Planar Cationic Imine Radical Iodide Charge Transfer Complexes: Key Intermediates for Iodine-(co)catalyzed Homogenous Asymmetric Hydrogenation of N-Arylimines

The mechanism of why both reactivity and enantioselectivity remarkably improves by adding an iodide and an acid in Ir-catalyzed asymmetric hydrogenation of N-arylimines is a decades’ mystery. And the excellent efficiency of iodine as a sole soft Lewis acid catalyst in the transfer hydrogenation of N-arylimines is also unclear at the molecular level. Via a systematic analysis of experimental data in reported small molecular iodine imine complexes and iodine-poly(Schiff base)s complexes, as well as iodine catalysis involved N-arylimines, planar cationic imine radical iodide charge transfer complexes were proposed as more reactive intermediates, in comparison with the twisted N-arylimines. Via the active species, a hydride transfer that occurs first in the outer-sphere mechanism was demonstrated to be more probable. The new understanding is insightful for the rational design of iodine catalysis and iodine additive for asymmetric catalysis.