The synthesis of 3,4-diamino-2,5-dibromopyridine was improved based on the method reported in literature [18]. The procedure was as follows: in a 250 mL three-necked round-bottomed flask equipped with a magnetic stirrer, pyridine-3,4-diamine (2 g, 18.3 mmol) was mixed with 48% aqueous hydrobromic acid (30 mL). After heating the mixture to 100 °C, bromine (2.5 mL) was added slowly and the reaction solution was subsequently stirred at 135 °C for 5 hours. After completion of the reaction, the mixture was cooled to room temperature and a yellow precipitate was obtained by adding aqueous sodium thiosulfate solution, aqueous sodium carbonate solution and distilled water sequentially. The precipitate was separated by filtration and washed three times with distilled water. Finally, recrystallization was carried out using a solvent mixture of toluene and tetrahydrofuran (5:1, v/v) to give a white flocculent solid in 50% yield. The structure of the product was confirmed by 1H NMR (DMSO, 400 MHz, ppm) and 13C NMR (DMSO, 101 MHz, ppm): 1H NMR δ = 7.53 (s, 1H), 5.99 (s, 2H), 5.03 (s, 2H); 13C NMR δ = 139.93, 139.13, 129.54, 126.67 , 106.22 (see Figure S1 in the Supporting Information for details).
