Understanding the Selectivity in Hydrogenation of α,β-Unsaturated Aldehydes:  A Water-Assisted Mechanism

The mechanisms of the competitive CC and CO hydrogenations of the α,β-unsaturated aldehydes by [RuH2(PH3)3] are investigated by means of calculations based on density functional theory. The classical mechanism via double-bond insertion into the Ru−H bond followed by σ-bond metathesis does not explain the selectivity to the unsaturated alcohol, since the CC hydrogenation is found to be easier than the CO hydrogenation, both thermodynamically and kinetically. A new water-assisted mechanism is presented in which the CO hydrogenation is clearly favored. This allows us to explain the preferred formation of the unsaturated alcohol, particularly true in biphasic media.