Amination of Benzylic CH Bonds by Arylazides Catalyzed by Co^II–Porphyrin Complexes: A Synthetic and Mechanistic Study

Abstract

Co(II)-porphyrin complexes catalyze the reaction of aromatic azides (ArN(3)) with hydrocarbons that contain a benzylic group (ArR(1)R(2)CH) to give the corresponding amines (ArR(1)R(2)C-NHAr). When at least one of the R substituents is hydrogen, the catalytic reaction proceeds further to give the imine ArRC=NAr in good yields. The reaction mechanism has been investigated. The reaction proceeds through a reversible coordination of the arylazide to the Co(II)-porphyrin complex. This unstable adduct can either react with the hydrocarbon in the rate-determining step or decompose by a unimolecular mechanism to afford a putative "nitrene" complex, which reacts with more azide, but not with the hydrocarbon, to afford the byproduct diaryldiazene. The kinetics of the catalytic reaction have been investigated for a range of azides and substituted toluenes. Arylazides with electron-withdrawing substituents react at a faster rate and a good correlation is found between the log(k) and the Taft parameters. On the other hand, an excellent correlation between the logarithm of the rate for substituted toluenes relative to that of toluene and a radical parameter (sigma*JJ) alone was found, with no significant contribution by polar parameters. An explanation has been proposed for this anomalous effect and for the very high isotopic effect (k(H)/k(D)=14) found.

The (Porphyrin)ruthenium‐Catalyzed Aziridination of Olefins Using Aryl Azides as Nitrogen Sources

“…Any attempt to spectroscopically observe the species [(ArN 3 )Ru(TPP)(CO)] failed. Evidence that an imidometal complex is not always the group transfer reagent has already been provided by the amination of benzylic groups and by the aziridination of styrenes cata-lyzed by [Co II (porphyrin)] [55,56] and [Mn III (corrole)] [64] complexes, respectively.…”

“…Moreover, it is important to point out that the aryl group on the aziridine nitrogen atom should be considered as part of the molecule that can play an important role in further aziridine rearrangements. [53,54] Aryl azides react with benzylic C-H bonds to form amines and imines, [55,56] with conjugated dienes to give Naryl-2-vinylaziridines [54] and with styrenes to form N-arylaziridines. [57] The amination reactions of benzylic com-pounds are catalyzed by [Co(porphyrin)] complexes and the aziridination reactions are efficiently catalyzed by [Ru(porphyrin)CO] complexes.…”

Metal‐Free Conversion of Methane and Cycloalkanes to Amines and Amides by Employing a Borylnitrene

“…[10] Only very electrophilic nitrenes yield the C À H insertion product with hydrocarbon solvents in appreciable amounts. [11][12][13][14] The current state of the art [15] in the amidation of C À H bonds thus relies on nitrene surrogates, for example iminoiodanes PhI=NR [16] and aryl azides, [17] in transition-metalmediated reactions, although metal-free conversions are also known. [18] The groundbreaking work goes back to Breslow and co-workers, [19] who demonstrated in 1982 that cyclohexane can be amidated in 3-6 % yield based on the iminoiodane in the presence of metal porphyrins.…”

Oxidation of p‐Cresol to p‐Hydroxybenzaldehyde with Molecular Oxygen in the Presence of CuMn‐Oxide Heterogeneous Catalyst

“…[21,22] In fact, we have reported that cobalt supported on carbon was a highly active and selective catalyst in the catalytic oxidation of o-cresol with molecular oxygen to form salicylaldehyde. [23] However, when the cobalt catalyst was adopted in the oxidation of p-cresol, the selectivity for p-hydroxybenzaldehyde was relatively low.…”