Cyclometalated Complexes of Ruthenium, Rhodium and Iridium as Catalysts for Transfer Hydrogenation of Ketones and Imines

Abstract A library of organometallic compounds derived from primary and secondary amines cyclometalated by ruthenium(II), rhodium(III) and iridi‐ um(III) was tested in the asymmetric transfer hydrogenation of a number of ketones and imines. All compounds displayed high catalytic activity for the reduction of ketones under mild conditions. The most enantioselective catalysts were based on secondary amines containing two asymmetric carbon atoms bound to the nitrogen atom. For the reduction of aryl alkyl ketones [Ar(CO)R where R=CH3 or CH2R′] the cyclometalated ruthenium and rhodium derivatives of the (2R,5R)‐2,5‐diphenylpyrrolidine ligand displayed the best results with respect to activity and selectivity (ees up to 97%). However, for the reduction of aryl tert‐alkyl ketones [Ar(CO)R′′ in which R′′ is a tertiary alkyl group] the best catalyst was a ruthenium compound derived from bis[(R)‐1‐phenylethyl]amine, allowing the reduction of isobutyrophenone and cyclohexyl phenyl ketone which were both reduced with high enantioselectivities (ees up to 98%). This shows that the cyclometalated compounds have a high substrate specificity. In addition, acyclic and cyclic imines were reduced with good selectivities by both rhodium(III) and iridium(III) metalacycles built up with (2R,5R)‐2,5‐diphenylpyrrolidine.