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Synlett 2008(9): 1418-1422  
DOI: 10.1055/s-2008-1072597
LETTER
© Georg Thieme Verlag Stuttgart · New York

Immobilization of Dipyridyl Complex to Magnetic Nanoparticle via Click Chemistry as a Recyclable Catalyst for Suzuki Cross-Coupling Reactions

Guanghua Lv, Wenpeng Mai, Rizhe Jin, Lianxun Gao*
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Changchun 130022, P. R. of China
Fax: +86(431)85685653; e-Mail: lxgao@ciac.jl.cn;
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Publication History

Received 20 December 2007
Publication Date:
07 May 2008 (online)

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Abstract

A magnetic nanoparticle (MNP)-supported di(2-pyr­idyl)methanol palladium dichloride complex was prepared via click chemistry. The MNP-supported catalyst was evaluated in Suzuki coupling reaction in term of activity and recyclability in DMF. It was found to be highly efficient for Suzuki coupling reaction using aryl bromides as substrates and could be easily separated by an external magnet and reused in five consecutive runs without obvious loss of activity.

Key words

Suzuki coupling - supported catalysis - palladium - di(2-pyridyl)methanol - magnetic nanoparticles

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Preparation of 4 Silica-capped MNP (1 g; prepared exactly according to the literature method30) and 3-azidopropyltriethoxysilane 3 (0.5 g, 2.02 mmol) was refluxed in anhyd toluene for 24 h, then the mixture was submitted to external magnetic field, and washed sequentially with toluene (3 × 10 mL), Et2O (3 × 10 mL). The loading was determined to be 0.684 mmol g-1 by elemental analysis. FT-IR: 2104, 1635, 1487, 1101, 969, 798, 585 cm-1.

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Synthesis of Di(2-pyridyl)methyl Propargyl Ether (2) To a stirred solution of 1 (0.93 g, 5 mmol) in anhyd DMF (10 mL) at 0 °C under Ar was added NaH (0.24 g, 6 mmol) in batches, the mixture was stirred at 0 °C for 1 h. After the dropwise addition of propargyl bromide (0.65 mL, 6 mmol) over 10 min, the reaction mixture was raised to r.t., and stirred for another 10 h. Then the system was quenched with MeOH (10 mL) and submitted to flash chromatography on silica gel (PE-EtOAc, 1:1) to afford 2 as dark brown solid (0.6 g, 60%). 1H NMR (600 MHz, CDCl3): δ = 8.57 (d, 2 H, J = 4.8 Hz), 7.72 (t, 2 H), 7.62 (d, 2 H, J = 7.8), 7.19 (t, 2 H), 5.93 (s, 1 H), 4.28 (d, 2 H, J = 2.4), 2.45 (t, 1 H). 13C NMR (100 MHz, CDCl3): δ = 159.4, 149.3, 136.8, 122.8, 121.9, 83.8, 75.1, 56.6, 29.7. Anal. Calcd for C14H12N2O: C, 74.98; H, 5.39; N, 12.49. Found: C, 74.96; H, 5.36; N, 12.39.

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Preparation of Di(2-Pyridyl)methanol-Functionalized MNP 5 via Click Reaction Compounds 2 (0.25 g, 1.12 mmol) and 4 (1 g) were mixed with CuI (0.008 g, 3.77 × 10-2 mmol) in DMF-THF (1:1, 10 mL) under Ar atmosphere, to this was injected DIPEA (1 mL), and the reaction mixture was stirred with a mechanic stirring at r.t. The reaction was monitored by FT-IR as was indicated by the almost complete disappearance of IR signal of 2104 cm-1 which stands for the azide group. Then the reaction mixture was submitted to magnetic separation, and the MNP were washed sequentially with Et2O (3 × 20 mL), H2O (3 × 20 mL), then acetone (3 × 20 mL), finally dried under vacuum. FT-IR: 1646, 1597, 1475, 1434, 1091, 794, 588, 463 cm-1.

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Preparation of 6 To the freshly prepared PdCl2 (MeCN)2 (0.16 g, 0.62 mmol) solution in toluene (10 mL) was added 5 (1 g), the mixture was mechanically stirred and refluxed for 12 h, the reaction system was magnetically separated, the MNP were washed with H2O (3 × 20 mL) and MeOH (5 × 20 mL) then dried in vacuo.

29

Typical Procedure for Suzuki Coupling A mixture of aryl halide (2 mmol), arylboronic acid (2.6 mmol), 6 (0.004 mmol), K2CO3 (4 mmol), and DMF (5 mL) was added to a flask and stirred at 100 °C, open to the air, for the desired time until complete consumption of the starting substrates as judged by TLC. After completion of the reaction, the reaction mixture was cooled to r.t., and to the solution was added Et2O (30 mL), then a magnet was used to separate the catalyst, washed with H2O (3 × 10 mL), then the organic phase was evaporated and the residue was purified by flash chromatography to afford the desired coupling products. The MNP-supported catalyst 6 was washed with Et2O, H2O, dried, and reused directly in the next run. All the coupling products could be found in literature.