Computational Study of the Ni-Catalyzed C-H Oxidative Cycloaddition of Aromatic Amides with Alkynes.

Abstract

The mechanism of Ni-catalyzed ortho C(sp2)–H oxidative cycloaddition of aromatic amides with internal alkynes containing 2-pyridinylmethylamine directing group was investigated using density functional theory (DFT) calculations. The C–H cleavage step proceeds via σ-complex-assisted metathesis (σ-CAM) with an alkenyl-Ni(II) complex. This is in contrast to the more common carboxylate/carbonate-assisted concerted metalation–deprotonation mechanism in related Ni-catalyzed C–H bond functionalization reactions with N,N-bidentate directing groups. In this reaction, the alkyne not only serves as the coupling partner, but also facilitates the σ-CAM C–H metalation both kinetically and thermodynamically. The subsequent functionalization of the five-membered nickelacycle proceeds via alkyne insertion into the Ni–C bond to form a seven-membered nickelacycle. This process proceeds with high levels of regioselectivity to form a C–C bond with sterically more encumbered alkyne terminus. This unusual regioselectivity is due to steric repulsions with the directing group that is coplanar with the alkyne in the migratory insertion transition state. The C–N bond reductive elimination to form the isoquinolone cycloadduct is promoted by PPh3 complexation to the Ni center and the use of flexible 2-pyridinylmethylamine directing group. The origin of the cis–trans isomerism of alkene byproduct was also explained by computations.