Step 1: Synthesis of 1-(difluoromethyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole
To a 100 mL round bottom flask was added 4-pyrazole boronic acid pinacol ester (1.0 g, 5.15 mmol), 18-crown-6 (0.27 g, 1.03 mmol) and anhydrous acetonitrile (25 mL). The mixture was stirred until a colorless homogeneous solution was formed and then sodium difluorochloroacetate (0.94 g, 6.18 mmol) was added. The reaction mixture was heated to reflux and held for 18 hours. After completion of the reaction, the mixture was cooled to room temperature. Insoluble solids were removed by filtration through a diatomaceous earth pad and the filter cake was washed with ethyl acetate (3 x 20 mL). The filtrate and washings were combined, filtered through a hydrophobic glassine and concentrated under reduced pressure to give a light yellow oily crude product. The crude product was purified by fast silica gel column chromatography (gradient elution, hexane to 33% ethyl acetate/isohexane) to afford 1-(difluoromethyl)-4-boronic acid pinacol ester-based-1H-pyrazole as a colorless solid (1.06 g, 84% yield). The mass spectrum (electrospray positive ion mode) showed the molecular ion peak (M+H)+ consistent with the target compound.
