A hexane solution of 1.6 M n-butyllithium (3.75 mL, 6.0 mmol) and anhydrous tetrahydrofuran (5 mL) were added to a flame-dried flask under nitrogen protection. The reaction system was cooled to -78 °C and 2,4-dichloropyridine (6.0 mmol) was added slowly dropwise under stirring, keeping the stirring at -78 °C for 30 min. Subsequently, iodomethane (0.374 mL, 6.0 mmol) was added slowly and dropwise at the same temperature. The reaction mixture was continued to be stirred at -78 °C for 1 h. Then glacial acetic acid (0.114 mL, 2.0 mmol) was added to adjust the pH to 5-6 (wet pH paper test). The reaction mixture was dissolved in ether (100 mL), the organic phase was washed sequentially with water (10 mL) and saturated brine (10 mL), and dried over anhydrous magnesium sulfate. The solvent was removed by concentration under reduced pressure to give the crude product as an oil. Purification by fast column chromatography with the eluent being petroleum ether:dichloromethane (50:50, v/v) to petroleum ether:dichloromethane:ethyl acetate (50:47:3, v/v/v) afforded 2,4-dichloro-3-methylpyridine as a white solid (589 mg, 72% yield). Note: 2,4-dichloro-3-methylpyridine sublimates readily under reduced pressure.1H NMR (CD3OD, 400 MHz) δ 8.15 (d, 1H), 7.46 (d, 1H), 2.50 (s, 3H).LRMS (ESI) m/z: Calculated value C6H5Cl2N [M+H]+ 160.98, measured value 161.9.
