The role of the metal-bound N–H functionality in Noyori-type molecular catalysts

Noyori-type catalysts have found numerous applications in research and industrial settings. The central mechanistic component of such catalysts is a metal centre coordinated to a N–H moiety. This amino moiety has traditionally been thought to participate directly in catalytic reactions by serving as a H+ donor, with the resulting amido group then serving as a H+ acceptor. This traditional understanding has been supplanted by more recent studies that instead suggest that the N–H group(s) (or N–Ma group(s) obtained in the reaction with a base of an alkali metal Ma) serve to stabilize rate-determining transition states through non-covalent N–X···O interactions (X？=？H or Ma). Thus, N–X bonds are actually not cleaved or formed in many catalytic cycles. This Review describes examples of metal–ligand bifunctional catalysts relevant to reactions involving H2 or its equivalents, emphasizing systems that have been applied in industry. Subsequently, a summary of our present understanding of the Noyori–Ikariya and Noyori reaction mechanisms is presented, which we compare to topical related reactions such as MeOH dehydrogenation, ester and carboxamide hydrogenation and the dehydrogenative coupling of primary alcohols with other alcohols and amines. This mechanistic understanding allows us to identify the design principles that may potentially afford improved molecular catalysts and that may unravel a distinct mechanism for H2 production by the diiron hydrogenase enzymes