Selective reduction of carbonyl compounds with 8-oxyquinoline dihydroboronite

Abstract

The amine-borane complex prepared from 8-hydroxyquinoline and borane-dimethylsulfide complex in equimolar ratio in THF, designated herein 8-oxyquinoline dihydroboronite, was reacted with aldehydes, ketones and selected carbonyl compounds in order to explore its reducing properties and synthetic utility. In the absence of Lewis acids, this reagent reacted with carbonyl compounds very slowly as observed in other amine-borane complexes. However, in the presence of BF$_3$, this reagent was very effective for selective reduction of aldehydes and other carbonyl compounds. Aldehydes can be reduced with high selectivity in the presence of enones and ketones except unhindered cyclohexanones. Unhindered cyclohexanones could be also reduced selectively even in the presence of less reactive ketones such as acyclic ketones. Acid, amide, ester, anhydride, and double bond do not react with this reagent. Benzoyl chloide as acid chloride gives 17.7\% conversion to a mixture of benzaldehyde and benzyl alcohol in a ratio of 2:1. Accordingly, it was possible to reduce aldehyde, ketone and enone selectively in the presence of acid, ester, amide, anhydride, double bond, and acid chloride. Other Lewis acids were found to be less effective than BF$_3$. Two model compounds, p-bromophenacyl 6-oxohexanoate and 10-oxopentadecanal, were reduced to show the effectiveness of this reagent.