General procedure for the synthesis of 1,3,5-trimethyl-1H-pyrazole-4-boronic acid pinacol ester from 3,5-dimethylpyrazole-4-boronic acid pinacol ester and iodomethane: 3,5-dimethyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole (500 mg, 2.25 mmol) was suspended in acetone (5 mL) in acetone (5 mL), potassium carbonate (622 mg, 4.5 mmol) and iodomethane (0.21 mL, 3.37 mmol) were added. The reaction mixture was heated to 60 °C and kept for 4.5 hours. After completion of the reaction, it was cooled to room temperature, water was added, the aqueous phase was separated and extracted three times with ethyl acetate (EtOAc). The organic phases were combined, washed with water, dried over anhydrous magnesium sulfate (MgSO4), and concentrated in vacuo to afford 1,3,5-trimethyl-1H-pyrazole-4-boronic acid pinacol ester (403 mg, 76%) as a light yellow oil.LCMS (ES+) m/z 237 (M+H)+, retention time (RT) 3.35 min (Method 1).
