a) Dissolve 2,5-dichloropyridine (10 g, 67.57 mmol) in THF (17 mL) under nitrogen protection and slowly add dropwise to a mixture of n-BuLi isohexane solution (33.8 mL, 67.57 mmol) and diisopropylamine (9.63 mL, 67.57 mmol) in THF (68.0 mL) that has been pre-cooled to -78°C. . After the dropwise addition, the reaction mixture was continued to be stirred at -78°C for 30 min. Subsequently, a solution of THF (17.0 mL) with I2 (17.49 g, 68.92 mmol) was added slowly dropwise, and after completion of the dropwise addition, the reaction was kept at -78°C for 1 hour. At the end of the reaction, the reaction was quenched with deionized water (75 mL) and slowly warmed to room temperature. The reaction mixture was extracted with ether (3 x 100 mL), the organic phases were combined, dried with anhydrous MgSO4 and concentrated under reduced pressure. The residue was ground with dichloromethane to give the solid product 2,5-dichloro-4-iodopyridine (9.72 g, 53% yield). The concentrated residue of the filtrate was purified by silica gel column chromatography using a gradient elution with a 50-100% isohexane solution of dichloromethane. The product-containing fraction was collected, concentrated under reduced pressure, and the residue was ground with methanol to give a second batch of 2,5-dichloro-4-iodopyridine (5.74 g, 31% yield).1H NMR (300 MHz, DMSO) δ 7.85 (1H, s), 8.34 (1H, s).
