N-Fluorobenzenesulfonimide → 2-(4-Ethyl-2-fluorophenyl)-4,4-dimethyl-4,5-dihydrooxazole

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The 4-ethylphenyloxazoline (2.03 g, 10 mmol) was dried in a vacuum oven at 60° C. for 2-3 hours, dissolved in 50 mL of dry THF, and charged to a 300 mL round bottom flask equipped with a thermometer, nitrogen inlet, and magnetic stir bar. The mixture was cooled under nitrogen to −70° C. in a dry ice/acetone bath. Butyl lithium in hexane (7.5 mL, 0.012 mmol) was added in two portions and warmed to −25° C. over 2 hours. The mixture was cooled again to −65° C. and N-fluorobenzenesulfonimide (3.79 g, 0.012 mmol) was added in three portions. The mixture was allowed to warm to room temperature and was stirred overnight. The reaction mixture was quenched with 100 mL of saturated NH4Cl, and transferred to a separatory funnel with ethyl ether washes. 25% NaOH was added slowly and mixed until an aqueous phase with pH=10 was achieved. The aqueous phase was extracted with ether and the ether was washed with a small volume of water. The ether extracts were dried over MgSO4 and evaporated to give 2.51 g of 2-(2-fluoro-4-ethyl-phenyl)-4,4-dimethyl-4,5-dihydro-oxazole as a brown oil. In a second experiment, a rate of 70% fluorination was achieved (highest) with a reactant ratio of 1:1.5:1.5 oxazoline:BuLi:N-fluorobenzenesulfonimide. 1H NMR (300 MHz, CDCl3) δ (ppm): 7.0 (d, 2H), 7.78 (t, 1H), 4.1 (s, 2H), 2.7 (q, 2H), 1.39 (s, 6H), 1.2 (t, 3H).

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