Enantioselective Polymerization Of Epoxides: A Highly Active And Selective Catalyst For The Preparation Of Stereoregular Polyethers And Enantiopureepoxides

摘要

The development of chiral catalysts for the preparation of enantiopure building blocks and stereoregular macromolecules are two vibrant areas of stereoselective synthesis. Asymmetric catalysts provide invaluable routes to enantiopure pharmaceutical starting materials and intermediates, while stereoselective polymerization catalysts are the most commercially viable means for the synthesis of isotactic polymers. An interesting intersection of these important subfields of stereoselective synthesis is enantioselective polymerization (also known as stereoelective or kinetic resolution polymerization), where chiral, racemic monomers are kinetically resolved during polymerization. Significant advances in enantioselective polymerization of racemic lactones, alkenes, and thiiranes have been achieved. Although spectacular successes have been reported regarding the kinetic resolution of racemic epoxides by catalytic ring-opening with small molecules, highly enantioselective polymerization catalysts for the resolution of racemic epoxides do not exist (Scheme 1). Such a reaction would yield two desirable products: enantiopure epoxides and stereoregular polyethers. Thus the development of a catalyst for this transformation became a primary goal of our research.rnIt has long been postulated that some metal catalyzed epoxide polymerizations occur via a bimetallic reaction where one metal center coordinates the epoxide and another delivers the polymer alkoxide; recent studies involving homogeneous catalysts have substantiated these hypotheses. Furthermore, elegant studies by Jacobsen and co-workers have delineated the bimetallic nature of enantioselective epoxide hydrolysis using chiral cobalt salen catalysts.