To a solution of 1-methylpyrazole (4.1 g, 50 mmol) in tetrahydrofuran (THF, 100 mL) was slowly added n-butyllithium (n-BuLi, 2.2 M solution in THF, 55 mmol) at 0 °C. The reaction mixture was stirred at room temperature for 1 hour and then cooled to -78 °C. Subsequently, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (12.3 mL, 60 mmol) was added to the reaction system. The reaction was carried out at -78 °C for 15 min and then gradually warmed up to 0 °C, taking 1 hour. Upon completion of the reaction, the reaction was quenched with saturated ammonium chloride (NH4Cl) solution and extracted with dichloromethane (DCM). The organic phases were combined, washed with distilled water (2 × 100 mL), dried over anhydrous sodium sulfate (Na2SO4), and concentrated under reduced pressure to afford the brown solid product 1-methyl-1H-pyrazole-5-boronic acid pinacol ester (8.0 g, 77% yield), which could be used for the subsequent reaction without further purification. The product was detected by liquid chromatography-mass spectrometry (LCMS, ES), showing m/z 127 (M+H)+ [RB(OH)2]; nuclear magnetic resonance hydrogen spectroscopy (1H NMR, CDCl3, 400 MHz) showed δ 7.57 (s, 1H), 6.75 (s, 1H), 4.16 (s, 3H) and 1.41 (s, 12H).
