New Solid-Phase Catalysts for Asymmetric Synthesis: Cross-Linked Polymers Containing a Chiral Shiff Base-Zinc Complex

A cross-linked polystyrene resin containing chiral primary amino alcohol moieties bound through the ether linkage to some of its p-methylene-substituted aromatic rings is a useful regenerable chiral auxiliary in the enantioselective catalytic alkylation of aldehydes. The primary amino groups of the chiral amino alcohols react with the aldehydes to form Schiff bases, which catalyze the addition of dialkylzinc to aldehydes leading to optically active secondary alcohols having enantiomeric purity of up to 99%. A series of polymeric amino alcohols were synthesized by two methods involving either attachment of a chiral moiety as a side chain onto a reactive cross-linked polystyrene, or the terpolymerization of a chiral monomer with styrene and a cross-linking agent. New cross-linking agents affording more flexibility to the chiral catalysts were used in the preparation of the chiral polymers and found to provide excellent performance. An interesting extension of the method is its adaptation to a continuous-flow system where diethylzincand aldehyde are supplied continuously to a column filled with the chiral polymeric catalyst. Large amounts of chiral products and high turnovers may be obtained by this method.