Asymmetric Reactions in Water Catalyzed by L-Proline Tethered on Thermoresponsive Ionic Copolymers

Abstract

L-Proline was covalently tethered on thermoresponsive ionic block copolymers that formed micelles in aqueous solutions. The block copolymers consisted of a poly(N-isopropylacrylamide) (PNIPAAm) segment and an anionic or cationic polymer segment. These copolymers exhibited lower critical solution temperature (LCST) behavior at ca. 35-40°C, and achieved thermal stimuli-induced formation and dissociation of micelles. The copolymer generated micelles in aqueous solution at a higher temperature, where a catalytic aldol reaction proceeded with high diastereo- and enantioselectivities. The micelles dissociated at lower temperature to form a clear solution such that the products could be efficiently extracted from the aqueous reaction mixture. Extraction of the aldol product with an organic solvent from the aqueous solution of the anionic copolymer was more efficient than from the nonionic copolymer solution.