Unraveling the Pathway of Gold(I)-Catalyzed Olefin Hydrogenation: An Ionic Mechanism

The reaction mechanism of olefin hydrogenation catalyzed by the bimetallic gold catalyst {(AuCl)(2)[(R,R)-Me-DuPhos]} was studied by means of density functional theory calculations. This catalyst is enantioselective for the homogeneous hydrogenation of olefins and imines. The reaction mechanism involves activation of the H-2 molecule. This process takes place heterolytically, generating a metal-hydride complex as the active species and releasing a proton (formally EtOH2+) and a chloride ion to the medium. The hydrogenation reaction proceeds through an ionic mechanism in which the gold catalyst provides a hydride and the proton comes from the solvent. The reaction mechanism ends up with H-2 coordination and subsequent heterolytic cleavage, regenerating the gold(I)-hydride active species. Significant differences were found in the reaction mechanism depending on the nature of the substrate (ethene, cyclohexene, or diethyl 2-benzylidenesuccinate) and the character of the catalyst (mono- or bimetallic). Our data suggest that for prochiral substrates, the step that determines the enantioselectivity within the ionic mechanism involves a proton transfer.