General procedure for the synthesis of 1-phenyl-1H-pyrazole-4-boronic acid pinacol ester from 1-phenyl-1H-pyrazole-4-boronic acid pinacol ester: To a solution of 1-phenyl-1H-pyrazole-4-boronic acid pinacol ester (0.6 g, 3.1 mmol) in acetone/water (30 mL, v/v = 1:1) was added sodium periodate (NaIO4, 2.0 g, 9.3 mmol) and ammonium acetate (NH4OAc, 0.54 g, 7.1 mmol). The reaction mixture was stirred at room temperature overnight. After completion of the reaction, the mixture was concentrated to give a residue. The residue was purified by column chromatography (eluent: methanol/dichloromethane = 1:5) to afford the target product 1-phenyl-1H-pyrazole-4-boronic acid (0.3 g, 89% yield) as a light yellow solid. Mass spectrometry (MS) analysis showed a molecular ion peak (m/z) of 189 ([M+H]+).
