Htellurachromenes

Tellurochromones

3-Benzotellurepines

Preparation

The intramolecular version of the anti-hydrotelluration of alkynes has been applied to synthesize 1-benzotellurepines, 2-alkylidene-2H-tellurochromenes and tellurochromones. The starting acetylenic substrates are prepared by the Pd2⫹-induced coupling of the alkynes with the appropriate o-substituted bromobenzenes.39

Br Br

H R PdCl2(PPh3)2.CuI benzene / piperidine

1) t - BuLi, THF, -80°C, 1 h R 2) Te, -40°C, r.t., 1 h

Br (77-93%)

3) K3FeCN6

NaBH4

Te]2

R

(55-83%)

+ TeH

R

Te (8-48%)

R

Te (5-51%)

R

R = Me, n- Bu, t - Bu, n- hex, c - hex, n- oct

(Z)-4-alkyl-1-(o-bromophenyl)-1-buten-3-ynes (general procedure).39 Bis(triphenylphospbine)palladium dichloride (350 mg, 0.5 mmol) and copper(I) iodide (200 mg, 1.05 mmol) were added to a stirred mixture of (Z)--o-dibromostyrene (13.1 g, 50 mmol) and an alkylacetylene (60 mmol) in benzene–piperidine (1:1) (100 mL) at room temperature; the mixture was heated at 50–60°C with stirring. In the case of gaseous methylacetylene, a steady stream of this gas, generated in situ by treatment of 1,2-dibromopropane with potassium hydroxide, was passed through the mixture during the period of the reaction. The reaction was followed in terms of the disappearance of the spot of the starting material on TLC and was complete in 8–10 h. After cooling, cold water (100 mL) was added to the reaction mixture with stirring. The layers were separated and the aqueous layer was extracted with benzene (3 ⫻ 70 mL). The combined organic layer was successively washed with water (3 ⫻ 100 mL), 5% H2SO4 (2 ⫻ 100 mL), saturated aqueous NaHCO3 (2 ⫻ 100 mL) and brine, then dried over MgSO4 and concentrated in vacuo. The resulting reddish yellow oily residue was vacuum-distilled to give the butenines as pale yellow oils. Di[o-(1-buten-3-ynyl)phenyl] ditellurides (general procedure). A tert-butyllithium hexane solution (1.5 M, 14.7 mL, 22 mmol) was added dropwise over a 15-min period to a stirred solution of the above-obtained butenine (10 mmol) in anhydrous THF (50 mL) at ⫺ 80°C

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5. TELLUROHETEROCYCLES

under an argon atmosphere; the mixture was stirred for 1 h at the same temperature. The reaction mixture was allowed to warm to ⫺40°C and tellurium powder (1.27 g, 10 mmol) was added in one portion; then the mixture was warmed to room temperature over a 1-h period with stirring. Stirring was continued for an additional 1 h, then an aqueous solution (120 mL) of potassium ferricyanide (4.0 g, 12 mmol) was added to the reaction mixture. The whole was stirred for 30 min and extracted with ether (3 ⫻ 100 mL). The combined extract was washed with brine, dried and concentrated in vacuo. The residue was chromatographed on silica gel with hexane–benzene (1:1) to give the ditellurides as viscous oils. 1-Benzotellurepines and 2-akylidene-2H-tellurochromones (general procedure). NaBH4 (200 mg, ca. 5 mmol) was added in small portions to a stirred solution of the ditelluride (1 mmol) in THF–EtOH (1:1) (20 mL) at room temperature under an argon atmosphere, and then the solution was heated at 55–60°C with stirring. The reaction was followed in terms of the disappearance of the starting material on TLC and was complete in 5–10 h. After cooling, ice water (50 mL) was added to the reaction solution and the whole was extracted with hexane (3 ⫻ 50 mL). The combined extract was washed with brine, dried and concentrated in vacuo. The residue was chromatographed on silica gel with hexane to give successively the benzotellurepine and the tellurochromenes. The treatment of o-bromophenyl ethynyl ketones with sodium hydrogen telluride follows a similar pathway giving tellurochromone.40 O

O Cl

+ R

Br O

TeH

H

PdCl2(Ph3P)2.CuI

Br

NaHTe (NaBH 4 + Te R DMF)

O R

R = Me, n - Bu, t -Bu, n - hex, n - oct, Ph Te R (28-63 %)

Preparation of tellurochromones (typical procedure).40 To a mixture of a 1-alkyne (0.1 mol) and o-bromobenzoyl chloride (21.95 g, 0.1 mol) in Et3N (160 mL) and benzene (40 mL) were added PdCl2[Ph3Ph]2 (100 mg) and CuI (100 mg). The mixture was stirred at room temperature under argon for 12–15 h. After addition of MeOH (10 mL), the solvent was removed under reduced pressure. Benzene (300 mL) and water (200 mL) were added to the residue and the aqueous layer was extracted with benzene (2 ⫻ 200 mL). The combined organic extract was washed with water (5 ⫻ 200 mL), 5% H2SO4 (3 ⫻ 200 mL), saturated NaHCO3 (2 ⫻ 200 mL) and brine (2 ⫻ 200 mL), and then dried (MgSO4). Benzene was removed in vacuo. The red residual oil was purified by chromatography (silica gel, n-bexane) to give pure o-bromophenyl ethynylketone. In the case of R ⫽ Me, a slow stream of methylacetylene, which was prepared from 1,2-dibromopropane and KOH in refluxing BuOH, was immediately passed through the mixture without isolation. A solution of o-bromophenyl ethynyl ketone (10 mmol) in DMF (20 mL) was slowly added to a stirred solulion of NaHTe (12 mmol), which was prepared from tellurium

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powder (1.54 g) and NaBH4 (0.54 g) in DMF (40 mL), at ⬃100°C for 1 h under argon. The mixture was stirred under these conditions for 2–5 h. After the addition of water (100 mL), the mixture was filtered. The filtrate was extracted with benzene (3 ⫻ 100 mL). The combined organic extracts were washed with water (3 ⫻ 200 mL) and brine (2 ⫻ 200 mL), dried (MgSO4) and concentrated. The resulting residue was purified by chromatography (silica gel, n-hexane/acetone, 50:1) to give the tellurochromone. Crystalline products were recrystallized from n-hexane/acetone. 3-Benzotellurepine has been prepared by the addition of Na2Te to o-diethynylbenzene in the presence of hydrazine hydrate, under PTC conditions. The compound is quite unstable but is converted into the more stable dihalides by treatment with SO2Cl2 or Br2. The dihalides regenerate the tellurepine by reduction with Na2S.41 H

Na2Te

r.t. -Te

Te

H2NNH2.H2O + H benzeno / H2O / MeOct3N ]Cl Na2S X = Cl, Br Hex / H2O

SO2Cl2 or Br2 Te

X X

(50-70 %)

5.5.2

Reactions

The tert-butyl-1-benzotellurepine (R ⫽ t-butyl) has been submitted to a variety of interesting synthetic manipulations.42 MeMgI NiCl2dppp

Me

Me

SO2Cl2 X2

Te X X X = Cl, Br, I

Na2S

H eO ,M OH BA H Na H O CP m- , Na NaO S l, NC C or t-BuO or

Flash vaccum pyrolisys 650°C

Te

1) n-BuLi 2) H2O

1) n-BuLi 2) MeI

H

Li I

H

Ac2O Te O

Te AcO OAc

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5. TELLUROHETEROCYCLES

BENZO-[c ]-TELLUROPHENES

5.6 5.6.1

Preparation

Benzo-[c]-tellurophenes have been synthesized starting from o-bishalomethyl benzenes, as depicted in the following scheme.43 X X

R

Te / NaI* MeO(CH2)2OH

Te R

CO2R ROC(O)Cl CO2R

Et3N Li Te

R SO2C6H4Me - p Te

(p -

C

Me

Te R

X = Br R = NO2

Te R

X F CCO Ag 3 2 X

)2 SO 2 Li 4 H 6

Et3N

O2CCF3 O2CCF3

X = Cl; R = H (78 %) X = Br; R = H, MeO

(62 %)

2 n -BuLi / THF -78°C R=H

R

Te +

Te R minor product

R SO2C6H4Me - p *see ref. 44

The following features are noteworthy. In the case of R ⫽ H and MeO, all the attempts to convert the diiodide into the tellurophene by direct elimination of Hl with a base, failed. Such conversion was attained via the bistrifluoroacetate. However, as expected, the nitro derivative (R ⫽ NO2) undergoes facile dehydroiodination upon treatment with Et3N. Benzotellurophene (R ⫽ H) is stable only in benzene solution at low temperature, but the diester and disulphonyl derivatives (obtained by the bis-lithiation and subsequent reaction with alkyl chloroformates and p-toluenesulphonic anhydride) as well as the nitroderivative are more stable. Synthesis of 1,3-dihydro-2,2-bis(trifluoroacetoxy)-benzo[c]tellurophene.43 A mixture of 2,2-diiodo-1,3-dihydrobenzo[c]tellurophene (4.86 g, 10 mmol) and silver trifluoroacetate (4.42 g, 20 mmol) in benzene (200 mL) was stirred at room temperature for 2 h. After filtration, the filtrate was concentrated to give 1,3-dihydro-2,2-bis(trifluoroacetoxy)benzo[c]tellurophene (4.17 g, 91%), m. p. 160°C (dec.). Benzo[c]tellurophene.43 A mixture of 1,3-dihydro-2,2-bis(trifluoroacetoxy)-benzo[c]tellurophene (0.916 g, 2 mmol), triethylamine (2.02 g, 20 mmol) in degassed benzene (200 mL) was refluxed under argon for 15 min. The resulting solution was washed with deionized water (2 ⫻ 200 mL) and dried over sodium sulphate. Removal of solvent in vacuo at room temperature resulted in a mixture (0.36 g) of benzo[c]tellurophene and 1,3-dihydrobenzo[c]tellurophene (78%, 89% of 2-benzotellurophene).

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5.7 5.7.1

301

TELLURO[3,4-c]THIOPHENE

Preparation

A similar protocol applied to 2,5-biscarboxymethyl-3,4-bis bromomethyl thiophene gives rise to a thiophene–tellurophene diannellated compound A, which is the first telluriumcontaining diheteropentalene. It cannot be isolated since it gives a dimeric product B and the reduced compound C.45 The dimer heated with dimethylacetylene dicarboxylate (DMAD) gives the adduct D, resulting from the addition to the tellurophene moiety of A. The adduct E resulting from the addition to the thiophene moiety of A was not detected. CO2Me

CO2Me Br 1) Te / NaI, DME Br 2) F CCO Ag 3 2

S

S

CO2Me

CO2Me

OCOCF3 Te OCOCF3 CO2Me

Et3N

S

CO2Me

D

DMA

MeO2C

CO2Me

-Te

S MeO2C (11%) D

A

X

CO2Me

CO2Me Te

MeO2C S

E

S

CO2Me S

Te

CO2Me CO2Me + (29%) B CO2Me

CO2Me

MeO2C

Te

Te

S

CO2Me

Te CO2Me

C

(15%)

5.8 5.8.1 5.8.1.1

DIBENZOTELLUROPHENES

Preparation From tellurium

Tellurium replaces sulphur dioxide in dibenzothiophene dioxide,46 and in thianthrene tetraoxide47 and 2,2⬘-biphenylmercury.48 Te , -SO2

S O

Te

O

Te + 1/4 R R = H, Me

Te , -SO2

Hg

R 4

270°C vacuum, -Hg

R

O O S S O O

Te (79, 82 %)

R

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Dibenzotellurophene.46 Tellurium powder (6 g, 47 mmol) and dibenzothiophene S,S-dioxide (8 g, 37 mmol) are mixed thoroughly, the mixture is carefully heated under an atmosphere of carbon dioxide until evolution of sulphur dioxide commences, and the temperature is then regulated to achieve a steady evolution of sulphur dioxide. From time to time the sublimed dibenzothiophene dioxide is melted and allowed to flow back into the reaction mixture. After 36 h, the mixture is cooled to 20°C and extracted with boiling acetone. The extract is evaporated to dryness, the solid residue is washed several times with cold ethanol, and the washings are collected and evaporated. The residue is steam distilled and the product is recrystallized from light petroleum ether. Yield: 1.0 g (10%); m.p. 93°C. 3,7-Dimethyldibenzotellurophene.48 Powder tellurium (0.94 g 7.4 mmol) is thoroughly mixed with 2.61 g (6.1 mmol) of pure 4,4-dimethyl-2,2⬘-biphenyldiyl mercury, the mixture is placed into a sublimation apparatus equipped with a cold finger, and slowly heated under vacuum in a metal bath to 260–270°C. Colourless needles begin to sublime at 228°C. The temperature is held at 260°C for 12 h. The sublimate is recrystallized from a mixture of 300 mL of methanol and 10 mL of carbon tetrachloride. Yield: 1.38 g (79%); m.p. 158°C. Dibenzotellurophene was similarly obtained in 82% yield. 5.8.1.2

From tellurium dichloride48 TeCl2 +

ether, -78°C Te (54%)

Li Li

Dibenzotellurophene.48 In an uncommon use as synthetic reagent, tellurium dichloride (10.6 g; 53.4 mmol) was added to 2,2⬘-dilithiobiphenyl (61.6 nmmol), in dry ether. The stirred mixture is allowed slowly to warm to 20°C. After 15 h the mixture is hydrolysed, filtered and the organic phase is separated. The ether is evaporated, the pasty residue is dissolved in petroleum ether (b.p. 40–60°C) and the solution is chromatographed on a column of neutral alumina. Yield: 8.0 g (54%); m.p. 94°C. 5.8.1.3

From tellurium IV halides49 heat

TeX4 +

Te X X

X = Cl, Br

The reaction of TeCl4 with two or more molar equivalents of 2,2⬘-dilithiobiphenyl gives dibenzotellurophene and the formulated telluronium chloride as by-product.48 The scheme rationalizes this result.

Li Li

Et2O

TeCl4 + Li Li

Te X X

-2LiCl

Te

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303

Et2O

TeCl4 + 2

+ Te

+HCl

Te

Cl-

Li Li

2-Biphenyl tellurium trichloride by heating in nitrobenzene cyclizes to dibenzotellurophene dichloride.50 210°C TeCl3

-HCl

Te Cl Cl

The corresponding tribromide furnishes similar results when heated at its melting point (181°C). 5.8.1.4

From bis[2,2⬘⬘-biphenyldiyl]tellurium

By heating, the title compound produces dibenzotellurophene in 76% yield.48

260°C

Te

Te

5.8.2 5.8.2.1

Reactions Halogenation

Dibenzotellurophene adds halogen to give the corresponding dihalides which can be submitted to further conversions.

Te

2AgX / acetone -2AgCl

inert solvent X2

(ref. 51)

Te X X X = o - NO2, o - ClO3, OCOCH3

NaOH

Te X X

(ref. 51) Te O

X = Cl (ref. 48) X = Br (ref. 50) X = I (ref. 48,50)

RONa

(ref. 52) Te RO OR

OHC R=

,

, N

N

CH3

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5. TELLUROHETEROCYCLES

The dihalides in turn are reduced to dibenzotellurophene by KHSO349,50 and Na2S·9H2O.48,53 5.8.2.2 (a)

Cleavage of Te–C bond

Conversion into 2,2⬘-bis(trichlorotelluro)biphenyl54 Cl Cl + TeCl4

Te Cl Cl

(b)

TeCl3 TeCl3 (87%)

Conversion into the corresponding thiophenes55 R

R

R

R R

330°C

+ S R

Te

R R = H, F

(c)

R

R

R

R

R

R S R

R R

Conversion into 2,2⬘-dilithiobiphenyl56

+ 2 n-BuLi

ether

Te Cl Cl

5.9

Li Li

NAPHTOTHIA-, NAPHTOSELENA- AND NAPHTODITELLUROLE

Te Y Y = S, Se, Te

1,2-Diselenolo-1,2-ditellurolonaphtacene bis(1,2-ditellurolo)naphtacene (Y ⫽ Se, Te)

Te Te

Y

Y

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2H-1,3-DITELLUROLES

305

Preparation

The following scheme depicts the preparation of the title compounds. Br

Br

Li

Li

2 n- BuLi R

R1

R

R1

Te Te 1) Te / THF -78°C 2) air/ H2O

R

R1

R1

R, = H (10%) (ref. 57,58) R, R1 = CH = CH (14%) (ref. 59) Br

Br

Li

Br

n- BuLi YLi TeLi

Y

Y = S (2%) Y = Se (4%)

air

SH Cl

SLi TeLi

SLi Li n- BuLi THF, -78°C

n - BuLi

Te

H2O

Te THF, -78°C

YLi Li

YLi Br Y THF, -78°C

(ref. 58,60)

S

Te

H2O

Te THF

(ref. 58,60)

air (60%)

Cl

Te Te

Cl Na2Te2

(ref. 61)

DMF Cl

Cl

Cl

Cl

Te Te Te Te Na2Te

(ref. 62)

DMF Se Se

Se Se

Bis(1,2-ditellurolo)naphtacene.61 Tellurium powder (7.65 g, 60 mmol), 1.38 g (60 mmol) of sodium and 100 mL of dry dimethylformamide are placed in an argon-flushed flask fitted with a magnetic stirrer. The mixture is stirred and heated at 100°C for 1 h and then cooled to 55°C whereupon 5.0 g (14 mmol) of 5,6,11,12-tetrachlorononaphthacene followed by 100 mL of dimethylformamide are added. The resultant mixture is heated at 50°C for 20 h, poured into brine and filtered. The solid is dried, extracted with acetone and benzene and the solid is recrystallized from chlorobenzene. Yield: 1.28 g (13%); m.p. >360°C. 5.10

2H-1,3-DITELLUROLES Te Te

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5.10.1 5.10.1.1

5. TELLUROHETEROCYCLES

Preparation From alkalimetal ethynetellurolates

The procedure is based on the reaction of lithium ethynyltellurolates (generated by the successive treatment of acetylenes with n-BuLi and elemental tellurium) with iodochloromethane giving ethynylchloromethyl tellurides. Subsequent reaction with lithium telluride forms lithium(ethynyltelluro) methane tellurolates which cyclize via an intramolecular addition of the tellurolate to the ethynyltelluro moiety.63

R

Li

Te / THF

Li2Te

R

R

TeLi

ICH2Cl THF

R

TeCH2TeLi

TeCH2Cl

R Te

Te (4 - 65%)

