On the mechanism of reduction of benzophenones by aluminium hydride and alkoxyaluminium hydrides

Abstract

The rates of reduction of a group of pp′-disubstituted benzophenones by lithium tri-t-butoxyaluminium hydride have been determined together with the relative rates of reaction with aluminium hydride. The Hammett plots obtained show that the carbonyl group acquires carbanion character in the former reaction but that there is little charge separation in the latter; this is consistent with the cyclic transition state assumed for aluminium hydride reductions. Only a small proportion of the hydrogen atoms of aluminium hydride were available for the reduction of benzophenones. Alkoxyaluminium hydrides produced as intermediates in the reduction of benzophenones, or by exchange between aluminium hydride and alkoxides, reduce aliphatic ketones preferentially.