Autocatalytic Role of Molecular Hydrogen in Copper-Catalyzed Transfer Hydrogenation of Ketones

Catalytic transfer hydrogenation of ketones and aldehydes is generally accepted to follow a dehydrogenation–hydrogenation mechanism on copper, which makes the increased hydrogenation rate and selectivity rather puzzling. Using first-principles microkinetics on a Cu(111) surface, we show that, rather than a dehydrogenation–hydrogenation mechanism, there is also direct proton transfer between the sacrificial alcohol and the reacting ketone. The ketone is hydrogenated to a stable alkoxy intermediate using surface hydrogen. This alkoxy intermediate is subsequently hydrogenated to the alcohol product via direct proton transfer from the sacrificial alcohol, also forming a sacrificial alkoxy intermediate. To close the catalytic cycle, the sacrificial alkoxy species dehydrogenates, forming its corresponding ketone. We also observed a surprising catalytic effect of molecular hydrogen, which can be explained by the rate-controlling step in transfer hydrogenation: the direct hydrogenation of the ketone to its alkoxy intermediate by surface hydrogen. Under all realistic reaction conditions, this step has the highest degree of rate control.