General procedure for the synthesis of 3-chloro-1H-pyrazole from 3-aminopyrazole: Concentrated hydrochloric acid (20 mL) was slowly added to a solution of acetonitrile (600 mL) containing 1H-pyrazol-3-amine (20.0 g, 241 mmol), followed by the addition of copper(II) chloride (65.0 g, 481 mmol) at 0 °C. The reaction mixture was stirred at 0 °C for 30 min followed by dropwise addition of isoamyl nitrite (56.4 g, 481 mmol). The reaction mixture was continued to be stirred at room temperature for 2 days. After completion of the reaction, the reaction was quenched with 10% aqueous ammonia solution (1 L). The aqueous phase was extracted with ethyl acetate (5 x 500 mL) and the combined organic phases were washed with saturated brine, dried over anhydrous sodium sulfate, filtered and concentrated. The residue was purified by silica gel column chromatography (eluent: hexane/ethyl acetate=20:1) to afford the target product 3-chloro-1H-pyrazole (10.3 g, 42% yield) as a green oil.1H NMR (400 MHz, CDCl3): δ 12.84 (br s, 1H), 7.62 (s, 1H), 6.29 (s, 1H).
