General procedure for the synthesis of 2,3-dibromoquinoline from 3-bromoquinoline: To a stirred solution of 3-bromoquinoline (306 mg, 1.47 mmol) in dichloromethane (3 mL) was added m-chloroperoxybenzoic acid (m-CPBA, 358 mg, 85% max, 1.76 mmol) at 0 °C and the reaction was stirred overnight at room temperature. Additional dichloromethane (11.6 mL) was added to the reaction mixture. Phosphorus tribromide oxide (POBr3, 516 mg, 1.76 mmol) was added to the resulting solution at 0 °C, followed by dropwise addition of N,N-dimethylformamide (DMF, 57 μL, 0.74 mmol) under argon protection. The reaction mixture was gradually warmed to 25 °C and stirred for 2 h, after which additional dichloromethane (10 mL) was added. Saturated aqueous sodium carbonate solution was slowly added to the reaction mixture and the pH was adjusted to 7-8. The resulting mixture was separated and the aqueous phase was thoroughly extracted with dichloromethane. The organic phases were combined, washed with brine, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure. The crude product was purified by column chromatography (petroleum ether: ethyl acetate = 100:1) to afford 2,3-dibromoquinoline (335 mg, 79% yield) as a white solid.
