Direct Conversion of Aromatic Aldehydes into Benzamides via Oxidation with Potassium Permanganate in Liquid Ammonia

Abstract

Oxidation of aromatic aldehydes by KMnO₄ in liquid ammonia gives amides directly. The reaction proceeds satisfactorily when the aldehydes are activated by electron-withdrawing substituents on the ring.

Key words

aldehydes - amides - aminals - oxidation - liquid ammonia

• References and Notes

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• 12 General Procedure for the Oxidation of Aldehydes to Amides in Liquid AmmoniaUnder an argon atmosphere, liquid NH₃ (25 mL) was condensed in a two-neck round-bottom flask immersed in a dry ice cooling bath and equipped with a dry ice reflux condenser. Aldehyde (7.34 mmol) was added, and the resulting solution (or suspension) was stirred for 1 h. KMnO₄ (7.34 mmol, 1.16 g) was added, the cooling bath was removed, and the reaction mixture was stirred for another hour with gentle reflux of NH₃. Na₂SO₃ (22.0 mmol, 2.78 g) was added, the reflux condenser was removed, and the NH₃ was allowed to evaporate spontaneously. The dark-brown residue was treated with 6 M HCl (30 mL), and the resulting precipitate was filtered, washed with H₂O (100 mL) and sat. aq NaHCO₃ (20 mL). All products were recrystallized from EtOH.3-BromobenzamideColorless crystals; mp 152–156 °C (lit.^6d 157–159 °C). IR (KBr): ν_max = 3353, 3175, 1659, 1623, 1564, 1427, 1389, 1123, 1066, 901, 794 cm^–1. ¹H NMR (400 MHz): δ = 7.44 (1 H, t, J = 7.8 Hz), 7.56 (1 H, br s), 7.73 (1 H, ddd, J = 7.8, 2.0, 1.0 Hz), 7.90 (1 H, dm, J = 7.8 Hz), 8.08 (1 H, t, J = 1.8 Hz), 8.15 (1 H, br s). ¹³C NMR (100 MHz): δ = 122.5, 127.5, 131.1, 131.4, 134.9, 137.4, 167.3.
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