To a 250-mL three-necked reaction flask, 2.3 g of magnesium flakes were added and heated to dryness under a nitrogen atmosphere. Subsequently, 0.1 g of iodine, 12.8 g of bis(dimethylamino)chloroborane and 30 mL of tetrahydrofuran were added sequentially. A mixed solution consisting of 2.4 g of 2-chloro(trifluoromethyl)benzene and 20 ml of tetrahydrofuran was added slowly dropwise, and stirring was continued for 30 minutes after completion of the addition. Next, a solution consisting of 12.0 g of 2-chloro(trifluoromethyl)benzene and 100 ml of tetrahydrofuran was added slowly and the temperature of the reaction was maintained at 20-30°C. The addition process was completed in about 2 hours. After completion of the reaction, the mixed solution was cooled to 0°C and 50 ml of saturated ammonium chloride solution was added with continuous stirring for 1 hour. After standing and layering, the aqueous phase was extracted with 70 ml of ethyl acetate. The organic layers were combined, dried with magnesium sulfate and concentrated under reduced pressure. The concentrated residue was dissolved in 80 ml of hot ethyl acetate at 65°C and subsequently cooled to room temperature within 2 hours. 300 ml of n-hexane was added and stirring was continued for 3 h. Filtration gave 7.9 g of white solid 2-trifluoromethylphenylboronic acid in 52% yield.
