Evaluating the Effect of Catalyst Nuclearity in Ni-Catalyzed Alkyne Cyclotrimerizations

An evaluation of catalyst nuclearity effects in Ni-catalyzed alkyne oligomerization reactions is presented. A dinuclear complex, featuring a Ni-Ni bond supported by a naphthyridine-diimine (NDI) ligand, promotes rapid and selective cyclotrimerization to form 1,2,4-substituted arene products. Mononickel congeners bearing related N-donor chelates (2-iminopyridines, 2,2'-bipyridines, or 1,4,-diazadienes) are significantly less active and yield complex product mixtures. Stoichiometric reactions of the dinickel catalyst with hindered silyl acetylenes enable characterization of the alkyne complex and the metallacycle that are implicated as catalytic intermediates. Based on these experiments and supporting DFT calculations, the role of the dinuclear active site in promoting regioselective alkyne coupling is discussed. Together, these results demonstrate the utility of exploring nuclearity as a parameter for catalyst optimization.