Chiral NHC Ligands for the Copper-Catalyzed Asymmetric Conjugate Addition of Grignard Reagents



In this paper, we disclose our recent advances in the copper-catalyzed enantioselective conjugate addition of Grignard reagents to 3-substituted cyclic enones. Several new N-heterocyclic carbene (NHC) ligands have been synthesized in high yields by starting from inexpensive materials. These new NHCs were evaluated as ligands for copper, for the Michaeladdition of ethylmagnesium bromide leading to chiral 3,3-disubstituted cyclohexanones (up to 93 % ee). The best ligand was then engaged in conjugate additions of various Grignard reagents, allowing the formation of quaternary centers with high levels of regio- and enantioselectivity with only 0.75 mol-% catalyst loading. The reaction is also robust to scale-up. The addition of ethylmagnesium bromide for the construction of a 3,3-disubstituted cyclopentanone (up to 86 % ee), a synthetically useful chiral synthon, is noteworthy. A tentative mechanism for the catalytic cycle is also discussed.