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

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 intermediate consisting of a partly hydrogenated acetophenone species and nonhydrogenated acetophenone. Based on the spectroscopic assignment of the reaction intermediate, we conclude that H atom can be attached either to the C=C bond of the enol part, or to the strongly weakened C=O bond of the ketone part of the ketone-enol dimer. In both cases, the formation of ketone-enol dimer species was found to be a crucial step in acetophenone hydrogenation. Observed phenomena provide atomistic level insights into the mechanisms of heterogeneously catalyzed hydrogenation of simple carbonyl compounds and can be employed for purposeful modification of catalysts with functional groups capable of stabilizing the enol species.