The general procedure for the synthesis of 2,4-difluoro-3-methylbenzaldehyde from 2,6-difluoro-3-bromotoluene and N,N-dimethylformamide was as follows: a solution of anhydrous THF (36 ml) of 2,4-difluoro-3-methylbromobenzene (2.000 g; 9.661 mmol) was cooled at -78 °C, and then a solution of hexane with 1.6 M n-BuLi ( 6.04 ml; 9.661 mmol), and the dropwise addition time was controlled within 10 min while keeping the reaction temperature below -70 °C. After the dropwise addition, the mixture was continued to be stirred at -78 °C for 2 min. Subsequently, anhydrous DMF (1.49 ml; 19.326 mmol) was added dropwise at the same temperature for a controlled dropwise addition time of 10 min while keeping the temperature below -70 °C. After completion of dropwise addition, the reaction mixture was continued to be stirred at -78 °C for 1 h 30 min. Upon completion of the reaction, the reaction was quenched with saturated aqueous NH4Cl solution (10 ml) at -78 °C and then the reaction mixture was allowed to warm up slowly to room temperature. Extraction was carried out by adding ether (50 ml) and water (20 ml), and the organic layer was separated, dried over anhydrous MgSO4, filtered, and concentrated to dryness under reduced pressure (Note: the rotary evaporation bath temperature was controlled at 30°C to prevent volatilization of the aldehydes). The crude product was purified by column chromatography (DCM as eluent) to afford pure 2,4-difluoro-3-methylbenzaldehyde as a light yellow oil (1.250 g; 83% yield).
