4-Fluoro-2-methoxyphenylboronic acid was synthesized as follows: 2-bromo-5-fluoroanisole (5.9 g, 29 mmol) and anhydrous tetrahydrofuran (50 mL) were added to a dry round-bottomed flask under argon protection. After cooling the reaction system to -78°C, n-butyllithium (17.4 mL, 43.5 mmol) was added slowly and dropwise. After stirring the reaction at -78°C for 30 minutes, trimethyl borate (4.9 mL, 43.5 mmol) was added, followed by a slow warming of the reaction mixture to room temperature (~1.3 hours). After completion of the reaction, the mixture was cooled to 0°C and acidified by adding 10% aqueous hydrochloric acid (20 mL). Tetrahydrofuran was removed by distillation under reduced pressure and saturated saline (50 mL) was added. The aqueous phase was extracted twice with ether. The organic phases were combined and washed sequentially with 10% aqueous hydrochloric acid and saturated saline. The organic layer was dried over anhydrous magnesium sulfate and concentrated under reduced pressure to give the yellow solid product 4-fluoro-2-methoxyphenylboronic acid (4 g, 82% yield). The product was characterized by 1H NMR (400 MHz, acetone-d6): δ 7.84 (t, 1H, J = 8 Hz, C6-H), 6.85 (dd, 1H, J = 12.2 Hz, C3-H), 6.76 (td, 1H, J = 8, 2 Hz, C5-H).
