Chiral Phosphine−Phosphite Ligands with a Substituted Ethane Backbone. Influence of Conformational Effects in Rhodium-Catalyzed Asymmetric Olefin Hydrogenation and Hydroformylation Reactions

Abstract

A family of chiral (3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-2,2′-biphenol-derived) phosphine−phosphite ligands (P−OP) with a substituted ethane backbone has been synthesized and the performance of these ligands in the Rh-catalyzed enantioselective hydrogenation and hydroformylation of several representative olefins analyzed. Corresponding cationic rhodium complexes provide highly enantioselective catalysts for the hydrogenation of methyl (Z)-α-acetamidocinnamate (MAC) and dimethyl itaconate. The catalyst comparison indicates that, for the two substrates, product configuration is determined by the configuration of the phosphite. Regarding matching and mismatching effects in these hydrogenations, small effects were observed in the reduction of MAC, while for the itaconate the bigger difference between the matched and mismatched cases was of 21% ee. On the other hand, Rh catalysts based on P−OP ligands showed good levels of activity and regioselectivity in the hydroformylation of styrene and allyl cyanide, while moderate enantioselectivities were obtained. Participation of the two stereogenic elements has been observed in these reactions, and their mismatched combination leads to cancellation of enantioselectivity. To further investigate the influence of the ligand backbone in the course of these reactions, structures of rhodium model complexes Rh(Cl)(CO)(P−OP) were analyzed by DFT methods. The results obtained indicate the existence of two types of preferred conformations, whose relative stability depend on the backbone nature. Comparison of structures of the more stable conformers for each ligand indicates that the orientation of the biaryl phosphite group with respect to the coordination plane does not vary substantially along the series. Differently, the position of the phenyl phosphine substituents greatly depends on the backbone. On the basis of these observations it has been concluded that chiral induction in the hydrogenation is very predominantly due to the phosphite part of the ligand. Alternatively, conformation of the phosphine group has a great influence on enantioselectivity in the hydroformylation reactions, and even reversal of product configuration was observed between catalysts with an opposite axial equatorial arrangement of Ph phosphine substituents.