Dynamically Bifurcating Hydride Transfer Mechanism and Origin of Inverse Isotope Effect for Heterodinuclear AgRu-Catalyzed Alkyne Semihydrogenation

The mechanism and heterodinuclear cooperative effects for AgRu-catalyzed alkyne semihydrogenation were analyzed with density-functional theory (DFT) and experiment. This combined effort revealed the following: (1) AgRu-catalyzed diphenylacetylene hydrogenation initially gives a kinetic mixture of cis-stilbene and trans-stilbene by an ionic Ag–H hydride transfer transition state and post-transition state bifurcation, which was identified by quasiclassical direct dynamics simulations. (2) The hydrogenation reaction exhibits an unexpected inverse kinetic isotope effect (KIE u003c 1) resulting from an inverse equilibrium isotope effect (EIE) for heterodinuclear H2/D2 activation. (3) The Ag–Ru heterodinuclear cooperative effective is critical for both H2 activation and vinylsilver protonolysis reaction steps. (4) Rate studies and computational analysis show that electron-donating groups accelerate catalysis.