Isotope Effects and the Mechanism of Chlorotrimethylsilane-Mediated Addition of Cuprates to Enones

Abstract

Kinetic isotope effects were determined for the chlorotrimethylsilane-mediated reactions of cyclohexenone with lithium dibutylcuprate in tetrahydrofuran and with lithium butyl(tert-butylethynyl)cuprate in ether. For the reaction in tetrahydrofuran, the observation of a significant carbonyl oxygen isotope effect (16k/17k = 1.018−1.019) and small olefinic carbon isotope effects (12k/13k = 1.003−1.008) is consistent with rate-limiting silylation of an intermediate π-complex. Theoretically predicted isotope effects for model reactions support this conclusion. Rate-limiting silylation is also supported by relative reactivity studies of chlorotrimethylsilane versus chlorodimethylphenylsilane. The absence of a significant butyl-group carbon isotope effect on product formation indicates that the cuprate butyl groups are nonequivalent in the intermediate leading to the product-determining step. In diethyl ether the isotope effects revert to values similar to those found previously in reactions of cyclohexenone wit...