DFT Study of Ni-Al Bimetallic-Catalyzed Selective meta-C(sp2)-H Alkenylation of Pyridine: Mechanism and Regioselectivity.

The mechanism of Ni-Al bimetallic-catalyzed selective meta-C(sp2)-H bond alkenylation of pyridine has been investigated using DFT calculations. The reaction constructs macrocyclic intermediates using a bifunctional NHC ligand and a pyridine substrate, which selectively generates the meta-C(sp2)-H alkenylated pyridine products. Calculations show that the pyridine meta-C(sp2)-H bond activation involves a ligand-to-ligand hydrogen transfer (LLHT) mechanism, rather than a stepwise oxidative addition mechanism, and the rate-determining step of the reaction is the reductive elimination process. In addition, to disclose the regioselectivity for the C(sp2)-H bond alkenylation, the reaction mechanisms for the disfavored pyridine para-C(sp2)-H and ortho-C(sp2)-H alkenylation reactions have also been calculated. It was found that the rate-determining step for both the para- and ortho-substituted pathways is likewise the reductive elimination process. Studies reveal that the regioselectivity is governed by the stability of macrocyclic templates regulated by appropriate levels of ring strain.