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

Abstract

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 EtOH(2)(+)) 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.