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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 Else_TOS-PETRAGNANI_ch001.qxd 12/4/2006 5:46 PM Page 1 –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 Else_TOS-PETRAGNANI_ch001.qxd 2 12/4/2006 5:46 PM Page 2 1. INTRODUCTION 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. Else_TOS-PETRAGNANI_Ch002.qxd 12/4/2006 1:56 PM Page 3 –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 Else_TOS-PETRAGNANI_Ch002.qxd 4 12/4/2006 1:56 PM Page 4 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. Else_TOS-PETRAGNANI_Ch002.qxd 2.3 12/4/2006 1:56 PM Page 5 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 Else_TOS-PETRAGNANI_Ch002.qxd 6 12/4/2006 1:56 PM Page 6 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. Else_TOS-PETRAGNANI_Ch002.qxd REFERENCES 12/4/2006 1:56 PM Page 7 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. Else_TOS-PETRAGNANI_Ch003.qxd 12/11/2006 3:58 PM Page 9 –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 Else_TOS-PETRAGNANI_Ch003.qxd 10 12/11/2006 3:58 PM Page 10 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; Else_TOS-PETRAGNANI_Ch003.qxd 12/11/2006 3:58 PM Page 11 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 Else_TOS-PETRAGNANI_Ch003.qxd 12 12/11/2006 3:58 PM Page 12 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. Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 13 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 Else_TOS-PETRAGNANI_Ch003.qxd 14 (ii) 12/11/2006 3:58 PM Page 14 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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). Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 15 DIORGANYL TELLURIDES 15 (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 Else_TOS-PETRAGNANI_Ch003.qxd 16 12/11/2006 3:58 PM Page 16 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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). Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 17 DIORGANYL TELLURIDES 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 Else_TOS-PETRAGNANI_Ch003.qxd 18 12/11/2006 3:58 PM Page 18 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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 Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 19 DIORGANYL TELLURIDES 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 Else_TOS-PETRAGNANI_Ch003.qxd 20 12/11/2006 3:58 PM Page 20 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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 Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 21 DIORGANYL TELLURIDES 21 (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 Else_TOS-PETRAGNANI_Ch003.qxd 22 12/11/2006 3:58 PM Page 22 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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)] Else_TOS-PETRAGNANI_Ch003.qxd 12/11/2006 3:58 PM Page 23 REFERENCES 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. Else_TOS-PETRAGNANI_Ch003.qxd 24 12/11/2006 3:58 PM Page 24 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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%)). Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 25 DIORGANYL TELLURIDES 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 Else_TOS-PETRAGNANI_Ch003.qxd 26 12/11/2006 3:58 PM Page 26 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM (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). Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 27 DIORGANYL TELLURIDES 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 Else_TOS-PETRAGNANI_Ch003.qxd 28 12/11/2006 3:58 PM Page 28 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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%) Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 29 DIORGANYL TELLURIDES 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%) Else_TOS-PETRAGNANI_Ch003.qxd 30 3.1.3.3 12/11/2006 3:58 PM Page 30 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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 Else_TOS-PETRAGNANI_Ch003.qxd 3.1 12/11/2006 3:58 PM Page 31 DIORGANYL TELLURIDES 31 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) Else_TOS-PETRAGNANI_Ch003.qxd 32 12/11/2006 3:58 PM Page 32 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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 Else_TOS-PETRAGNANI_Ch003.qxd 12/11/2006 3:58 PM Page 33 REFERENCES 33 (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. Else_TOS-PETRAGNANI_Ch003.qxd 34 12/11/2006 3:58 PM Page 34 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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. Else_TOS-PETRAGNANI_Ch003.qxd 12/11/2006 3:58 PM Page 35 REFERENCES 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) Else_TOS-PETRAGNANI_Ch003.qxd 36 12/11/2006 3:58 PM Page 36 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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. Else_TOS-PETRAGNANI_Ch003.qxd 3.2 4. 5. 6. 7. 12/11/2006 3:58 PM Page 37 DIORGANYL DITELLURIDES 37 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. Else_TOS-PETRAGNANI_Ch003.qxd 38 12/11/2006 3:58 PM Page 38 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM (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%)). Else_TOS-PETRAGNANI_Ch003.qxd 3.2 12/11/2006 3:58 PM Page 39 DIORGANYL DITELLURIDES 39 ( 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 Else_TOS-PETRAGNANI_Ch003.qxd 40 3.2.2 12/11/2006 3:58 PM Page 40 3. PREPARATION OF THE PRINCIPAL CLASSES OF ORGANIC TELLURIUM 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). Else_TOS-PETRAGNANI_Ch003.qxd 3.2 12/11/2006 DIORGANYL DITELLURIDES 3:58 PM Page 41 41 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 an...
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