Modular Chiral Dendritic Monodentate Phosphoramidite Ligands For Rh(Ⅱ)-catalyzed Asymmetric Hydrogenation: Unprecedented Enhancement Of Enantioselectivity

摘要

Modular chiral dendrimers with monodentate phosphoramidite ligands located at the core were synthesized and applied in the Rh-catalyzed asymmetric hydrogenations, afforded unprecedented enhancement of enantioselectivity.rnThe search for ideal chiral catalysts which combine the advantages of both homogeneous and heterogeneous catalysis is the center of many present investigations on the transition metal catalyzed asymmetric synthesis. Due to the well-defined and tunable molecular architectures as well as nano-scale size, metallodendrimers have been emerging as a promising class of catalysts as demonstrated in the pioneering work by van Koten in 1994. So far, a number of organo-metallic dendrimers with catalytic sites at the core or at the periphery have been reported. Among them, however, examples of positive dendrimer effect, in which the dendrimer played an active role in enhancing catalytic reactivity and/or selectivity, are still limited. For asymmetric catalysts, subtle conformational changes may significantly influence their enantioselectivity because the catalytic reaction is governed by small increments of free enthalpy of activation. Thus, attachment of chiral catalyst onto the well-defined dendrimer framework is expected to provide a unique tool for fine-tuning catalytic performance through adjusting their microenviron-ment.