R = H, Me, Bu, Ph, Me3Si

2H-1,3-ditellurole.63 Under an atmosphere of argon, 0.23 g (2.4 mmol) of trimethylsilylacetylene are dissolved in 5 mL dry tetrahydrofuran. The solution is cooled to ⫺70°C. n-Butyl lithium (1.0 mL, 2.4 M, 24 mmol) is dropped into the stirred solution. Then 0.20 g (2.0 mmol) of tellurium powder is added. The mixture is warmed to 20°C and kept at this temperature for 2 h. To this mixture, cooled again to ⫺70°C, is added a solution of 0.35 g (2.0 mmol) of chloroiodomethane in 1 mL of tetrahydrofuran. The mixture is stirred for 15 min and then quenched with 50 mL water. The product is extracted with three 15 mL portions of dichloromethane. The combined extracts are washed with brine, dried with anhydrous sodium sulphate and filtered. The filtrate is concentrated to give trimethylsilylethynyl chloromethyl tellurium as a pale-yellow oil. Tellurium powder (0.125 g, 1.0 mmol) is added to 2 mL of a 1 M solution (2.0 mmol) of lithium triethylborohydride in ethanol. The mixture is stirred at 20°C for 2 h under an atmosphere of argon. Then 2 mL of 1 M sodium ethoxide in ethanol are added followed by 0.27 g (1.0 mmol) of trimethylsilylethynyl chloromethyl tellurium dissolved in 2 mL dimethylformamide. The mixture is stirred for 15 h at 20°C, then diluted with 25 mL water and extracted with three 15 mL portions of dichloromethane. The combined extracts are dried with anhydrous sodium sulphate, filtered and the filtrate concentrated. The residue is chromatographed on silica gel with hexane/dichloromethane (1:1) as mobile phase. The fractions containing the product are concentrated and recrystallized from methanol; 65% yield, m.p. 85°C. Another method uses the dimerization of ethynyltellurolates promoted by trifluoroacetic acid.64

2 Ph

TeNa

Te

TFA Ph

Te

Ph H

(5% cis; 7% trans)

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5.10.2 5.10.2.1

307

Reactions Lithiation and reaction with electrophiles

2H-1,3-ditellurole and 4-phenyl-2H-1,3-ditellurole are lithiated by LDA giving respectively the 4- and 2-lithio derivatives. These are stable at ⫺70°C, and below this temperature can be submitted to several manipulations.65 Te

Te

Te

LDA Te THF, -70°C

Li

PhCHO THF

Te

HO

Te Ph (77%) Te

MeI HMPA

Me

Te (40%) Te

MeOD

Te

D

(95%) Te Ph

LDA Te THF, -70°C Ph

Te

PhCHO THF

Li Te

Ph

Te

OH

Te

Ph

(70%) MeI HMPA

Te Me Ph

Te (68%) Te

MeOD Ph

5.11

TETRATELLURAFULVALENES Te

Te

Te Te TTeF

5.11.1

D

Te (91%)

Te

Te

Te Te HMTTeF

Preparation

Tetratellurofulvalene (TTeF) and hexamethylenetetratellurofulvalene (HMTTeF) have been prepared in low yield (10% and 36%) by the reaction of the appropriate dilithium victellurolates with tetrachloroethylene.66,67 TeLi 2

Cl

Cl

+ TeLi

Cl

Cl

THF, -78°C to r.t.

Te

Te

Te

Te

(ref. 66)

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5. TELLUROHETEROCYCLES

TeLi 2

Cl

Cl

Cl

Cl

+

TeLi

THF, -80°C to r.t.

Te

Te

Te

Te

(ref. 67)

Several tetratellurofulvalenes, such as the following ones, have been prepared by the same method.

Te

Me

Te

Me Te

Te (ref. 68)

Te Te (10%)

S

S Te Te (75%)

Me

(ref. 69)

Me

Bis[dimethylthienol]1,4,6,8-tetratellurafulvalene.69 3,4-Dibromo-2,5 dimethylthiophene in tetrahydrofuran is treated at ⫺78°C with 2 equiv of tert-butyllithium. After 2 h, 1 equiv of tellurium powder is added. The mixture is slowly warmed to 0°C and kept at 0°C until all the tellurium has dissolved. The mixture is cooled again to ⫺78°C, treated with tertbutyllithium and then with tellurium at 0°C. The ditellurolate solution is cooled to ⫺78°C, mixed with 0.5 equiv of tetrachloroethene, stirred for 18 h and allowed to warm to 20°C. The brown solid is isolated by filtration and extracted with carbon disulphide. The extract is evaporated and the residue recrystallized from 1,1,2-trichloroethane to give bronzecoloured crystals. Yield: 75%; m.p. 295–298°C. TTeF exhibits interesting electrochemical properties: by cyclic voltametry two reversible one-electron oxidations are detected. The assessed ionization potential suggests that organic metals can be formed from TTeF. Several charge-transfer complexes of HMTTeF with TeNQs, p-quinone and other accepting -acceptors have been prepared and electrical resistivities measured with compaction samples. Some of these exhibit very low resistivities and low activitation energies. Intermolecular interaction through tellurium atoms plays an important role in the conduction.67b

5.12

TELLURIN AND DERIVATIVES OF 4-H-TELLURINS AND 4-OXO-4-H-TELLURINS (TELLUROPYRAN-4-ONES) R

Te

R

R

Te

R

O

5.12.1

Preparation

2,6-Disubstituted tellurins were obtained by reduction (with DIBAL-H) of the the 4-oxo-4H-tellurins, prepared by the addition of alkali metal tellurides to

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309

bis(organylethynyl)ketones. O R

a) Li2Te / THF

R

(Te/ Et3BH) R (50, 5%) (ref. 70)

Te R

O

R = Me, Me3C, Ph

b) Na2Te / THF DIBAL (Te, NaOEt, NaBH4) (ref. 72) THF, 0°C (37- 67%) (ref. 71)

R

Te R R = Me3C, Ph

(62, 65%)

5.12.1.1

Method a

2,6-Diphenyl telluropyran-4-one (typical procedure).70 120 mL (0.12 moI) of a 1.0 M solution of lithium triethylborohydride in tetrahydrofuran are added to 7.65 g (60 mmol) of powdered tellurium under nitrogen, and the mixture stirred at 20°C for 4 h. A solution of sodium ethoxide (prepared from 5.52 g (0.24 moI) of sodium and 240 mL of absolute alcohol) is added to the dilithium telluride, 13.8 g (60 mmol) of bis(phenylethynyl) ketone are dissolved in a mixture of 150 mL of tetrahydrofuran and 150 mL of 1 M sodium ethoxide in ethanol; this solution is poured as quickly as possible into the deep-purple-coloured dilithium telluride solution. The flask containing the reaction mixture is immediately placed in a water bath at 50°C and the temperature slowly increased over 30 min until ethanol begins to condense on the side of the flask. The water bath is removed and the mixture is stirred overnight at 20°C. Dichloromethane (400 mL) is then added, the resultant mixture is washed with 800 mL of water, and the organic phase is separated and concentrated to an oil. The oil is dissolved in 600 mL of dichloromethane, and the solution is filtered through a pad of sand. The filtrate is washed with 200 mL of 2% aqueous sodium chloride solution, dried with anhydrous sodium sulphate, filtered and evaporated. The brownish solid residue is triturated with 20 mL of butanenitrile and the fine yellow solid is collected by filtration; yield: 10.9 g (51%); m.p. 126–129°C (from acetonitrile). 5.12.1.2

Method b

Typical procedure.71 A 1 L flask equipped with a water-cooled reflux condenser, a magnetic stirring bar and a dry-N2 inlet were charged with 12.76 g (0.100 mol) of tellurium shot and 300 mL of 0.5 M NaOEt in EtOH. NaBH4 (3.80 g, 0.100 mol) was added in three portions at 15 min intervals (mildly exothermic) to a gently refluxing mixture. After the final addition, the mixture was stirred at ambient temperature for 2 h until a clear colourless solution was obtained. 1,5-Diphenyl-1,4-pentadiyn-3-one (23.0 g, 0.100 mol) was dissolved in 200 mL of 0.05 M NaOEt in EtOH and was immediately added in one portion to the colourless solution of disodium telluride. The solution was stirred for 1 h at room temperature. The mixture was poured into 1 L of H2O. The product was extracted with CH2Cl2 (3 ⫻250 mL). The combined organic extracts were washed with H2O (250 mL), 10% aqueous NH4Cl (250 mL) and brine (250 mL), dried over Na2SO4 and concentrated.

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The residue was purified by chromatography on silica gel, eluted with 10% EtOAc in CH2Cl2 to give, after recrystallization from MeCN 24.1 g (67%) of the product as a yellow crystalline solid: m.p. 127–129°C (Lit. m.p. 127.5–129°C). 2,6-Di-tert-butyl-4H-tellurin.72 A solution of 1.60 g (5.0 mmol) of 2,6-di-tert-butyl-4-oxo-4Htellurin in 15 mL dry tetrahydrofuran is placed into a flame-dried, 50 mL, two-necked flask equipped with a rubber septum cap and a dry-argon inlet. The flask is cooled to 0°C; 7.5 mL of a 1.5 M solution (10.5 mmol) of diisobutyl aluminium hydride in toluene are dropped to the cold solution with the help of a syringe. The reaction is quenched by addition of 10 mL of moist diethyl ether. The resulting solution is poured into 100 mL diethyl ether. The organic phase is washed with three 25 mL portions of 5% hydrochloric acid and then with two 50 mL portions of brine, dried with anhydrous sodium sulphate, filtered and the filtrate concentrated. Cold pentane (5 mL) is added to the residual oil and the mixture is cooled to ⫺20°C. The pentane is decanted from the yellow crystals. The decantate is chromatographed on silica gel with dichloromethane as the mobile phase to give a yellow oil. Yield: 62%. 5.12.2

Reactions

By treatment with halogens, the 4-oxo-4H tellurins are converted into the corresponding dihalides.73 R

Te R

R

CH2Cl2

+ X2

X X Te R

or CCl4

O

O

R = Me3C X = Cl (79%); Br (71%), I (40%) R = Ph X = Br (79%)

Telluropyran-4-thiones are prepared by treating the pyranones with Lawesson reagent.71 S R

Te

O

R Me

P

S S P S

Me

R = Me, Me3C, Ph

R

Te

R

S (63 - 96%)

The thiones, obtained as depicted above undergo copper-assisted dimerization giving (telluropyranyl)telluropyranes.71

R

Te R

Cu toluene /

R

R

Te

Te

R R S R = Ph (66%), Me (32%), t - Bu (22%)

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311

General procedure for the preparation of chalcogenopyran-4-thiones. Preparation of 2,6diphenyltelluropyran-4-thione.71 A mixture of the pyranone (3.60 g, 0.0100 moI) and the Lawesson reagent (3.6 g) in 50 mL of toluene was heated at reflux under N2 for 1 h. The mixture was cooled to room temperature and filtered through a pad of Celite®. The Celite® pad was washed with CH2Cl2 (2 ⫻ 50 mL), and the combined organics were concentrated. The residue was purified by chromatography on silica gel eluted with 10% EtOAc in CH2Cl2 to give 3.61 g (96 %), m.p. 120–123°C. Preparation of 4,4⬘– 2,2⬘,6,6⬘-tetraphenyl-4-(tellurapyranyl)-4H-tellurapyran.71 The thione (0.99 g, 2.6 mol) and copper powder (1.0 g) were slurried in 20 mL of toluene. The mixture was warmed for 4 h under reflux and filtered through a 2.5 cm pad of Celite® while hot. The Celite® pad was washed with several portions of CH2Cl2, and the combined organic filtrates were concentrated. The residue was warmed in boiling acetonitrile and filtered while hot to give 0.60 g (66%) as a green solid: m. p. 273–274°C. The first (E1) and second (E 2) oxidation potentials (versus saturated calomel electrode (SCE)) have been determined by voltammetry.71 The electrochemistry of the 4-(telluropyranyl)-4H-telluropyran has been examinated and compared to the O, S and Se analogues. 5.13

2H-1-BENZOTELLURIN Te

5.13.1

Preparation

2H-1-benzotellurin has been prepared from 4-hydroxy-3,4-dihydro-2H-1-benzotellurin.74 Te

KHSO4

Te

, -H2O OH

(87%)

2,3,4,7,8-Monomethyl derivatives have similarly prepared in 75–85%. 5.14

4H-1-BENZOTELLURINS AND 4-OXO-4H-1-BENZOTELLURINS Te

Te

O

5.14.1

Preparation

4H-1-benzotellurins are achieved by the reduction of 4-oxo-4H-1-benzotellurins with DIBAL.72 The oxo derivatives are the result of the cyclization of 2-chlorocarbonyl or

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2-carboxy-1-ethenyl aryl tellurium compounds.75 R1

Te

R

O R2

Cl

a) x = Cl AlCl3 CH2Cl2 -78°C

R1

Te R

DIBAL

R2 O

b) x = OH

a) R = Ph

R2 MeO; R2 = H R = H (75%), Me3C (82%), Ph (35%)

R2 = MeO (4%) R2 = F (5%)

R1 = MeO

b)

Te R

R1 =

P2O5/ MeSO3H

R1 = H

R1

R2 = H (73%) R2 = MeO (90%)

R = H, Me, Ph R1 = MeO (62 - 90%) R2 = H, MeO

General procedure for the cyclization of -(aryltelluro)propenoyl chlorides with aluminium chloride.75 The propenoyl chloride derivatives were dissolved in methylene chloride (1 g, 10 mL) under a nitrogen atmosphere. The solution was cooled to ⫺78°C, and 1.1 equiv of aluminium chloride was added. The cooling bath was removed and the reaction was allowed to warm to room temperature. After stirring for 1 h at room temperature, the reaction mixture was poured into ice-water, and the products were extracted with several portions of methylene chloride. The combined methylene chloride extracts were dried over sodium sulphate and concentrated. The residues were recrystallized from methanol if 1H NMR spectroscopy showed a single product. Typical procedure for the cyclization of -(aryltelluro)propenoyl acids with P2O5. 7Methoxy-4-oxo-4H-1-benzotellurin.75 2-Carboxyethenyl 3-methoxyphenyl tellurium (1.20 g, 3.92 mmol) is added to a solution of 1 g of phosphorus pentoxide in 10 mL of distilled methanesulphonic acid and the resultant mixture is stirred at 20°C for 4 h. Saturated aqueous sodium hydrogen carbonate solution (250 mL) is then added dropwise, the mixture is extracted with three 50 mL portions of dichloromethane and the combined extracts are dried with anhydrous sodium sulphate, filtered and concentrated. The residue is recrystallized from methanol. Yield: 0.76 g (67%); m.p.103°C. The 4-oxo-4H-1-benzotellurins have also been prepared by the cyclization of 2-(3dimethylamino propenoyl)-phenyl tellurenyl bromide by heating with hypophosphorous acid in pyridine.36 TeBr

O R = H, Me

R

NMe2

H3PO2 py

Te R

O (45%)

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TELLUROXANTHENES/TELLUROXANTHONES (AND DERIVATIVES)

5.15

313

TELLUROXANTHENES/TELLUROXANTHONES (AND DERIVATIVES) O

Te

5.15.1

Te

Preparation

Telluroxanthene is prepared from bis(2-lithiophenyl)methane and elemental tellurium or by the cyclization of 2-(phenylmethyl)phenyl tellurium trichloride promoted by AlCl3. + Te Li Li

ether reflux

(ref. 76) Te (47%)

AlCl3

(ref. 77)

Cl

TeCl3

Te Cl Cl

Cl

Telluraxanthene.76 Bis(2-bromophenyl)methane (3.33 g, 1.02 mmol) is dissolved in 300 mL of absolute diethyl ether and, under dry nitrogen, 22 mL (36.1 mmol) of a 15% solution of n-butyllithium in hexane are added dropwise. The mixture is heated under reflux for 0.5 h, cooled to 20°C and 1.8 g (14.1 mmol) of finely powdered tellurium are added. The resultant mixture is heated under reflux for 2 h and then poured into ice/water. The mixture is extracted with chloroform, the extract is filtered and the solvent is evaporated in a rotatory evaporator at 20°C under aspirator vacuum. The residue is recrystallized from diethyl ether/petroleum ether after addition of activated charcoal. Yield: 1.42 g (47%); m.p. 151°C. By sequential treatment with n-BuLi and CO2, telluroxanthene is converted into 9-carboxytelluroxanthene.77 CO2H

Li

Te

CO2

n- BuLi benzene reflux

Te (65%)

Te

Telluroxanthone is prepared from Na2Te and the diazonium salt obtained from bis(2-aminophenyl)ketone. Owing to the low yield (2%), this is not a useful preparative procedure.78 O

O

O 2Cl-

NH2 NH2

N+2

+N 2

Na2Te EtOH / H2O rongalite method

Te (2%)

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5.15.2

5. TELLUROHETEROCYCLES

Reactions

Telluroxanthene and telluroxanthone react with halogens in inert solvent to give the corresponding dihalides, which are reduced back to the parent compound with sodium sulphite or sodium disulphite. Y

Y X2 reduction

Te Y = H2 Y=O

Te X X

X = Cl, Br, I (ref. 76-78) X = Cl, Br (ref. 76,77)

Telluroxanthone can be reduced by treatment with AlLiH4 and Zn, respectively, to the 9-hydroxy telluroxanthone79 and to bis(telluro-9-xanthenyl)80 and react with Grignard reagents giving the expected 9-organo-9-hydroxy derivative.79

Te (77%)

Te

O

OH AlLiH4 ether

R = Me, benzyl, p-MeC6H4, o-MeC6H4, p-MeOC6H4, p- BrC6H4, 1-naphthyl

Te RMgX

Zn HOAc Te (74%)

HO R

Te (53-98%)

9-Hydroxytelluraxanthene.79 Lithium aluminium hydride (1.5 g, 39 mmol) is suspended in 100 mL of absolute diethyl ether; 9.24 g (30 mmol) of thoroughly ground telluraxanthone are added in small portions to the stirred suspension kept at 20°C, and the resultant mixture is stirred for 2 h. Ethyl acetate (100 mL) is then added to the cooled, vigorously stirred reaction mixture, followed by a saturated aqueous ammonium chloride solution. The hydrolysed mixture is filtered, the organic layer is separated, the aqueous phase is extracted twice with diethyl ether, and the organic solutions are combined, washed with water, dried with anhydrous sodium sulphate, filtered and evaporated. The residue is chromatographed on aluminium oxide with benzene as the mobile phase. Yield: 7.2 g (77%); m.p. 110–112°C. Bis-[tellura-9-xanthenyl].80 Zinc powder (1.0 g, 15 mmol) is added to a refluxing solution of 1.5 g (4.9 mmol) of telluraxanthone in 30 mL of glacial acetic acid. Then, 1.5 mL of concentrated hydrochloric acid are added to the stirred, refluxing mixture over 1 h. The resultant mixture is poured into 200 mL of 6 M hydrochloric acid, and stirred until the zinc has dissolved. The mixture is filtered, and the precipitate is washed with water and dried. Yield: 1.0 g (74%); m.p. >300°C (from chlorobenzene).

