Outer sphere hydrogenation catalysis

In the title catalysts, the substrate, typically a ketone, imine or N-heterocycle, remains in the outer sphere (OS). The catalyst transfers hydride and a proton to the unbound substrate either by a concerted or by a stepwise path. These include catalysts not always considered together, such as Bullock's ionic hydrogenation catalysts, bifunctional catalysts in the tradition of Shvo and Noyori and Stephan's frustrated Lewis pair catalysts. By omitting the oxidative addition, insertion and reductive elimination pathways of conventional inner sphere (IS) catalysts, these OS pathways are in principle equally open to inexpensive metals and even nonmetal catalysts. These OS pathways lead to useful selectivity properties, particularly Noyori's asymmetric catalysis, but much more remains to be done in this rapidly developing field.