In recent years, radical C–C bond cleavage reactions have been increasingly understood and used to perform transformations that complement traditional ionic processes. However, to date radical C–C bond cleavage/functionalization reactions have not been the subject of a dedicated review. Herein we summarize the most recent and significant developments in the radical activation and functionalization of carbon–carbon bonds, with an emphasis on both synthetic outcomes and reaction mechanisms, and highlight how these radical C–C bond cleavage reactions enable challenging transformations.

Cleavage of carbon-carbon bonds by radical reactions.

Visible-light photoredox catalysis enables easy access to acyl radicals under mild reaction conditions. Reactive acyl radicals, generated from various acyl precursors such as aldehydes, α-keto acids, carboxylic acids, anhydrides, acyl thioesters, acyl chlorides, or acyl silanes, can undergo a diverse range of synthetically useful transformations, which were previously difficult or inaccessible. This review summarizes the recent progress on visible-light-driven acyl radical generation using transition-metal photoredox catalysts, metallaphotocatalysts, hypervalent iodine catalysts or organic photocatalysts. 1 	Introduction 2 	The Scope of This Review 3 	Aldehydes as a Source of Acyl Radicals 4 	α-Keto Acids as a Source of Acyl Radicals 5 	Carboxylic Acids as a Source of Acyl Radicals 6 	Anhydrides as a Source of Acyl Radicals 7 	Acyl Thioesters as a Source of Acyl Radicals 8 	Acyl Chlorides as a Source of Acyl Radicals 9 	Acyl Silanes as a Source of Acyl Radicals 10 	Conclusions and Future Outlook

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Acyl Radical Chemistry via Visible-Light Photoredox Catalysis.

Photogenerated acyl/alkoxycarbonyl/carbamoyl radicals for sustainable synthesis

A mild and facile visible-light-mediated strategy has been proposed for the synthesis of 3-acyl quinoxalin-2(1H)-ones through acridine red catalyzed aerobic oxidative decarboxylative acylation of α-oxo-carboxylic acids with quinoxalin-2(1H)-ones at room temperature in air. Various 3-acyl quinoxalin-2(1H)-ones could be efficiently obtained in good yields with excellent functional group compatibility.

Visible-light-promoted acridine red catalyzed aerobic oxidative decarboxylative acylation of α-oxo-carboxylic acids with quinoxalin-2(1H)-ones

Visible-light-mediated metal-free decarboxylative acylations of isocyanides with α-oxocarboxylic acids and water leading to α-ketoamides

Carbamoyl Radicals via Photoredox Decarboxylation of Oxamic Acids in Aqueous Media: Access to 3,4-Dihydroquinolin-2(1 H)-ones.

Visible-Light-Induced α-Amino C–H Bond Arylation Enabled by Electron Donor–Acceptor Complexes

An efficient silver-catalyzed tandem decarboxylative radical addition/cyclization of oxamic acids with alkenes has been developed. This method provides a novel and straightforward protocol toward a variety of 4-aryl-3,4-dihydroquinolin-2(1H)-ones, 4-(α-carbonyl)-3,4-dihydroquinolin-2(1H)-ones, and quinolin-2(1H)-ones in aqueous solution.

Gaofeng Feng

Papers

Carbamoyl Radicals via Photoredox Decarboxylation of Oxamic Acids in Aqueous Media: Access to 3,4-Dihydroquinolin-2(1 H)-ones.

Visible-Light-Induced α-Amino C–H Bond Arylation Enabled by Electron Donor–Acceptor Complexes

Silver-Catalyzed Decarboxylative Radical Addition/Cyclization of Oxamic Acids with Alkenes towards Quinolin-2-ones