radicalphotoredox

Radical photoredox, or radical photoredox catalysis, is a class of chemical transformations that use visible light to excite a photocatalyst in order to generate radical intermediates through single-electron transfer events. This approach enables many radical reactions under mild conditions, expanding the range of feasible substrates and enabling difficult bond-forming processes without harsh reagents or high temperatures.

In typical radical photoredox cycles, a photocatalyst absorbs light and reaches an excited state capable of oxidizing or reducing substrates to form radical species, while the catalyst is simultaneously transformed to a counter redox state. The catalyst is then regenerated by a sacrificial electron donor or acceptor, completing the catalytic cycle. Some systems proceed via energy transfer instead of electron transfer, and others involve radical chain processes that propagate beyond the initial photochemical event. A wide variety of photocatalysts are used, including transition-metal complexes such as Ru(bpy)3^2+ and Ir(ppy)3, as well as organic dyes like eosin Y and 9,10-dicyanoanthracene.

Applications span carbon–carbon and carbon–heteroatom bond formations, including decarboxylative alkylations, arylations, cross-couplings with nickel co-catalysts, hydrofunctionalizations, and heteroatom functionalization (amines, sulfonyls, boronates). Radical additions to alkenes (Giese-type additions) and to carbonyl compounds are common motifs, enabling late-stage functionalization of complex molecules. While many systems operate under visible light with mild conditions and broad scope, limitations include sensitivity to oxygen and moisture, potential catalyst cost for metal-based systems, and, in some cases, the need for inert atmosphere. Ongoing developments emphasize metal-free organic photocatalysts and synergistic dual-catalysis to access new transformations.