General procedure for the synthesis of 4-bromo-1,3-dimethyl-1H-pyrazole from 1,3-dimethyl-1H-pyrazole: Bromine (2.0 mL, 40.80 mmol) was added slowly and dropwise to a solution of 1,3-dimethyl-1H-pyrazole (3.92 g, 40.80 mmol) in acetic acid (10 mL) at room temperature. The reaction mixture was heated to 70 °C and maintained for 1 hour. Upon completion of the reaction, it was cooled to room temperature and concentrated under vacuum to give the crude product. The pH of the crude product was adjusted with saturated sodium carbonate solution to 7~8. Subsequently, extraction was carried out with ethyl acetate (50 mL x 3), the organic phases were combined and dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated under vacuum to give the residue. The residue was purified by column chromatography (eluent: ethyl acetate/petroleum ether = 1/6 to 1/4) to afford the target compound 4-bromo-1,3-dimethyl-1H-pyrazole (3.30 g, yield: 46%) as an oil. The product was detected by ESI-MS and showed (M + H)+ peak located at 175.1. 1H NMR (400 MHz, CDCl3) data were as follows: δ 7.30 (s, 1H), 3.83 (s, 3H), 2.23 (s, 3H).
