Transition metal hydrides as active intermediates in hydrogen transfer reactions

Catalytic hydrogen transfer reactions involving transfer hydrogenation of ketones, imines as well as Oppenauer-type oxidation of alcohols occur via active transition metal hydride intermediates. In the RuCl2(PPh3)3-catalyzed hydrogen transfer reaction a dramatic rate enhancement by base was observed. It was found that the role of base is to generate a highly active dihydride catalyst RuH2(PPh3)3. The mechanism of Ru-, Rh-, and Ir-catalyzed hydrogen transfer was probed by using α-deuterated alcohol as hydrogen donor and measuring the amount of deuterium transferred to the keto carbon of the hydrogen acceptor. Two different mechanisms are proposed for the transition metal-catalyzed hydrogen transfer, one via a monohydride (giving a high D-content) and another via a dihydride (giving about half of D-content).