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

Studies for controlling rate and enantioselectivity of the asymmetric hydro genation with Rh-diphosphine catalysts by cooperative attractive interactio ns within the framework of catalyst-substrate complexes are represented. In strong contrast to a Rh(I)[three-1,4-bis(diphenylphosphino)butane-2,3-diol ] catalyst an extremely fast reaction took place applying the complex of th e analogue erythro ligand. This is likely due to a strong intramolecular hy drogen bond between the vicinal HO groups impeding the hemilabile coordinat ion of one of the hydroxy groups on the metal center during the hydrogenati on. When a substrate with strong hydrogen bond acceptor properties such as (E)-methyl 3-dimethoxyphosphorylbut-2-enoate was hydrogenated even the thre e catalyst exhibited a fast reaction. The product was obtained in 83% ee. I n contrast, when in the ligand both HO groups were replaced by MeO groups o nly poor conversion and 24% ee were achieved. While with the complex bearin g HO groups in methanolic solution the corresponding diastereomeric catalys t substrate complexes were dominant, analogue MeO groups bearing catalyst s ubstrate complexes could not be detected by NMR. For the latter, only the R h(I)-solvent complex was found revealing the high importance of the additio nal functional groups on this equilibrium. (C) 2000 Elsevier Science S.A. A ll rights reserved.