CONVERSION OF TELLUROL ESTERS TO ENOL SILYL ETHERS OF ACYLSILANES

Tellurol esters having an anion stabilizing group at the position alpha to the carbonyl, such as aryl-, (phenylthio)-, and (benzyloxy)ethenetelluroates, gave enol silyl ethers of the corresponding acylsilanes in good to excellent yields upon treatment with 2 equiv of (n)BuLi in the presence of chlorosilanes. This reaction was stereoselective, and Z-isomers were obtained as sole or major products from a variety of chlorosilanes, such as trimethyl-, triethyl-, dimethylphenyl-, and tert-butyldimethylsilyl chlorides. Control experiments revealed that the reaction comprises the following consecutive processes: (i) alpha-proton abstraction, (ii) chlorosilane-trapping, (iii) lithium-tellurium exchange, (iv) 1,2-silyl migration, and (v) chlorosilane-trapping. Intramolecular rearrangement of (alpha-siloxyvinyl)lithiums to lithium enolates (step iv) was very fast even at -105 degrees C, and the former could not be trapped intermolecularly either with acetic acid or with coexisting trimethylsilyl chloride.