A General Mechanism for the Copper- and Silver-Catalyzed Olefin Aziridination Reactions: Concomitant Involvement of the Singlet and Triplet Pathways

Abstract

The olefin aziridination reactions catalyzed by copper and silver complexes bearing hydrotris(pyrazolyl)borate (Tp(x)) ligands have been investigated from a mechanistic point of view. Several mechanistic probe reactions were carried out, specifically competition experiments with p-substituted styrenes, stereospecificity of olefins, effects of the radical inhibitors, and use of a radical clock. Data from these experiments seem to be contradictory, as they do not fully support the previously reported concerted or stepwise mechanisms. But theoretical calculations have provided the reaction profiles for both the silver and copper systems with different olefins to satisfy all experimental data. A mechanistic proposal has been made on the basis of the information that we collected from experimental and theoretical studies. In all cases, the reaction starts with the formation of a metal-nitrene species that holds some radical character, and therefore the aziridination reaction proceeds through the radical mechanism. The silver-based systems however hold a minimum energy crossing point (MECP) between the triplet and closed-shell singlet surfaces, which induce the direct formation of the aziridines, and stereochemistry of the olefin is retained. In the case of copper, a radical intermediate is formed, and this intermediate constitutes the starting point for competition steps involving ring-closure (through a MECP between the open-shell singlet and triplet surfaces) or carbon-carbon bond rotation, and explains the loss of stereochemistry with a given substrate. Overall, all the initially contradictory experimental data fit in a mechanistic proposal that involves both the singlet and the triplet pathways.

Chemoselective silver-catalyzed nitrene insertion reactions

“…Aziridine yields using the less expensive AgOTf/phen catalyst system were high for all substrates studied [14]. After establishing the aziridination activity of these catalysts, 4 Proposed mechanism for Ag(I)-catalyzed amination of alkenes invoking nitrene intermediates [13].…”

“…The mechanism of Ag(I)-catalyzed olefin aziridination was investigated by Perez to determine whether nitrene insertion proceeds via a concerted triplet radical pathway or a stepwise singlet pathway [13]. Initial studies were performed on a variety of Z-and E-olefins to determine the extent to which the stereochemistry of the olefin was eroded during the course of the reaction.…”

“…Although the results of these initial studies implied a concerted mechanism for Ag-catalyzed aziridination, the radical contribution indicated by the Hammett study suggested there was some radical character to the reaction pathway [13]. To clarify this issue, DFT calculations were carried out and the calculated free energy profiles for different points on the reaction coordinate in the aziridination of E-2-hexene showed that the most favorable transition state contained a triplet diradical, implying a stepwise pathway.…”

Silver-Mediated Direct sp3 C–H Bond Functionalization

“…In 2014, Tang and coworkers reported a silver-mediated difluorination of benzylic C-H bonds [40]. In this reaction, they used AgNO 3 as catalyst, Na 2 C for several hours. Moderate to excellent yields were achieved (42-93%, Scheme 13).…”

Substrate Specific Silver(I)-Catalyzed Cycloisomerization of Diene Involving Alkyl Rearrangements: Syntheses of 1,2,5,6-Tetrahydrocuminic Acid, <i>p</i>-Menth-3-en-7-ol, and <i>p</i>-Menth-3-en-7-al

“…In comparison with the catalytic reaction using a triple bond, the silver catalyzed reactions of simple alkene moieties are limited. [35][36][37][38][39][40][41][42] However, for example, the silver cation is often used in ion chromatography to separate a mixture of unsaturated fatty acids containing similar multiple double bond units. [43][44][45][46][47][48][49][50] Thus, we hypothesized that the coordination of the silver ion with two intramolecular alkenes might cause a novel cyclization under high temperature.…”