Light-assisted functionalization of aryl radicals towards metal-free cross-coupling

The many synthetic possibilities that arise when using radical intermediates, in place of their polar counterparts, make contemporary radical chemistry research an exhilarating field. The introduction of photocatalysis has helped tame aryl radicals, leading to a resurgence of interest in their chemistry, and an expansion of viable coupling partners and attainable transformations. These methods are more selective and safer than classical approaches, and they utilize new radical precursors. Given the importance of sustainability in current organic synthesis and our interest in light-assisted metal-free transformations, this Review focuses on recent advances in the use of aryl radicals in photoinduced cross-couplings that do not rely on metals for the crucial bond-forming event, and it is structured according to the key step that the aryl radicals engage in. Our improved understanding of how to tame aryl radicals means they are now used in many transformations to access high-value products. This Review provides a summary of contemporary approaches towards the photogeneration of aryl radicals and their use in metal-free cross-coupling reactions. Visible light has been used for the generation of aryl radicals under mild reaction conditions utilizing a variety of strategies, from direct photoexcitation and single-electron transfer to electron donor-acceptor complexes and energy-transfer catalysis.Aryl radical intermediates have been used in photocatalytic, metal-free protocols for the construction of C(2)-C(2), C(2)-C(3) and C(2)-C() bonds.Photogenerated aryl radicals can engage in a number of different mechanistic pathways, such as addition to -systems, hydrogen-atom transfer and halogen-atom transfer.A large number of stable precursors has been used to generate aryl radicals under photocatalytic conditions, and efforts towards the design of new ones are ongoing.The photogeneration of aryl radicals may be coupled with other known approaches, such as organocatalysis, to further expand their synthetic utility and to unlock asymmetric reactions.