Under nitrogen protection, 1-bromo-2-chloro-4-nitrobenzene (57.4 g, 0.243 mol) was dissolved in tetrahydrofuran (THF, 500 mL) and cooled to -40°C. At this temperature, vinylmagnesium bromide (1 M solution of THF, 728 mL, 0.728 mol) was added slowly and dropwise. After the dropwise addition was completed, the reaction mixture was kept stirred at -40°C for 1 hr. Subsequently, the reaction was quenched by addition of aqueous ammonium chloride (NH4Cl) and gradually warmed to room temperature. The reaction mixture was extracted with ethyl acetate, the organic layers were combined, dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated. The crude product was purified by column chromatography (using 5-bromo-6-chloro-1H-indole as reference) to afford the target product 5-bromo-6-chloro-1H-indole (8.40 g, 15% yield).
