4-Carboxy-phenylboronic acid, concentrated sulfuric acid, and nitric acid were added to a three-necked flask equipped with a reflux condenser and a thermometer. The mixture was magnetically stirred to initiate the reaction. 4-Carboxy-phenylboronic acid began to dissolve at 103°C, and the temperature was then increased to 120°C for 2 hours. The reaction progress was monitored by thin-layer chromatography. After the reaction was complete, the mixture was cooled, filtered, and the filter cake was washed with diethyl ether, filtered again, and dried to obtain gray needle-like crystals of 4-carboxy-2-nitrophenylboronic acid.
