Step 1: Preparation of tert-butyl 4-bromo-1H-pyrazole-1-carboxylate
Di-tert-butyl dicarbonate (Boc2O, 2.34 mL, 10.2 mmol) was slowly added to a solution of 4-bromo-1H-pyrazole (1 g, 6.8 mmol) in acetonitrile (20 mL) at 0 °C. Subsequently, triethylamine (3.3 mL, 23.8 mmol) and 4-dimethylaminopyridine (DMAP, 0.166 g, 1.36 mmol) were added sequentially to the reaction mixture. The reaction mixture was stirred at room temperature for 2 hours. After completion of the reaction, the reaction mixture was diluted with water and extracted with ethyl acetate (100 mL × 3). The organic phases were combined, washed sequentially with water (100 mL) and saturated saline (100 mL), dried over anhydrous sodium sulfate, and concentrated under reduced pressure to remove the solvent. The crude product was purified by fast column chromatography (eluent: 10% ethyl acetate/hexane) to afford tert-butyl 4-bromo-1H-pyrazole-1-carboxylate (1.65 g, 98% yield) as a colorless liquid.
1H NMR (400 MHz, CDCl3) δ 8.09 (s, 1H), 7.66 (s, 1H), 1.65 (s, 9H).
