Development of Dynamically Chiral Helical Polymer Catalysts Based on Post-Polymerization Modification of Boronyl Pendants

Catalytic asymmetric synthesis is one of the most efficient methods for the preparation of chiral compounds with high optical purity. Although a variety of robust chiral scaffolds have been developed to achieve efficient chiral catalysts which show high catalytic activities and stereoselectivities, use of dynamic chiral scaffold is desired for the development of advanced chiral catalysts featuring stimuli-responsive properties such as chirality-switching and asymmetric amplification. We have developed chirality-switchable helical polymer ligands bearing monodentate phosphine pendants, which exhibit high enantioselectivities based on solvent-dependent dynamic screwsense control of pol(y quinoxaline-2,3-diyl)s(PQXs)having chiral side chains. However, the application has been limited to palladium-catalyzed asymmetric reactions. This article describes the development of helically chiral polymer catalysts based on post-polymerization modification of bornyl pendants(-B(OH) 2)attached to the PQXs. Introduction of 2,2'-bipyridine pendants to chiral PQXs through Suzuki-Miyaura coupling at the boronyl pendants led to the development of chirality-switchable helical polymer ligands for copper-catalyzed asymmetric reactions. In addition, introduction of 4-aminopyridine pendants into the chiral PQXs provided chirality-switchable efficient asymmetric nucleophilic catalysts for kinetic resolution of secondary alcohols and asymmetric Steglich-type acyl rearrangements. Furthermore, by using the boronyl pendants as molecular recognition sites for screw-sense induction to achiral PQXs, a chiral-guest-responsive helical polymer ligand bearing monodentate phosphine pendants was developed for the palladium-catalyzed asymmetric reaction. Using an amino alcohol as a chiral additive, the screw-sense induction proceeded with asymmetric amplification, giving the corresponding product with higher optical purity than that of the chiral additive.