General procedure for the synthesis of 3-thiophene boronic acid from 3-bromothiophene: Triphenylphosphine (0.131 g, 0.5 mmol, 20 mol%), p-iodoiodobenzene (0.585 g, 2.5 mmol), and triethylamine (1.78 mL, 12.5 mmol) were added sequentially to a 50 mL round-bottomed flask (equipped with a side arm, a condenser, and a stir bar). The reaction system was degassed by alternating vacuum and argon filling three times. Palladium dichloride (0.023 g, 0.13 mmol, 5 mol%) was added under positive argon pressure. After stirring for 15 minutes at room temperature, diisopropylaminoborane (5 mL, 1 M THF solution, 5 mmol) was added and the reaction mixture was again degassed by alternating vacuum and argon filling three times. The reaction solution was heated to reflux and kept at reflux for 12 hours. Upon completion of the reaction, the reaction solution was cooled to 0 °C and 6 mL of methanol was slowly added (note: this is an exothermic reaction, which releases hydrogen). After stirring for 15 minutes, all solvent was removed by distillation under reduced pressure to give a black solid. The solid was dissolved in 3M sodium hydroxide solution (8 mL) and subsequently washed with hexane (3 x 10 mL). The aqueous layer was cooled to 0°C (ice bath) and acidified with concentrated hydrochloric acid to pH ≤ 1, at which point 3-thiopheneboronic acid precipitated as a white solid. The aqueous layer was extracted with ether (3 x 10 mL), the organic phases were combined, dried with magnesium sulfate and filtered. Finally, the solvent was removed by distillation under reduced pressure to obtain 3-thiopheneboronic acid in white solid form.
