Telluroesters

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

R = Ph, p-MeOC0H4, p-MeC0H4, PhCH2, 1-Napht, 2-Napht, C17H35 R = Me, /-pr, n-Bu 

Typical procedure Phenyltellurotrimethylsilane (2.10 mL, 10.9 mmol) was added into astirred solution of benzoyl chloride (1.42 g, 10.1 mmol) in 30 mL of dry tetrahydrofuran under an argon atmosphere. The mixture was stirred for 3 h at room temperature and concentrated under a reduced pressure. The residual Te-phenyl tellurobenzoate was purified by recrystallization from pentane to give yellow needles. Yield 2.71 g (87%). 

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The acyl anion chemistry of acylzirconocene chlorides has also been applied to the stereoselective preparation of ( )-a,(3-unsaturated selenoesters and telluroesters (Scheme 5.35) [38]. Although no carbon—carbon bond was formed, this reaction reflects the synthetic interest in ( )-a,(3-unsaturated selenoesters and telluroesters, which are well-known precursors of acyl radicals and acyl anions, respectively. 

Telluroesters have also been employed as starting materials in teUuro vinylation reactions. Thus, by treatment of telluroesters with aryl propiolates in the presence of KjCOj, (Z)-/(-acyltellurocinnamates are formed in high yields. This procedure involves the addition of potassium aryltellurolate anions derived from the telluroesters. " ... 

Otherwise, telluroacylation of terminal alkynes is achieved by treatment of aUcynes with telluroesters in the presence of Cul and triethylamine. ... 

By treatment with dialkylcuprates, telluroesters are easily converted into ketones in high yields. 

By hydroxy group - hydrolysis of telluroesters to carboxylic acids and esters... 

Telluroesters can be easily hydrolysed or converted into the corresponding oxygenated esters by treatment respectively with CuCl2 dihydrate in acetone or anhydrous CuClj in the appropriate alcohols. 

Elimination of phenyltellurol from the telluroesters intermediate explains the formation of a, -unsaturated carbonyl compounds. 

Acyl- and aroyllithiums are generated by submitting telluroesters, easily accessible from the corresponding acid chlorides and tellurolate anions (see Section 3.11), to the above-described lithium-tellurium exchange reactions. ... 

Telluroesters (acyl tellurides) have been recognized as excellent sources of acyl radicals upon photolysis with a 250 W tungsten lamp, or thermal process (benzene at reflux) in the... 

Photolytic reaction of telluroesters typical procedure for photolysis in the presence of TEMPO f Photolysis of the telluroester (Ar = T-FCgH ) (34 mg, 0.1 mmol) and TEMPO... 

S-phenyl thiobenzoate typical procedure in the presence of diphenyl disulphide. Photolysis of the telluroester (Ar = 4-FCgH4) in the presence of 6 equiv of diphenyl disulphide according to the standard for the formation of selenoesters gave 71% of the thiobenzoate after chromatography on silica gel (eluent hexane/ ethyl acetate, 9 1). The product had a melting point and spectral characteristics in accordance with those described in the literature. 

Photolysis of the telluroester in the presence of thiophenol isolation of benzaldehyde. A solution of the telluroester (Ar = 1-naphthyl) (15 mg, 41.7 jjmol) and thiophenol (17 mg, 154 jjmol) in CDCI3 (0.6 mL) was photolysed with a 250 W white light source at 8°C for 2 h. Preparative TLC on silica gel of the reaction mixture, eluting with ethyl acetate/hexa-nes, 1 9, gave benzaldehyde (3.5 mg, 80%). 

Typical procedure for rearrangement with aryl tellurium group transfer (formation of 3-[(aryltelluro)methyl]chromanones)f In a 5 mL flask fitted with a reflux condenser the telluroester (0.1 mmol) was photolysed under argon in benzene (3 mL) with a 250 W GE sunlamp from a distance of 15 cm. Either no cooling bath was used and the heat from the lamp caused the solution to reflux or the reaction was maintained at 8°C by means of a circulating cold water bath. After completion (TEC control) the solvent was removed in vacuo and the residue analysed by NMR spectroscopy. 

Apart from aryl esters and amides of carboxylic acids, aryl carbonates, and carbamates, other types of organic compound have been reported to undergo PFR. They include oxalates, formiates, sulfonates, sulfonamides, thioesters, selenoesters, and telluroesters. 

Although the photochemistry of aryl selenoesters and aryl telluroesters is not fully developed, there are some photoreactions of these compounds that resemble a photo-Fries process. Se-/ ara-tolyl selenobenzoate (212) gives, upon irradiation, selenocresol (214), benzaldehyde, and the benzophenone 213 (Scheme 56), which is clearly a photo-Fries product [157,158], Starting from Se-phenyl 2-chlorosele-... 

There is one important class of exchange which is unique to tellurium telluroesters such as 181 undergo tellurium-lithium exchange to generate the highly unstable acyllithiums such as 183.147 These must be formed in the presence of an electrophile - typically pinacolone (t-butyl methyl ketone 182) or they decompose. 

Another access to alkyl and aryl acyllithiums is based on a tellurium-lithium exchange58. Thus, telluroesters 44 reacted with n-BuLi at —105 or —78 °C in the presence of pinacolone or chlorotrimethylsilane to afford products 45 or 46, respectively (Scheme 11). However, selenoesters and acylstannanes did not give the corresponding adducts under similar reaction conditions. 

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