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315

9-(4-Fluorophenyl)-9-hydroxytelluraxanthene.79 A solution of 12 mmol of 4-fluorophenyl magnesium bromide in diethyl ether is added dropwise over 30 min to 3.08 g (10 mmol) of telluraxanthene dissolved in 50 mL of diethyl ether/benzene (1:1, v/v). The mixture is stirred during the addition and for a further 2 h. The resultant mixture containing a red precipitate is hydrolysed with saturated aqueous ammonium chloride solution, the organic layer is separated, the aqueous layer is extracted with two 30 mL portions of diethyl ether, and the organic layers are combined, washed with water, dried with anhydrous magnesium sulphate, filtered and the filtrate is evaporated. The residue is chromatographed on aluminium oxide with benzene as the mobile phase. Yield: 2.3 g (57%); m.p. 158°C. 5.16

PHENOXATELLURINS Te O

5.16.1

Preparation

Phenoxatellurium dichlorides are prepared by heating 2-phenoxyphenyl tellurium trichlorides81,82 or a mixture of equimolar amounts of TeCl4 and diphenylethers.81–83 O R

TeCl3 (80, 62%)

R=H R1 = Me, CO2H

O R1

R

Te Cl Cl

O R1

(18-54%)

+ TeCl4 R

R1

R, R1 = H, Me, F, Cl R = H; R1 = Cl, Br, F R = Me, R1 = Cl, F

Dichlorophenoxatellurine.81 Tellurium tetrachloride (11.1 g) and diphenyl ether (97.0 g) were heated together in a flask with a long air-condenser, carrying a moisture guard-tube, and a slow current of nitrogen being bubbled through the melt (this was not essential, however). The temperature was raised slowly from 100°C to 240°C over a period of 13 h, when hydrogen chloride was evolved, mainly during two maximal phases, the first occurring near 120°C and the second near 200°C. The melt gradually thickened and after ⬃8 h partly solidified even at 190°C, but remelted as the temperature rose. The cooled melt finally set to a crystalline cake, which was ground to a loose brown powder. The powder was stirred with ether, filtered off, dissolved in acetone, and filtered from 0.8 g of free tellurium. Evaporation of the acetone left 9.4 g of the fairly pure cyclic dichloride (62%); but, on further purification of the product in a Soxhlet apparatus with toluene, the yield fell to 48%. 2-Methylphenoxatellurin-10,10-dichloride.82 2-(4-Methylphenoxy)-phenyl tellurium trichloride (16.8 g, 40 mmol) are heated in a large test tube at 200–240°C in an oil bath for 0.5 h with occasional stirring. The deep red solid is cooled, ground and recrystallized from acetone to give pale yellow plates. Yield 12.2 g (80%), m.p. 275°C.

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5. TELLUROHETEROCYCLES

Phenoxatellurin dihalides are reduced to phenoxatellurins by treatment with potassium disulphite81,84 or sodium sulphide monohydrate.85 The reverse reaction is accomplished by treatment of phenoxatellurins with halogens.81,83 The 10-oxide is formed by treatment of the dichloride with water81 or Ag2O.84 Hydrogen peroxide oxidizes phenoxatellurins to the 10,10-dioxide.82,86 O

H2O

O

Te O

or Ag2O

Te X X

O

reduction X2

O

H2O2

Te

Te O O

Phenoxatellurin, in addition to the other phenoxachalcogens, forms a crystalline complex, as donor, with 7,7,8,8-tetracyano-para-quinodimethane (TCNQ) whose stoichiometry was found to be 1:1. The equilibrium content and the activation coefficients are relatively small, suggesting that the complex formed in dicloromethane solution is relatively weak, probably due to the – charge transfer type. Voltametry yields the one electron oxidation potential (E½): pTe ⫹ 0.76 V. The large difference between the first oxidation potential of the phenoxatellurine donors and the first reduction potential of the acceptor (TCNQ ⫹ 0.14 V) suggests the complex will possess a non-ionic ground state.87

5.17

PHENOTHIATELLURINS/PHENOSELENOTELLURINS Te Y = S, Se

Y

5.17.1

Preparation

The title compounds are prepared respectively from phenyl-2-aminophenyl sulphide88 and selenide89 in accordance with the accompanying scheme. Y NH2

TeCl4

NaNO2 HCl/ H2O

Y TeCl3 Y = Te (86%) Y = Se (100%)

Y N2+]Cl-

Y

1) HgCl2 2) Cu/ acetone

HgCl Y = Te (54%) Y = Se (22%) Y

(Y = S) 250°C (Y = Se) AlCl3 /

Cl Cl

Te Cl Cl Y = Te (42%) Y = Se (33%)

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317

Y

Na2S.9H2O

Te Y = Te (96%) Y = Se (100%)

o-Thiophenoxyphenyl mercury chloride.88 Sodium nitrite (12 g, 0.166 mole) was added to a mixture of 40 g (0.166 mol) o-thiophenoxyaniline hydrochloride (prepared reducing 2-nitrodiphenyl sulphide with Fe/acetic acid; m.p. 36°C), 75 mL concentrated hydrochloric acid and 80 g ice, keeping the temperature at 0–5°C. To the filtrate was added, dropwise, with stirring, a solution of 45.1 g (0.166 mol) of mercury chloride in 100 mL cold hydrochloric acid (50%). During the addition the temperature must be maintained at 0°C. An orange-yellow precipitate formed and stirring was continued for 30 min. The product was filtered, washed with water, with alcohol and finally with ether, yielding the diazonium salt mercury chloride complex (75 g, 88.7%). A suspension of the complex in 270 mL acetone was cooled to ⫺60°C, treated with 18.2 g metallic copper (freshly prepared), and the temperature maintained at ⫺60°C for 12 h and then slowly brought to room temperature. The brown product was filtered, extracted with acetone and the yellow solution allowed to crystallize, yielding 54.5% (33 g). Recrystallization from alcohol furnished colourless plates, m.p. 135–136°C. o-Thiophenoxyphenyltellurium trichloride. A solution of the above-obtained product 8.4 g (0.02 mol) and TeCl4 (5.4 g, 0.02 mol) was refluxed in pure dioxane (40 mL). The dioxane mercury chloride complex that precipitates on cooling is removed by filtration. The dark yellow solution was concentrated, and the obtained viscous oil solidified by adding petroleum ether (50–70°C), 9.6 g (80.6%) of the crude trichloride and yellow needles from glacial HOAc, m. p. 213–215°C. 10,10-Dichlorothiophenoxytellurine. o-Thiophenoxyphenyltellurium trichloride (2.1 g, 0.005 mol) was heated at 240–250°C in a glass tube with stirring for 30 min. Evolution of HCl was noted. On cooling, the dark liquid solidified to a dark yellow mass (1.9 g). This was dissolved in acetone, the solution filtered from a small residue of tellurium and crystallized, yielding needles (0.8 g, 42%), m.p. 265–270°C (at 230°C, a change in the crystalline form was noted). Thiophenoxytellurine. To 0.8 g (0.002 mol) of the dichloride, 7.2 g (0.03 mol) of hydrated sodium sulphide were added and heated at 100°C for 15 min. The reaction mixture was diluted with water and the product filtered and dried (0.6 g, 96%). Recrystallization from ethanol gave pale yellow needles, m.p. 122–123.5°C.

5.18

TELLURANTHRENES Te Te

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5.18.1 5.18.1.1 (a)

Preparation From tellurium

By heating elemental tellurium with polyhalogenated arenes X

X

X

I + Te

X

300°C

I

X

X X

X

X

Te Te X

X X

X = F (30%) (ref. 90) X = Cl (70%) (ref. 91)

Octafluorotelluranthrene.90 Tellurium powder (5.0 g, 39.2 mmol) and 10.0 g (24.9 mmol) of 1,2-diiodotetrafluorobenzene are placed in a 20 cm test tube, and the tube evacuated to 0.2 torr and sealed. The sealed tube is placed in an oven at 300°C for 1 day, cooled and carefully opened. The solid and liquid in the tube are extracted with dichloromethane, the mixture is filtered, and the filtrate is washed first with 50 mL of concentrated aqueous sodium thiosulphate solution and then with 50 mL of distilled water. The solvent is removed under vacuum, the residual dark brown oil is dissolved in chloroform, and bromine is added until the colour of bromine persists. The precipitate is filtered and washed with chloroform to give 5,5,10,10-tetrabromoperfluorotelluranthrene. Yield: 3.5 g (29%); m.p. 281°C. Tetrabromoperfluorotelluranthrene (0.5 g, 0.57 mmol) and excess sodium sulphide nonahydrate are placed in a flask and the flask is heated in a water bath at 90°C for 20 min. The resultant reaction mixture is cooled to 20°C and extracted with portions of diethyl ether (2⫻25 mL). The combined ether extracts are dried with anhydrous sodium sulphate, filtered, the filtrate is evaporated under vacuum, the residue is sublimed, and the sublimate is recrystallized from absolute methanol. Yield: 0.065 g (20%); m.p. 119°C. (b)

By heating elemental tellurium with mercury compounds X X

X = H 250°C X = Cl 320°C + 6 Te -6 Hg

Hg

X X

n

X X

X X = H (57%) (ref. 92) X X = Cl (70%) (ref. 91)

X Te

X

Te

X X

Telluranthrene.92 Finely powdered tellurium (2.3 g, 18 mmol) and 2.5 g (9.0 mmol) of o-phenylenel mercury are intimately mixed by grinding in a mortar, the mixture is placed in the well of a sublimation apparatus, which is then evacuated to <1 torr. The well of the sublimation apparatus is heated at 250°C in a bath of Wood’s metal for 10 h, and the cold finger is cooled with dry ice. The sublimed solids are dissolved in chloroform, the solution is decanted through a filter, the solvent is evaporated, and the residue is recrystallized from carbon tetrachloride or ethanol/benzene. Yield: 1.05 g (57%). m.p. 179°C.

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5.18.1.2

319

From sodium telluride

By treating 1,2-dichloro- or 1,2-dibromoarenes with Na2Te (from NaH and Te) in DMF at reflux.93 Cl

2

Te + 2 Na2Te

Cl

Te (3%)

Br

Te + 2 Na2Te

Br

Te (5%)

Telluranthrene.93 A 1 L, round-bottomed flask, fitted with a reflux condenser and a stirrer, is charged with 147 g (1.0 mol) of 1,2-dichlorobenzene, 127.6 g (1.0 mol) of tellurium powder, 65 g (2.7 mol) of sodium hydride (60% suspension in mineral oil), and 250 mL of dry dimethylformamide. The mixture is heated under reflux with stirring for 12 h and is then poured into 1.5 L of water. The mixture is filtered, the brownish-black filter cake is extracted with seven 200 mL portions of diethyl ether, and the remaining 105 g of solid is air dried. The solid is placed in the well of a sublimation apparatus with a dry ice-cooled cold finger, and the apparatus is evacuated with a water aspirator and heated at 350°C in a bath of Wood’s metal for ⬃1.5 h. The semi-solid yellow sublimate is dissolved in diethyl ether and bromine is added to this solution until no more precipitate forms. The telluranthrene 5,5,10,10-tetrabromide is filtered, washed with diethyl ether and transferred to a flask containing 50 mL of concentrated aqueous sodium sulphide nonahydrate solution. This mixture is heated on a boiling water bath for 15 min, filtered and the solid is chromatographed on silica gel with dichloromethane as the mobile phase. The fraction corresponding to the yellow band is collected, the solvent is evaporated, and the residue is sublimed as described above. Yield: 6.0 g (3%); m.p. 170°C. Despite the low yield, the method deserves some use since the non-reacted tellurium can be recovered. 5.19

BIS-THIENO-1,4-DITELLURINS Te S

S Te

5.19.1

Preparation

A member of the class has been prepared from TeCl4 and 3,4-dilithio-2,5-dichlorothiophene.94 Cl 2

+ TeCl4

S Cl

Cl

Li Li

ether

Te

S Cl

Cl S

Te

Cl

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5. TELLUROHETEROCYCLES

5.20

1,4-TELLURINO-1,4-TELLURINS Te Te Te Te

5.20.1 Preparation This compound is formed as by-product in the described preparation of TTef (see Section 5.11).66 TeLi 2

Cl

Cl

Te Te

Cl

Cl

Te Te

+ TeLi

A similar reaction has been performed with a cyclalkene tellurolate.95 TeLi

Cl

Cl

Te Te

Cl

Cl

Te Te

+

2 TeLi

A related compound was prepared by the reaction of 1,2-diiodoacenaphtalene with sodium telluride under aprotic conditions.96

I

I Na2Te

2

DMF, r.t.

Te Te (35%)

5.21

5.21.1

BENZENE-FUSED HETEROCYCLES CONTAINING TELLURIUM, SELENIUM AND SULPHUR Preparation

The reaction of the stannoles Aa and Ab with TeCl4 affords the substitution of the dimethyltin moiety by the dichlorotelluro group to give the thio- and selenodichloro telluroles Ba and Bb. The spiro tellurole C is obtained by treatment of Aa, respectively, with 0.5 equiv. of TeCl4 or with Ba. When Ba is treated with aqueous THF at reflux, a mixture of C and tetrathiocins D and E is formed in ratios that depend from the heating time.

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1,5-DITELLURACYCLOOCTANE AND 5H,7H-DIBENZO[b,g][1,5]

321

Even more Bb, submitted to the same treatment, yields a mixture of F and G, tetrachalcogenines containing both sulphur and selenium in the eight-membered ring.97 TBDMS

TBDMS

S Me ether Sn + TeCl4 -15°C, 30 min X Me Aa X = S Ab X = Se

Ba X = S (84%) Bb X = Se (62%) TBDMS

83%

Aa + 1 / 2 TeCl4

S Cl Te X Cl

S S Te S S C

THF, 0°C, 1 h Aa + Ba

75%

10% aq. THF Ba reflux

Bb

TBDMS S S

TBDMS +

C +

10% aq. THF

TBDMS S S

S S

S S

D

E

TBDMS S S

reflux, 20 h

TBDMS +

5.22.1

TBDMS

TBDMS S Se

Se Se

Se S G

F

5.22

TBDMS

TBDMS

1,5-DITELLURACYCLOOCTANE AND 5H,7H-DIBENZO[b,g][1,5] TELLUROTHIOCIN

Preparation

The title compounds A and B are synthesized in accordance with the following scheme:98,99 Br 2 Na2Te Br

TeNa TeNa

Br Br

Te Te A

Br

1)Mg / Et2O

Te

2)Te

+

Te Te

C 1) Br2 /Et2O

Br2 / Et2O

2) o- MeC6H4MgBr Br Br Br Te

Br Br Te

NBS hυ

D Br

Na2S.9H2O

Te S B

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5. TELLUROHETEROCYCLES

Synthesis of compound A.98 A solution of 1,3-dibromopropane (10.1 g, 0.05 mol) in benzene (100 mL) was added to a solution of sodium telluride (Na2Te; 17.4 g, 0.1 mol) in ethanol (700 mL). After 3 h sodium borohydride (3.8 g, 0.1 mol) was further added to the mixture to produce sodium propane-1,3-ditellurolate, and then to the mixture was added a solution of the dibromopropane (10.1 g, 0.05 mol) in benzene (100 mL). The whole mixture was stirred at room temperature for 2 h. After usual work-up, the crude products were purified by silica gel column chromatography (eluent: n-hexane/benzene) to afford compound A, which was further purified by preparative liquid chromatography. Synthesis of compound B.99 Bis(2-methylphenyl)telluride C was obtained by the reaction of 2-methylphenylmagnesium bromide with tellurium. Treatment of the telluride C with bromine in Et2O gave bis(2-methylphenyl)tellurium dibromide, which was irradiated using a high-pressure mercury lamp after addition of NBS in CCl4 to afford a tetrabromide D. Compound D (2.59 g. 4.13 mmol) in CH2Cl2 (100 mL) was treated with Na2S·9H2O (2.18 g, 9.08 mmol) in EtOH (300 mL) using a high-dilution technique at room temperature under an Ar atmosphere. The mixture was stirred at room temperature overnight. After usual work-up, the crude product was purified by silica gel column chromatography (eluent: hexane/CHCl3) to give B which was recrystallized from CHCl3. The transannular bond formation between Te–Te and Te–S bonds giving the dications E and F is afforded by treatment of compounds A and B with the oxidizing agents nitrosyl tetrafluoroborate and hexafluorophosphate or with D2SO4. A

2 NO+XCH2Cl2 /MeCN

X = BF4, PF6 B

+ Te Te + E

2 NO+PF6-

2 X-

Te+

or D2SO3

in D2SO4

S+ F

X = PF6

The dications, characterized by spectroscopic means, are useful oxidizing agents as shown by the oxidation of thiophenol to diphenyl disulphide by dications E and F as well as of diphenyl hydrazine to azobenzene by dication F. In addition, the telluroxide G prepared by hydrolysis of the dibromide derived from B reacts with acetic anhydride and trifluoroacetic anhydride to give the tellurane H. The tellurothio dication I is formed by treatment of G with triflate anhydride.100 O Ac2O or (F3CCO)2O

Te

XO OX Te

CH2Cl2, r.t.

