Photocatalytic Cross-Dehydrogenative Amination Reactions between Phenols and Diarylamines

Abstract

The direct intermolecular aryl C–N coupling reaction from precursors without preactivated C–H and N–H bonds has been challenging. Herein, an oxidative system combining a catalytic amount of organic photocatalyst with stoichiometric amount of persulfate was developed to enable the successful cross-dehydrogenative-coupling amination between phenols and acyclic diarylamines in a nonmetallic method. This protocol precludes both coupling partners from prefunctionalization and achieved single regioselectivity of amination products under genuinely simple and benign conditions. Broad scopes of substrates were evaluated with moderate to high efficacy, and the reaction efficiency of electron-deficient phenothiazine and phenol was highly improved. A radical/radical cross-coupling pathway was proposed based on mechanistic studies, wherein a radical chain propagation process was involved.

Mechanism of the Dehydrogenative Phenothiazination of Phenols

“…Indeed, as soon the hydrogen‐atom transfer step occurs at the phenol coupling partner, the resulting phenol radical would be captured by the excess amount of the accumulating persistent PTZ radical. In view of these considerations, the herein proposed mechanism might well be operative in some of the other variants of this reactions –. This high heterocoupling specificity may, moreover, facilitate the design of future radical cross‐dehydrogenative coupling methods.…”

“…[9] The latter,h owever,s uffer from the usual requirement for large amounts of Cu salts or onerous Pd salts (Buchwald-Hartwig reaction) [10] and, moreover,o ften require expensive and/ors ensitivel igand architectures. In 2015, we proposed an atom-and step-efficient direct cross-dehydrogenative [11] coupling pathway that enabled the direct N-arylation of phenothiazines with phenols [12,13] under an O 2 atmosphere [14] (Scheme 1). This reaction is unusual on many accounts, notably because of its metal-free, [15] halidefree, and preactivation-free character.M oreover,t he capture of oxidizedP TZs with phenols [16] seems to be as urprisinglyf avorable process.…”

“…Moreover, the capture of oxidized PTZs with phenols seems to be a surprisingly favorable process. In the present study, we explore the mechanism, of these reactions by utilizing both theoretical and experimental methods. The precise nature and intermediacy of the oxidized PTZ is investigated, as well as its reactivity with phenols.…”

Recent Advances in Photocatalytic C–N Bond Coupling Reactions

“…Xia's group reported another cross-dehydrogenative amination reaction between phenols and acyclic diarylamines using persulfate as the oxidant (Scheme 16). 22 The reaction was proposed to proceed via a chain propagation pathway to generate a phenoxenium radical. Afterwards, a radical-radical cross-coupling reaction would furnish the product.…”

Nature Inspires an Aerobic Coupling of Phenol and Amine

“…3 Visible-light-promoted versions have also been reported by Xia and colleagues. 4,5 However, these reactions worked only with phenothioazoles and diarylamines. A direct coupling of unprotected phenols with simple amines, including aliphatic primary and secondary amines, represents an unmet challenge in the gold rush of C-H functionalization reactions.…”