Tellurium in Organic Synthesis. Preparation of Z-Vinylic Cuprates from Z-Vinylic Tellurides and Their Reaction with Enones and Epoxides

Z-Vinylic tellurides, obtained with 100% stereoselectivity by the hydrotelluration of acetylenes, are easily transformed into Z-vinylic higher order cyanocuprates by reaction with preformed Me2Cu(CN)Li2, n-Bu2Cu(CN)Li2, or n-Bu(2-Th)Cu(CN)Li2, with total retention of the double-bond configuration. The resulting vinylic higher order cyanocuprates react with unhindered enones to give the corresponding 1,4-addition products in good yields. Reaction of the vinylic higher order cyanocuprates with monosubstituted epoxides at 0 °C gives the homoallylic alcohols resulting from the attack to the less-substituted carbon atom, while the disubstituted epoxides failed to react. Allylic epoxides react at −78 °C with the vinylic higher order cyanocuprates to give mixtures of 1,2- and 1,4-opening products, the 1,4-product predominating. In all cases the double-bond configuration of the original vinylic telluride was preserved. The vinylic cuprates derived from simple vinylic tellurides and conjugated 1-telluroenynes react with epoxides at 0 °C, while vinylic cuprates derived from conjugated 1-tellurodienes required the addition of 1 equiv of BF3·Et2O to give the homoallylic alcohols on reaction with epoxides. The opening of optically pure epoxides through tellurium/copper transmetalation is stereospecific, giving one single stereoisomer of the corresponding homoallylic alcohol.