Asymmetric Hydrogenation with Rhodium(I)–Chiral Diphosphinites. The Effect of the Dimethylamino Group of the Ligand on the Asymmetric Induction

Abstract New chiral diphosphinites were prepared starting from (+)-diethyl tartrate. The asymmetric hydrogenation of dehydroamino acids, itaconic acid and dehydrodipeptides was studied using Rh(I)-diphosphinite catalysts. In the hydrogenation of dehydroamino acid derivatives, an introducion of ω-(dimethylamino)alkyl group in the ligands did not raise the optical yield. By the use of Rh(I)-diphosphinite having 3-(dimethylamino)propyl group, the inversion of the preferred product was observed. Itaconic acid can be hydrogenated in a good optical yield (76% e.e.) by use of the diphosphinite having 2-(dimethylamino)ethyl group. The diphosphinites having ω-dimethylamino group gave high optical yields in the case of dehydrodipeptides with a chiral center. Especially Rh(I)-catalysts coordinated by diphosphinites with 4-methyl or 4-methoxyl group on the phenyl ring of the ligand gave excellent stereoselectivities (more than 98% d.e.) in the case of Ac–ΔPhe–(S)-Phe–OH. The effect of these new chiral diphosphinites on the asymmetric hydrogenation of dehydroamino acids, itaconic acid and dehydrodipeptides was discussed in terms of the stereocontrol induced by electrostatic interaction between the dimethylamino group of the ligand and the substrate and by the steric interaction between the ligand and the substrate.