Scope and mechanism of enantioselective Michael additions of 1,3-dicarbonyl compounds to nitroalkenes catalyzed by nickel(II)-diamine complexes

Readily prepared Ni(II)-bis[(RR)-N,N-dibenzylcyclohexane-1,2-diamine]Br-2 was shown to catalyze the Michael addition of 1,3-dicarbonyl compounds to nitroalkenes at room temperature in good yields with high enantioselectivities. The two diamine ligands in this system each play a distinct role: one serves as a chiral ligand to provide stereoincluction in the addition step while the other functions as a base for substrate enolization. Ligand modification within the catalyst was also investigated to facilitate the reaction of aliphatic nitroalkenes, 1,3-diketones, and beta-ketoacids. Ni(II)-bis[(R,R)-N,N-di-p-bromo-benzylcyclohexane-1,2diamine]Br-2 was found to be an effective catalyst in these instances. Furthermore, monodiamine complex, Ni(II)-[(R,R)-N,N-dibenzylcyclohexane-1,2-diamine]Br-2, catalyzed the addition reaction in the presence of water. The proposed model for stereochernical induction is shown to be consistent with X-ray structure analysis.