4-Hydroxybenzonitrile (III) (5.0 g, 0.042 mol) was dissolved in acetonitrile (50 mL) under nitrogen atmosphere. The reaction mixture was cooled to -15 °C and trifluoromethanesulfonic acid (CF3SO3H) (3.7 mL, 6.3 g, 0.042 mol) was slowly added. Maintaining the reaction temperature at -15 °C, N-bromosuccinimide (NBS) (8.2 g, 0.046 mol) was added in 6 additions in batches. After addition, the reaction mixture was returned to room temperature and continued to be stirred under nitrogen atmosphere for 4 h. The reaction process was monitored by thin-layer chromatography (TLC) (unfolding reagent: hexane/ethyl acetate = 75:25, detection method: UV and KMnO4 color development). When the raw materials were completely consumed, the reaction mixture was diluted with aqueous sodium carbonate (Na2CO3) and extracted with methyl tert-butyl ether (MTBE) (3 x 50 mL). The organic layers were combined, dried with anhydrous sodium sulfate, and concentrated under reduced pressure to remove the solvent to give 3-bromo-4-hydroxybenzonitrile (IV) as a white solid (7.7 g, 93% yield).1H NMR (DMSO-d6, 300 MHz, 300 K): δ=8.04 (d, J=2.2Hz, 1H), 7.63 (dd, J=2.2Hz, 8.5Hz, 8.5 Hz. 1H), 7.04 (d, J=8.5Hz, 1H).
