Helical Polymer-Based Polycarboxylate as Chiral Ligands in Cu-Catalyzed Enantioselective Diels-Alder Reaction in Water.

Abstract

Water-soluble poly(quinoxaline-2,3-diyl)s (PQXlac) bearing chiral carboxylic acid side chains derived from l-lactic acid were used for Cu-catalyzed enantioselective Diels-Alder reaction in water. The reaction of azachalcone with cyclopentadiene in the presence of PQXlac (1000mer, 1 mol% monomer unit) and Cu(NO₃)₂ (1 mol% Cu) proceeded at 5 °C in good yield with high enantioselectivity up to 99% ee. The utilization of the corresponding oligomers shorter than 10mer resulted in low chemical yield with low enantioselectivity, suggesting that a tight helical structure is essential for the formation of an effective catalyst. A comparison of the random and block copolymer of PQXlac and PQXteg, which has achiral tetra(ethylene glycol) side chains, indicated that block copolymer, in which the carboxylate groups are more densely packed, showed higher chemical yields along with higher enantioselectivity. These results suggest that the active catalyst has a copper dicarboxylate structure in which two carboxylate groups on every other monomer unit coordinate to the Cu center to form an effective chiral reaction environment.