Stoichiometric, catalytic and enantioface-selective hydrogenation of carbon-nitrogen double bonds by an ionic mechanism

Optically pure ruthenium hydrides CpRu(P-P*)H stoichiometrically transfer hydride asymmetrically to iminium cations with moderate enantioface-selectivity. Catalytic hydrogenation of iminium cations occurs in the presence of hydrogen, producing the same enantiomeric excess and absolute configuration of product as the analogous stoichiometric hydride transfer. The turnover-limiting and enantioselectivity-determining steps are independent of hydrogen pressure, implicating hydride transfer as the key step in the catalytic reaction. Monitoring the catalytic reaction also indicates hydride transfer is turnover-limiting.; Study of the mechanism of stoichiometric hydride transfer shows a first-order dependence of the rate on iminium cation concentration. Acetonitrile, which coordinates to the ruthenium metal center, has no effect on the rate of stoichiometric hydride transfer. Hydride transfer to iminium cations in the initial presence of acetonitrile proceeds without an intermediate.; Steric bulkiness of the iminium cation tends to inhibit the rates of stoichiometric and catalytic hydrogenation. Regioselective hydrogenation of C=N bonds in preference to C=C bonds is not feasible. Hydrogenation of dialkyl iminium cations proceeds with no enantiomeric excess in the product.