Butyl lithium (1.6 M in hexane, 3.75 mL) was slowly added dropwise to a stirred solution of 2-methyl-5-bromoanisole (5 mmol) in tetrahydrofuran (20 mL) at -78 °C for 45 min. Subsequently, triisopropyl borate (1.75 mL, 7.5 mmol) was added to the reaction system. The reaction mixture was allowed to slowly warm to -30°C, and then 2.5 N hydrochloric acid (10 mL) was added. The mixture was gradually warmed to room temperature with stirring and diluted with ethyl acetate. The organic and aqueous layers were separated and the aqueous layer was extracted twice with ethyl acetate. The organic layers were combined, washed with brine, dried over anhydrous sodium sulfate, filtered, and concentrated in vacuum. Purification of the crude product using fast column chromatography (elution gradient: 0-40% ethyl acetate/hexane) gave 3-methoxy-4-methylphenylboronic acid as a white powder.
