Theoretical Mechanistic Study of Nickel(0)/Lewis Acid Catalyzed Polyfluoroarylcyanation of Alkynes: Origin of Selectivity for C–CN Bond Activation

The
cooperative mechanism of Ni(0)/Lewis acid catalyzed carbocyanation
of alkyne with 2,3,5,6-tetrafluorobenzonitrile was investigated using
the DFT method. Our calculations indicate that the most feasible catalytic
cycle consists of the oxidative addition of a C–CN bond to
the Ni(0) center, alkyne insertion into the C­(aryl)–Ni­(II)
bond, and reductive elimination. Notably, the Lewis acid (LA) interacting with the cyano nitrogen atom of the substrate can have
a significant effect on activating the C–CN bond while suppressing
C–H and C–F bond activations. The origin of lower C–CN
activation barrier in the presence of LA can be attributed
to the remarkably enhanced charge transfer (CT) amount from the Nicat 3d orbital to C-CN σ* + π* antibonding molecular
orbital and the little decrease of interaction energy between Ni-catalyst
and substrate. In the C–F and C–H bond activations in
the presence of LA, on the contrary, the significant
decrease of interaction energy between Ni-catalyst and substrate and
almost no change of charge transfer amounts are the origin of the
larger bond activation barrier. Thus, LA is essential
to make the C–CN bond weaker than other bonds, which agrees
well with the experimental observation. Electronic processes as well
as interaction energy analyses are discussed in detail.