Theoretical Studies on Palladium-Mediated Enantioselective C-H Iodination.

DFT calculations have been carried out to investigate the reaction mechanism for Pd(II)-mediated enantioselective C-H iodination. Iodination of the aryl ortho C-H bond of benzylamines catalyzed by Pd(II) diacetate complexes in the presence of the L-MPAA ligand experiences three main steps: first, C-H bond activation; second, oxidative addition of iodine on Pd(II) and reductive elimination of iodobenzene; third, catalyst regeneration through ligand exchange. The C-H bond activation is found to be the rate-determining step of the overall iodination due to higher activation energy. The reaction barrier for the formation of iodinated (R)-benzylamine is lower than that of (S)-benzylamine, which confirms the R enantioselectivity in iodination at room temperature. The retainment of the coordination of one acetic acid on Pd(II) and the chelating MPAA ligand during the catalyzed reaction are suggested to give space economy to facilitate the C-H bond activation. The NHTf functional group on the substrate is found to be very important for ortho C-H iodination at ambient condition. Our calculated results are consistent with the experimental observations.