Hydrochloric Acid → 3,4-Diaminothiophene dihydrochloride

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Concentrated HCl (230 mL) and 2,5-dibromo-3,4-dinitrothiophene (11, 12.8 g, 38.0 mmol) were added into a flask, cooled in an ice bath, and mixed together. A metal tin was slowly added for 1 hour while maintaining a temperature of 25 to 30. After the addition was complete, the reactant was stirred until tin was consumed, and stored in a fridge for one night. A solid precipitate was vacuum filtered, collected and washed with diethylether and acetonitrile. The product 2H+ salt 12 is significantly stable; however, a free-amino product is significantly sensitive to oxidation. Due to the above reason, this precursor was generally stored as the product 2H+ salt. In the case of conversion to free-diaminothiophene, the product 2H+ salts was dissolved into water of 600 mL and cooled in an ice bath. This solution was basified with 4N Na2CO3, extracted with diethylether, dried with anhydrous sodium sulfate, followed by concentration in a rotary evaporator, thereby obtaining 3,4-diaminothiophene as white crystalline product (2.6 g, 61%). 1H NMR (CDCl3, 400 MHz, d/ppm): 6.16 (s, 2H, 2×CRS), d 3.36 (s, br, 4H, 2×NH2). 13C NMR (CDCl3, 100 MHz, d/ppm): 137.2, 101.7. C4H6N2S (114.17): Theoretical value C, 42.08; H, 5.30; N, 24.54; S, 28.09; Experimental value C, 42.18; H, 5.50; N, 24.66; S, 27.92.

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