Phosphorus oxychloride → 2,3-Dichloro-5-iodopyridine

A 50 ml 3-necked round bottom flask, equipped with a thermometer, a reflux condenser topped with a N2 inlet, a stopper and a magnetic stirrer, was charged with 3.50 g (0.0137 mole) of 3-chloro-5-iodo-2-pyridinol, from STEP 1 above, 3.00 g (0.0144 mole) of phosphorus pentachloride and 5-10 ml of phosphorus oxychloride. The reaction mixture was heated to reflux and the solid dissolved resulting in a golden brown solution. The reaction mixture was stirred under N2 at reflux for 4 hours and then cooled to room temperature. The reaction mixture was poured into crushed ice and allowed to stand for about 1 hour. A tan precipitate, which was shown to be product and unreacted starting material, formed and was isolated by filtration and washed with pentane. The aqueous layer was also washed with pentane. The combined organic layers were washed with deionized water and saturated NaCl, dried over anhydrous MgSO4, filtered and evaporated to dryness to give 0.4 g of a dark oil that crystallized upon standing.