Self-Assembled Dinuclear Cobalt(II)-Salen Catalyst Through Hydrogen-Bonding and Its Application to Enantioselective Nitro-Aldol (Henry) Reaction

Novel chiral dinuclear Co(II)-salen catalysts self-assembled through the pyridone/aminopyridine hydrogen-bonding pair have been developed. The self-assembled dinuclear Co(II)-Salen catalyst results in significant rate acceleration (48 times faster rate) as well, as excellent enantioselectivity (96% ee vs 55% ee) in a nitro-aldol reaction, compared to the corresponding monomeric catalyst. A bimetallic mechanism is suggested by the kinetic experiment showing that the rate taw is second order in [catalyst]. The self-assembly through hydrogen bonding was confirmed by X-ray structure and by the H-1 experiments revealing that the dimerization constant of the novel hydrogen-bonding capable salen (ligand in 25% v/v CD3NO2/CDCl3 is 53 +/- 21 M-1. This result proves the validity of self-assembly based approaches toward the efficient construction of chiral bimetallic catalyst system.