Photocatalytical and Photochemical Generation of Imidoyl Radicals: Synthetic Applications

Abstract

In the last few decades, the development in the field of radical chemistry has gained great attention. The majority of these radical strategies mainly depend on the in situ generation of reactive radical species by employing radical initiators such as oxidants and metal salts or methods such as photocatalysis and electrochemistry etc. This review covers the literature on the generation of imidoyl radicals using photoredox catalysis. In the majority of the reactions, the spontaneity of the underlying SET transitions is correlated by the compiled redox potential data of the reacting species for a better understanding of the readers. Besides, the applications of these reactive imidoyl radicals to access a variety of heterocycles such as benzothiazoles, indoles, quinoxalines, phenanthridines, isoquinolines, pyridines, and amides are also addressed.

Iron‐Catalyzed Oxidative Decabonylation/Radical Cyclization of Aliphatic Aldehydes with Biphenyl Isocyanides: A New Pathway For the Synthesis of 6‐Alkylphenanthridines

“…The tert ‐butoxyl radical abstracts the aldehyde hydrogen atom to provide the acyl radical A , and the acyl radical A undergoes decarbonylation to yield the alkyl radical B . Then, the addition of radical intermediate B to the biphenyl isocyanides 1 afforded the imidoyl radical C [11b] . Subsequently, the imidoyl radical C cyclized to the arene to form cyclized radical intermediate D .…”

The Dark Side of Isocyanides: Visible-Light Photocatalytic Activity in the Oxidative Functionalization of C(sp3)–H Bonds

“…36 The crude material (reaction time: 48 h) was purified by column chromatography (n-hexane/ethyl acetate 96:4) to give the product as an offwhite amorphous solid (6.9 mg, 39% yield). 1 H NMR (400 MHz, CDCl 3 ): δ 7.29−7.25 (m, 2H), 6.85 (t, J = 7.3 Hz, 1H), 6.74 (d, J = 8.0 Hz, 2H), 6.38 (br s, −NH), 3.73 (s, 2H), 2.98 (s, 3H), 1.33 (s, 9H); 13 N-((3s,5s,7s)-Adamantan-1-yl)-2-(methyl(phenyl)amino)acetamide (23). The crude material (reaction time: 48 h) was purified by column chromatography (n-hexane/ethyl acetate 9:1) to give the product as an off-white solid (6.7 mg, 28% yield).…”

“…Recently, with the advent of the visible-light photocatalysis era, − the somophile reactivity has been investigated in a plethora of transformations involving intramolecular cyclization of 2-isocyanobiphenyls and related analogues, ,, as well as in two- or multicomponent reactions leading to amides, keto-amides, and other interesting molecular scaffolds. − More in detail, these processes involve the formation of an imidoyl radical intermediate upon the addition of a radical species to isocyanide (Figure c). The imidoyl radical, depending on the reaction conditions and the species involved, can meet different fates such as oxidation to nitrilium ions, α-fragmentation (α-FGM) to give back isocyanide and the radical species, β-fragmentation (β-FGM) to nitrile (Figure a), and intramolecular interception of a radical acceptor (e.g., phenyl ring in the ortho -position of 2-isocyanobiphenyl, Figure b).…”

“…9−14 On the other hand, the presence of a lone pair on the (carbenic) isocyanide carbon atom makes them excellent geminal radical acceptors in reactions involving the formation of open-shell species (somophile reactivity, Figure 1c). 15−18 Recently, with the advent of the visible-light photocatalysis era, 19−22 the somophile reactivity has been investigated in a plethora of transformations involving intramolecular cyclization of 2-isocyanobiphenyls and related analogues, 16,17,23 as well as in two-or multicomponent reactions leading to amides, keto-amides, and other interesting molecular scaffolds. 24−26 More in detail, these processes involve the formation of an imidoyl radical intermediate upon the addition of a radical species to isocyanide (Figure 1c).…”

Rationally designed conjugated microporous polymers for efficient photocatalytic chemical transformations of isocyanides

“…5 In addition, isocyanides can function as radical acceptors to generate imidoyl radicals, which are fascinating reaction intermediates and usually lead to the formation of nitrogen heterocycles via radical cascade reactions. 6 Another typical reaction of isocyanides is the insertion reaction including insertion into heteroatom–hydrogen bonds, carbon–halogen bonds, carbon–hydrogen bonds, and metal carbenes. 7 In particular, isocyanide insertion into N–H bonds is a representative insertion reaction of isocyanides and was initially developed in the presence of copper catalysts in 1966, 8 but since then only a few studies have been reported on this reaction.…”