Achiral Benzoic Acid Appended Helical Polymers for Copper-catalyzed Asymmetric Diels-Alder Reaction and Michael Addition with Excellent Enantioselectivity and Recyclability

Abstract

One-handed helical polyisocyanides bearing achiral benzoic acid pendants were readily synthesized via the asymmetric polymerization of achiral isocyanides using chiral Pd(II) catalysts. Despite the inexistence of any stereogenic centers, these polymers showed significant optical activity owing to the one-handed helicity. Two adjacent carboxyl groups on the polymer backbone coordinate with a single Cu2⁺, producing helical polyisocyanide-Cu complexes that act as highly active and recyclable chiral catalysts for asymmetric Diels–Alder and Michael addition reactions. Under optimized conditions, the block copolymer S-poly(350-b-230-b-350)/CuCl2 delivers products in high yield (>77%) with outstanding stereocontrol (up to 98% ee). For Michael addition reactions, the same polymer gives up to 98% ee with yields >80%. Employing the enantiomeric backbone under the same conditions afforded the opposite R-configured product. Regulating the backbone helicity could control the direction of enantioselectivity and enantiomeric products could be facilely obtained using helical polyisocyanide with opposite handedness. Importantly, catalyst can be readily precipitated from the reaction mixture by addition of a poor solvent and reused for five consecutive cycles without significant loss of yield and enantioselectivity. After copper leaching and re-coordination, they can be cyclically applied to distinct asymmetric reactions.