Synthesis and bifunctional asymmetric organocatalysis of helical poly(phenylacetylene)s bearing cinchona alkaloid pendants via a sulfonamide linkage

摘要

Four novel helical poly(phenylacetylene)s with amino-functionalized cinchona alkaloid pendant groups connecting to the phenyl rings through a sulfonamide linkage were synthesized by the polymerization of the corresponding phenylacetylene monomers using Rh~+(2,5-norbornadiene) [(η~6-C_6H_5)B-(C_6H_5)_3] (Rh(nbd)BPh_4) as the catalyst. The optically active sulfonamide-linked polymers adopted a helical conformation with an excess of one-handedness as supported by the appearance of the induced Cotton effects in the main-chain chromophore regions, and efficiently catalyzed the enantioselective methanolytic desymmetrization of a cyclic anhydride and aza-Michael addition of aniline to chalcone, thereby producing the corresponding optically active products up to 86% enantiomeric excess. However, their enantioselectivities from the methanolytic desymmetrization were slightly lower than those catalyzed by the corresponding cinchona alkaloid-bound monomers. On the other hand, during the asymmetric aza-Michael addition, a unique enhancement of the enantioselectivity was observed for several sulfonamide-linked helical polymers, and thus affording a remarkably higher enantioselectivity compared to those of the corresponding monomers and nonhelical polymers bearing the identical cinchona alkaloid residues.