The general procedure for the synthesis of 2-fluoro-4-iodo-5-methylpyridine from 2-fluoro-5-methylpyridine was as follows: diisopropylamine (92 mL) was added to tetrahydrofuran (THF, 1.2 L) under nitrogen protection and the mixture was cooled to -18 °C. A hexane solution of 2.69 M n-butyllithium (224 mL) was slowly added dropwise to this solution. After the dropwise addition was completed, the temperature of the reaction system was raised to -5 °C within 20 min under stirring and subsequently cooled to -73 °C. At this temperature, a THF solution (240 mL) of 2-fluoro-5-methylpyridine (61 g) was slowly added dropwise and the reaction was stirred at -75 °C for 3.5 hours. Subsequently, a THF solution (24 mL) of iodine (139 g) was added dropwise to the reaction solution and stirring was continued at -75 °C for 1 hour and 55 minutes. After the reaction was completed, water (220 mL) was added at the same temperature and stirred for 5 minutes. After returning the reaction system to room temperature, water (1.2 L) was added. Next, an aqueous solution (300 mL) of sodium thiosulfate pentahydrate (136 g) was added and stirred for 10 minutes. The mixture was extracted with methyl tert-butyl ether (MTBE, 1.2 L) and the organic layer was washed with saturated brine (500 mL). The combined aqueous layers were then extracted with MTBE (1 L). All organic layers were combined and dried with anhydrous magnesium sulfate. The desiccant was removed by filtration and the filtrate was concentrated under reduced pressure. To the residue n-heptane was added, cooled and filtered to collect the precipitated solid. The solids were washed with n-heptane and the filtrate was cooled and filtered again to collect the precipitate. The process was repeated 5 times to finally obtain 2-fluoro-4-iodo-5-methylpyridine (109.69 g). The product was characterized by 1H-NMR (400 MHz, CDCl3) and ESI-MS to confirm the correct structure.
