Photochemical transformation of S-aryl 2-benzoylbenzothioates to 3-phenyl-3-arylthiobenzofuranones involving aryl migration

Photochemical reactions of S-aryl 2-benzoylbenzothioates 1 in solution media have been investigated, These thioesters undergo photoinduced cyclization to 3-aryl-3-arylthioisobenzofuranones 2 in yields of 14-46% upon direct irradiation. The photocyclization reaction is accompanied by a subsequent homolytic cleavage reaction of the isobenzofuranone leading to the dihydroisobenzofuranone dimer, (+/-)- and meso-3,3'-dioxo-1,1'-diphenyl-1,1',3,3'-tetrahydro-1,1'-bi(isobenzofuranyl) 5. The formation of the dihydroisobenzofuranone 2 can be explained by a stepwise mechanism involving (i) intramolecular cyclization to a zwitterionic intermediate and (ii) subsequent aryl migration. Furthermore, sensitization and quenching studies reveal that the photochemical process occurs from the singlet excited states of the thioesters 1.