Optically active helical polymers bearing cinchona alkaloid pendants: an efficient chiral organocatalyst for asymmetric Henry reaction

Abstract

Inspired by the highly efficient and enantioselective reactions catalyzed by biomacromolecules, the development of artificial helical polymer-supported catalysts is an attractive and meaningful field. In this work, a series of helical polymers, poly-1_ns, with controlled molecular mass (M_ns) and narrow molecular mass distribution (M_w/M_ns) bearing cinchona alkaloid pendants were obtained by asymmetric polymerization of the corresponding monomer. The poly-1_ns exhibited an intense positive Cotton effect at 364 nm, indicating that a preferred right-handed helix was formed in their backbone. Due to the catalytic groups on the pendants and helix in the backbone, the poly-1_ns exhibited satisfactory catalytic efficiency in the asymmetric Henry reaction. Compared to small molecule (1) with a similar structure, the enantioselectivity of the Henry reaction was significantly enhanced using poly-1_n as catalyst. The enantiomeric excess (ee) value of the Henry reaction could be up to 75%. Furthermore, the helical polyisocyanide catalyst could be recovered and reused facilely for at least five cycles without apparent significant loss of its enantioselectivity.