C2 symmetric chiral NHC ligand for asymmetric quaternary carbon constructing copper-catalyzed conjugate addition of Grignard reagents to 3-substituted cyclohexenones

The asymmetric construction of quaternary carbon centers by conjugate addition of Grignard reagents to 3-methyl- and 3-ethylcyclohexenones was realized in a maximum enantioselectivity of 80% by using a C(2) symmetric chiral N-heterocyclic carbene (NHC)-copper catalyst, generated from (4S,5S)-1,3-bis(2-methoxyphenyl)-4,5-diphenyl-4,5-dihydro-1H-imidazol-3-ium tetrafluoroborate and copper(II) triflate. The stereostructures of the NHC-Au complexes were analyzed by X-ray crystallography, which rationalized the good stereocontrolling ability of N-aryl NHCs.