The asymmetric hydrogenation of α-keto esters catalyzed by rhodium(I) complexes with chiral diphosphine ligands. On the catalytic cycles and the mechanism of asymmetric induction

Asymmetric hydrogenations of n-propyl pyruvate and ketopantoyl lactone catalyzed by rhodium complexes with (—) DIOP and BPPM were carried out under a variety of conditions. It was found that i) the Schrock-Osborn mechanism was not operative in these reactions at all since no acceleration of the reaction rate by the addition of water (1%) was observed, ii) there was a clear difference between cationic and neutral (in situ) rhodium catalysts in their enantioselectivity, and iii) there was a remarkable solvent effect on the extent and direction of asymmetric induction. Possible mechanisms are discussed on the basis of these results.