General procedure for the synthesis of 3-fluoro-4-methylphenylboronic acid from 2-fluoro-4-iodotoluene: In a 50 mL round-bottomed flask (equipped with a side arm, condenser, and stirring bar) triphenylphosphine (0.131 g, 0.5 mmol, 20 mol%), 2-fluoro-4-iodotoluene (0.585 g, 2.5 mmol), and triethylamine (1.78 mL, 12.5 mmol). 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 atmosphere. 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 mixture was heated to reflux and kept at reflux for 12 hours. Upon completion of the reaction, the mixture was cooled to 0 °C and methanol (6 mL, note: this is an exothermic reaction accompanied by hydrogen release) was slowly added. 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 washed with hexane (3 x 10 mL). The aqueous phase was cooled to 0°C (ice bath) and acidified with concentrated hydrochloric acid to pH ≤ 1, at which point 3-fluoro-4-methylphenylboronic acid precipitated as a white solid. The aqueous phase was extracted with ether (3 x 10 mL), the organic phases were combined, dried with magnesium sulfate and filtered. The solvent was removed by distillation under reduced pressure to give a white solid product.
