5-Fluoroindole is prepared from 2-(5-fluoro-2-nitrophenyl)-5-acetonitrile.
10% Pd/C (110 mg) was transferred into a round-bottom flask under a nitrogen atmosphere, followed by the addition of 2-(5-fluoro-2-nitrophenyl)-5-acetonitrile (564 mg, 3.13 mmol) in anhydrous ethanol (25 mL). The mixture was degassed and backfilled with hydrogen gas for a total of three cycles, then stirred overnight. After replacing the hydrogen with nitrogen, a second portion of 10% Pd/C (110 mg) was added to the reaction mixture, and hydrogen gas was reintroduced. The completion of the reaction was monitored by ¹⁹F NMR. Subsequently, the hydrogen gas was replaced with nitrogen, and chloroform (CHCl₃) was added to quench the unreacted Pd/C. The reaction mixture was concentrated under reduced pressure, then partitioned between dichloromethane (DCM) and water. After separation, the aqueous phase was back-extracted, and the combined organic layers were washed with brine, dried over Na₂SO₄, and concentrated under reduced pressure. Column chromatography (mobile phase: DCM) afforded 5-fluoroindole as a white solid.
Yield: 342 mg, 2.53 mmol (81% yield based on 2-(5-fluoro-2-nitrophenyl)-5-acetonitrile).
¹H NMR (400 MHz, CDCl₃) δ: 7.27 (m, 3H), 6.96 (t, 1H), 6.53 (t, 1H).
¹⁹F NMR (376 MHz, CDCl₃) δ: –100.22.
