General procedure for the synthesis of 2-chloro-4-bromobenzaldehyde from (4-bromo-2-chlorophenyl)methanol: Oxalyl chloride (6.9 g, 54 mmol, 1.3 eq.) was dissolved in dichloromethane (DCM, 153 mL) and the solution was cooled to -78 °C. A solution of DCM (57 mL) in dimethyl sulfoxide (DMSO, 4.72 mL, 66.5 mmol, 1.6 eq.) was slowly added dropwise to the cooled solution at -78 °C and stirred for 15 min. (4-Bromo-2-chlorophenyl)methanol (Compound 43, 9.2 g, 41.5 mmol, 1.0 eq.) was dissolved in DCM (116 mL) and added drop-wise to the above cooled solution, keeping the temperature at -78 °C. Stirring was continued at -78 °C for 2 hours. Subsequently, triethylamine (TEA, 28.7 mL, 207 mmol, 5 eq.) was added to slowly warm the reaction mixture to room temperature. After stirring for 30 min at room temperature, the reaction mixture was diluted with DCM (300 mL), washed sequentially with saturated ammonium chloride (NH4Cl) solution and brine, dried with anhydrous sodium sulfate (Na2SO4), filtered and concentrated in vacuum to afford 2-chloro-4-bromobenzaldehyde (Compound 44) in 96% yield (8.8 g).
