(668f) Enhanced Cooperativity Via Design: Pendant Co(III)-Salen Polymer Brush Catalysts for the Hydrolytic Kinetic Resolution of Epichlorohydrin

The Co(III)-salen catalyzed hydrolytic kinetic resolution (HKR) of racemic epoxides has emerged as a highly attractive and efficient method of synthesizing chiral C3 building blocks for intermediates in larger, more complex molecules. The HKR reaction involves a bimetallic mechanism and shows a second order dependency of activity upon the concentration of Co(III) sites. Thus, proper design of supported catalysts can enhance reaction rates if the design enhances Co-Co cooperativity. Here, the utilization of pendant Co(III)-salen catalysts on silica supported polymer brushes as catalysts for the HKR of epichlorohydrin is reported. The catalyst was prepared by grafting an atom transfer radical polymerization (ATRP) initiator onto Cab-O-Sil silica. Asymmetric styryl-modified salen monomers were synthesized and copolymerized with styrene from the functionalized silica support via ATRP, forming the polymer brush catalyst. The novel polymer brush architecture provides a unique framework for promoting site-site interactions as required in the bimetallic transition state of the HKR mechanism. Furthermore, the polymer brushes mimic the environment of soluble polymer based catalysts, while the silica support permits facile recovery and reuse of the catalyst. The polymer brush catalyst is 20x more active than the homogeneous Jacobsen Co-salen catalyst and is observed to retain high enantioselectivity (>99%) after each of five reactions.