Keto–Enol Tautomerization as a First Step in Hydrogenation of Carbonyl Compounds

Abstract

Keto–enol tautomerization of carbonyl compounds to their enol form is theoretically predicted to enable a low-barrier pathway for hydrogenation of normally very stable C═O bond. In the scope of this anticipated mechanism, the reaction can proceed via two consecutive steps, including the formation of enol followed by an H insertion into the enolic C═C bond, and exhibits a lower activation barrier than the direct H insertion into the carbonyl group. Here, we present an experimental study on atomistic level details of hydrogenation of a simple carbonyl compound acetophenone over Pt(111) providing experimental evidence that keto–enol tautomerization plays a crucial role in this reaction. By employing a combination of spectroscopic and imaging techniques, we show that acetophenone forms ketone–enol dimers, in which the normally unstable form of enol is stabilized by H-bonding with the carbonyl group of the neighboring acetophenone molecule. These ketone–enol dimers can attach an H atom to form a reaction inter...