Pyridine → 3-Bromo-4-fluorobenzyl Bromide

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Pyridine (0.7 cm3) and a solution of 3-bromo-4-fluorobenzyl alcohol (5 g) in dry toluene (50 cm3) were added to a stirred solution of phosphorus tribromide (2.64 g) in dry toluene (50 cm3) under an atmosphere of nitrogen, the temperature of the reaction mixture being maintained at -10° C. during the addition. The mixture was allowed to warm to the ambient temperature (ca. 20° C.) and was stirred for one hour. The mixture was poured into ice and the products extracted into diethyl ether. The combined organic extracts were washed with brine and dried over anhydrous magnesium sulphate to give an orange-red oil, shown by thin layer chromatography to contain two components. NMR spectroscopy of the mixture suggested the presence of a phosphite intermediate and the required product. The oil was redissolved in dry toluene (50 cm3) and further phosphorus tribromide (0.2 g) and pyridine (0.7 cm3) added. The mixture was stirred at 50° C. for 4 hours and then stood at the ambient temperature for 17 hours. The mixture was poured into ice and sodium bicarbonate solution added to neutralise the resulting acid mixture. The products were extracted into diethyl ether, and the combined organic extracts washed with brine and dried over anhydrous magnesium sulphate. Evaporation of the solvent gave a white solid which was purified by flash column chromatography on a silica gel support, eluting with petroleum ether containing 5% by volume diethyl ether, to give the title compound (4.6 g) as a colourless crystalline solid.

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