Under stirring conditions, 2,3-difluorobenzaldehyde (XVIII) (75.0 g, 528 mmol, 1.0 eq.) was dissolved in concentrated sulfuric acid (565 mL) and heated to 60°C. The mixture was then dissolved in a mixture of 2,3-difluorobenzaldehyde (XVIII) and 2,3-difluorobenzaldehyde (XVIII). Subsequently, N-bromosuccinimide (NBS) (113 g, 633 mmol, 1.2 eq.) was added in batches. The reaction mixture was stirred continuously at 60 °C for 12 h. The completion of the reaction was confirmed by LC/MS monitoring. The reaction mixture was slowly poured into a mixture of ice water (500 mL) and petroleum ether (500 mL) and stirred for 10 min before separating the organic layer. The organic layer was concentrated under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography (eluent: 100% petroleum ether) to afford 5-bromo-2,3-difluorobenzaldehyde (XIX) (120 g, 543.0 mmol, quantitative yield). The product was analyzed by electrospray ionization mass spectrometry (ESIMS) and the molecular formula was confirmed as C7H3BrF2O, m/z 221.1 ([M+H]+).
