Cooperative attractive interactions in asymmetric hydrogenations with dihydroxydiphosphine Rh(I) catalysts — a competition study

Studies for controlling rate and enantioselectivity of the asymmetric hydrogenation with Rhdiphosphine catalysts by cooperative attractive interactions within the framework of catalyst–substrate complexes are represented. In strong contrast to a Rh(I)[threo-1,4-bis(diphenylphosphino)butane-2,3-diol] catalyst an extremely fast reaction took place applying the complex of the analogue erythro ligand. This is likely due to a strong intramolecular hydrogen bond between the vicinal HO groups impeding the hemilabile coordination of one of the hydroxy groups on the metal center during the hydrogenation. When a substrate with strong hydrogen bond acceptor properties such as (E)-methyl 3-dimethoxyphosphorylbut-2-enoate was hydrogenated even the threo catalyst exhibited a fast reaction. The product was obtained in 83% ee. In contrast, when in the ligand both HO groups were replaced by MeO groups only poor conversion and 24% ee were achieved. While with the complex bearing HO groups in methanolic solution the corresponding diastereomeric catalyst substrate complexes were dominant, analogue MeO groups bearing catalyst substrate complexes could not be detected by NMR. For the latter, only the Rh(I)–solvent complex was found revealing the high importance of the additional functional groups on this equilibrium.