Transition States and Origins of 14-Asymmetric Induction in Alkylations of 226-Trialkylpiperidine Enamines

摘要

The asymmetric C-alkylation of chiral enamines derived from terminal epoxides and lithium 2,2,6-trialkylpiperidides has previously been shown to provide α-alkylated aldehydes by intermolecular nucleophilic substitution in good levels of asymmetric induction. We now report a computational study of the origins of asymmetric induction in these reactions. Computational modeling with density functional theory (B3LYP/6–31G(d)) agrees closely with the experimental observations. This stereoselectivity is attributed to a preferential conformation of the enamine and the piperidine ring that places the C-6 alkyl substituent in an axial position due to A^1,3 strain. Preferential attack occurs away from the axial group, for steric reasons. The effects of changing the C-6 substituent from methyl to isopropyl were studied, and twist transition states were found to contribute significantly in the latter alkylations.