Transfer Hydrogenation - Organometallic Catalysts

Organometallic Catalysts

In the area of organic synthesis, a useful family of hydrogen-transfer catalysts have been developed based on ruthenium and rhodium complexes with diamine and phosphine ligands. A representative catalyst precursor is derived from (cymene)ruthenium dichloride dimer and the tosylated diphenylethylenediamine. These catalysts are mainly employed for the reduction of ketones and imines to alcohols and amines, respectively. The hydrogen-donor (transfer agent) is typically isopropanol, which coverts to acetone upon donation of hydrogen. Transfer hydrogenations can proceed with high enantioselectivities when the starting material is prochiral:

RR'C=O + Me₂CHOH → RR'C*H-OH + Me₂C=O

where RR'C*H-OH is a chiral product. A typical catalyst is (cymene)R,R-HNCHPhCHPhNTs, where Ts = SO₂C₆H₄Me and R,R refers to the absolute configuration of the two chiral carbon centers. This work was recognized with the 2001 Nobel Prize in Chemistry to Ryōji Noyori.

Another family of hydrogen-transfer agents are those based on aluminium alkoxides, such as Aluminium isopropoxide, but the activities are relatively low by comparison with the transition metal-based systems.