Reduction of Carbonyl Compounds by Cross-Linked Poly(4-vinylpyridine)-Borane in Acidic Media

Cross-linked poly(4-vinylpyridine)-borane was successfully used for the reduction of aldehydes and ketones to the corresponding alcohols. Reduction was carried out in protic and aprotic acidic media at room temperature. Polymer reactivity depends on solution pH. Due to the relative high hydrolytic stability of the boranebound polymer, reduction can be carried out in pH 1 buffer solution and in acetic acid. A low degree of cross-linking did not affect the polymer reactivity. Complete reduction of carbonyl compounds can be reached in the presence of excess of borane-bound polymer at a 1:l reactants molar ratio. Only two of the borane hydrides are available for reduction; the third one is consumed by hydrolysis of the partially reacted borane derivatives formed during and after reduction. The chemoselectivity in the competitive reduction of cyclohexanone and other ketones as well as acetophenone and benzaldehyde with cross-linked poly(4-viny1pyridine)- borane was investigated. The cross-linked polymer borane was found to be a much better chemoselective reduction agent than its low molecular weight analog pyridine-borane. Chemoselectivity of this polymer is as good as that of available low molecular weight chemoselective reduction agents.