S G TF2O -20°C

S H X = Tf, Ac

Te+ S+ I

2 TFO-

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DITELLURANE DERIVATIVES

5.23 5.23.1

323

DITELLURANE DERIVATIVES

Preparation

1,2-Ditellurane A is an elusive compound which has been detected only in organic solvent solution. The spiro ditellurolane B was obtained by the reaction of 3,3-bis(dichloromethyl)oxetane C with potassium tellurocyanide in DMSO.101 O

Te Te A O Cl

Cl C

Te Te B

KTeCN DMSO, r.t. (65%)

O

Te Te + H D

OTs OTs E

Te Te F

Unequivocal confirmation of its structure was furnished by X-ray crystallography. Compound B forms a blue solution in a variety of solvents. Its solution in benzene shows electron spin resonance (ESR) signal. An unusual feature of B is a -type conjugation between the two tellurium atoms, in accordance with the observation that the blue solution of the compound in MeCN becomes orange upon addition of a strong acid. These observations are consistent with protonation of the Te–Te bond to form the new symmetrical -complex D. The protonation is reversible, the blue ditelluride colour reappearing by the addition of pyridine. The carbocyclic analogue F was synthesized in low yield, by analogous method starting from E. 1.2-Ditellurolane A.101 A solution of KTeCN was prepared from tellurium (5.2 g, 0.04 mmol) and KCN (2.6 g, 0.04 mol) in dry DMSO (60 mL) under nitrogen. A solution of 1,3-dibromopropane (4.08 g, 0.02 mol) in DMSO (10 mL) was added to the cooled KTeCN solution. The mixture was stirred overnight. Water (60 mL) was added, and the brown precipitate was filtered and washed with more water. The solid was suspended in benzene (100 mL), treated with aqueous NaOH solution (10%, 100 mL), and gently warmed. The blue benzene layer was separated. The lower layer was extracted with more benzene until no more blue colour was extracted. The combined benzene layer was washed, dried, filtered and evaporated to give a brown gum (1.2 g). Extracts of this gum in common organic solvents (benzene, CS2, CH2Cl2 and CHCl3) contain A and were blue. 6,7-Ditelluraspiro[3,4]octane F. To a cooled solution of KTeCN in DMSO (30 mL) under nitrogen, which was generated from tellurium (1.30 g, 0.01 mol) and KCN (0.69 g, 0.011 mol), was added a solution of 1,1-bis[(tosyloxy)methyl]cyclobutane E (1.50 g, 0.0035 mol). The mixture was heated overnight at 90–100°C under nitrogen and then stirred at room temperature for 1 day. The dark suspension was poured into dilute aqueous NaOH solution (100 mL, 1.5%). The precipitate was filtered, washed, dried and extracted into CH2Cl2 using a Soxhlet extractor, until no more blue colour was extracted. Concentration

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5. TELLUROHETEROCYCLES

of the blue extract, followed by addition of cyclohexane, yielded ditellurolane derivative (0.21 g, 21% yield) in two crops: m.p. 98°C dec.

5.24

REDUCTIVE DIMERIZATION OF TELLURO- AND SELENOXANTHONE

By treating an equimolar mixture of telluroxanthone and selenoxanthone with Zn powder in boiling HOAc and concentrated HCl (100:1.25) for 1 h, a reductive dimerization takes place giving the dimerized products in the ratio 21:35 in addition to telluroxanthene and selenoxanthene.102 Te

Se

Se

Te

Te

+

+

O

+

O Se A

Se C

Te B Se

+

Te +

D

E

The observed distribution of the products, compared to the statistical distribution 25:25:50, clearly indicates that the Te/Te bridged B is preferred over the Se/Te bridged C, and Se/Se bridged A, while C is preferred over A.

5.25

TELLUROSTEROIDS

A tellurolactone of estrone series was synthesized starting from the estratien-17-one in accordance with the following scheme:103 O 1) Bayer-Villiger 2) HBr/AcOH 3) SOCl2

Br

COCl

Te

NaHTe (Te/NaBH4) EtOH AcO

AcO

(60%)

5.26

21,21-DIHALO-21-TELLUROPORPHYRIN

The title compound, a heterocycle with several interesting structural features, was recently synthesized by the condensation of 2,5-bis(1-phenyl-1-hydroxymethyl)tellurophene, pyrrole,

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325

4-methoxybenzaldehyde promoted by toluenesulphonic acid, followed by oxidation with pchloranil.104 CHO Ph

Te OH

Ph +

Ph +

OH

OMe

N H

TsOH p-chloroanil CHCl3

Ph

Te N

N H N

MeO

OMe (18%)

air fluorescent light THF Ph

Te

Ph

CH2Cl2 HCl (two phase) Ph

Te Cl Cl

Ph

O

(94%)

(76%)

Preparation of 5,10-Diphenyl-15,20-di-4-methoxyphenyl-21-telluraporphyrin.104 A solution of 2,5-bis(1-phenyl-1-hydroxymethyl)tellurophene (0.783 g, 2.00 mmol), pyrrole (0.72 mL, 11 mmol) and 4-methoxybenzaldehyde (0.84 mL, 5.5 mmol) in 450 mL of CHCl3 was degassed under a stream of argon bubbles. The reaction vessel was covered with aluminium foil. p-Toluenesulphonic acid (0.39 g, 2.0 mmol) was added, and the resulting solution was stirred at room temperature for 1 h. p-Chloranil (2.26 g, 9.2 mmol) was added, and the resulting mixture was heated at reflux for 1 h. The reaction mixture was cooled to ambient temperature and concentrated in vacuo. The residue was purified via chromatography on basic alumina eluted with CH2Cl2 in a foil-wrapped column. The first bands were rechromatographed on silica gel eluted with CH2Cl2, and the first, redbrown band was collected. The dark purple product was washed with 1:1 acetone/methanol to give 283 mg (18%), m.p. >300°C. The molecular structure was determined by X-ray crystallography. Air oxidation of a CH2Cl2 solution under fluorescent lighting, and treatment with HCl in a two-phase system, give respectively the corresponding telluroxide and dichlorotelluro derivatives. REFERENCES 1. (a) Morgan, G. T.; Drew, H. D. K. J. Chem. Soc. 1920, 117, 1456; 1921, 119, 610; 1924, 125, 731, 1601. (b) Morgan, G. T.; Reeves, H. G. J. Chem. Soc. 1923, 123, 444. (c) Morgan, G. T.; Elvins, O. C. J. Chem. Soc. 1925, 127, 2625. (d) Morgan, G. T.; Taylor, C. J. A. J. Chem. Soc. 1925, 127, 797. 2. Farrar, W. V.; Gulland, J. M. J. Chem. Soc. 1945, 11. 3. McCullough, J. D. Inorg. Chem. 1965, 4, 862.

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–6– Toxicology of Organotellurium Compounds

Toxicity data on organotellurium compounds are still scarce in the literature, in contrast with those of organoselenium compounds. Therefore, in this brief outline, the two series of compounds will be frequently compared. Although some authors have informed that organotellurium compounds are less toxic than their selenium co-partners, further data indicated in opposition that organotellurium compounds are more toxic than organoselenium compounds.1–3,4–7 Inorganic and organic tellurium compounds are highly toxic to the central nervous system of rodents. In contrast to selenium, the methylated products of tellurium are considered more toxic for mammals. Organotellurium compounds such as dimethyltellurium dichloride and dimethyltelluride have been reported as potential inhibitors of squalene monooxygenase, causing a dramatic reduction in the rate of cholesterol biosynthesis and leading to degradation of the myelin sheath. There is evidence that organotellurium compounds react with vicinal cysteine sulphhydryl groups on squalene monooxygenase,8 plausibly because of the formation of unstable intermediates in which the tellurium atom is bonded to sulphhydryl groups in squalene monooxygenase. Dimethyltelluride, dimethyltellurium dichloride and trimethyltelluronium chloride inhibit squalene monooxygenase in Schwann cells in culture. Tellurium-induced demyelination seems to be a result of squalene monooxygenase inhibition, and dimethyl tellurium dichloride may be the neurotoxic species presented to Schwann cells in vivo. The inhibitory effect of diphenyl ditelluride (compared to diphenyl diselenide) on [3H]glutamate, [3H]MK-801 and total [3H]GMP-PNP binding ex vivo and in vitro to synaptic membrane preparations of rat brain has been studied. This shows that diorganylditellurides are more reactive than structurally related selenium compounds, because the dose of diphenyl diselenide used was about eight times higher than that of diphenyl ditelluride to cause similar effect. These results can be related to the higher electronegativity associated to a larger atomic volume of tellurium.

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However, the reactivity in vitro towards biological systems is not always in agreement with this role, because [3H]glutamate binding was more sensitive to diphenyl ditelluride than diphenyl diselenide, and other bindings were similarly affected by both compounds.5 It was also demonstrated that acute exposure to diphenyl ditelluride did not change [3H]glutamate release and uptake by rat brain synaptosomes. In contrast, in vitro experiments indicate that 100 m of diphenyl ditelluride clearly inhibit [3H]glutamate uptake by brain synaptosomes and synaptic vesicles. These results also suggested that the inhibitory effect on glutamate uptake may be related, at least in part, to the ability of this compound to oxidize thiol groups.6 Another line of investigations has been devoted towards the effect of diphenyl ditelluride on 45Ca influx into rat brain synaptosomes.9 Since changes in 45Ca movements can interfere in a variety of neurophysiologic processes, the high neurotoxicity of diphenylditelluride can be linked to its effect on 45Ca fluxes at the presynapse. In addition to neurotoxic effects, diphenyl ditelluride has been reported as a strong toxic compound. This effect was investigated for the route of administration (intraperitoneal or subcutaneous) and for the animal species.

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It is well recognized that, glutathione plays a central role in endogenous antioxidant defence as a reducing agent and nucleophile as well as a substrate for the glutathione peroxidases and transferases. Consequently, glutathione peroxidase-mimetic compounds are under intensive investigation. Organotellurium compounds are readily oxidized from the divalent to tetravalent state. Consequently, this property makes tellurides attractive as scavengers of reactive oxidizing agents such as hydrogen peroxide, hypochloride and peroxyl radicals. It has been reported that substitution of selenium by tellurium in a series of diarylchalcogenides results in a pronounced increase of antioxidant activity. Therefore the development of new and potent antioxidants is an important goal. Diaryl tellurides exhibit potent glutathione-peroxidase-like activity. O N Se Ebselen

The most active diaryl telluride, bis(4-aminophenyl)telluride, demonstrated 348%, 530%, 995% and 900% of the catalytic activity of ebselen for the glutathione-dependent reduction of H2O2, t-butylhydroperoxide (TBH), cumene hydroperoxide and linoleic acid peroxide, respectively.10 Diorganyl tellurides are able to catalyse the reaction of H2O2 with thiols. p-Electrondonating substituents (OH, NH2, NMe2 and NHPh) enhance such activity, whereas the electron-withdrawing CF3 group has the opposite effect. p,p⬘-Dihydroxydiphenyl telluride and p-hydroxydiphenyl telluride present a similar performance, suggesting that only one p-OH group substituent is sufficient to offer a highly potent catalyst. 2,2⬘-Disubstituted compounds are generally less active.11 p-Substituted diaryl tellurides have been recognized as inhibitors of peroxidation in classical and biological systems. The bis-(p-dimethylamino)phenyl telluride is the most potent antioxidant in the microsomal system ever reported.12

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Based on mechanistic studies, the antioxidant property of diaryl tellurides is related to their conversion into 4,4⬘-disubstituted telluroxides. Also has been described to retard peroxidation of linolenic acid in methanol.13 2-Substituted-1-naphthols are the most potent 5-lipoxygenase inhibitors known. 2-Phenyltelluro-1-naphthol inhibits stimulated LTB4 biosynthesis in human neutrophiles, and acts as a catalytic peroxide decomposer as well as a catalytic chain-breaking antioxidant.14 OH Te

Several diaryl tellurides exhibit protection against TBH-induced cell death in lung fibroblast cultures. Besides, the same compounds prevent leucocyte-mediated cell damage in Caco-2 cells and protect rat kidney tissue against oxidative damage caused by anoxia and reoxygenation.15 4,4⬘-Disubstituted diaryl tellurides (R ⫽ OH, NH2 and NMe2), at low concentration, present a protective effect on DNA damage without modifying the hemolysis rate; however, conversely, at high concentrations they are not able to protect DNA against breakage and they produce a marked genotoxic effect.16 Cyclodextrinyl ditelluride has been recognized as an excellent glutathione peroxidase mimic, revealed in the mitochondrial damage system induced by ferrous sulphate/ascorbate.17 Another class of compounds, dihydrotellurophenes, exhibits antioxidant activity together with the oxygen, sulphur and selenium analogues.18 HO Te

Water-soluble organotellurium compounds have been demonstrated as protectors against peroxynitrite-induced oxidation in solution. The p-CF3-substituted member is less reactive than the unsubstituted co-partner.19 Te R

SO3Na S

Te

SO3Na

Te

SO3Na

R = H, Me2N, F3C

The p-Me2N-substituted compounds possess significant activity towards H2O2, peroxynitrite and the H2O2-induced hydroxyl radical in cortical synaptosomal systems from gerbil brain.20 Notably peroxynitrite can be formed in vivo and can induce DNA damage as well as initiate lipid peroxidation in biomembranes, can cause tyrosine nitration of proteins and inactivate a variety of enzymes. In support of this evidence, it was demonstrated that organotellurium compounds exhibiting glutathione-peroxidase-like activity, also protect

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against peroxynitrite-mediated oxidation and nitration reactions. 4,4⬘Bis(aminophenyl) telluride is the most efficient against peroxynitrite-mediated oxidation.

7.1

CHEMOPREVENTIVE ACTIVITY

Almost 20 years ago the ammonium tellurolate AS101 was demonstrated to possess immunomodulating properties and to mediate anti-tumour effects in rats.21 The same compound stimulates human lymphoid cells to proliferate and produce lymphokines,22 tumour necrosis factor (TNF) and other cytokines in vitro.23 NH4+ O O TeCl Cl Cl (AS 101)

In addition AS101 seems to increase DNA repair mechanisms, which could restore the function of UVB-damaged cells.24 Organotellurium compounds, such as telluranthrene, diphenylditelluride and some substituted derivatives, demonstrated bacterial cytotoxicity and capacity to induce apoptotic cell death in eukaryotic HL-60 cells.25 An additional topic for anticancer research involving organotellurium compounds was thioredoxin reductase. Since in several human cancers thioredoxin expression is increased,26 several organotellurium compounds have been tested. 4,4⬘-Disubstituted diorganyl tellurides have been shown to be effective inhibitors of thioredoxin reductase and also to inhibit the growth of human cells in culture.27 Because of their increased solubility in water, 4-sulphopropyl-substituted derivatives such as Li, Na, K and Me4N salts were found to be the most efficient tellurium-based inhibitors of thioredoxin reductase. Another active diorganyl telluride is characterized by p-OMOM, p-OTHP and (OCH2CH2)3OTHP, groups. Finally the analogue of vitamin E28 shows interesting inhibition characteristics towards thioredoxin reductase. Me HO

C16H33(phytyl) Te Me

Me Me

Exploring a further feature of organotellurium compounds, the bioactivation of an organotellurium cysteine derivative has been evaluated.29 The results demonstrated that it is bioactivated into its corresponding tellurol.

Te

CO2H NH2

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REFERENCES This chapter was written on the basis of the review “Organoselenium and Organotellurium Compounds: Toxicology and Pharmacology”, Nogueira, C. W.; Zeni, G.; Rocha, J. B. T. Chem. Rev. 2004, 104, 6255. We are greatly indebted with the authors for this fundamental support. 1. Nogueira, C. W.; Meotti, F. C.; Curte, E.; Pilissão, C.; Zeni, G.; Rocha, J. B. T. Toxicology 2003, 183, 29. 2. Meotti, F. C.; Borges, V. C.; Zeni, G.; Rocha, J. B. T.; Nogueira, C. W. Toxicol. Lett. 2003, 143, 9. 3. Farina, M.; Soares, F. A.; Zeni, G.; Souza, D. O.; Rocha, J. B. T. Toxicol. Lett. 2004, 146, 227. 4. (a) Maciel, E. N.; Flores, E. M. M.; Rocha, J. B. T.; Folmer, V. Bull. Environ. Contam. Toxicol. 2003, 70, 470. (b) Jacques-Silva, M. C.; Nogueira, C. W.; Broch, L. C.; Flores, E. M. M.; Rocha, J. B. T. Pharmacol. Toxicol. 2001, 88, 119. 5. Nogueira, C. W.; Rotta, L. N.; Perry, M. L.; Souza, D. O.; Rocha, J. B. T. Brain Res. 2001, 906, 157. 6. Nogueira, C. W.; Rotta, L. N.; Zeni, G.; Souza, D. O.; Rocha, J. B. T. Neurochem. Res. 2002, 27, 283. 7. Borges, V. C.; Nogueira, C. W.; Zeni, G.; Rocha, J. B. T. Neurochem. Res. 2004, 29, 1505. 8. Laden, B. P.; Porter, T. D. J. Lipid Res. 2001, 42, 235. 9. Moretto, M. B.; Rossato, J. I.; Nogueira, C. W.; Zeni, G.; Rocha, J. B. T. J. Biochem. Mol. Toxicol. 2003, 17, 154. 10. Andersson, C.-M.; Hallberg, A.; Brattsand, R.; Cotgreave, I. A.; Engman, L.; Persson, J. Bioorg. Med. Chem. Lett. 1993, 3, 2553. 11. Engman, L.; Stern, D.; Pelcman, M. J. Org. Chem. 1994, 59, 1973. 12. Andersson, C. M.; Brattsand, R.; Hallberg, A.; Engman, L.; Persson, J.; Moldeus, P.; Cotgreave, I. Free Radical Res. 1994, 20, 401. 13. Engman, L.; Persson, J.; Vessman, K.; Ekstrom, M.; Berglund, M.; Andersson, C.-M. Free Radical Biol. Med. 1995, 19, 441. 14. Engman, L.; Stern, D.; Frisell, H.; Vessman, K.; Berglund, M.; Ek, B.; Andersson, C.-M. Bioorg. Med. Chem. 1995, 3, 1255. 15. Wieslander, E.; Engman, L.; Svensjo¨, E.; Erlansson, M.; Johansson, U.; Linden, M.; Andersson, C. M.; Brattsand, R. Biochem. Pharmacol. 1998, 55, 573. 16. Tiano, L.; Fedeli, D.; Santroni, A. M.; Villarini, M.; Engman, L.; Falcioni, G. Mutat. Res. 2000, 464, 269. 17. Ren, X.; Xue, Y.; Zhang, K.; Liu, J.; Luo, G.; Zheng, J.; Mu, Y.; Shen, J. FEBS Lett. 2001, 507, 377. 18. Engman, L.; Laws, M. J.; Malmstron, J.; Schiesser, C. H.; Zugaro, L. M. J. Org. Chem. 1999, 64, 6764. 19. Jacob, C.; Arteel, G. E.; Kanda, T.; Engman, L.; Sies, H. Chem. Res. Toxicol. 2000, 13, 3. 20. Kanski, J.; Drake, J.; Aksenova, M.; Engman, L.; Butterfield, D. A. Brain Res. 2001, 911, 12. 21. (a) Sredni, B.; Caspi, R. R.; Klein, A.; Kalechman, Y.; Danziger, Y.; Ben Ya’akov, M.; Tamari, T.; Shalit, F.; Albeck, M. Nature 1987, 330, 173. (b) Sredni, B.; Caspi, R. R.; Lustig, S.; Klein, A.; Kalechman, Y.; Danziger, Y.; Ben Ya’akov, M.; Tamari, T.; Shalit, F.; Albeck, M. Nat. Immun. Cell Growth Regul. 1988, 7, 163–168. (c) Schlesinger, M.; Kalechman, Y.; Ben Ya’akov, M.; Kazimirsky, G.; Sredni, B.; Caspi, R. R.; Klein, A.; Shani, A.; Catane, R.; Tichler, T.; Michlin, H.; Tamari, T.; Shalit, F.; Albeck, M. J. Allergy Clin. Immunol. 1989, 83, 226. 22. Shani, A.; Tichler, T.; Catane, R.; Gurwith, M.; Rozenszajn, L. A.; Gezin, A.; Levi, E.; Schlesinger, M.; Kalechman, Y.; Michlin, H.; Shalit, F.; Engelsman, E.; Farbstein, H.; Farbstein, M.; Albeck, M.; Sredni, B. Nat. Immun. Cell Growth Regul. 1990, 9, 182.

