General method: synthesis of 1-benzyl-4-chloro-3,5-dimethyl-1H-pyrazole. In a 16 mL reaction flask equipped with a magnetic stirring bar, 1-benzyl-3,5-dimethyl-1H-pyrazole (196 mg, 1.05 mmol), water (0.7 mL) and ethyl acetate (0.3 mL) were added sequentially. Subsequently, sodium chloride (123 mg, 2 mmol) was added and the reaction flask was placed in a room temperature water bath to control the exothermic nature of the reaction. Next, Oxone (322 mg, 0.52 mmol KHSO5) was added and the reaction vial was sealed. The reaction was carried out under continuous vigorous stirring and the progress of the reaction was monitored by TLC (about 1 h) until the feedstock was completely consumed. Upon completion of the reaction, the remaining oxidant was reduced using solid sodium bisulfite until starch iodide paper tested negative. The reaction mixture was diluted with the addition of water (5 mL) and extracted with a 1:1 hexane/ether solvent mixture (3 x 5 mL). The organic phases were combined, dried with anhydrous magnesium sulfate and concentrated under reduced pressure to give the crude product. The crude product was purified by fast column chromatography (column size: 14×1 cm, eluent: 9:1 hexane/ethyl acetate) to afford pure 1-benzyl-4-chloro-3,5-dimethyl-1H-pyrazole as a light yellow oil (215 mg, 93% yield).
