Step 1: Isopropylmagnesium chloride (19.2 mL, 1 M in tetrahydrofuran) was slowly added to a mixture of 1,2-dibromo-5-chloro-3-fluorobenzene (5 g, 17.5 mmol) in hexane (12 mL) and tetrahydrofuran (20 mL) at -45 °C. The reaction temperature was maintained at -40°C. Subsequently, N,N-dimethylformamide (6.4 g, 87.5 mmol) was slowly added dropwise to the above reaction mixture and stirred at the same temperature for 30 min. After completion of the reaction, the reaction was quenched with hydrochloric acid (15 mL, 2.0 M). The mixture was gradually warmed to room temperature and diluted with ethyl acetate (80 mL) to separate the organic phase. The organic phase was washed with saturated brine, filtered and the filtrate concentrated under reduced pressure. The residue was purified by fast column chromatography (petroleum ether/ethyl acetate = 10/1) to afford 2-bromo-4-chloro-6-fluorobenzaldehyde (3 g, yield: 73%) as a yellow solid.
