Hydride transfer reactions of benzylic alcohols catalyzed by acid faujasites

Abstract

AbstractReaction of five benzylic alcohols in solution at 78°C has been performed in the presence of a series of HY zeolites at three different levels of Na+ ‐to‐NH4+ exchange (HY‐21, HY‐50 and HY‐100) and framework Si‐to‐Al ratio (HY‐100, HY‐200 and HY‐400). Dehydration to form a CC double bond is the main primary process for 1‐phenyl‐ and 1,1‐diphenylethanol. Nevertheless, the initial alkenes are unstable, giving rise to the corresponding phenylethanes and, in the case of styrene, also to dimers. In contrast, benzyl alcohol and diphenylmethanol gave equimolar amounts of phenylmethanes and phenyl ketones, by an intermolecular hydride transfer not observed under conventional homogeneous acid catalysis. This process was found to be highly demanding from the point of view of the acid strength of the sites involved. Finally, triphenylmethanol showed a remarkable reluctance to undergo any change, although small amounts of triphenylmethane, benzophenone and 9‐phenyl‐9‐hydroxyfluorene were detected after long reaction times.