Chiral Copper Carbene Catalysts

By creating a novel set of chiral imidazolium salts, Kang-sang Lee and Amir H. Hoveyda of Boston College have extended the already versatile range of N-heterocyclic carbenes (NHCs) as ligands for transition-metal catalysts (J. Org. Chem., DOI: 10.1021/jo900589x). Lee and Hoveyda designed the imidazole rings of the reagents to be dissymmetric by placing a different substituted aryl group on each nitrogen atom. They in turn created chiral copper-NHC catalysts (one shown) by treating the imidazolium salts with CuBr or CuCl. To test the catalysts, Lee and Hoveyda focused on enantioselective conjugate addition reactions of aryl or alkenyl groups to the ring double bond of cyclic enones. They showed that the in situ-generated catalysts in conjunction with in situ-generated aryl- or alkenyltetrafluorosilicates is an efficient reaction system for adding aryl or alkenyl groups to five-, six-, seven-, or eight-membered-ring enones, yielding cyclic ketones in up to 97% yield and 97% enantiomeric excess. "These studies pave the way for development of additional catalytic methods that are of substantial potential utility and involve chiral NHC catalysts, organosilanes, and other important classes of electrophiles," the researchers write.