3,6-Bis(p-dimethylaminophenyl)-1,2-dithiin versus 3,4-Bis(p-dimethylaminophenyl)-1,2-dithiete – A Mechanistic Probe for the Photoinduced Behavior of the 1,2-Dithiin System©

Abstract

Whereas 3,4‐bis(p‐dimethylaminophenyl)‐1,2‐dithiete (18) is in equilibrium with its ring‐opened valence isomer 19/19′, which can also exist as a solid, the analogously disubstituted 1,2‐dithiin 20 is stable when protected from light, but on exposure to light undergoes immediate ring transformation to 2,5‐bis(p‐dimethylaminophenyl)thiophene‐3‐thiol (30). To a lesser extent, it also undergoes sulfur extrusion to form 2,5‐bis(p‐dimethylaminophenyl)thiophene (29). The ring‐opened valence isomer 21/21′ and dihydrothiophene episulfide 28 are suggested as key intermediates. X‐ray crystallography of the disulfide 31 obtained from the main product 30 unequivocally confirms the structure. The synthesis of 20 is based on the nucleophilic addition of 1,1‐dimethylethanethiol to 1,4‐bis(p‐dimethylaminophenyl)butadiyne (23a) with ensuing functional group conversion. An efficient variant of this method using ethoxycarbonylsulfenyl chloride as a deprotecting reagent is described, leading to the synthesis of 3,6‐diphenyl‐1,2‐dithiin (27).