The straightforward capture of oxidized phenothiazines with phenols under aerobic conditions represents a unique cross‐dehydrogenative C−N bond‐forming reaction in terms of operational simplicity. The mechanism of this cross‐dehydrogenative N‐arylation of phenothiazines with phenols has been the object of debate, particularly regarding the order in which the substrates are oxidized and their potentially radical or cationic nature. Understanding the selective reactivity of phenols for oxidized phenothiazines is one of the key objectives of this study. The reaction mechanism is investigated in detail by utilizing electron paramagnetic resonance spectroscopy, cyclic voltammetry, radical trap experiments, kinetic isotope effects, and solvent effects. Finally, the key reaction steps are calculated by using density functional theory (DFT) and broken‐symmetry open‐shell singlet DFT methods to unravel a unique biradical mechanism for the oxidative phenothiazination of phenols.
In a highly specific intermolecular dehydrogenative C−N bond forming process, phenols can be coupled to phenothiazines under mildly oxidizing reaction conditions. Since the serendipitous discovery of this click‐like dehydrogenative coupling reaction in 2015, several research groups have independently studied the scope of this reaction, some even performing the coupling at room temperature. This work focuses on the radical‐type mechanism of this operationally simple C−N coupling protocol. For more information, see the Full Paper by B de Bruin, F. W. Patureau, et al. on page 11936 ff.
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