At room temperature and under nitrogen protection, 6-bromo-2-methylquinoline (10 g, 45.0 mmol) was dissolved in 1,4-dioxane (100 mL), followed by the addition of selenium dioxide (6.00 g, 54.0 mmol). The reaction mixture was heated to 75°C and maintained at this temperature for 5 hours of reaction. Upon completion of the reaction, the mixture was filtered through a diatomaceous earth pad and the filter cake was washed with ethyl acetate. The filtrate was concentrated under reduced pressure and the resulting residue was ground with hexane to promote product precipitation. The solid product was collected by vacuum filtration and dried overnight under high vacuum to give the final 6-bromoquinoline-2-carbaldehyde (8.86 g, 37.5 mmol, 83% yield) as an off-white solid. The structure of the product was analyzed by 1H NMR (400 MHz, DMSO-d6) δ 10.11 (d, J=1.0 Hz, 1H), 8.56 (d, J=8.5 Hz, 1H), 8.43 (d, J=2.3 Hz, 1H), 8.14 (d, J=9.0 Hz, 1H), 8.05-8.02 (m, 1H), 8.02-8.00 ( m, 1H) and LCMS (ESI) m/z 235.9,237.9 [M+H]+ (calculated value C10H7BrNO 236.0) confirmed.
