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As discussed in Section 2.2, the strong electrophilic character of amidyl and sulfamidyl radicals means that very effective 1,5‐HAT transpositions can be achieved.

A powerful example based on SET reductions can be found in the work by Wang, who reported the remote sp³ C–H allylation of amides (Scheme 2.8) [31]. In this case, the authors used the highly electron‐poor O‐aryl hydroxyamides 28 ( = −0.9 V vs. saturated calomel electrode [SCE]) that were previously identified as powerful electrophores for SET by the photoexcited state of the organic dye eosin Y (* = −1.1 V vs. SCE). This event led to a fragmentation across the weak N—O bond, thus delivering the key amidyl radical 29. A very facile 1,5‐HAT process was used to generate the distal carbon radical 30, which underwent allylation by reaction with a variety of sulfone reagents (e.g. 31). This step provided the product 32 with concomitant release of PhSO₂^·. The authors proposed a redox‐neutral photoredox process, whereby N,N‐diisopropylethylamine (DIPEA) closed the cycle by SET with the oxidized eosin Y. However, as quantum yield determination was not performed, a potential radical chain mechanism cannot be ruled out [32]. This reaction enabled the modification of secondary and tertiary centers and also displayed broad functional group compatibility.

The ability of amidyl radicals to undergo 1,5‐HAT has also been exploited as part of a strategy, leading to remote sp³ C–H arylated products 35. Yu and coworkers demonstrated that a site‐selective Minisci‐type reaction [33] can be achieved using the electron‐poor O‐acyl hydroxy‐amides 33 as amidyl radical precursors and several N‐heteroarenes 34 (Scheme 2.9) [34]. Upon visible light irradiation, the excited dye 3CzCIIPN promoted SET reduction of 33 (in analogy to the O‐acyl oximes), thus enabling access to the corresponding nitrogen radicals 36. A 1,5‐HAT process gave the carbon radical 37, which underwent addition to an activated heteroarene to give 38. The authors suggested that under basic conditions, a proton and electron transfer took place, thus leading to the desired product 39. This reactivity displayed remarkable functional group tolerance in terms of the compatible heterocycles and was used for the modification of biologically active systems.

Scheme 2.8 Remote C(sp³)–H allyation of amides using organic photocatalyst.

Source: Modified from Wu et al. [31].

Scheme 2.9 Site‐selective remote C(sp³)–H heteroarylation of amides.

Source: Modified from Chen et al. [34].

Scheme 2.10 Visible‐light‐promoted C(sp³)–H amidation and chlorination of N‐chlorosulfonamides.

Source: Modified from Quin and Yu [35].

The generation of amidyl radicals through photoinduced SET reduction is not restricted to electrophores embedded into a N–O systems but can also be performed on N‐Cl derivatives. This was demonstrated by Yu and coworkers in the HLF‐type reactivity of N‐Cl–N‐Ts amines 40 by using a heteroleptic Ir(III) photocatalyst under white light irradiation (Scheme 2.10) [35]. In this case, after generation of a very electrophilic sulfamidyl radical, the 1,5‐HAT took place, and then the corresponding carbon radical was chlorinated, most likely, as part of a radical chain propagation. This protocol has also been used in the assembly of many pyrrolidines 41 and also in late‐stage functionalizations of some bioactive materials, demonstrating the strong functional group compatibility.