Mechanistic Studies on Chiral Zinc Complex-Catalyzed Asymmetric Alkylation

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In the presence of a catalytic amount of (-) -3-exo- (dimethylamino) isoborneol (DAIB), reaction of dialkylzincs and aldehydes is accelerated markedly to give, after hydrolysis, the corresponding S alcohols in high enantiomeric purity. In this enantioselective alkylation, nonlinear relationship between optical purities of the chiral source and products. For example, reaction of benzaldehyde and diethylzinc in the presence of 8 mol% of (-)-DAIB in 15% ee leads to (S)-1-phenyl-1-propanol in 95% ee. The mechanisms of chirality multiplication as well as amplification have been elucidated on the basis of the kinetic measurement, alkyl scrambling experiments, single crystal X-ray analysis, 1H-NMR study, molecular-weight determination of certain key intermediates, systematic investigation using DAIB and its stereoisomers. This unusual phenomenon has been understood as a result of a marked difference in chemical properties of the diastereomeric dinuclear complexes formed from dialkylzincs and the DAIB auxiliary, proposing importance of the self and nonself recognition of the asymmetric catalysts. This view is supported the ab initio molecular orbital calculations of the model systems.