Asymmetric hydrogenation by heterogenized cationic rhodium phosphinite complexes

Abstract

Chiral cationic rhodium(I) complexes with bisphosphinite ligands have been immobilized on sulphonated ion exchangers based on styrene-divinylbenzene copolymers, thus allowing the formation of highly active and enantioselective heterogenized catalysts. It was found that low-crosslinked supports are suitable for the preparation of such catalysts, certainly as far as the asymmetric hydrogenation of N-acylated dehydroamino acid esters is concerned. The activity can be increased to optimum values by using gel polymers crosslinked with O.5% divinylbenzene and by preloading the polymers with lithium or ammonium ions. Swelling volumes have been checked and an unexpectedly high ratio of activity to swelling volume was found for the ammonium ion. When the rhodium complex of 4,6- O-benzylidene-2,3- O-bis(diphenylphosphino)-β-D-glucopyranoside (Ph-β-glup) was heterogenized on acid supports it gave higher ee values (95%) in comparison to the homogeneous catalyst (91% ee). This is due to solvolysis catalyzed by the acid function of the polymer which leads to a splitting off of the benzylidene unit of the rhodium complex as verified by immobilization of an analogous complex prepared independently. Recycling experiments displayed high constancy in optical induction which remained stable in all the runs until activity ceased. This was not true for the aminophosphine-phosphinite complex PROPRAPHOS, however, which exhibited a steady reduction of optical induction with activity. Rhodium leaching was found to be < O.5% using the protonated exchangers, but increased when salt forms of the exchangers were employed.