Isoquinoline (13a) (1.83 mL, 15.50 mmol) was slowly added dropwise to concentrated sulfuric acid (17 mL) at 0 °C while maintaining stirring. The reaction mixture was cooled to -25 °C, followed by the addition of sodium borohydride (6.34 g, 35.65 mmol) in batches, with the rate of addition controlled to maintain the reaction temperature between -25 °C and -20 °C. The reaction was continued for 1 hour at this temperature. After the reaction was continued at this temperature for 1 h, the reaction mixture was allowed to warm up naturally to room temperature. The reaction mixture was carefully poured into crushed ice and the pH was adjusted with concentrated aqueous ammonia solution to 7.0. The resulting slurry was stirred at 0°C for 1 hr and subsequently filtered and washed with ice water. The crude product was air dried and purified by column chromatography to afford 5,8-dibromoisoquinoline (13b) (2.5 g, 56%). Mass spectrum (EI) m/z: 286 ([M+1]+). 1H NMR (400 MHz, DMSO-d6) δ: 9.48 (s, 1H), 8.78 (d, J = 6 Hz, 1H), 8.06 (d, J = 8 Hz, 1H), 7.98 (dd, J = 6 Hz, J = 8 Hz, 2H).
