Helical polyether-immobilized chiral aza-bis(oxazolines): Synthesis and synergistic effect on the enantioselectivity of Zn-catalyzed Henry reaction

Abstract

Novel polyether-immobilized aza-bis(oxazolines) ligands derived from optically pure epichlorohydrin and aza(bisoxazolines) were prepared by anionic polymerization. Through chiral optical studies, it is determined that the chiral polyether ligands can maintain stable helical conformation in the solvent, with controllable molecular weights and low polydispersity index of 1.02–1.06. Asymmetric Henry reactions of aldehydes were performed to investigate the relationship between the catalytic activity and the helical conformation of these polyethers, gave the adducts in up to 97 % yield with high enantiomeric excess(98 % ee). In particular, there is a synergistic effect between the helical chirality of the polyether backbone and the point chirality of the pendant aza-bis(oxazolines) in the asymmetric Herny reaction. Helical polyether-immobilized chiral aza-bis(oxazolines) ligands can be easily recycled at least 10 times without losing their enantioselectivity. It was also found by MALDI-TOF-MS that zinc(II) combined with polyether probably result in a double spiral species in the presence of more than 2.5 mol% Zn(OAc)2, which lead to negative effect on catalytic performance of the polyether.