Thianthrene Radical Cation as a Transient<scp>SET</scp>Mediator: Photoinduced Thiocyanation and Selenocyanation of Arylthianthrenium Salts^†

Comprehensive Summary A novel transient SET mediator approach has been developed for the photoinduced radical‐radical cross coupling reaction. Using the in‐situ generated thianthrene radical cation as the transient SET mediator, the thiocyanation and selenocyanation of aryl thianthrenium salts have been realized under the mild conditions without the need for photocatalyst or single electron donor. In comparison with the photocatalyst enabled process, the protocol features mild conditions, simple manipulation, a broad substrate scope, excellent functional group and heterocycle tolerance. Due to the feasible accessibility of aryl thianthrenium salts, this method has also been applied in the efficient synthesis of a bioactive molecule, and the late‐stage functionalization of complex arenes.