General procedure for the synthesis of 2-bromo-3-thiophenecarboxylic acid from 3-thiophenecarboxylic acid: 3-thiophenecarboxylic acid (2.00 g, 15.6 mmol) and anhydrous tetrahydrofuran (THF; 30 mL) were added to a pre oven-dried 100 mL round bottom flask. The reaction system was cooled to -78.0 °C and a 2.50 M hexane solution of n-butyllithium (12.6 mL, 31.5 mmol) was added slowly and dropwise under nitrogen protection. The reaction mixture was stirred at -78.0 °C for 30 min before adding bromine (0.86 mL, 16.4 mmol) dropwise. Stirring was continued at -78.0 °C for 1 h. The reaction system was then slowly warmed to room temperature. After stirring overnight, dilute 1M HCl solution (50.0 mL) was added to quench the reaction, followed by concentration of the reaction solution. The resulting mixture was extracted with ether and water and the organic layer was dried with anhydrous Na2SO4. The product was recrystallized twice by H2O/ether mixed solvent to give pale yellow needle-like crystals of 2-bromo-3-thiophenecarboxylic acid (2.13 g, 66.0% yield). The structure of the product was confirmed by 1H NMR (500 MHz, DMSO-d6): δ 7.63-7.62 (d, J=5.8 Hz, 1H), 7.32-7.31 (d, J=5.8 Hz, 1H).
