General procedure for the synthesis of 4-bromo-2-cyanobenzaldehyde from 5-bromo-2-(bromomethyl)benzonitrile: To a stirred solution of trimethylamine N-oxide dihydrate (16.98 g, 152.8 mmol) in DMSO (60 mL) was added 5-bromo-2-(bromomethyl)benzonitrile (from step 1, 10.5 g, 38.19 mmol) in DCM (30 mL) to the solution in DCM. The resulting mixture was added slowly at 5 °C, followed by warming to 22 °C and continued stirring for 6 hours. After completion of the reaction, the reaction mixture was diluted with DCM (70 mL), washed sequentially with brine (20 mL × 3), dried over anhydrous Na2SO4, filtered and concentrated in vacuum. The residue was purified by silica gel column chromatography (eluent: 0-16% EtOAc in petroleum ether solution) to afford 4-bromo-2-cyanobenzaldehyde (4.2 g, 52% yield) as a white solid.1H NMR (400 MHz, CDCl3) δ 10.31 (s, 1H), 7.98 (s, 1H), 7.92 (s, 2H).
