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Scheme 3.23 Electrochemical oxidative C–H amination of phenols.

      Source: Modified from Tang et al. [35].

3.3 Amination via N‐atom Nucleophilic Addition

As introduced in Section 3.2, construction of C—N bonds via N‐radical addition pathway relies on N‐centered radical (or radical cation) species, generated by oxidation of N—H bonds, as key intermediates. On the contrary, aminations via N‐atom nucleophilic addition pathway proceed through the oxidation of the C–H partners, and the amines usually serve as nucleophiles to attack the generated C‐centered cationic intermediates during the C—N bonds forming processes.

      3.3.1 Aromatic C(sp²)—H Bond Amination

      As documented in the literature precedents, direct oxidation of (hetero)arenes is feasible under photo‐ or electrochemical conditions, and the resulting radical cation intermediates could then be attacked by nucleophiles such as –NHR, –OH, and –CN⁻. Yet, among the electrochemical amination reactions via arene radical cation intermediates, methods directly employing simple N–H nucleophiles are still not common. Nevertheless, notable progress has been advanced via photoredox catalysis, wherein strongly nucleophilic azoles usually serve as the amine partners.

In 2015, a photocatalytic site‐selective CDC amination of unfunctionalized arenes with azoles was described by Nicewicz's group using an organic catalyst system consisting of an acridinium photocatalyst and a nitroxyl radical (Scheme 3.24) [37]. Upon irradiation with visible light, the photoexcited acridinium photocatalyst Mes‐Acr⁺* is able to directly oxidize arene 130 into the corresponding radical cation 134, which is subsequently attacked by the amine nucleophile 131 to form cationic radical 135. Deprotonation of 135 affords radical intermediate 136, which further undergoes aromatization via a TEMPO‐mediated HAT process to furnish the final product 133. Alternatively, intermediate 136 can also be trapped by O₂ to form 1,3‐cyclohexadienyl peroxyl radical 137, which then converts into product 133 after the internal elimination of the hydroperoxyl radical HOO^·. The closure of two catalytic cycles is indicated by the regeneration of ground‐state photocatalyst Mes‐Acr⁺ from the SET oxidation of Mes‐Acr^· by O₂ or HOO^·, and TEMPO from a HAT process between TEMPO‐H and O₂ or O‐radical species such as O₂^·− and HOO^·. In the scope exploration, a variety of aromatic compounds including complex drug‐like structures are firstly investigated, with the corresponding amination products obtained in moderate to good yields. A series of heterocyclic nucleophiles including pyrazoles, triazoles, tetrazoles, imidazoles, and benzimidazoles are proven as suitable coupling partners for the CDC amination with arenes. Particularly, reactions between a commercially available ammonium carbamate H₄N⁺H₂NCO₂⁻ (132) and various (hetero)arenes directly furnish primary aniline products.