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Synlett 2004(10): 1814-1816  
DOI: 10.1055/s-2004-829552
LETTER
© Georg Thieme Verlag Stuttgart · New York

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

Shifang Liu, Takahide Fukuyama, Masaaki Sato, Ilhyong Ryu*
Department of Chemistry, Faculty of Arts and Sciences, Osaka Prefecture University, Sakai 599-8531, Japan
Fax: +81(72)2549695; e-Mail: ryu@ms.cias.osakafu-u.ac.jp;
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Publication History

Received 23 April 2004
Publication Date:
15 July 2004 (online)

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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.

Key words

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

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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.