1,2-DITHIINS AND PRECURSORS - 18 - 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

Whereas 3,4-bis(p-dimethylaminophenyl)-1,2-dithiete (18) is in equilib rium with its ring-opened valence isomer 19/19', which can also exist as a solid, the analogously disubstituted 1,2-dithiin 20 is stable whe n protected from Light, but on exposure to light undergoes immediate r ing 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 synthes is of 20 is based on the nucleophilic addition of 1,10-dimethylethanet hiol to 1,4-bis(p-dimethylaminophenyl)butadiyne (23a) with ensuing fun ctional group conversion. An efficient variant of this method using et hoxycarbonylsulfenyl chloride as a deprotecting reagent is described, leading to the synthesis of 3,6-diphenyl-1,2-dithiin (27).