Further studies on the reduction of benzylic alcohols by hypophosphorous acid/iodine

Hydrogen iodide, generated (and regenerated) in situ by the action of iodine upon hypophosphorous acid, was found to reduce a variety of aryl carbinols to the corresponding diphenylmethanes under mild conditions. The generality of the reaction was explored by extension to several diols, heterocyclic alcohols, and allylic alcohols. Several lines of evidence, including occasional isolation of alkyl iodides, dependence of the rate of reduction upon substituent effects, and isolation of ethers and esters in the early stages of reduction, reveal that the first step in the reduction process is conversion of the alcohol to a carbocation. Isolation of dimers from cinnamyl alcohol and 1-phenyl-2-propen-1-ol demonstrates the intermediacy of radical intermediates in at least some reductions.