Arylthianthrenium salts for triplet energy transfer catalysis

Sigma bond cleavage through electronically excited states allows synthetically useful transformations with two radical species. Direct excitation of simple aryl halides to form both aryl and halogen radicals necessitates UV-C light, so undesired side reactions are often observed, and specific equipment is required. Moreover, only aryl halides with extended pi systems and comparatively low triplet energy are applicable to synthetically useful energy transfer catalysis with visible light. Here we show the conceptual advantages of arylthianthrenium salts (ArTTs) for energy transfer catalysis with visible light in high quantum yield as compared to conventional aryl(pseudo)halides, and their utility in arylation reactions of ethylene. The fundamental advance is enabled by the low triplet energy of ArTTs that may originate in large part from the electronic interplay between the distinct sulfur atoms in the tricyclic thianthrene scaffold, which is neither accessible in simple (pseudo)halides nor other conventional sulfonium salts.