Under nitrogen protection, 3,5-dibromochlorobenzene (27 g, 100 mmol) was dissolved in anhydrous isopropyl ether (300.0 mL) in a dry flask. The reaction mixture was cooled to -78 °C and stirred continuously under nitrogen atmosphere. A 2.6 M hexane solution of n-butyllithium (40 mL, 100 mmol) was slowly added dropwise to the above solution. After the dropwise addition was completed, the reaction mixture was continued to be stirred at -78 °C for 30 minutes. Subsequently, N,N-dimethylformamide (DMF, 7.5 g, 100 mmol) was added dropwise while keeping the temperature below -78 °C. After the reaction was completed, 80 mL of ammonium chloride solution was added to the mixture and stirring was continued for 15 minutes. The ether layer was separated, washed sequentially with water (2 x 250 mL), dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure. Finally, the crude product was purified by silica gel fast column chromatography to afford the target compound 3-bromo-5-chlorobenzaldehyde (16 g, 67% yield).
