The general procedure for the synthesis of 3-fluoro-2-thiophenecarboxylic acid from thiophene-2-carboxylic acid was as follows: 2-thiophenecarboxylic acid (1.7 g, 13.3 mmol) was dissolved in anhydrous tetrahydrofuran (30 mL), stirred under argon protection and the solution was cooled to -78 °C. Hexane solution of n-butyllithium (18.3 mL, 29.3 mmol) was slowly added to the above solution and stirring was continued at -78 °C for 30 min. Subsequently, a solution of tetrahydrofuran (30 mL) of N-fluorobenzenesulfonimide (5 g, 15.9 mmol) was added, and the reaction mixture was stirred at -78 °C for 4 hours, followed by a slow warming up to room temperature over 6 hours. Upon completion of the reaction, the reaction mixture was diluted with ether (100 mL), cooled to 0 °C, and the reaction was quenched by the addition of 1N hydrochloric acid (15 mL) to form a two-phase mixture. The aqueous layer was separated and washed with ether (3 x 50 mL). The organic layers were combined, dried with anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to give an orange colored oil. The oil was purified by silica gel column chromatography using 1:1 hexane/ethyl acetate as eluent to give 3-fluoro-2-thiophenecarboxylic acid (0.777 g, 40% yield) as a light brown solid. Thin layer chromatography (1:1 ethyl acetate/hexane) Rf value was 0.17; 1H NMR (CDCl3) δ 10.7 (s, 1H), 7.53 (dd, J = 5.4, 3.6 Hz, 1H), 6.89 (d, J = 5.4 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 166.2 (d, J = 3.5 Hz), 161.5 (d, J = 3.5 Hz) 161.5 (d, J = 278 Hz), 132.0 (d, J = 10 Hz), 118.9 (d, J = 24.7 Hz), 113.6; 19F NMR (282 MHz, CDCl3, CFCl3) δ -65.2 (d, J = 6 Hz); and EI-MS m/e 145.9838 (the calculated value for C5H3FO2S is 145.9838).
