The unexpected formation and structure of 4,6-dimethylthieno[3,4-c] thiophene-1(3H)-thione

The reaction of 2,5-dimethylthiophene-3,4-dicarboxaldehye (1) with excess Lawesson's Reagent afforded the unexpected 5,5-fused ring thienothione derivative, 4,6-dimethylthieno[3,4-c]thiophene-1(3H)-thione (2), in good yield (79%). The desired non-classical thiophene, 1,3-dimethylthieno[3,4-c]thiophene (3), was observed by GC/MS as a minor product (approximately 5%) along with thione 2. The thienothione 2 was characterized spectroscopically and its structure was confirmed by X-ray crystallography. The formation of compound 2 is proposed to occur through a 1,3-hydride shift.