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.
21Polyoxometalate 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.
22Amides 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).
