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As an example for the direct amination of benzylic C(sp³)—H bonds, Pandey's group disclosed a photocatalytic CDC amination of alkylbenzenes 196 with N‐methoxyamides 197 under metal‐free and external oxidant‐free conditions (Scheme 3.34) [48]. The 410 nm wavelength visible light source is obtained by using a combination of Pyrex and a CuSO₄:NH₃ solution filter from a 450 W Hanovia medium pressure lamp. Compound 9,10‐dicyanoanthracene (DCA) is employed as a photosensitizer, which facilitates an array of C(sp³)–H amination reactions between 196 and 197 with good functional group tolerance (198a–198f). Notably, this protocol is also applicable to intramolecular C—N bond construction to provide five‐membered rings (198g). The mechanistic proposal is depicted in Scheme 3.34b. First, the single‐electron oxidation of 197 by the photoexcited DCA* yields the N‐methoxyamide radical cation 199, which then converts into radical intermediate 200 after deprotonation. The following HAT process from the benzylic position of alkylbenzene 196 to radical 200 regenerates 197 and meanwhile gives benzylic radical 201. The subsequent oxidation of 201 generates benzylic cation 202, which further reacts with 197 to deliver the desired amination product 198. The results of control experiments and some experimental observations add credit to this putative mechanism. For example, the existence of N‐centered radical intermediate 200 is suggested by the formation of its dimeric product, while benzylic radical 201 is captured by TEMPO in a radical trapping experiment. To demonstrate the intermediacy of benzylic cation 202, the authors performed the model reaction using a mixture of MeCN/AcOH (9 : 1) as the solvent, which provides the desired 198 in diminished yield, along with the by‐product benzyl acetate in 10% yield.

Scheme 3.34 Visible‐light‐enabled direct benzylic C(sp³)–H amination.

Source: Modified from Pandey and Laha [48].

Alkyl‐substituted arenes with available benzylic C(sp³)—H bonds tend to undergo direct C–H functionalization at the benzylic position via the preliminarily formed arene radical cation intermediate upon single‐electron oxidation. In 2016, Pandey et al. developed a visible‐light‐mediated cross‐dehydrogenative amination of benzylic C(sp³)—H bonds with azole derivatives, employing an Ir‐based photocatalyst and bromotrichloromethane (BrCCl₃) as an oxidative quencher (Scheme 3.35) [49]. Upon photoexcitation, the excited catalyst (Ir^III*) first undergoes a single‐electron oxidation by BrCCl₃, affording the high‐valence catalyst Ir^IV and a trichloromethyl radical ^·CCl₃. Subsequently, arene substrate 203 is oxidized by the Ir^IV species to generate radical cation 206, along with the regenerated ground‐state Ir^III photocatalyst. Then, radical ^·CCl₃ as a competent H‐atom abstractor engages in a HAT process with intermediate 206 to provide benzylic cation 207, which further reacts with nucleophile 204 to deliver the final product 205. Moreover, it is possible to trap cation 207 with moisture to in situ generate the benzylic alcohol intermediate, which is prone to further oxidation to furnish the corresponding carbonyl compound.

Scheme 3.35 Benzylic C–H amination via visible‐light photoredox catalysis.

Source: Modified from Pandey et al. [49].