Asymmetric Hydrogenation of Prochiral Olefins Catalyzed by Rhodium Complexes with Chiral Pyrrolidinodiphosphines. Crucial Factors for the Effective Asymmetric Induction

Remarkable effects of hydrogen pressure on the stereoselectivity were observed in the asymmetric hydrogenations of itaconic acid, α-(acylamino)acrylic acid, and their derivatives catalyzed by rhodium complexes of chiral pyrrolidinodiphosphines. Effects of added triethylamine on the pressure dependency of stereoselectivity were also studied. Marked differences in the direction of asymmetric induction were found in the asymmetric hydrogenations of the methylsuccinic acid precursors itaconic acid, citraconic acid, and mesaconic acid. This result clearly indicates that the chiral recognition by the rhodium catalyst is extremely sensitive to the stereochemistry of the prochiral olefins. Possible mechanisms are discussed on the basis of these results. 31Pj1H| NMR studies on the key intermediates in asymmetric hydrogenations have revealed that the mode of the bidentate complexation of the prochiral substrates is extremely regioselective. The “induced-fit” phenomena of the chiral rhodium complex were observed. A possible mechanism for the chiral recognition of enantiofaces in the chiral coordination sphere is proposed.