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23. (a) Kalechman, Y.; Albeck, M.; Oron, M.; Sobelman, D.; Gurwith, M.; Seghal, S. N.; Sredni, B. J. Immunol. 1990, 145, 1512. (b) Sredni, B.; Kalechman, Y.; Shalit, F.; Albeck, M. Immunology 1990, 69, 110. (c) Sredni, B.; Kalechman, Y.; Albeck, M.; Gross, O.; Aurbach, D.; Sharon, P.; Sehgal, S. N.; Gurwith, M. J.; Michlin, H. Immunology 1990, 70, 473. 24. Shohat, B.; Kozenitzki, L.; David, M.; Albeck, M.; Sredni, B. Nat. Immun. 1993, 12, 50. 25. Sailer, B. L.; Prow, T.; Dickerson, S.; Watson, J.; Liles, N.; Patel, S. J.; Fleet-Stalder, V. V.; Chasteen, T. G. Environ. Toxicol. Chem. 1999, 18, 2926. 26. Grogan, T. M.; Fenoglio-Prieser, C.; Zeheb, R.; Bellamy, W.; Frutiger, Y.; Vela, E.; Stemmerman, G.; Macdonald, J.; Richter, L.; Gallegos, A.; Powis, G. Hum. Pathol. 2000, 31, 475. 27. Engman, L.; Cotgreave, I.; Angulo, M.; Taylor, C. W.; Paine-Murrieta, G. D.; Powis, G. Anticancer Res. 1997, 17, 4599. 28. Malmström, J.; Jonsson, M.; Cotgreave, I. A.; Hammarström, L.; Sjödin, M.; Engman, L. J. Am. Chem. Soc. 2001, 123, 3434. 29. Rooseboom, M.; Vermeulen, N. P.; Durgut, F.; Commandeur, J. N. Chem. Res. Toxicol. 2002, 15, 1610.

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8.1

SOME ADDITIONAL APPLICATIONS OF TeCl4

8.1.1

Preparation of -methylene ketones

The reaction of TeCl4 with siloxycyclopropanes, followed by treatment with DMSO, or bases such TMEDA, Et3N and pyridine, gives -methylene ketones via the intermediacy of -oxoalkyl- or bis(-oxoalkyl)tellurium trichlorides or dichlorides.1 Me3SiO

TeCl4 (1 equiv)

R R1

CH2Cl2, 0°C

1) base 2) Me3SiCl 3) CH2I2 / Zn-Ag

O R

TeCl3 R1

DMSO) or bases) 5 equiv, 0°C, 10 min R (71-93%)

; R1 = H R = t-Bu, R = Ph; R1=Me R, R1= (CH2)5, (CH2)10,

R1 TMEDA (2 equiv) 0°C, 30 min (94%)

O R

O

O R1

TeCl4 (0.5 equiv) CH2Cl2, 0°C

R

O Te R1 Cl Cl R1

TeCl3

R = t-Bu; R1 = H

Similar dehydrotrichlorostannation of -trichlorostannyl ketones by DMSO requires prolonged heating at 60°C. Preparation of -methylene ketones (typical procedure).1 Siloxycyclopropane (from cyclododecene) (1.34 g, 5 mmol) is added to a suspension of TeCl4 (1.34 g, 5 mmol) in CH2Cl2 (10 mL) at 0°C. After stirring for 10 min DMSO (1.8 mL, 25 mmol) is added, and stirring continued for a further 10 min. The black precipitate is filtered off and aqueous treatment (pentane/H2O) is performed. The organic phase is dried (MgSO4) and evaporated under vacuum. The residue is chromatographed on SiO2 to give -methylene cyclododecanone (0.903 g (93%)).

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Olefin inversion by syn-chlorotelluration/anti-dechlorotelluration

The inversion of the olefin configuration can be attained by employing a one-pot procedure involving a sequential syn-chlorotelluration/anti-dechlorotelluration.2 Reaction of the appropriate alkene with 1 equiv of tellurium tetrachloride in chloroform or acetonitrile gives a 2-chloroalkyltellurium trichloride adduct in a high yield (see Section 3.5.1.2). Reduction of this adduct with aqueous sodium sulphide results in the instantaneous separation of elemental tellurium and formation of the starting alkenes which, in most cases, exhibit an inverted configuration (with relevant variations depending on the solvent and olefin used). The proposed mechanism involves an intermediate epitelluride. R

H

H

R1

TeCl4 / MeCN syn addition

R H H R1 Cl

TeCl3

Na2S

R H

1 Cl R H

H2O

(E )-1-D-1-decene (Z )-2-butene (E )-2-butene

Te-

R

R

Te

H

R1 H

R1

-Te H H (E )/(Z ) ratio: 26/ 74; 81/19; 3/97

Inversion of olefins (typical procedure).2 (E)-2-butene (0.038 g) is added to a solution of TeCl4 (0.220 g, freshly sublimed) in MeCN (5 mL). The mixture is stirred at 0°C for 3 h and then evaporated at room temperature under vacuum. Na2S·9H2O (14% aqueous solution, 10 mL) is added dropwise (through a septum), producing evolution of gas and precipitation of elemental Te. Analysis of the gaseous 2-butene shows an E/Z ratio at 3:97. 8.1.3 Tellurium tetrachloride as a catalyst for dithioacetalization and ketalization Tellurium tetrachloride is an efficient Lewis acid catalyst for the thioacetalization of aldehydes and thioketalization of aliphatic ketones.3 R R1

TeCl4

O + 2 R2SH Cl

Cl, r.t.

R

SR2

R1

SR2 (80-99%)

R O + HS

( )n SH

R1

TeCl4 Cl

Cl, r.t.

R

S

R1 S

( )n

R = H; R1 = Ph, p -HOC6H4, p -MeC6H4, p -ClC6H4, PhC=CH2; R2 = Et and n = 2, 3 R, R1= (CH2)5; n = 2 Ac ;n=2 R, R1 =

R, R1 =

C8H17 ; n = 2, 3

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Selective ketalization is achieved as illustrated. O CHO + HS

O

TeCl4

SH

S

Cl, r.t.

Cl

S

(76%) O

O + HS

TeCl4

SH

O

Cl, r.t.

Cl

S S

(68%)

TeCl4-catalysed dithioacetalization (or dithioketalization) (general procedure).3 To a solution of the carbonyl compound (5 mmol) and the thiol (10 mmol) or dithiol (5 mmol) in ClCH2CH2Cl (10 mL) is added powdered TeCl4 (0.25 mmol); the resulting mixture is stirred for 2–3 h at room temperature, during which period black tellurium gradually separates. The reaction is quenched by adding NaHCO3 (~0.2 g), and the insoluble material is filtered off. The filtrate is dried (Na2SO4) and the solvent evaporated under vacuum, giving the crude dithioacetal, which is purified by SiO2 chromatography or by Kügelrohr distillation. Benzaldehyde dimethyl acetal reacts similarly.

C6H5C

OMe + HS OMe

SH

S

TeCl4 CH2Cl2, r.t.

C6H5C

S (99%)

8.1.4 Tellurium tetrachloride as reagent for the conversion of alcohols into alkyl chlorides and as a Lewis acid catalyst for aromatic alkylation Treatment of alcohols with TeCl4 in solvents affords the corresponding chlorides in good yields.4 ROH + TeCl4 Solvent -HCl

[R-OTeCl3]

-TeOCl3-

[R+]

TeOCl3-

RCl -TeOCl2 (65-86%) R = PhCH2CH2, Ph2CH, PhCHCH3, Ph3C, 1-C8H17, 2-C8H17 Solvent: toluene, CH2Cl2, Cl(CH2)Cl

In the presence of tellurium tetrachloride, aromatic hydrocarbons are alkylated with reactive alkylating agents such as benzylic or t-butyl alcohols and chlorides.4 The yields are high with toluene but only moderate for benzene, p-xylene and anisole. Equivalent and catalytic amounts of tellurium tetrachloride, respectively, are required for the alcohols and

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chlorides. Tellurium tetrachloride therefore works as a reagent for the conversion of the alcohols into chlorides, and then as a Lewis catalyst for the aromatic substitution (paraisomers are formed preferentially to ortho-isomers).

ROH + TeCl4

[R-OTeCl3]

-HCl

RCl

-

-TeOCl3

ArH, r.t. TeCl4 cat.

[R+]

TeOCl3-TeOCl2

RCl

Ar-R

R = PhCHCH3, PhCH2, t -Bu; ArH = toluene (83, 87, 34%) R = PhCHCH3; ArH = benzene, p -xylene, anisole (3, 40, 59%) RCl + ArH

TeCl4 cat.

Ar-R

R = PhCHCH3, t-Bu; Ar = toluene (93, 71%)

TeCl4-catalysed Friedel-Crafts aromatic alkylation (typical procedure).4 To a solution of 1-phenylethanol (3.7 g, 30 mmol) in toluene (30 mL) is added slowly TeCl4 (9.7 g, 36 mmol), keeping the temperature at 25°C (exothermic reaction). Small amounts of white precipitate appear immediately, and after a few minutes the colour of the mixture becomes dark brown. The mixture is stirred for 3 h and then quenched with H2O (20 mL). The organic layer is separated, washed with brine (2×20 mL) and dried (MgSO4). Evaporation of the solvent leaves an oily residue which is distilled under vacuum, giving a mixture of 1-phenyl-1-tolylethanes (4.9 g (83.3%); b.p. 117–128°C/1 torr). GLC analysis (silicone OV-101, 0.24 mm ⫻ 30 m capillary column at 100–260°C, 4°C min⫺1) reveals an ortho/para ratio of 12:88. 8.1.5 Tellurium-tetrachloride-promoted rearrangement of cycloheptatriene to benzylic alcohols The title reaction has been performed with various cycloheptatrienes as depicted in sequence.5 R

R1

R1 CCl4 + TeCl4 (47-78%) R2

Cl R + TeCl2 + HCl R2

Te° + TeCl4 R, R1, R2 = H, Me (20, 2 h; 47, 78%) R = Me, Ph; R1, R2 = H (12, 5 h; 63, 70%)

General procedure.5 A solution of 10 mmol of the cycloheptatriene and 1.9 g (7 mmol) of TeCl4 in 30 mL of dry CCl4 or dry CH2Cl2 is stirred at 0–5°C until reaction is complete. The progress of the reaction may be followed by GLC (OV-1, 3%, 2 m column, 110°C,

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and SE-30, 20%, 2 m column, 170°C). In CH2Cl2 solution the reaction times are considerably shorter. Following addition of water, removal of precipitated tellurium compounds by filtration, and evaporation of the solvent from the dried organic layer, the rearrangement product (benzylic halide) may be distilled. The products were identified by GLC and NMR comparison with authentic samples. 8.1.6 Tellurium tetrachloride as a catalyst for cationic oligo- and polymerization reactions Tellurium tetrachloride, a source of TeCl3+ ions, behaves as a cationic catalyst for oligoand polymerization reactions. Two types of monomer, phenyl-substituted ethylenes and benzyl chlorides, have been submitted to these reactions.6 The termination of polymerization of substituted ethylenes is by an internal FriedelCrafts reaction, whereas that of the substituted benzyl chlorides is by the reaction with chloride ions. -HYDROXYALKYLATION OF ,-UNSATURATED CARBONYL COMPOUNDS

8.2

Diisobutylaluminium phenyl tellurolate, a highly air- and moisture-sensitive reagent, prepared by reaction of diisobutylaluminium hydride with diphenyl ditelluride, undergoes an in situ addition to ,-unsaturated carbonyl compounds, leading to the corresponding -phenyltelluroaluminium enolate. This intermediate is hydrolysed by aqueous HCl into -phenyltelluro compounds or smoothly affords an aldol reaction with aldehydes to give -hydroxyalkyl--phenyltellurocarbonyl compounds. These adducts, submitted to MCPBA oxidation, undergo telluroxide elimination, regenerating the original C⫽C bond. The overall transformation therefore provides -hydroxyalkylation of ,-unsaturated carbonyl compounds.7 PhTeTePh + 2 i-Bu2AlH O + PhTeAlBu2 -i

THF

2 PhTeAlBu2-i + H2 OAlBu2-i HCl / H O 2

THF

(47-84%)

TePh

O TePh

RCHO O

HO

HO MCPBA (47-89%)

R

PhTe

R

O

enone O

O O

O

O H

O enone

n( )

n = 1, 2; R = n - C3H7, Ph

n( ) n = 1-3 OR

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8.3

CONVERSION OF ALLYLSILANES INTO ALLYLAMINES VIA PHENYLTELLURINYLATION

Allylamines are obtained by sequential treatment of allylsilanes8 with benzenetellurinyl trifluoroacetate in the presence of BF3·Et2O and then with an alkylamine.9 The reaction intermediate is an unstable and not isolated phenyl allyl telluroxide. Arylamines behave similarly, but require longer reaction times. SiMe3

PhTe(O)OCOCF3 Cl, 3 h BF3.Et2O, Cl

Te(O)Ph

alkyl amines, 3 h RNHR1, 65°C aryl amines, 6 h (72-96%) R = H; R1= Ph, p -ClC6H4, p -NO2C6H4, n-C8H17 NRR1 R = Me; R1 = Ph R, R1 = (CH2)5, (i-prCH2)2

Substituted allylsilanes react with aryl- and alkylamines, giving -trans- and -transsubstituted allylamines, respectively. This selectivity is independent of the cis/trans geometry and the - or -isomerism of the starting allylsilanes. This result is consistent with an equilibrium between an initially formed -substituled allyl telluroxide (resulting from the well-known regio-controlled electrophilic attack at the -carbon) and the -isomer (which is really the main product, as detected by NMR). In the subsequent reaction with amines, the more nucleophilic alkylamines attack the central carbon atom, giving rearranged products, whereas the less nucleophilic arylamines attack the terminal carbon, giving the thermodynamic -product. R R1

SiMe3

PhTe(O)OCOCF3 -15°C (γ attack)

R R1 TePh O

R R Representative examples

Ar 2 R Ph

R R1 Ph

Te

O

R a) ArNHR2,3 h (57-89%)

R

Te O Ph

R2NH2 amine b) or a) p-ClC6H4, PhNHMe R22NH, 1 h b) n-C4H9NH2, cyclo-C6H11NH2, piperidine R R1 a) p-ClC6H4 NHR2 b) n-C4H9NH2 (NR 2)

R = H; R1 = n-C6H13 (cis / trans = 84 / 16)

R, R1 = (CH2)5

2

Allylamines from allylsilanes (typical procedure).9 To a solution of PhTe(O)OCOCF3 (1 mmol) (generated from (PhTeO)2O (0.229 g, 0.5 mmol) and (F3CCO)2O (0.120 g,

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REFERENCES

343

0.55 mmol) in ClCH2CH2Cl (4 mL)) are added BF3·Et2O (0.156 g, 1.1 mmol) and then allyltrimethylsilane (0.125 g, 1.1 mmol) in ClCH2CH2Cl (2 mL). The solution is stirred at room temperature for 1 h. p-Chloroaniline (0.32 g, 2.5 mmol) is then added and the mixture heated at 65°C with stirring for 6 h. The reddish-black mixture is poured into saturated aqueous NaHCO3 and extracted with CH2Cl2. The extract is dried (MgSO4) and evaporated under vacuum. The residue is chromatographed on SiO2 (eluting with hexane/EtOAc, 95:5), giving N-allyl-p-chloroaniline as a pale yellow oil (0.151 g (90%)).

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

Nakahira, H.; Ryu, I.; Han, L.; Kambe, N.; Sonoda, N. Tetrahedron Lett. 1991, 32, 229. Backwall, J. E.; Engman, L. Tetrahedron Lett. 1981, 22, 1919. Tani, H.; Masumoto, K.; Inamasu, T. Tetrahedron Lett. 1991, 32, 2039. Yamaguchi, T.; Hattori, K.; Mizutaki, S.; Tamaki, K.; Uemura, S. Bull. Chem. Soc. Jpn. 1986, 59, 3617. Albeck, M.; Tamari, T.; Sprecher, M.; Ohe, K. J. Org. Chem. 1983, 48, 2276. Albeck, M.; Tamari, T. J. Organomet. Chem. 1982, 238, 357. Sasaki, K.; Aso, Y.; Otsuba, T.; Ogura, F. Chem. Lett. 1989, 607. Colvin, E. W. Silicon Reagents in Organic Synthesis, p. 25. Academic Press, London, 1988. Hu, N. X.; Aso, Y.; Okubo, T.; Ogura, F. Tetrahedron Lett. 1988, 29, 4949.

