Hydrochloric Acid → 4-Bromo-2,3-dihydro-1H-indole

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A mixture of 4-bromooxindole (2.77 g, 0.01 mol) and a solution of BH3 in THF (2 M, 40 mL) was stirred at room temperature overnight. The mixture was cooled to 0° C. and diluted with 30 mL of methanol, followed by addition of 12 N HCl (7.5 mL). The resulting mixture was stirred at room temperature for 1 hour, adjusted to pH 8-9 with 10% aqueous NaOH. Water was added to the mixture and extracted with ethyl acetate (3×100 mL). The organic layer was dried and concentrated to get the crude product which was washed through a silica gel column (ethyl acetate in petroleum ether (10%). The crude product was dissolved in 10% HCl (3×10 mL). The aqueous layer was adjusted to pH7 with NaHCO3, extracted with ethyl acetate (3×20 mL). The organic layer was dried and concentrated to get 4-bromoindoline (1.16 g, 45%) as an oil. LCMS: 200 [M+1]+. 1H-NMR (400 MHz. DMSO-d6) δ 2.90 (t, J=8.8 Hz, 2H), 3.46 (t, J=8.8 Hz, 2H), 5.86 (s, 1H), 6.43 (m, 1H), 6.64 (m, 1H), 6.83 (t, J=8.0 Hz, 1H).

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