Using 2,2'-bithiophene (5.40 g, 32.4 mmol) as starting material, the reaction was stirred with N-bromosuccinimide (NBS, 11.57 g, 64.8 mmol, 2 eq.) in a solvent mixture of chloroform/acetic acid (1:1 v/v, 300 mL) for 4 hours at 70 °C. Upon completion of the reaction, the reaction mixture was diluted with dichloromethane (50 mL) and saturated aqueous sodium carbonate solution (100 mL). The organic phase was separated and the aqueous phase was extracted with dichloromethane (3 x 50 mL). The organic phases were combined, dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to give a light yellow solid. The solid was washed with acetone to give the pure product 5,5'-dibromo-2,2'-bithiophene (1a, 10.42 g, 98% yield). To further improve the purity, it was recrystallized from tetrahydrofuran (THF). The structure of the product was confirmed by the following characterization data: 1H NMR (DMSO-d6, 600 MHz) δ: 7.23-7.22 (d, J = 3.9 Hz, 2H), 7.15-7.14 (d, J = 3.9 Hz, 2H); 13C NMR (DMSO-d6, 100 MHz) δ: 136.86, 131.60, 136.86, 131.60, 135.22, 110.84 125.22, 110.84; MS (ESI+): m/z = 324.05; UV-Vis (DMSO) λmax = 325 nm; IR (ATR): 3069 cm-1 (Ar-H stretching), 1683 cm-1 (conjugated C=C stretching), 1416 cm -1 (R1-C=C-R2 cis stretching), 1293 cm-1 (C=C bending). Elemental analysis results: calculated C 29.65, H 1.24, S 19.79; measured C 30.03, H 1.40, S 18.13.
