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.