Mechanisms of the H2-hydrogenation and transfer hydrogenation of polar bonds catalyzed by ruthenium hydride complexes

The catalytic cycles for the H-2-hydrogenation (H) and transfer hydrogenation (T) of C=O and C=N bonds catalyzed by over 100 ruthenium hydride complexes in organic and aqueous media can be classified into two main classes: the hydride transfer step is inner sphere (1) or outer sphere (O). Important subclasses of these mechanisms are cases where an ancillary ligand may assist in the hydride transfer step (IL or OL, respectively). The types of hydride complexes and their reactivity toward C=O (ketones, aldehydes, CO2) and C=N (imines) bonds is examined. Features of the different types of catalytic cycles are described. It is clear that the ligand assisted cases lead to very efficient catalysts for the selective hydrogenation of these polar bonds (e.g. Noyori's metal-ligand bifunctional catalysis). (C) 2004 Elsevier B.V. All rights reserved.