Novel bifunctional chiral squaramide-amine catalysts for highly enantioselective addition of mono and diketones to nitroalkenes

Abstract

Novel bifunctional chiral squaramide-amine organocatalysts have been developed by rational combination of pyrrolidine and a cinchona alkaloid. The catalysts promoted the enantioselective Michael addition of both mono-and diketones to a broad range of nitroalkenes providing the corresponding products in moderate to high yields with excellent enantioselectivities and diastereoselectivities (up to 96% yield, 96% ee, 98:2 dr) under mild conditions. These results demonstrate that the assembly of two chiral privileged skeletons, pyrrolidine and cinchonine with a squaramide linker is a useful strategy to reach a wider substrate scope, high reaction efficiency and enantioselectivity. The match of the chiralities between two backbones embedded in the catalysts is also critical for improving enantioselectivity.