Cobalt(II)-Salen-Linked Complementary Double-Stranded Helical Catalysts for Asymmetric Nitro-Aldol Reaction

Double-helical, bimetallic chiral Co(II)-salen complexes stabilized by chiral amidinium–carboxylate salt bridges efficiently catalyzed the asymmetric nitro-aldol (Henry) reaction, producing products with up to an 89% enantiomeric excess (ee); the reactivity and enantioselectivity were higher than those catalyzed by the corresponding single strands. The key role of the chiral double-helical framework for the supramolecular bimetallic catalysis has been revealed by a double-helical catalyst carrying achiral Co(II)-salen units that promoted the Henry reaction, yielding the product with a 50%–45% ee, while the corresponding single strands showed poor or no enantioselectivity.