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

Abstract

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.

Factors in Determining the Chemo-Selective Reactions at Thymine in One-Electron Oxidation of DNA: Identification of an Intramolecular Proton Coupled Electron Transfer Pathway via a Full-Carbon Spin Centre Shift

“…It is generally a consensus 10,37 that the charge and spin density is largely located at guanine (G) radical cations and adenosine (A) radical cations. The stability and charge transfer property of the radical cation A or G is also consistent with the proposed stabilized N-aryl group containing planar cationic imine radical iodide charge transfer complexes being involved in multiple asymmetric hydrogenation reactions in my recent systematic mechanistic work 38 , and the very recent discovery 39 of these highly conductive, long organic wires fully consisting of N-aryl radical cations (Figure 2). Due to the fact 10 that the population of the thymine radical cations in an A/T base pair will be less than 1% of that at the adenine, a shorter distance between the hydrogen bondings of A base and T base would likely result in a more efficient charge transfer from A base to T base, facilitating a high reaction probability at the T base.…”

“…Either the cobalt (I) species or the cobalt (II) species (the existence of all three cobalt (0), cobalt(I) and cobalt (II) species was evidenced in the reported work 55 ) could be able to induce one-electron oxidation of enamide 46, possible forming a radical cation intermediate. A hydride addition to the radical cation like the proposed hydrogenation mechanism in my previous work 38 , would generate a captodative radical 48. A reduction of the radical in 48 by Zn dust (or cobalt (0), or cobalt(I)) and a subsequent proton abstraction resulted in chiral product 47, completing a catalytic cycle.…”

“…An exception 53 is the anti-Markovnikov selective addition of water to a steroid derivative, forming dehydroepiandrosterone (67% yield), in which a tertiary steric aliphatic carbon radical as a stabilized radical or a persistent radical 40,41 would be generated as a potential intermediate. My recent systematic study 38 on multiple hydrogenations of N-aryl imines with hydrogen gas also supporting a hydride addition to imine radical cation occurring first mechanism. Planar cationic imine radical iodide charge transfer complexes with a highly conjugated resonance structure 42 (Scheme 9) is the possible active intermediates.…”

“…5,6,10 , as well as the dihydroxyl products 39 51 , and the hydration products 40 53 . The excellent enantioselectivity was also demonstrated in asymmetric hydrogenation 38 via type I RCIA activation mode. Due to the reversible hydration, the enantioselective control can be very challenging for asymmetric hydration (Scheme 10A) via type II activation mode and no reported success has been confirmed to proceed via this mode, to my best of knowledge.…”

“…On the contrary, an increase of conjugation usually will increase the reactivity of an addition reaction by a nucleophile tutorially. My recent systematic study 38 on multiple hydrogenations of N-aryl imines with hydrogen gas also supported a hydride addition to imide radical cation 15 occurring first in the mechanism. It was also reported 42,43 that the separation and localization of a positive charge and an unpaired electron play the most important role as the controlling factors in the reactivities of the unimolecular isomerization and the oxidation of stilbene derivatives via radical cations.…”