Photocatalytic Aryl Radical Chemistry Through Proton-Coupled Electron Transfer via an Organic Photocatalyst

Aryl radicals serve as essential intermediates in contemporary organic synthesis, facilitating diverse carbon-carbon and carbon-heteroatom bond-forming reactions under mild conditions. Aryl halides, widely available and frequently employed as precursors for photocatalytic aryl radical generation, typically require photocatalysts with strong reducing capabilities to overcome their high reduction potentials. Herein, we report an innovative approach for the photocatalytic generation of aryl radicals using an organic photocatalyst 5,5'-bis((4-methoxyphenyl)ethynyl)-2,2'-bibenzo[d]thiazole (dBAP) that leverages the proton-coupled electron transfer (PCET) mechanism. This strategy allows the selective generation of aryl radicals at the ortho and para positions of aryl halides bearing PCET handles, despite dBAP's intrinsic reducing power being insufficient for direct electron transfer to the aryl halides, in contrast to the prevalent approaches. Mechanistic investigations highlight the role of PCET in lowering the activation barrier for aryl radical formation, enabling efficient C-H, C-C, and C-B bond-forming reactions with high regioselectivity and functional group tolerance. This work underscores the potential of PCET-driven photocatalysis using organic photocatalysts as a sustainable and versatile platform for expanding the synthetic utility of aryl radicals under ambient conditions.