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Transition states and origins of 1,4-asymmetric induction in alkylations of 2,2,6-trialkylpiperidine enamines.

ABSTRACT: The asymmetric C-alkylation of chiral enamines derived from terminal epoxides and lithium 2,2,6-trialkylpiperidides has previously been shown to provide alpha-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.

SUBMITTER: Um JM

PROVIDER: S-EPMC3049728 | biostudies-literature | 2010 Jun

REPOSITORIES: biostudies-literature

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Transition states and origins of 1,4-asymmetric induction in alkylations of 2,2,6-trialkylpiperidine enamines.

Um Joann M JM Kaka Naeem S NS Hodgson David M DM Houk K N KN

Chemistry (Weinheim an der Bergstrasse, Germany) 20100601 21

The asymmetric C-alkylation of chiral enamines derived from terminal epoxides and lithium 2,2,6-trialkylpiperidides has previously been shown to provide alpha-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 stereoselectivi ...[more]

PMID: 20411546

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