31230-17-8

57097-81-1

S2. 1.9 g of 3-amino-5-methyl-1H-pyrazole (2), 15 mL of concentrated hydrobromic acid and 2.8 g of cuprous bromide were added to a 100 mL three-necked flask and heated to 70°C. About 1.5 g of sodium nitrite was dissolved in 5 mL of water and slowly added dropwise to the three-necked flask through a constant pressure funnel. After the dropwise addition was completed, the reaction mixture was stirred at 70°C for 30 minutes. After completion of the reaction, it was cooled to room temperature and 20 ml of tetrahydrofuran (THF) and 20 ml of water were added. The mixture was extracted three times with 30 mL of ethyl acetate. The organic layers were combined, washed with aqueous sodium thiosulfate, dried over anhydrous magnesium sulfate and silica gel filtered. Finally, the solvent was removed by distillation under reduced pressure to give 3-bromo-5-methylpyrazole (3) in 62% yield.