The reaction of B-alkyl-9-borabicyclo[3.3.1]nonanes with aldehydes and ketones. A facile elimination of the alkyl group by aldehydes

Certain B-alkyl-9-borabicyclo[3.3.1]nonanes (9-BBN) reduce benzaldehyde to benzyl alcohol under exceptionally mild conditions. Factors which contribute to a high rate of reaction include: an increase in the degree of substitution at the position β to the boron (isobutyl > 1-butyl >> ethyl), the ability of the alkyl group to form a syn-planar B-C-C-H conformation (cyclopentyl {reversed tilde equals} norbornyl > sec-butyl >> cyclohexyl), and the presence of an electron-withdrawing para-substituent on the benzaldehyde (p-Cl > p-H > p-CH₃O). The B-alkyl group is transformed into an olefin as the benzaldehyde is reduced. Elimination takes place predominantly if not exclusively towards the more highly substituted β hydrogen. The reaction obeys second order kinetics. The observations are consistent with a cyclic mechanism rather than a dehydroboration-reduction pathway.