Polymer-Assisted Asymmetric Reactions. 4. Polymer-Bound Ephedrine, Its Use and Limitations in Supported LiAlH4 Reductions

Article abstract of DOI:10.1021/jo00368a011

A cross-linked polystyrene resin containing (1R,2S)-(-)-ephedrine moieties bound through nitrogen to some of its p-methylene-substituted aromatic rings is a useful regenerable chiral auxiliary in the enantioselective reduction of acetophenone by the chiral polymer-bound complexes of lithium aluminium hydride and an added achiral phenol.Evidence is presented to explain the capacity-dependent behavior of the polymer in the formation of chiral complexes and its effect on the enantioselectivity of the reduction of acetophenone.At high capacities, both unbound achiral and m ultiply bound chiral complexes are formed while numerous chiral ligands appear to be inaccessible to the hydride; under such conditions the enantioselectivity of the reaction is poor.In contrast, at low capacities corresponding to 8-10percent substitution of the styrene repeating units, the polymer-bound chiral moieties can act independently from one another and are fully accessible to the hydride.The reduction then proceeds with a high enantioselectivity, comparable to that of similar small chiral molecules.This mechanism is consistent with and explains the phenomena observed with other polymer-supported hydride reagents.