Robust and Recyclable Self-Supported Chiral Nickel Catalyst for the Enantioselective Michael Addition

A simple chemical modification of a chiral diamine ligand may produce a robust and recyclable enantioselective catalyst. Metallopolymers based on chiral cyclohexyldiamine-containing ditopic ligands and nickel(II) complexes have been readily prepared and applied in catalytic enantioselective Michael additions of 1,3-dicarbonyl compounds to nitroalkenes. High yields and good enantioselectivities have been obtained and the catalytic systems have been recycled up to 11 times without loss of either activity or enantioselectivity at a low catalyst loading of 0.75mol%. Moreover, the nickel metallopolymers were found to be air- and moisture-stable, which enabled this chemistry to be carried out on the bench without the use of any air-free techniques and with non-degassed solvents. Finally, the nature of the catalyst was studied by non-linear effect experiments, giving a negative non-linear effect (NLE) as a consequence of an in situ decrease in the ee of the active species consistent with the trapping of homochiral aggregates.