Polyoxometalate–Ionic Liquid-Catalyzed Ritter Reaction for Efficient Synthesis of Amides

Abstract

A series of polyoxometalate-ionic liquid catalysts that combine the features of a polyoxometalate and an ionic liquid, with the introduction of acidity and miscibility, have been developed to promote the Ritter reaction. Among them, [BSmim]CuPW₁₂O₄₀ [BSmim = 1-methyl-3-(4-sulfobutyl)-1H-imidazol-3-ium] displayed the highest activity for the amidation of a variety of alcohols with nitriles, delivering the corresponding amide products in good to excellent yields. Furthermore, the reaction can be easily scaled up to a gram scale without losing efficiency. This process therefore provides an appealing way to prepare amides by a Ritter reaction using a polyoxometalate–ionic liquid-based catalyst.

Key words

alcohols - nitriles - amides - ionic liquids - Ritter reaction - organocatalysis

• References and Notes

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• 21 Polyoxometalate Ionic Liquids ([BSmim]MPW₁₂O₄₀) A mixture of 1-methylimidazole and an equimolar amount of 1,4-butane sultone was stirred magnetically at 313 K for 24 h to form a white precipitate of BSmim. The solid product was washed with Et₂O (×3) to remove residual precursors, and then dried under a vacuum. The appropriate metal carbonate (10 mmol transition-metal ion) was dispersed in deionized H₂O (20 mL), and the dispersion was added dropwise to a solution of H₃PW₁₂O₄₀·5 H₂O (10 mmol; 20 mL) with agitation under ambient conditions. The resulting mixture was then stirred at rt for 6.0 h. MHPW₁₂O₄₀ (M = Mn, Co, Ni, Cu) was obtained after removing H₂O and vacuum drying at 353 K for 24 h. Aq MHPW₁₂O₄₀ was added dropwise to an equivalent molar quantity of BSmim and the mixture was stirred at rt for 24 h. The solvent was then removed under vacuum at 353 K for 24 h.
• 22 Amides 1–24; General Procedure An oven-dried Schlenk tube equipped with a stirrer bar was charged with the appropriate secondary alcohol (1 mmol), [BSmim]CuPW₁₂O₄₀ (100 mg), CH₃CN (2 mL), and H₂O (50 μL) under Ar. The pressure tube was sealed, and the mixture was stirred at 130 °C for 4 h then slowly cooled to rt. The aqueous reaction mixture was concentrated under reduced pressure, and the residue was diluted with CH₂Cl₂ (5 mL) with stirring for 3 min. The yield was then determined by GC before workup. The residue was purified by flash column chromatography (silica gel).
• 23 N-[1-(4-Fluorophenyl)ethyl]acetamide (2) Prepared by the general procedure and purified by flash column chromatography (silica gel, 10–50% gradient EtOAc–hexanes) to give a white solid; yield: 160.7 mg (92%); mp 148–150 °C. ¹H NMR (700 MHz, CDCl₃): δ = 7.26 (dd, J = 8.5, 5.4 Hz, 2 H), 6.99 (d, J = 8.6 Hz, 2 H), 5.73 (s, 1 H), 5.08 (pent, J = 7.1 Hz, 1 H), 1.96 (s, 3 H), 1.45 (d, J = 7.0 Hz, 3 H). ¹³C NMR (175 MHz, CDCl₃): δ = 169.09, 162.68, 161.28, 139.02, 139.00, 127.84, 127.80, 115.50, 115.38, 77.21, 77.03, 76.85, 48.14, 23.44, 21.76. HRMS (EI⁺): m/z [M + H]⁺ calcd for C₁₀H₁₃FNO: 182.0976; found: 182.1017.