Subject Index

Acenaphthene 119 3-Acetamidocycloheptene 182 Acetolysis 177 Acetoxy-2-thiopyridone 263 N-Acetoxy-2-thiopyridone 263 Acetoxyalkyl ditellurides 176 Acetoxymercuriation 290 Acetoxytelluride 174 Acetoxytellurium tribromides 177 N-Acetoxythiopyridone 262 Acetylation 290 Acetylenes 71, 88, 111 Acetylenic ketones 82 Acetylenic phosphonates 82 Acetylenic selenides 96 Acetylenic tellurides 78, 97, 99, 208, 258 o-Acetylphenyl tellurenyl bromide 293 Acrylonitrile 197 Activated acetylenes 88 Active methylene compounds 149 Acyl radicals 266, 270 a-Acyl radicals 275 Acyl tellurides 266, 267 Acylhydrazides 165 Acylhydrazines 165 Acyltellurides 66, 275,279 (Z)-/3-Acyltellurocinnamates 76 Addition of tellurium tetrachloride 48 Addition to enones 241 Alcohols 115, 129 Alcoxycarbonyl tellurophenes 290 Aldehydes 115, 148, 153, 154, 220, 221,276 Aliphatic aldehydes 120 Aliphatic ditellurides 11 Alkali 2-thienyl tellurolates 137 Alkali O,O-diethyl phosphorotellurolates 128 Alkanes 212 Alkenes 48, 58, 60, 197, 252, 281 Alkenyl telluride 251 345

346

Alkenylaluminium 249 a-Alkenyl-substituted fl-dicarbonyl compounds 192 Alkenylzinc 248 Alkinyl esters 84 Alkinyl tellurides 260 fl-Alkoxyalkyltellurium trichlorides 43 Alkoxytrihalotellurination 48 Alkyl aryl sulphones 151,152 Alkyl aryl tellurides 27, 29 Alkyl aryltellurides 270 Alkyl ethynyltellurides 109 Alkyl phenyl tellurides 26, 207, 208, 210, 212, 213, 217 Alkyl phenyl telluroxides 210 Alkyl vinyl tellurides 73, 80 (Z)-4-Alkyl-l-(o-bromophenyl)-l-buten-3-ynes 297 Alkylamines 342 Alkylaryltellurium dichlorides 62 Alkylethynyl tellurides 109 Alkylidenation of aldehydes 151 a-Alkylidene fl-ketosulphones 142 a-Alkylidene fl-oxosulphones 143 Alkylidene malonates 150 Alkylidene malonic esters 224 2-Alkylidene-2H-tellurachromenes 297 Alkyltellurium trichlorides 60 Alkyltelluro groups 242 Alkyltellurocyanocuprates 242 t-Alkylthiols 169 Alkylvinyl tellurides 71, 72 Alkynes 83, 273, 281 Alkynil zinc 258 Alkynyl Grignard 108 Alkynyliodonium salts 109 Alkynyllithium 110 Alkynylphenyl tellurides 109 Alkynylselenolate anions 97 Alkynylzinc reagents 257 a-Allenic acids 185 Allenic butyl tellurides 111 Allenic tellurides 112 a-Allenoic acid 185 Allenyl tellurides 113 Allyl alcohols 130 Allyl ethers 160 Allyl phenols 187 Allyl telluronium salts 224 Allyl telluroxide 342 Allylamines 127, 342 Allyldiisobutyltelluronium bromide 225 Allylic alcohols 104, 130, 215 Allylic amines 202 Allylic bromides 248, 261 Allylic dibromides 135 Allylic ferrocenyl tellurides 217 Allylic phenyl tellurides 202

SUBJECT INDEX

SUBJECT I N D E X

Allylic radicals 261 Allylic telluroacetate 177 Allylides 221 o-Allyloxy-substituted telluroesters 267 N-Allyl-p-chloroaniline 343 Allylphenols 187, 188 o-Allylphenols 189 Allylsilanes 342 Aluminium telluride 45 Amido olefins 182 Amidotellurinylation of olefins 194 Amines 161 Aminotellurination of olefins 180 Aminotellurinylation 181,182 Aminotellurinylation reactions 187 Ammonia 120 Ammonium fluoride 203 Ammonium tellurolate 333 Anhydrides 172 Anilines 121,122, 123, 124 Anisyl phosphorotellurolate esters 55 Anisyltellurenyl chloride 55 Anti-2-chloroalkyltellurium dichlorides 60 Anti-acetolyses 174 Anti-debromination 135

Anti-dechlorotelluration 338 Anti-hydrotelluration 292, 297 Antioxidants

331

Anti-tellurosulphonates 83 Anti-tellurosulphonation 83 Anti-tumour effects 333 Arbuzov reaction 170 Arene tellurolate anions 80 Arene tellurolates 27, 81 Arenediazonium chlorides 59 Arenediazonium fluoroborates 20 Arenediazonium salts 63 Arenes 58, 61 Arenetellurenyl halides 31 Arenetellurinic anhydrides 54, 171 Arenetellurinic mixed anhydrides 54 Aromatic aldehydes 143 Aromatic ditellurides 11 Aromatic telluroesters 235 Aromatic thioketones 121 Aroylchlorides 66 Aroyllithiums 235 Aryl acetylenes 77 Aryl alkyl tellurides 25 Aryl boronic acids 44 Aryl butyltellurides 50 Aryl ditellurides 27 Aryl halides 39 Aryl iodides 258 Aryl ketones 116

347

348

Aryl phenyl sulphones 227 Aryl sulphonyl chlorides 151 Aryl tellurocyanates 56 Aryl tellurocyanides 21 Aryl tellurocyclohexane 69 Aryl telluroesters 67 Aryl telluroformates 68, 270 Aryl tellurolates 30 Aryl tellurotris(trimethylsilyl)silane 69 Aryl vinyl tellurides 229 Arylacetylenes 265 Arylation of styrene 197 Arylbenzyl tellurides 63 Arylboronic acids 33 Aryldichlorotellurium butyrolactones 183 Aryldichlorotellurolactones 184, 185, 186 Arylethynyl tellurides 33 N-Arylhydroxyl amines 122 Aryllithiums 229 Arylmagnesium bromide 21 Arylmercury chlorides 50, 58 a-Arylpropanoic acids 210 Aryltellurenyl bromides 56, 133 Aryltellurenyl halides 55, 185 Aryltellurenyl iodides 86 Aryltellurinic 174 Aryltellurinic acids 53, 54 Aryltellurinic anhydrides 54 Aryltellurinyl acetates 187 Aryltellurium anhydrides 53, 54 Aryltellurium tribromides 33, 60, 61, 84, 85, 189 Aryltellurium trichlorides 29, 49, 50, 62, 183, 184, 188, 199, 204, 206 Aryltellurium trihalides 12, 53 Aryltellurium triiodides 54 fl-Aryltelluro vinyl aldehydes 80 (Z)-(fl-Aryltelluro)-a, fl-enones 84 2-((Aryltelluro)methyl)chromanone 268 3- [(Aryltelluro)methyl]chromanones 268 fl-(Aryltelluro)propenoyl acids 312 fl-(Aryltelluro)propenoyl chlorides 312 Aryltellurolates 109 Aryltellurols 45, 73, 115 Azides 126 Aziridine sulphonates 127 Azo compounds 121,123 1,1-Azobis(cyclohexane-1-carbonitrile) 275 2,2'-Azobisisobutyronitrile 69 Azoxybenzenes 122, 125

Bacterial cytotoxicity 333 Baldwin rules 187 Benzalacetophenone 218 Benzaldehyde 117, 150, 168, 238, 267

SUBJECT INDEX

SUBJECT INDEX

Benzenetellurinic anhydride 174, 175, 180, 181 Benzenetellurinyl acetate 173 Benzenetellurinyl trifluoroacetate 173, 180, 342 Benzenetellurinyl trifluoromethanesulphonate 173 Benzhydryl ethyl sulphides 144 Benzo[c]tellurophene 300 Benzo- [c]-tellurophenes 300 Benzoin 117, 168 Benzonitrile 173 Benzophenone 117, 276 Benzophenone hydrazone 165 p-Benzoquinone 168 1-Benzotellurepines 297 3-Benzotellurepines 297, 299 1-Benzotellurepines and 2-akylidene-2H-tellurochromones 298 2H-1-Benzotellurin 311 4H-1-Benzotellurins 311 Benzotellurophene dichlorides 296 1-Benzotellurophenes 292 Benzoyl-l-naphthyl telluride 267 Benzydryl group 127 Benzyl 274 Benzyl bromide 208 Benzyl carboxylates 155 Benzyl dodecyl ether 117 Benzyl ester 184 Benzyl groups 10 Benzyl p-methylphenyl telluride 278 Benzyl sulphones 226 Benzyl tellurocyanate 208 2-Benzyl-1, 4-benzoquinone 278 Benzylic alcohols 168, 173 Benzylic tellurides 235 Benzylic thiols 169 1-(Benzylseleno)-3-undecyl (phenyltelluro)formate 271 Benzylselenoalkyl-substituted telluroformates 271 Biaryls 195, 197 Bicyclic telluroethers 187 2,2'-Binaphthyl 195 2-Biphenyl tellurium trichloride 303 2,2'-Biphenylmercury 301 Bis vinylic tellurides 240 Bis(1,2-ditellurolo)naphtacene 305 2,5-Bis(1-phenyl-l-hydroxymethyl)tellurophene 325 Bis(2,4,6-trimethylphenyl) telluride 23 Bis(2-aminophenyl)ketone 313 Bis(2-arylethenyl)tellurides 230 Bis(2-methoxy-3-phenylpropyl) ditelluride 43 Bis(2-methylphenyl)telluride 322 Bis(2-oxomethyl)tellurium dichlorides 201 Bis(2-phenyl-l,2-dioxolan-2-yl)methyl telluride 14 Bis(2-thienyl) ditelluride 135, 140 Bis(4-aminophenyl)telluride 331 Bis(4-carbomethoxy)butyl ditelluride 38

349

350

4,4'-Bis(aminophenyl) telluride 333 Bis(arenediazonium) hexachlorotellurates 59 Bis(/3-acylvinyl) tellurides 80 Bis(/3-oxoalkyl)tellurium trichlorides 337 Bis(butyl tellurium)methyl sulphide 238 Bis(butyltelluro)ketene acetals 98 Bis(chloroalkyl)tellurium dichlorides 48 Bis(m-methoxyphenyl) ditelluride 40 Bis(p-bromophenyl)ditelluride 249 Bis(p-chlorophenyl)tellurium diiodide 185, 191 Bis-(p-dimethylamino)phenyl telluride 331 Bis-(phenylethynyl) telluride 23 1,1-Bis(phenyltelluro)-l-pentene 90 (E)- 1,2-Bis(phenyltelluro)-3-hydroxyprop- 1-ene 89 Bis(p-methoxyphenyl) diteiluride 40, 41 Bis(p-methoxyphenyl) tellurium diacetate 167 Bis(p-methoxyphenyl) telluroxide 162, 166 Bis(p-methoxyphenyl)tellurium dichloride 59 Bis(telluro-9-xanthenyl), 314 2,2'-Bis(trichlorotelluro)biphenyl 304 1,4-Bis(trimethylsilyl)-l,3-butadiene 286 Bis(triphenylstannyl) telluride 17, 18, 137, 145 Bis[2-(2,3-dihydrobenzofuranyl)methyl] ditelluride 190 Bis[2-(2,3-dihydrobenzofuranyl)methyl]tellurium dichloride 190 Bis[2,2'-biphenyldiyl]tellurium 303 Bis[dimethylthienol] 1,4,6,8-tetratellurafulvalene 308 Bis-[tellura-9-xanthenyl] 314 Bis-acylethylene 201 1,2-Bis-alkyltelluroacetylenes 109 Bis-allylic telluride 261 2,5-Biscarboxymethyl-3,4-bis bromomethyl thiophene 301 Bischloromethylcarbinols 130 Bis-ethynyl tellurides 108 o-Bishalomethyl benzenes 300 Bis-organyl telluromethanes 46 Bis-phenylethynyl telluride 111 Bis-phenyltelluroalkenes 88 Bis-phosphonium halotellurate 92 Bis-silylated hydroquinones 279 Bis-silylation 279 Bis-stannylated 238 a-Bis-sulphenylated carbonyl 152 Bis-tellurenyl ferrocenes 238 Bis-tellurobutadiene 288 Bis-thieno- 1,4-ditellurins 319 Bisvinylic tellurides 72 Bis-vinylic tellurides 240, 246 Bis-ylide 201 Bromine 207 (E)-3-Bromo-l-trimethylsilylprop-l-ene 223 2-Bromo-3-(p-tolyl)propene 249 Bromoacetates 148 Bromoacetic esters 147 Bromoacetylenes 108 Bromoalkynes 247

SUBJECT INDEX

SUBJECT INDEX

Bromoallyl silane 223 Bromoazobenzene 125 3-Bromobenzotellurophene 293, 296 3-Bromobenzotellurophene dichloride 293 Bromochlorination 203 3-Bromocyclohexene 261 2-Bromodecyl phenyl telluroxide 214 Bromodetelluration 204 a-Bromoester 219 2-Bromoethyl benzoate 159 a-Bromoketones 207, 219 Bromomagnesium 110 Bromomagnesium cuprate 254 p-Bromonitrobenzene 125 2-Bromooctyl phenyl telluroxide 214 p-Bromophenacyl bromide 140 o-Bromophenyl ethynyl ketone 298 1-(4-Bromophenyl)-3-(4-ethoxyphenyltelluro)-2-propen-l-one 81 ct-Bromophenylacetic acid 140 Bromophenylacetylene 247 N-Bromosuccinimide 99, 203 Bromotetrahydrofurans 191 a-Bromovinyl tellurides 101 1,3-Butadienes 251 (Z,Z)-(1,3)-Butadienes 253 Butenines 297 t-Butyl alcohols 339 t-Butyl hydroperoxide 204 t-Butyl hypochlorite 65 Butyl phenylethynyl telluride 107 Butyl telluroesters 67 Butyl vinyl telluride 229 Butyl-2-ethynylphenyl telluride 296 3-t-Butylcyclohexanone 242 Butyltellurenyl bromide 77, 111 2-Butyltelluro aldehydes 104 E- 1-(Butyltelluro)- 1-phenyl-3,3-dimethyl- 1-butene 248 1-(Butyltelluro)heptyne 208 Butyltelluro-l,3-butadienes 230 (Z)- 1-Butyltelluro- 1,4-(bis-hydroxymethyl)but- 1-en-3-yne 75 1-Butyltelluro- 1-halo ethenes 100 (E)-l-Butyltelluro-l-phenylthio-l-alkenes 103 Butyltelluroacetylene 99 Butyltellurobutenine 291 (Z)-Butyltellurobutenines 288, 289 Butyltelluroferrocene 238 Butyltellurophenylthioethene 75 N-Butyltin hydride 277

Carbalcoxy group dealkylation 156 Carbamate group 193 Carbodetelluration 195 Carbohydrate anisyl tellurides 262 Carbon monoxide 200, 259

351

352

Carbonates 159 o-Carbonyl benzotellurophenes 294 Carbonyl compounds 120 Carbonylative cross-coupling 198 Carbotelluration 111 2-Carboxy- 1-ethenyl aryl tellurium 312 2-Carboxybenzotellurophene 294 fl-Carboxylalkyltellurium trichlorides 42 Carboxylmethyl tellurides 294 Catechol 166 Cesium carbonate 223 Cetyltrimethylammonium bromide 169 Chalcogenophenes 286 Chalcogenopyran-4-thiones 311 Charge-transfer complex 291 Chiral allylic amines 202 Chloramine-T, 154, 202 p-Chloranil 325 Chlorinating reagent 51 Chlorine 208 Chlorinolysis 51 2-Chloro-2-phenylethenyl tellurium trichlorides 294 2-Chloro-2-phenylethenyl-l-p-methoxyphenyl telluride 85 2-Chloro-2-phenylethenyl- 1-p-methoxyphenyltellurium dichloride Chloroacetates 148 2-Chloroalkyl-2-naphthyltellurium dichloride 61 2-Chloroalkyltellurium trichloride 338 fl-Chloroalkyltellurium trichlorides 48 o-Chlorobenzaldehyde 116 p-Chlorobenzaldehyde 226 3-Chlorobenzotellurophenes 294 1-Chlorocyclohexyltellurium trichloride 206 b-Chloroenals 287 fl-Chloroesters 149 a-Chloroketones 207, 208 Chloromethyl epoxides 130 Chloromethylation 290 N-Chloro-N-sodium-p-tolysulphonarnide 217 p-Chlorophenol 225 1-(p-Chlorophenyl)-4-(trimethylsilyl)-3-butyn-l-ol 226 p-Chlorostyrene 179 N-Chlorosuccinimide 65 Chlorotrimethylsilane 243 Chlorotriphenylstannane 134 (Z)-2-Chlorovinyltellurium trichlorides 203 Chromanone 88 Cinnamaldehyde 218 Cis-allylic alcohols 131 Cleavage of 2-haloethyl carboxylates 158 Cleavage of allyl carboxylates 157 Cleavage of aryl carbonates 160 Cleavage of aryl carboxylates 159 Cleavage of aryl haloacetates 160 Cleavage of phenacyl esters 157

SUBJECT I N D E X

85

SUBJECT INDEX

Cleavage of trichloro-t-butylcarbamates 161 Cross-coupling 252 Cross-coupling reaction 197 p-Cyanobenzyl bromide 148 o-Cyanobenzyllithium 235 Cyanocuprates 242, 254 Cyanomethylcarbinol 226 fl-Cyanosulphones 143 a-Cyanovinyl(telluro acrylonitriles) 90 Cyclalkene tellurolate 320 Cyclic organyltellurium trichloride 29 Cyclic tellurides 15 Cyclic tellurinyl ethers 192 Cyclic tellurium oxychlorides 85 Cyclic telluroethers 192 Cyclo-5,6-dihydrouridine 264, 265 Cycloalkanones 120 Cycloalkyl bromides 215 Cyclodextrinyl ditelluride 332 Cyclododecyl phenyl telluride 215 Cycloheptatrienes 340 Cycloheptyl phenyl telluride 209, 210 Cyclohexyl phenyl telluroxide 213 Cyclonucleosides 264 Cyclo-octanone 269 Cyclopropanation 224, 244 Cyclopropanation of a, fl-unsaturated carbonyl compounds Cyclopropane 150 Cyclopropyl ring 268 Cyclotelluroetherification 58, 187, 189 Cynnamic ester 253

Dealkylation of carboxylic esters 156 Deblocking reactions 155 Debromination 133 Debromination of vic-dibromides 132 Decarboxilative removal of phenyltellurolate 271 Dehalogenation 100, 139, 140 Dehalogenation of a-haloketones 139 Dehydrotrichlorostannation 337 Deoxygenation 128 Desoxybenzoin 117 Desulphonation 136 Desulphonylation of/3-ketosulphones 142 Desulphonylation 142 Detelluration 186, 191 Detellurative carbonylation 199 Detellurative methoxylation 209 Di(1-naphthyl) telluride 20 Di(2,4,6-tri-t-butylphenyl) ditelluride 42 Di(2-thienyl) ditelluride 139 Di(p-methoxy)phenyltelluride 196 Di-(p-methoxyphenyl telluride) 133

