Mechanism for the reduction of ketones to the corresponding alcohols using supercritical 2-propanol

Abstract

The mechanism for the reduction of ketones into the corresponding alcohols using supercritical 2-propanol under non-catalytic conditions was investigated. The studies of the kinetic-isotope effect and isotopic-labeling for the reduction of benzophenone and acetophenone were carried out using (CH 3) 2C D(OH), (CH 3) 2CH(O D), (C D 3) 2CH(OH), and (C D 3) 2C D(O D). It was clarified that the α- and hydroxyl hydrogens on 2-propanol, respectively, transfer to the carbonyl C and O in the rate-determining step. These isotope studies also suggested that this reaction proceeds via a six-membered cyclic transition state analogous to that of the Meerwein–Ponndorf–Verley reduction. The fact that Hammett's reaction constant for this reaction was low, i.e., ρ=0.33, and that the reduction of the prochiral ketones using optically active alcohols at supercritical or high temperature provided optically active products also supported the existence of a six-membered cyclic transition state.