Under ice-salt freezing conditions, a small amount of urea was added to colorless fuming nitric acid (50 mL, 1.20 M), and the mixture was cooled to -15 °C. Phenylated boric acid (9.02 g, 74 mmol) was slowly added over 1–2 hours to prevent the temperature from rising above -9 °C. The mixture was then stirred for another 15–30 minutes and poured onto ice, whereupon the product precipitated. The precipitated nitrophenylboronic acid was filtered under vacuum, recrystallized in a small amount of water, and decolorized charcoal was added. Hot vacuum filtration was then performed to remove the charcoal, followed by washing with water and then methanol. The original filtrate was cooled in an ice bath, neutralized with 12N NaOH to an orange/red solution, and then acidified with nitric acid. This solution was extracted with diethyl ether, washed with water, and concentrated at room temperature.
The two fractions from the post-processing were combined and separated by column chromatography (eluent C6H12:CH2Cl2; 80:20) to remove all trace impurities, yielding prismatic lemon crystals in 28% yield. 3-Nitrophenylboronic acid, 28% yield.