353

151

354

Di(p-methoxyphenyl) telluride 198 Di-(p-methoxyphenyl)tellurium dichloride 58 Di(p-tolyl) telluride 20 Di[o-(1-buten-3-ynyl)phenyl] ditellurides 297 Di-2-naphthyl ditelluride 39 Di-2-thienyl ditelluride 41 Diacetates 175 Diacetoxylation of 1,3-butadiene 177 Diacetoxylation of olefins 174, 175, 176 Diacetylenic ketones 82 Dialkyl ditellurides 32, 38, 39 Dialkyl ketones 120 Dialkyl tellurides 13, 14, 16, 18 Dialkylchlorophosphates 170 Dialkylcuprates 201 Dialkylditellurides 109 Dialkylmercury reagents 32 Dialkyltellurium diiodides 57 Dialkynyl tellurides 110 a,),-Diallyl-fl-ketoesters 193 Diallyl telluride 14 Diamines 123 Diaryl ditellurides 11, 21, 22, 26, 27, 31, 43, 44, 83, 196, 268 Diaryl ketones 116 Diaryl tellurides 19, 21, 22, 23, 197, 331,332 Diaryl tellurium dibromides 185 Diaryl telluroketals 46 Diaryl telluroxides 65, 171 Diarylchalcogenides 331 Diarylditelluride 86 Diarylditellurides 31 1,2-Diarylethylenes 226 Diaryltellurides 133 Diaryltellurium dibromides 133, 163, 191 Diaryltellurium dichlorides 36, 49, 58, 59, 199, 200 Diaryltellurium dihalides 61, 63 Diaryltellurium diiodides 191 Diazenes 122 DIBAL-H, 102 5H,7H-Dibenzo[b,g][1,5] tellurothiocin 321 Dibenzocyclooctanone B 269 Dibenzotellurophene 302, 303 Dibenzothiophene dioxide 301 Dibenzyl ditelluride 11, 37, 52, 208 Dibenzyl telluride 15, 16 2-Dibromo-l,2-diphenylethane 133 4-Dibromo-2-alkenes 135 1,1-Dibromoalkanes 207 5a, 6fl-Dibromocholestan-3fl-ol 135 Dibromodiaryltellurides 133 1,2-Dibromohexadecane 134 Dibromomalonates 150, 151 a, a-Dibromo-o-xylenes 135 1,2-Dibromotetrachloroethane 163

SUBJECT I N D E X

SUBJECT INDEX

Dibutyl telluride 17, 67, 150, 153,220 Dibutyl telluroxide 219 Dibutyl(alkylcarbamoylmethyl)telluronium halide 220 Dibutyldidecyl tellurium 266 N-Dibutylditelluride 74 Dibutyltelluronium benzylide 218 Dibutyltelluronium carbethoxy 218 a-Dichloroaryltelluroketones 207, 208 1,4-Dichloro-but-2-yines 286 Dichlorophenoxatellurine 315 Dichlorotelluroethers 189 Dichlorotellurolactones 185, 186 10,10-Dichlorothiophenoxytellurine 317 Diels-Alder 136 (Z,Z)-Diene 251 Dienes 135, 254, 257 1,5-Dienes 261 Diethyl ditelluride 37 O,O-Diethyl phosphorotellurolates 20, 152 Diethylchlorophosphorochloridrate 244 Diethyltelluride 13 Diethylzinc 248 o-Diethynylbenzene 299 1,2-Difluorides 154 Difunctionalization of alkynes 83 21,21-Dihalo-21-telluroporphyrin 324 Dihalocarbenes 70 Diheteropentalene 301 1,3-Dihydro-2,2-bis(trifluoroacetoxy)-benzo[c]tellurophene 1,3-Dihydro-2-benzotellurophene diiodide 57 Dihydrobenzoin 117 3,4-Dihydroiso-coumarins 235 Dihydronaphthalene 155 Dihydrotellurophenes 332 p,p'-Dihydroxydiphenyl telluride 331 Diines 74 1,2-Diiodoacenaphtalene 320 1,4-Diiodotetraphenyl-l,3-butadiene 287 Diisobutyl telluride 117, 223 Diisobutylaluminium benzenetellurolate 28 Diisobutylaluminium phenyl tellurolate 341 Diisobutylaluminium tellurolate 77 Diisobutyltelluronium trimethylsilylpropynylide 222 3,4-Dilithio-2,5-dichlorothiophene 319 2,2'-Dilithiobiphenyl 302, 304 1,4-Dilithiotetraphenyl-l,3-butadiene 287 Dilithium cyanocuprates 239 Dilithium dimethylcyanocuprate 247 Dilithium vic-tellurolates 307 1,4-Dilithium-1,4-diphenylbuta- 1,3-diene 229 Dimerization 324 1,2-Dimethoxy-l-phenylethane 179 Dimethyl ditelluride 37 Dimethyl telluride 17

355

300

356

Dimethyl tellurium dichloride 329 2,3-Dimethyl-l,3-butadiene 178 Dimethylacetylene dicarboxylate 301 3,7-Dimethyldibenzotellurophene 302 Dimethyltelluride 329 Dimethyltellurium dichloride 329 Dimethylzinc 257 Di-n-butyl ditelluride 40 Di-n-butyl telluride 16 Di-n-butyl-di-i-propyl tellurium 266 Di-n-octyl telluride 16 Dioctyl ditelluride 39 Diorganyl ditellurides 10, 37 Diorganyl tellurides 9, 10, 13, 62, 65, 200, 331 Diorganylditellurides 329 Diorganyltellurium dichlorides 11 Diorganyltellurium dihalides 35 2,4-Dioxopentanes 285 3,5-Dioxotellurane 285 Di-p-anisyl telluride 36 Di-p-anisyl telluroxide 65 p-Diphenols 165 Diphenyl disulphide 151,168, 173, 267 Diphenyl ditellurides 28, 32, 41, 42, 69, 88, 181,184, 280, 329, 330 Diphenyl phosphorochloridate 243 Diphenyl telluride 22 2,4-Diphenyl tellurophene 291 2,6-Diphenyl telluropyran-4-one 309 5,10-Diphenyl- 15,20-di-4-methoxyphenyl-21-telluraporphyrin 325 2,5-Diphenyl-3-iodo tellurophene 290 N,N'-Diphenylcarbodiimide 173 Diphenyldiazomethane 165 Diphenyldiselenide 267 Diphenyltellurium dichloride 58, 198 Diphenyltelluronium methyl tetrafluoroborate 221 2,5-Diphenyltellurophene 229 2,6-Diphenyltelluropyran-4-thione 311 N,N'-Diphenylthiourea 167, 173 Diphenylurea 164 N,N'-Diphenylurea 173 Di-p-methoxyphenyltellurium dichloride 200 Di-p-tolyl ditelluride 44 1,2-Diselenolo-1,2-ditellurolonaphtacene bis(1,2-ditellurolo)naphtacene 304 Distyryl telluride 252 (Z,Z)-Distyryl telluride 72 1,4-Disubstituted butadienes 291 4,4'-Disubstituted diaryl tellurides 332 Disubstituted dienes 229 4,4'-Disubstituted diorganyl tellurides 333 Disubstituted ferrocenes 239 2,6-Disubstituted tellurins 308 2,4-Disubstituted tellurophenes 287 2,4-Disubstituted tetrahydrofuran 272 2,5-Disubstituted tetrahydrofurans 193

SUBJECT INDEX

SUBJECT INDEX

357

Disubstituted vinylic tellurides 253 Disulphides 164, 166, 169, 170 1,5-Ditelluracyclooctane 321 1,2-Ditellurane 323 6,7-Ditelluraspiro[3,4]octane 323 Ditellurides 26, 40, 61,133, 139, 196 1.2-Ditellurolane 323 2H-1,3-Ditellurole 306, 307

2,6-Di-tert-butyl-4H-tellurin 310 2,6-Di-tert-butyl-4-oxo-4H tellurin 310 Dithienyl ditelluride 127 Dithioacetalization 339 Dithioketalization 339 Ditolyl disulphide 145 Ditosylate 167 N,N'-Ditosyltellurodiimide 154 Divinyl ditellurides 76 (Z)-Divinyl tellurides 255 Divinyl tellurides 72, 93,241 Divinyl tellurium dichlorides 198 Divinylbenzene styrene 166 Divinylic ditellurides 79 (E,E)-Divinylic tellurides 255 Divinylic tellurides 71 Diynes 111 DNA damage 332 DNA repair mechanisms 333 Dodecane thiol 171 Dodecyl methyl ether 210 Dodecyl phenyl telluroxide 210 N-Dodecylphenyltellurium dibromide

206

Ebselen 331 Electron transfer 279 Electro-oxidation of thioamides 168 Electrophilic mechanism 204 Electrophilic tellurium 107, 108, 183 Electrotelluration 83 Elemental tellurium 16, 22, 25, 40, 57, 71,127, 154, 261,306, 338 a-Elimination 204 Enamines 120 8-Endo-trig cyclization 269 Enediynes 255, 257 Enines 74 Enolsilyl ethers 236 Enones 201 (Z)-Enynes 255, 257 Epitelluride 130, 227,338 Epitelluronium intermediate 180 Epoxidation of olefins 174 Epoxides 128, 215,220, 245 /3-Epoxy ketones 129 a,/3-Epoxy ketones 129, 218

358

a, fl-Epoxyketones 129 1,2-Epoxyoctane 218 Erythro- 1, 133 Erythro-dibromides 132 Erythro-glycidol sulphonates 131 Ethenylphenyltelluride 87 Ethercyclization 189 Ethyl 5-telluro-(2E,4Z)pentanedienoate 254 Ethyl carbamate 180 Ethyl chromanones 267 Ethyl-2-tetralin carboxylate 136 Ethylalkynyl reagents 258 Ethylbenzene 120 Ethyldiazoacetate 33 Ethylenes 341 (1Z, 3Z)- 1-Ethylseleno-4-methoxy- 1,3-butadiene Ethynyl tellurides 228 Ethynylchloromethyl tellurides 306 5-Exo-trig cyclization 269 Extrusion 22

Ferrocenyltellurides 202 Five-membered rings 187, 269 9-(4-Fluorophenyl)-9-hydroxytelluraxanthene Formylation 289 2-Formylbenzotellurophene 295 Four-membered ring 154 Friedel-Crafts aromatic alkylation 340 o-Functionalization 243 Furan vinyltelluro derivatives 88 Furans 253 (2-Furylvinyl) phenyl tellurides 92

Gem-dihaloaryltelluro cyclopropanes 70 Geminal enedyines 256 a,D-Glucopyranosyl bromides 70 Glycidol 130 Glycosyl 274 Grignard reagents 110, 198, 252, 254 Group-transfer imidoylation 274 1-Halo-l-telluro ethenes 99 Haloacetonitrile 148 3-Halobenzotellurophenes 292, 294 2-Haloethyl carboxylates 158 2-Haloethyl ester 159 Halogenodetelluration 203 Halogenolysis 55 a-Haloketones 137 a-Halonitriles 126 2-Halovinyl tellurium trihalides 84

SUBJECT I N D E X

230

315

SUBJECT INDEX

359

1-Heptyne 255 Heteroaromatic tellurium dichlorides 256 Hetero-Cope rearrangement 163 Hexabutylditin 272 Hexadecylphenyltellurium dichloride 206 Hexamethylenetetratellurofulvalene 307 Higher-order cyanocuprates 239, 243, 247 Homoallylic alcohols 245 o-Homoallylic benzoyl telluride 269 Homocoupling 198 Homocoupling of vinyl tellurides 251 Homopropargylic alcohols 113 Horner-Emmons 89 Human neutrophiles 332 Hydrated sodium sulphide 43 Hydrazine hydrate method 39 Hydrazine hydrate 6, 17, 44 Hydride transfer reagent 16 Hydroalumination 102 Hydrobenzoin 168 Hydrogen telluride 6, 45, 115, 118, 120 Hydrolysis of telluroesters 202 Hydroquinone 166, 168 Hydrotelluration of alkynes 93 Hydrotelluration 71, 74, 76 3-Hydroxy alkynes 86 /3-Hydroxy ketones 129 Hydroxy tellurides 27 N-Hydroxy-2-thiopyridone 261 4-Hydroxy-3,4-dihydro- 2H- 1-benzotellurin 311 /3-Hydroxyalkyl tellurides 214 3-Hydroxyalkynes 85 p-Hydroxydiphenyl telluride 331 Hydroxylamines 125 Hydroxymethyl tellurophenes 290 /3-Hydroxynitriles 226 •-Hydroxyolefins 190 9-Hydroxytelluraxanthene 314 /3-Hydroxytellurides 272 N-Hydroxythiopyridone sodium salt 264 Hydroxyvinyl phenyl tellurides 259 Hydrozirconation 97

Imidazolyl cuprate 240 Imines 120 5-Imino-4, 5-dihydro- 1,2,4-thiadiazole Indene 179 Indoline 172 Internal acetylenes 111 Intramolecular cyclopropanation 151 Inversion of olefins 338 Iodination 207 Iodine 203, 208

167

360

(E)- 1-Iodo-2-aryltelluro-1-alkenes 86 Iodoarenes 203 Iodobromobenzene 292 Iodochloromethane 306 Iodocyclization of 4-pentenoic acid 185 Iodocyclization 291 Iododetelluration 203, 204 Iodonium ion 288 Iodonium salts 199 Iodotellurophene 291 3-Iodotellurophenes 288 Iodotetrahydrofuran 191 p-Iodotoluene 248 a-Iodovinyl tellurides 101 Ipso-substitution 204 Isocyanides 275 Isonitriles 274 Isophorone oxide 218 Isoprene 178 Isopropylidene malonates 119

Ketalization 339 Ketene butyltelluroacetal 256 Ketene stannyl(telluro) acetals 100, 101 Ketene telluro(seleno) 97 Ketene telluroacetal 98 Ketene telluroketals 89 Ketene 91 Ketones 60, 115, 129, 201,276, 338 /3-Ketosulphones 142 Knovenagel condensation 143

Lactonization 185 Lawesson reagent 310 Lewis acid 49 Lithiated ferrocene 238 2-Lithiobenzotellurophene 295 2-Lithiotellurophene 289 Lithium 2,2,6,6-tetramethylpiperidide 221 Lithium acetylenic 110 Lithium acetylides 258 Lithium amides 110 Lithium aryl tellurolates 42 Lithium butyl tellurolate 107 Lithium chloropalladate 251 Lithium diphenyltelluromethane 207 Lithium ethynyl tellurolates 107 Lithium ethynyltellurolates 306 Lithium n-butyltellurolates 81 Lithium phenyl tellurolate 116, 230 Lithium tellurolates 229 Lithium triethylborohydride 261,309

SUBJECT INDEX

SUBJECT INDEX

Lithium(ethynyltelluro) methane tellurolates Lithium-tellurium exchange 228 Lithium-tin exchange 237 Lutidine 171

361

306

(-)-Macrolactin A 247 Meldrum acids 119 Mercury lamp 206 Meso-stilbene dibromide 135 Mesylate 130 Meta-chloroperbenzoic acid 208 MeTe(CH2)3OH 25 Methacrylonitrile 197 Methanolysis 118 p-Methoxy phenyl p-tolyl telluride 31 (2-Methoxy)cyclohexyl phenyl telluride 209 7-Methoxy-4-oxo-4H-l-benzotellurin 312 (1Z,3Z)- 1-Methoxy-5-hydroxy-5-phenyl- 1,3-pentadiene 230 p-Methoxybenzaldehyde 116 Methoxycarbonylation of organic tellurides 259 (2-Methoxycyclohexyl)phenyltellurium dibromide 61 p-Methoxyphenyl tellurocyanate 56, 166 p-Methoxyphenyl tellurolate 78 p-Methoxyphenyl telluroxide 172 p-Methoxyphenyl(phenyl)iodonium tetrafluoroborate 199 (p-Methoxyphenyl)phenacyltellurium dichloride 60 (p-Methoxyphenyl)pinacolyltellurium dichloride 60 p-Methoxyphenyl-p-dimethylaminophenyltellurium dichloride p-Methoxyphenyltellurium tribromide 51 p-Methoxyphenyltellurium trichloride 49, 86 p-Methoxyphenyltellurium triiodide 52 Methoxytelluration of olefins 216 Methoxytellurenylation of olefins 178 Methyl (3-hydroxy)propyl telluride 25 Methyl 2-naphthyl telluride 25 Methyl anisyl telluride 262 Methyl bromoacetate 220 Methyl esters 200 Methyl ethers 208 Methyl o-methoxyphenyl telluride 30 Methyl phenyl telluride 26 Methyl phenylethynyl telluride 107 Methyl radical 262 1-Methyl-2-pyrrolidinone 117 2-Methyl-4,5-pentamethylene-2-oxazoline 182 Methylenation products 153 Methylenation reaction 153 Methylene cycloheptanone C 269 a-Methylene ketones 268, 337 2-Methylphenoxatellurin- 10,10-dichloride 315 a-Methylstyrene 119 fl-Methylstyrene 119 p-Methylstyrene 179

62

362

Methyltriphenylphosphonium iodide 93 Methylvinyl telluride 72 Monodesulphuration 144 Monosubstituted epoxides 272 Monosulphenylated fl-dicarbonyl compounds

SUBJECT INDEX

145

2-Naphthyltellurenyl iodide 55 1-Naphthyltellurium trichloride 51 2-Naphthyltellurium trichloride 49, 195 Naphtoditellurole 304 Nervous system 329 Neurotoxic species 329 Neurotoxicity 330 Nickel boride 129 Nickel tetracarbonyl 199 Nitriles 126, 167, 168 Nitro compounds 121 Nitroarenes 121 4-Nitrobenzaldehyde 220 Nitrobenzenes 122, 124 p-Nitrobenzenesulphonyl peroxide 184 a-Nitrocumene 141 Nitrocycloalkanes 123 (o-Nitrophenyl) tellurenyl bromide 56 m-Nitro-phenylboronic acid 198 Nitrosobenzene 165 4-Nitroso-N,N-dimethylaniline 295 (Z)-l-(1-Nonen)-3-ynyl benzene 255 Non-enolizable aldehydes 147 Non-stabilized ylides 220 Nucleophilic tellurium reagents 107

Octafluorotelluranthrene 318 N-Octyl (7-carbomethoxy)heptyl telluride 24 4-Octyl-3-oxaselenan-2-one 271 2-Octyltetrahydroselenophene 272 1-Octyne 280 Olefination reactions 89 Olefinic alcohols 188 Olefinic benzyl ethers 189 Olefins 122 Organic halides 204, 206 Organoboronic acids 198 Organostannanes 198 Organotellurenyl bromides 77, 90 Organotellurium compounds 331,332 Organotellurium cysteine 333 Organotellurium(IV) halides 206 Organotelluro 1,3-butadienes 74 Organotelluro 1,3-enines 74 (Z)-a-Organotelluro-a, fl-unsaturated carbonyl compounds fl-Organotellurobutenolides 185

100

SUBJECT I N D E X

Organotellurols 40 Organyl tellurenyl halides 108 Organyl telluroesters 29 Organyl telluroketals 46 Organyl tellurolate anions 82 Organyl tellurolates 25, 26, 30 Organyl tellurols 45 Organylmercury chlorides 62 Organyltellurium trichlorides 11, 29, 37, 47, 51 Organyltellurium trihalides 59 1-Organyltelluro-l,3-butadienes 75 Ortho-bromoethynylbenzene 292 2-Oxazolidinones 181 2-Oxazolines 182 Oxidation of arylhydrazines 165 Oxidation of diaryl tellurides 65 Oxidation reactions 162 Oxidation with bis(p-methoxyphenyl) telluroxide 165 Oxidation 213 Oxidative coupling of thiols 170 Oxidative functionalization 178 Oxiranes 128, 129 Oxo derivatives 162, 166 3-Oxo-2,3-dihydro benzotellurophene 293 3-Oxo-2,3-dihydro compound 294 3-Oxo-2,3-dihydrobenzotellurophenes 293,294, 295, 296 4-Oxo-4H- 1-benzotellurins 311, 312 4-Oxo-4H tellurins 310 4-Oxo-4H-tellurins 308 Oxoacyl radicals 270 Oxohalides 53

