Читать книгу Methodologies in Amine Synthesis - Группа авторов - Страница 46

In 2017, Adimurthy's group developed a regioselective C–H amination of heteroarenes with azoles using acridinium salt Mes‐Acr⁺ClO₄⁻ as the photocatalyst under visible light irradiation (Scheme 3.43) [57]. Quinoline amides 253 and imidazopyridines 254 serve as eligible substrates, which undergo regioselective amination at C5 and C3 position, respectively, with various azoles 255. Taking the amination of 253 as an example, it is proposed that the transformation proceeds through cross‐coupling of N‐radical cation 258 and C‐radical species 260 on the basis of a series of mechanistic investigations.

Scheme 3.43 Selective C–H amination of heteroarenes with azoles via an organic photoredox system.

Source: Modified from Samanta et al. [57].

More recently, Lei and coworkers disclosed an electrochemical C‐3 amination of imidazo[1,2‐a]pyridines 262 with amines 263 via an oxidative C–H/N–H cross‐coupling (Scheme 3.44) [58]. The reaction is supposed to proceed via a cross‐coupling process of electrochemically generated C‐ and N‐radicals (Scheme 3.44b). Additionally, an intramolecular C–N coupling through a Cp₂Fe‐mediated sequence of N‐radical formation and addition to the adjacent unsaturated moiety was also reported in the same article for the synthesis of 10H‐benzo[4,5]imidazo[1,2‐a]indole derivatives 266 from 265.

One more electrochemical C–H azolation was next developed by Feng and Chen and coworkers in 2019, wherein phenol and aniline derivatives are regioselectively aminated with azoles via a radical–radical cross‐coupling pathway (Scheme 3.45) [59]. During the investigation of the substrate scope, high ortho‐selectivity to the –XH group of this azolation is observed, and the sterically less‐hindered ortho‐position is preferred when asymmetrical phenols are applied (273b). In light of the mechanistic studies and reported work, the reaction mechanism is believed to involve a radical/radical cross‐coupling of 275 and 276, which are generated from anodic oxidation of substrates 271 and 272.

Scheme 3.44 Electrochemical oxidative C–H/N–H cross‐couplings for C—N bond formation with hydrogen evolution.

Source: Modified from Yu et al. [58].