Computational Study on Why and How of Nonconventional meta-C-H Arylation of Electron-Rich Arenes via Pd/Quinoxaline-Based Ligand/Norbornene Cooperative Catalysis.

By performing density functional theory (DFT) calculation, this work aims at understanding the nonconventional meta-C-H arylation reaction of electronic-rich arenes with aryl iodide via a Pd/quinoxaline-based ligand/norbornene cooperative catalysis. The reaction is indicated to be initiated either from the ortho-C-H carbopalladation to give the meta-monoarylation product via a sequence of subsequent steps, including norbornene insertion, meta-C-H activation, oxidative addition, and reductive elimination via the Pd(II)/Pd(IV)/Pd(II) redox cycle, norbornene extrusion, and protodepalladation, or from the para-C-H carbopalladation to form the meta-diarylation product via two sequential arylation processes following similar mechanisms. The initial carbopalladation process promoted by the ligand is characterized as the rate-determining step of the reaction. The calculated mechanism shows the distinct role of the norbornene as a transient mediator that enables the final C-H arylation at the same meta-position wherever the initial carbopalladation occurs at either ortho- or para-position. The Pd/ligand/norbornene cooperative catalysis is essential for achieving the exclusive meta-selectivity of the C-H arylation of electron-rich arenes.