[Bmim]NTf₂, a Low Viscosity Ionic Liquid is a Viable Reaction Medium for Pd-Catalyzed Cross-Coupling Reactions

Abstract

A low viscosity ionic liquid [bmim]NTf₂ was successfully used as a reaction medium in Pd-catalyzed Mizoroki-Heck, Suzuki-Miyaura and Stille coupling reactions. The yields are good to high, and comparable to those reported previously for high viscosity ionic liquids, such as [bmim]PF₆ and [bmim]BF₄. The products are readily separated and a reaction medium containing the Pd-carbene catalyst was successfully recycled.

low viscosity - ionic liquid - palladium - carbene complex - cross coupling reaction

References

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Preparation of Pd-Complex 1. A mixture of PdCl₂(CH₃CN)₂ (518 mg, 2 mmol), 1-butyl-3-methyl-imidazolium chloride (349 mg, 2 mmol), PPh₃ (514 mg, 2 mmol), and diisopropylamine (1 g, 10 mmol) in THF (50 mL) was heated at reflux under nitrogen for 10 h. After cooling to r.t., solvent was removed under reduced pressure. Resulting precipitates were dissolved in CHCl₃, washed with H₂O, and dried over MgSO₄. After evaporation, the precipitate was recrystallized from CHCl₃-hexane to give 1.08 g of Pd-carbene complex 1 as a white solid (94% yield, mp 257.5-258.0 °C). ¹H NMR (CDCl₃): δ = 7.55 (m, 6 H), 7.44 (m, 3 H), 7.32 (m, 6 H), 6.63 (s, 1 H), 6.61 (s, 1 H), 4.16 (m, 1 H), 3.67 (m, 1 H), 3.57 (s, 3 H), 1.81 (m, 1 H), 1.50 (m, 1 H), 1.27 (sextet, J = 7.3 Hz, 2 H), 0.86 (t, J = 7.3 Hz, 3 H). ¹³C NMR (CDCl₃): δ = 160.3, 134.2, 134.1, 131.2, 130.2, 128.6, 123.3, 121.3, 50.7, 37.6, 31.8, 20.0, 13.7. IR (KBr): 3103.7, 2959.1, 1479.5, 1435.2, 1097.6, 746.5, 694.4 cm^-1.

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Typical Procedure for Mizoroki-Heck Reaction. Pd-complex 1 (5.8 mg, 0.01 mmol) was dissolved in [bmim]NTf₂ (1.5 mL), and 2-iodotoluene (110 mg, 0.5 mmol), butyl acrylate (78 mg, 0.6 mmol), and tripropylamine (109 mg, 0.75 mmol) were sequentially added at r.t. The resulting mixture was heated to 100 °C. After stirring for 1 h, the reaction was quenched by the addition of degassed H₂O (1 mL), and cooled to r.t. The product was extracted with hexane (3 × 10 mL). The combined hexane phase was dried over MgSO₄, filtered, and concentrated under reduced pressure. The residue was purified by flash chromatography over silica gel (hexane-EtOAc = 3:1) to afford 74 mg (88%) of the desired product as a white powder. The IL phase containing Pd-catalyst was washed with H₂O (3 × 10 mL) first to remove the ammonium salt, then dried at 50 °C under vacuum for 4 h and employed in the next reaction.

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