General procedure for the synthesis of 4,7-dibromo-1H-indoles from 2,5-dibromonitrobenzene and vinylmagnesium bromide:
1. To a solution of 1,4-dibromo-2-nitrobenzene (18 g, 64.1 mmol) in tetrahydrofuran (THF, 200 mL) was slowly added vinylmagnesium bromide (1.0 M solution in THF, 199 mL, 199 mmol) at -40 °C (dry ice-acetonitrile bath).
2. The reaction mixture was stirred continuously at -40 °C for 1.5 hours.
3. the reaction was quenched with saturated aqueous ammonium chloride (NH4Cl) solution (500 mL) followed by stirring for 1 hour at room temperature.
4. pH was adjusted to 7 by adding 1 M hydrochloric acid (HCl).
5. Ethyl acetate (EtOAc) was added for extraction, the organic phase was separated, washed with brine, dried over anhydrous sodium sulfate, and concentrated under reduced pressure.
6. The crude product was purified by silica gel column chromatography (330 g silica gel) using ethyl acetate-hexane (5:95) as eluent.
7. The target fraction was collected and concentrated under reduced pressure to give a light yellow oil, which solidified to a solid on standing.
8. 4,7-dibromo-1H-indole (8.35 g, 47% yield) was finally obtained, LCMS analysis showed the main peak, no ionization was observed.
