Halogen Bonding-Mediated Radical Cyclisation of Aryl Halides

Aryl halides are convenient, stable precursors to synthetically versatile aryl radicals, but the generation of the latter typically requires either stoichiometric tin/silicon reagents or precious metal photocatalysts, in both cases typically under conditions of high dilution. Here, we show that tertiary amines can mediate reductive cyclisations of aryl halides under photochemical conditions by (i) facilitating generation of aryl radicals through formation of photo-active halogen bonding complexes, and (ii) acting as a source of mild terminal reductant. The absence of strong stoichiometric reductants allows the reactions to be conducted under untypically concentrated conditions without competing bimolecular aryl halide reduction, with associated benefits for productivity and sustainability metrics.