Synthesis of cinchona alkaloid squaramide polymers as bifunctional chiral organocatalysts for the enantioselective michael addition of β-ketoesters to nitroolefins

Abstract

Novel chiral polymers containing cinchona-based squaramides in their main chain structure were designed. We synthesized chiral squaramides bearing two cinchona moieties, which contain double bonds. Repeated Mizoroki-Heck reactions between the double bonds of the cinchona-based squaramide and aromatic diiodides proceeded smoothly to give cinchona squaramide chiral polymers. The catalytic activity of the chiral polymers was subsequently investigated. The asymmetric Michael addition of β-ketoesters to nitroolefins was successfully catalyzed by the polymeric organocatalysts to give the corresponding Michael adducts in good yields and excellent enantioselectivities of up to 99% ee. As the polymeric catalysts were insoluble in commonly used organic solvents, they were easily recovered from the reaction mixture and reused for further reaction without any loss of catalytic activity.