A 2M isopropylmagnesium chloride solution (10 mL, 20 mmol) was dissolved in 5 mL of tetrahydrofuran under ice bath conditions. To this solution was added a tetrahydrofuran solution of 1,3-dibromo-5-fluorobenzene (4.0 g, 15.70 mmol) and the reaction mixture was stirred for 2 hours. The reaction system was then warmed to 20°C and the reaction was continued for 30 minutes. After completion of the reaction, the system was cooled to 0°C, N,N-dimethylformamide (2.5 mL, 31.50 mmol) was slowly added dropwise and the reaction was stirred at this temperature for 1.5 hours. After that, the reaction mixture was stirred at room temperature for 12 hours. At the end of the reaction, the reaction was quenched by adding saturated ammonium chloride solution (20 mL). The reaction mixture was extracted with ethyl acetate (50 mL x 3), the organic phases were combined, dried over anhydrous sodium sulfate, filtered and the filtrate was concentrated under reduced pressure. Purification by silica gel column chromatography using eluent B afforded the target product 3-bromo-5-fluorobenzaldehyde (2.57 g, yellow liquid) in 81.0% yield.
