Electron Transfer in Nickel-Catalyzed Addition Reactions of Organozirconium Compounds to Unsaturated Ketones

The species formed by reacting Ni(acac)₂ with 1 equiv of Dibah have been studied with regard to their ability to catalyze carbon-carbon bond-forming reactions involving organozirconium compounds. Catalytic activity for these species is noted upon mixing the nickel and aluminum reagents; activity falls to almost nil after 24 h. Gas evolution studies monitoring the formation of isobutane suggest that these catalytically active species involve Ni(I). Cyclic voltammetric measurements and comparative investigations involving oxidative addition of aryl halides suggest that these catalyst species are good electron-transfer reagents. A mechanism for nickel-catalyzed conjugate addition of alkenylzirconium reagents to α,β-enones is proposed which involves one-electron reduction of the substrate by the Ni(I) catalyst. Comparisons between nickel-catalyzed and copper-catalyzed addition reactions of organometallics to conjugated dienones are described.