[Synthesis]
General procedure for the synthesis of 5-chloro-2-cyanopyridine from 2,5-dichloropyridine and zinc cyanide: In a 22-liter three-necked round-bottomed flask equipped with an overhead stirrer, a reflux condenser, and a thermometer, N,N-dimethylacetamide (DMAC, 6 L), 2,5-dichloropyridine (347.0 g, 2.34 mol), zinc cyanide (138.0 g, 1.17 mol ), bis(diphenylphosphino)dipalladium II CH2Cl2 complex (DPPF, 20.8 g, 0.02 mol) and zinc powder (1.6 g, 0.02 mol). The reaction mixture was slowly heated to 160 °C. When the temperature reaches 160°C, controlled exotherm may occur and the internal temperature may rise to 180-185°C. Remove the heat source and slowly cool the mixture to room temperature. Dilute the dark reaction mixture with 2L of water, add brine (2L), filter with diatomaceous earth and extract with ethyl acetate (4L). The process was repeated 3 times to ensure complete extraction, the organic phases were combined and dried with magnesium sulfate. Careful concentration at 25-30°C may give a dark colored liquid. (Note: the product may volatilize at higher temperatures, so keep the temperature low during concentration.) Add water (5L) to the concentrate, stir for 1 hour to precipitate a solid, filter, and wash the filter cake with 2L of water. The filter cake was dried to give 215 g of crude product. The aqueous filtrate was extracted with ether (8 L) and the organic phase was dried with magnesium sulfate and concentrated to give 51 g of crude product. The two batches of crude product were combined and purified by silica gel chromatography (Biotage 150; eluting first with 5% ethyl acetate/hexane and then 10% ethyl acetate/hexane) to give pure 5-chloro-2-cyanopyridine as a white solid (193 g, 59% yield).1H NMR (CDCl3) δ 8.68 (d, J = 2.0 Hz, 1H), 7.84 (dd , J = 2.7,8.6Hz, 1H), 7.66 (d, J = 8.3Hz, 1H). |