(B) A solution of trimethyl borate (25.98 g, 0.25 mol, Fluka) and 2,4-difluorophenylmagnesium bromide (0.25 mol) in tetrahydrofuran (200 mL) was added dropwise simultaneously to a reactor containing tetrahydrofuran (125 mL) over a period of 1 h under nitrogen protection, maintaining the reaction temperature at -15 °C. The reaction was carried out by stirring the reaction mixture for 15 min. After the dropwise addition, the reaction mixture was continued to be stirred at -15°C for 15 minutes. Subsequently, the temperature of the reaction system was raised to 0 °C and water (17.5 mL) and 10% sulfuric acid solution (125 mL) were slowly added over 10 and 15 min, respectively. After the reaction was complete, water (250 mL) was added to ensure complete dissolution of the product, which was subsequently extracted with ether (3 x 150 mL). The organic phases were combined and concentrated to dryness under reduced pressure to afford the crude 2,4-difluorophenylboronic acid (36.98 g, 90% purity, 84.3% yield), which was used directly in the subsequent cross-coupling reaction without further purification.
