N-Heterocyclic Nitreniums Can Be Employed as Photoredox Catalysts for the Single-Electron Reduction of Aryl Halides

Abstract

N-Heterocyclic nitrenium (NHN) salts, the analogues of N-heterocyclic carbenes, have attracted considerable interest. However, relatively little is known about their catalytic ability beyond their Lewis acid catalysis. Herein, we describe that NHNs can serve as catalytic electron acceptors for charge transfer complex photoactivations. We showcase that, under blue light irradiation, the NHN salts could catalyze the generation of aryl radicals from aryl halides.

Photoinduced N-Heterocyclic Nitrenium-Catalyzed Single Electron Reduction of Acyl Fluorides for Phenanthridine Synthesis

“…Note that no desired trapping adduct was observed without light (Figure C). According to these results and our previous study, one possible mechanism is proposed in Figure D. The ICTC NHN A can be excited by blue light irradiation to undergo a SET event, affording the NHN radical (pathway a).…”

“…We recently found that N-heterocyclic nitreniums (NHNs) could undergo a single electron transfer (SET) event in the presence of electron donors for the generation of NHN radicals, which enabled single electron reduction of Ar–X bonds (Figure B) . As part of our ongoing research in photoinduced NHN catalysis, we set out to explore the possibility of NHN-catalyzed photoreduction of acyl fluorides (Figure C).…”

Visible-Light Photocatalytic Reduction of Aryl Halides as a Source of Aryl Radicals

“…In addition, carbazolyl cyanophenyl (5CzBN) ( Figure 9 b) [ 43 ] could reduce activated aryl chloride to aryl hydride through photoinduced electron donor under visible light irradiation. Recently, Wang and Chen reported that under blue light irradiation, N -heterocyclic nitrenium (NHN) salt, as a catalytic electron acceptor for the photoactivation of charge transfer complexes, could catalyze the generation of aryl free radicals from aryl chlorine through single electron transfer in present of electron donor [ 44 ].…”

“…With the assistance of bases, 2,3,6,7-tetramethoxyanthrone anion (TMA − ) ( Figure 9 e) could reduce aromatic hydrocarbons without electron donors to the corresponding aromatic radical anion, which then reacted with CO 2 to obtain the corresponding carboxylic acid, providing a new opportunity for the valorization of aromatic hydrocarbons ( Figure 11 ) [ 48 ]. Guan and Shang found that o- phosphinophenol and o- phosphinothiophenol were photocatalysts with strong reduction ability, which could activate aryl chloride and bromide under visible light for boration, arylation and phosphorylation [ 44 ]. They also found that o- diphenylphosphine substituents could lead to narrow optical gaps and promote the cross entry into the triplet state between systems, thus promoting the role of phenolate and thiophenol as effective visible light photo redox catalysts.…”

Multimodal Reactivity of N–H Bonds in Triazanes and Isolation of a Triazinyl Radical

“…Stephan et al extended these structures to five-membered cyclic alkyl amino nitrenium ions ( B and C ), establishing their enhanced Lewis acidity compared to the corresponding triazolium species. All these findings opened a door to the emerging applications of these N-Lewis acids as non-innocent ligands, partners in the frustrated Lewis pair chemistry, and efficient catalysts for various transformations based on reduction, , electrophilic cyclization, and photoredox processes …”