To a solution of N,N-dimethylformamide (DMF, 10 mL) of 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole (1.0 g, 5.2 mmol) was added cesium carbonate (2.5 g, 7.7 mmol) and ethyl iodide (1.2 mL, 15 mmol). The reaction mixture was stirred at room temperature for 12 hours. After the reaction was completed, the reaction mixture was diluted with water (30 mL). The aqueous phase was extracted with dichloromethane (DCM, 30 mL x 3). The organic layers were combined and dried over anhydrous sodium sulfate (Na2SO4) and subsequently concentrated under reduced pressure. The residue was purified by silica gel column chromatography using petroleum ether (PE)/ethyl acetate (EtOAc) (2:1, v/v) as eluent to afford the colorless oily product 1-ethyl-1H-pyrazole-4-boronic acid pinacol ester (880 mg, 77% yield). Mass spectrometric (MS) analysis (electrospray ionization, cation mode) showed m/z: 223.25 [M + 1]+.
