Addition reactions of conjugated double bonds. Part II. The occurrence of 1,4- or 1,2-addition in the reduction of some aryl αβ-unsaturated ketones with metal hydrides

Abstract

Some substituted trans-chalcones, crotonophenone, benzylideneacetone, and 3-phenylcyclohex-2-enone have been reduced with sodium borohydride in three solvents. In diglyme and propan-2-ol the proportion of attack at the β-carbon atom (1,4-addition) is less in the last two compounds than in the others. Reduction of the chalcones or crotonophenone in pyridine led to almost exclusive double-bond saturation. The related allylic alcohols are also reduced under these conditions. The ease of borohydride reduction of allylic alcohols appears to be connected with the ease of the base-catalysed allylic alcohol–saturated ketone rearrangements, though the allylic alcohols are stable to rearrangement in pyridine solution. An explanation is suggested.