Pd(II)-catalysed arylation 196 4-Pentenoic acid 184, 185 4-Pentenol 191 Perfluoroalkyl halides 281 Perfluoroalkyltelluration of alkynes 281 Perfluoroalkyltelluration of olefins 281 Peroxynitrite-induced oxidation 332 Phase transfer catalysis 108, 142, 157 Phase transfer conditions 169 Phenacyl bromide 140, 157 Phenacyl esters 157 Phenolic function 160 Phenols 159, 160, 165 Phenoselenotellurins 316 Phenothiatellurins 316 Phenoxatellurin dihalides 316 Phenoxatellurium dichlorides 315 p-Phenoxyphenyltellurinic anhydride 173 p-Phenoxyphenyltellurium trichloride 189 Phenyl 1-(2-methoxy)octyl telluride 178 Phenyl alkyl tellurides 211

363

364

Phenyl dodecyl telluride 27 Phenyl isothiocyanate 164 Phenyl migration 209 Phenyl selenoformates 270 Phenyl tellurenate 125 Phenyl tellurenyl bromide 91 Phenyl telluride 209 Phenyl tellurium pentafluoride 154 Phenyl tellurocyanate 32 Phenyl telluroformate 270 Phenyl tellurol 118 Phenyl tellurones 209 S-Phenyl thiobenzoate 267 Phenyl(alkyltelluro)acetylenes 89, 108 Phenyl(p-methoxyphenyltelluro)acetylene 108 2-Phenyl- 1-(phenylthio)-5-(phenyltelluro)-2-pentene Phenyl-2-aminophenyl sulphide 316 4-Phenyl-2H-1,3-ditellurole 307 1-Phenyl-3,3-dimethyl-l-butenine 248 2-Phenyl-3-vinyloxirane 221 Phenylacetylenes 72, 120, 203, 265, 276, 292 Phenylbutyl telluride 258 N-Phenylhydroxylamine 122, 123, 165 Phenylmagnesium bromide 254 2-(Phenylmethyl)phenyl tellurium trichloride 313 3-Phenylpropanal 118 3-Phenylpropenal 118 a-Phenylseleno carboxylicesters esters 127 Phenylstyryl telluride 253 Phenyltellurenyl halide 204 Phenyltellurinic anhydride 175 Phenyltellurinyl acetate 193 Phenyltellurinyl trifluoroacetate 180, 181 Phenyltellurium tribromide 216 a-Phenyltelluro acrylonitriles 91 2-Phenyltelluro-l-naphthol 332 1-Phenyltelluro-l-trimethylsilylalkanes 217 1-Phenyltelluro-2-phenylethene 73 fl-Phenyltelluroaluminium enolate 341 Phenyltellurol 116, 119, 216 Phenyltellurotrifluoroacetimidates 282 Phenyltellurotrimethylsilane 116, 117, 118 Phenyltetrafluorotelluromethoxyde 154 a-Phenylthiocarbonyl compounds 151 N-Phenylthiourea 167 N-Phenyltrifluoromethanesulphonamide 244 Phenyltrimethylsilyl telluride 66 4-Phenylurazole 172 Phenylvinyl tellurides 239 Phosphine oxides 164, 166 Phosphines 166, 196 Phosphoric thiol esters 171 Phosphorochloridate 171 Photolytic cyclization 269

SUBJECT INDEX

280

SUBJECT INDEX

Photostimulation 30 Phthaloylhydrazide 172 Pinacol reaction 150 Piperidine 194 Poly(p-lithiostyrene) 166 Polymeric structure 174 Polymeric telluroxide 167 Polymerization 341 Polyprenyl quinol 278 Polysubstituted 193 Potassium aryltellurolate 76 Potassium ditelluride 39 Potassium hydrogen sulphite 42 Potassium tellurocyanate 21 Potassium tellurocyanide 323 Preparation of the polymeric telluride 166 Propargyl alcohol 203 Propargyl bromide 111, 112 Propargyl telluride 112 Propargyloxy compounds 272 (2-Propargyloxyalkyl) aryltellurides 88 (2-Propargyloxyphenyl)acyltellurides 88 Pyranones 310 2-Pyridyltelluro moiety 214 Pyrolitic tellurium extrusion 196 Pyrolysis 111,213 Pyrrolidine 172, 194

Quaternary ammonium salts 142 Quatemary a-nitrosulphone 144 Quinones 165, 166, 172, 277, 279

Racemic allyl alcohols 132 Radical addition 263 Radical captors 270 Radical cyclization 264 Radical mechanism 87, 211 Radical polymerization 283 Radical reaction 261,263 Radical scavenger 276 Radicalar addition 88 Radicalar reaction 282 Radicophilicity of tellurides 261 Raney nickel 195 Reduction 118 Reduction of acetylenic tellurides 89 Reduction of aromatic nitro compounds 123 Reduction of benzaldehyde 115 Reduction of carbonyl compounds 115 Reduction of diorganyltellurium dihalides 35 Reduction of ditellurides 73 Reduction of double bonds 119

365

366

Reduction of nitrobenzene 122 Reduction of organyltellurium trichlorides Reductive amination 121 Reductive desulphonylation 143 Reductive desulphuration 121 Reductive detelluration 192 Reformatsky reaction 148 Reformatsky-type reactions 147, 148 Removal of functional groups 137 Rongalite 6, 15, 19, 123, 130, 133, 152 Rongalite method 38 Rupe reaction 289

SUBJECT INDEX

42

Samarium diiodide 27, 35, 36 Schwartz reagent 95, 97 Secondary alcohols 225 Secondary amines 120 Selenodichloro telluroles 320 Selenophenes 253 Selenotelluration of phenylacetylene 281 Selenotelluration 279, 280 Selenotelluro adducts 280 Selenotelluroalkenes 279 Selenoxanthone 324 Selenoxide 166 Se-phenyl selenobenzoate 267 Sharpless kinetic resolution 130 Showdomycin 263 [2,3]-Sigmatropic rearrangement 202, 216 Siloxycyclopropanes 337 Silyl tellurides 279 Silyloxy amides 276 Silyloxytelluride 276 Silyltellurides 275 Six-membered carbocycles 263 Six-membered rings 187, 269 SN2 displacement 129, 155 SN2-type detellurative acetolysis 175 Sodium 2-thienyl tellurolate 161 Sodium acyltellurolates 66 Sodium aryltellurolate 109 Sodium ascorbate 35, 36, 44 Sodium bis(trimethylsilyl)amide 223 Sodium borohydride 35, 39, 158, 161 Sodium borohydryde 76 Sodium butyl tellurolate 74 Sodium butyltellurolate 77 Sodium ditelluride 37 Sodium dithionite 15 Sodium ethenetellurolates 287 Sodium formaldehyde sulphoxylate 286 Sodium hydride 38 Sodium hydrogen telluride 6, 17, 115, 118, 120, 129, 158, 159

SUBJECT INDEX

367

Sodium naphthalene 38 Sodium O,O-diethyl phosphoroteUurolate 137 Sodium periodate 65 Sodium phenyl tellurolate 124 Sodium phenyltellurolate 281 Sodium sulphide nonahydrate 296 Sodium sulphide 35 Sodium telluride 13, 14, 18, 19, 24, 39, 66, 115, 117, 143, 160, 286 Sodium tellurite 169 Sodium tellurolates 66 Sodium thienyl tellurolate 133 Sodium thiosulphate 176 Solution of o-acetylphenyl tellurenyl bromide 293 Sonogashira-type cross-coupling 255 Spiro ditellurolane 323 Squalene monooxygenase 329 Stabilized telluronium ylides 218, 225 Stannoles 320 Stannyl(telluro) acetals 101 Stannylacetylenes 100 Stilbenes 119, 227 Styrenes 119, 155, 179, 245,283 Styryl acetate 252 Styryl tellurides 251 (/3-Styryl) phenyl telluride 92 1-Styrylcyclopropane 280 (Z)-/3-Substituted cynnamic esters 253 p-Substituted diaryl tellurides 331 N-Substituted piperidines 120 2-Substituted tellurophenes 288 o~-Sulphenyl carbonyl compounds 152 Sulphinate anions 151 Sulphones 227 Sulphonium ylides 218 N-Sulphonylimines 154 Sulphuryl chloride 51 Symmetrical diaryl tellurides 18 Symmetrical disulphide 170 Symmetrical divinyl tellurides 92, 229 Symmetrical divinylic tellurides 79 Syn-chlorotelluration 338

Syn-diacetoxylation 175 Syn-elimination 209 Synthesis of biaryls

195

TeA1 exchange 249 Tellurane 322 Telluranthrene 318, 319 Telluraxanthene 313 Tellurenyl ferrocene 238 Tellurenyl halides 31, 55, 86, 97 Tellurenyl iodide 108 Tellurenylation 178

368

Tellurepine 299 Telluride anion 72 Tellurides 187 Tellurinates 27 Tellurinic anhydrides 172, 174, 187 1,4-Tellurino-l,4-tellurins 320 Tellurinyl acetate 188 Tellurinylated cyclic ethers 187 Tellurium anionic reagents 155 Tellurium dibromides 213, 216 Tellurium dichloride 171,302 Tellurium diiodides 57 Tellurium dioxide 4, 190, 293 Tellurium insertion 25, 40 Tellurium tetrabromide 21, 84 Tellurium tetrachloride 3, 4, 21, 42, 47, 49, 50, 149, 287, 338 Tellurium tetraiodide 21 Tellurium tribromides 51 Tellurium-A1 exchange 249 1,2-Tellurium-halogen shift 206 Tellurium-Zn exchange 248 Telluro fl-D-glucopyranosides 70 Telluro(seleno or thio)ketene acetals 95 Telluro(seleno)ketene acetals 95, 233 (E)-Telluro(seleno)ketene acetals 96 Telluro(stannyl) ketene acetals 100 Telluro(thio)ketene acetals 95, 102 Telluroacroleins 83 Telluroacylation 76, 84 Tellurobutenines 289 1-(Tellurobutyl)-l-(organyl)-ethenes 77 (Z)-l-(Tellurobutyl)-2-(organyl) ethenes 78 (E)-l-(Tellurobutyl)-2-(organyl)ethenes 77 Tellurobutyrolactone 184 Tellurocarbamates 19 Tellurocarbohydrates 262 Tellurochromones 297, 298, 299 Tellurocyanates 56, 61 Tellurocyclization 193 Tellurocyclofunctionalization 192 Tellurodibromide 100 Tellurodienes 83 Telluroesters 28, 66, 67, 76, 201,266, 267 Telluroetherification 188, 212 Telluroethers 191 Tellurofunctionalization 184 Telluroglycosides 273 Tellurohalide 177 Telluroindigo 296 Telluroketals 46 Telluroketene 256 Tellurol 76, 333 Tellurolactone 184, 185, 186 Tellurolactonization 184, 212

SUBJECT INDEX

SUBJECT INDEX

Tellurolate anion 27 Tellurolates 115 Tellurols 37, 40 Tellurones 173 Telluronium chloride 302 Telluronium ion 176 Telluronium salts 219, 224 Telluronium species 183 Telluronium ylide 220 Telluronium 174, 221 Tellurophenes 253, 286, 288, 300 Tellurophosphonium salt 93 Tellurophosphoranes 91 Telluropyran-4-thiones 310 4-(TeUuropyranyl)-4H-telluropyran 311 (Telluropyranyl)telluropyranes 310 Telluroseleno ethenes 96 Telluroseleno ketene 99 Tellurosteroids 324 Tellurosulphimino 217 Tellurosulphones 83 Tellurothioalkene 75 Telluroxanthene 313 Telluroxanthone 313, 314, 324 Telluroxide elimination 192, 213, 268, 341 Telluroxides 65, 163, 164, 167, 168, 172, 173, 209, 322 TEMPO 266, 273 Terminal acetylenes 76 Terminal alkenes 119 Tert-amines 171 Tert-butyl- l-benzotellurepine 299 Tertiary amine N-oxides 126 Tertiary amines 142 Tertiary nitro groups 141 Tertiary thiols 169 Tetraaryltellurium 22 1,2,3,4-Tetrabromoalkanes 135 Tetrabromoperfluorotelluranthrene 318 Tetrabutylammonium hydroxide 169 Tetrachalcogenines 321 Tetrachloroethylene 307 Tetracyanoethylene 291 Tetradecyltellurium 266 Tetrahydrofuran 190 Tetrahydronaphthalene 210 Tetrahydropyran 190 Tetrahydrotellurophene 35 Tetrakis-(triphenylphosphine) palladium 69 Tetralkyltelluriums 265 N,N,N',N'-Tetramethyl- 1,2-ethanediamine 244 Tetraorganyltellurium intermediates 225 Tetraorganyltin compounds 52 A4,4' 2,2',6,6'-Tetraphenyl-4-(tellurapyranyl)-4H-tellurapyran 311 Tetraphenyltellurophene 287, 291

369

370

Tetrasubstituted vinylic tellurides 244 Tetratellurofulvalenes 307, 308 1-Thia-4-telluranes 285 1,2,4-Thiadiazoles 167 Thiadiazoles 168 Thianthrene tetraoxide 301 2-Thienyl tellurolates 139 2-Thienyl vinyl tellurides 240 Thioacetalization 338 Thioamides 167, 168 Thiobenzamide 173 Thiocamphor 163 Thioesters 166, 172 Thioketalization 338 Thioketones 166 Thiolate 151 Thiolperoxidase 70 Thiols 164 Thiolysis 169 Thiones 310 Thionyl chloride 51 Thiophenes 140, 241,253 Thiophenol 168, 173, 267 o-Thiophenoxyphenyl mercury chloride 317 o-Thiophenoxyphenyltellurium trichloride 317 Thiophenoxytellurine 317 Thiotelluration of phenylacetylene 281 Thiotelluration 279, 280 Thiourea dioxide 15 Thiovinyl radical 280 a-Thiovinyl tellurides 90 Titanium(IV) chloride 117 [N-(p-Toluene-sulphonyl)imino] phenyliodinane 202 Tosylates 112 Trans-l,4-diacetoxylated adducts 177 Trans-2-ethoxycyclohexyltellurium trichloride 49 Transallylic alcohol 131 Trans-fl-bromostyrene 78 Trans-cinnamaldehyde 220 Transmetallation reactions 229, 240 Trans-olefins 227, 262 Trans-stilbenes 133, 154, 227, 253,262 Trialkyl boranes 32 Trialkyl phosphates 170 Trialkylphosphates 171 Triaryltelluronium halides 22 Tributyltin hydride 185 Trichloro-t-butylcarbamates 161 Triethyl phosphite 170 Triethylaluminium 249 Trifluoromethyl indole 282 Trifluoroperoxyacetic acid 209 Trihaloalkenes 208 Trimethyl phosphite 171

SUBJECT INDEX

SUBJECT INDEX

Trimethylsily methoxyde 154 3-Trimethylsilyl diisobutyltelluronium prop-2-enylide 222 Trimethylsilylallyl bromide 223 Trimethylsilylphenyl telluride 276 Trimethylsilylprop-2-enyl(di-isobutyl)telluronium bromide 223 3-Trimethylsilylprop-2-enyldiisobutyltelluronium bromide 223 Trimethylsilylvinyl cyclopropane 222 Trimethylsylilmethyl telluride 31 Trimethylsylilphenyl tellurium 33 Trimethyltelluronium chloride 329 Triphenyl phosphite 219 Triphenyltin hydride 211 Trisubstituted 1,3-butadienes 254 Trisubstituted olefins 253 Trisubstituted vinyl tellurides 94 Trithiocarbonates 166 TUDO 6, 35, 44, 189 TUDO method 38

Ubiquinone 278 ~,6-Unsaturated acids 186 a,/3-Unsaturated acyltelluride 269 Unsaturated alcohols 187 Unsaturated carbenes 102 a,/3-Unsaturated carbonyl compounds 118, 148 ?;5-Unsaturated carboxylic acids 183 a,/3-Unsaturated epoxides 221 a,/3-Unsaturated esters 147, 219 a,/3-Unsaturated ketones 224, 289 a,/~-Unsaturated methyl carboxylates 259 a,/3-Unsaturated nitriles 126, 144, 148 Unsymmetrical diaryltellurium dichlorides 62 Unsymmetrical diorganyl tellurides 31 Unsymmetrical disulphides 169, 170 Unsymmetrical tellurides 9, 24, 25, 29, 79, 86, 195 Ureas 164, 167

V~c-dibromides 134 V~c-dimesylates 136 V/c-ditosylates 136 V~c-phenyltelluration 88 Vinyl 2-thienyl telluride 241 Vinyl cyclopropanes 222, 279 Vinyl dihalogeno tellurides 101 (Z)-Vinyl organometallic reagents 71 Vinyl phosphates 243 Vinyl tellurides 90, 93, 104 Vinyl thienyl telluride 241 Vinyl triflates 244 (Z)-Vinyl zinc chloride 245 Vinylacetylene 72 Vinylchlorides 81

371

372

SUBJECT I N D E X

Vinylcyclopropane 280 Vinylic cuprates 239 Vinylic cyanocuprates 239 (Z)-Vinylic cyanocuprates 241,244 Vinylic disulphide 163 Vinylic ethers 214, 216 Vinylic magnesium bromide 79 Vinylic magnesium tellurolates 79, 80 Vinylic tellurides 70, 74, 78, 79, 86, 87, 103, 107, 111,246, 252, 268, 273 (Z)-Vinylic tellurides 71, 73, 82, 254, 255 Vinylic tellurides from vinylboranes 94 Vinyllithium intermediate 83 (E)-Vinyllithiums 229 (Z)-Vinyllithiums 229 Vinylphosphates 244 Vinylsilanes 217 Vinylstannanes 229 ]3-Vinyl-substituted ketones 239 Vinyltellurenyl iodides 87 Vinyltellurides 77 (Z)-Vinyltellurides 99 Vitamin K 278

Wittig olefination 263 Wittig-type olefination 153 Wittig-type reaction 201

Xanthene 121 Xenon difluoride 154 2,6-Xylylisonitrile 275

Zirconocene hydrochloride Zweifel reagent 102

100