Effective homogeneous hydrogenation of α-keto esters catalysed by neutral rhodium(I) complexes with phosphine ligands and application to the asymmetric synthesis of lactates

The hydrogenation of α-keto esters was found to be catalysed effectively by neutral rhodium(I) complexes with phosphine ligands, and the asymmetric hydrogenation of pyruvates was carried out using the rhodium(I) complexes with chiral diphosphines 2,3-O-isopropylidene-2,3-dihydroxy-1,4- bis(diphenylphosphino)butane (diop), and (2S,4S)-N-t-butoxycarbonyl-4- diphenylphosphino-2-diphenylphosphinomethylpyrrolidine (bppm), in dry benzene or tetrahydrofuran to give optically active lactates (36-76% enantiomeric excess) in nearly quantitative yields.