Synthesis of cinchona squaramide polymers by Yamamoto coupling polymerization and their application in asymmetric Michael reaction

Abstract

Yamamoto coupling polymerization has been used for the synthesis of polymeric chiral organocatalysts. Cinchona squaramide derivatives with dibromophenyl moiety were polymerized under the Yamamoto coupling conditions to afford the corresponding chiral polymers in good yields. Using this technique, novel cinchona alkaloid polymers containing the squaramide moiety were designed and successfully synthesized. In addition to the homopolymerization of cinchona squaramide monomers with a dibromophenyl group, achiral comonomers such as dibromobenzene were copolymerized with the cinchona monomers to yield chiral copolymers. These chiral polymers were successfully utilized as polymeric catalysts in asymmetric Michael addition reactions. Good to excellent enantioselectivities were observed for different types of asymmetric Michael reactions. Using the chiral homopolymer catalyst P4, almost perfect diastereoselectivity (>100:1) with 99% ee was obtained for the reaction between methyl 2-oxocyclopentanecarboxylate 25 and trans-β-nitrostyrene 17. The polymer catalysts developed in this study have robust structures and can be reused several times without a loss in their catalytic activities.