Step 1: To a solution of 2,5-dibromo m-xylene (2.0 g, 7.6 mmol) in dichloromethane (20 ml) was slowly added n-butyllithium (n-BuLi, 2.5 M hexane solution, 3 ml, 1.0 eq.) at -78 °C. The reaction mixture was stirred at -78 °C for 1 h. After that, N,N-dimethylformamide (DMF, 1.8 ml, 23 mmol, 3 eq.) was added, followed by slow warming of the reaction system to room temperature. Upon completion of the reaction, the pH was adjusted to 2 with 5% aqueous hydrochloric acid and then extracted with ether (3 x 50 ml). The organic phases were combined, dried with anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography with the eluent of petroleum ether (PE):ethyl acetate (EA) = 10:1 to afford 4-bromo-3,5-dimethylbenzaldehyde (0.76 g, 47% yield).
