The Magic of Triisopropyl-1,4,7-Triazacyclononane: Probing the Role of Ni(III) and Ni(I) Centers in Cross-Coupling Catalysis

The last two decades have experienced a dramatic development of nickel-catalyzed cross-coupling reactions involving alkyl substrates, including a wide range of stereoselective transformations. In these reactions, nickel complexes supported by N-donor ligands have been proposed to involve paramagnetic Ni(I) and Ni(III) species as the active intermediates during catalysis. Herein, we report the use of a bulky 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN) ligand that allowed the detection and isolation of uncommon organometallic Ni(I) and Ni(III) complexes, which are involved in well-defined oxidative addition, transmetallation, and reductive elimination steps of the proposed catalytic cycle. Moreover, (iPr3TACN)Ni(II) complexes were shown to be efficient catalysts for the alkyl-alkyl Kumada cross-coupling, while a beneficial effect of acetonitrile on the yield of the desired cross-coupled products was observed, likely due to the stabilization of transient Ni(I) species. Overall, these studies provide unambiguous evidence for the involvement of both Ni(III) and Ni(I) organometallic species in Ni-catalyzed alkyl-alkyl cross-coupling reactions.