Adsorption modes of aromatic ketones on platinum and their reactivity towards hydrogenation

Heterogeneous catalytic hydrogenation of ketones is an important synthetic route to alcohols, but the detailed reaction pathway of this common reaction is still unknown. In particular, different opinions exist concerning the nature of the surface intermediates that eventually react with the activated surface hydrogen: η1(O) and η2(C,O) adsorbed intermediates have been postulated by different authors, leading to different surface reaction pathways. Here we studied the hydrogenation of aromatic ketones activated by an ester group in α-position. Ethyl benzoylformate (1) was hydrogenated on Pt/Al2O3 under mild conditions, but insertion of one or two o-substituents into the aromatic ring diminished or completely eliminated the reactivity of the ketone. The dramatic difference between the reactivities of 1 and ethyl mesithylglyoxylate (5) prompted us to investigate the adsorption geometries and energies of the two ketones by electronic structure calculations on a model platinum (1 1 1) surface. The calculations revealed that the presence or absence of o-substitution on the phenyl ring strongly affects the interaction mode of the CO moiety with the metal surface. In particular, o-substitution suppresses adsorption modes where the keto-carbonyl group is bound to the metal in η2(C,O) mode. Following such observations the reactivity of aromatic ketones is discussed, and a correlation between adsorption mode and reactivity of the ketone towards hydrogenation is proposed that could be critical for the further investigation of a complete reaction mechanism.

Hydrogenation of aromatic α-ketoesters on Pt/Al2O3 under mild conditions showed that ethyl benzoylformate was smoothly hydrogenated, but insertion of two aryl substituents in o-position (mesitylglyoxylate and antracenylglyoxylate) completely eliminated the reactivity of the ketone. Theoretical calculations revealed that o-substitution constitutes an impediment to the formation of adsorbed intermediates having the keto-carbonyl group bound to the metal in η2 (C,O) mode.Figure optionsDownload as PowerPoint slide