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Synlett 2010(16): 2518-2522  
DOI: 10.1055/s-0030-1258545
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Recyclable NHC-Ni Complex Immobilized on Magnetite/Silica Nanoparticles for C-S Cross-Coupling of Aryl Halides with Thiols

Hyo-Jin Yoona, Jung-Woo Choia, Homan Kangb, Taegyu Kanga, Sang-Myung Leea, Bong-Hyun Jun*c, Yoon-Sik Lee*a,b
a School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea
b Interdisciplinary Program in Nano-Science and Technology, Seoul National University, Seoul, 151-747, Korea
c Department of Electrical Engineering, Seoul National University, Seoul 151-744, Korea
Fax: +82(2)8769625; e-Mail: yslee@snu.ac.kr;
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Publication History

Received 30 June 2010
Publication Date:
26 August 2010 (online)

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Abstract

A new type of magnetite/silica nanoparticle-supported N-heterocyclic carbene nickel catalyst (Mag-NHC-Ni) was developed from imidazolium with N-picolyl moieties and used as an efficient catalyst in the C-S coupling of various aryl halides with thiols. Moreover, the catalyst was easily recovered from the reaction mixture by simple filtration and recycled with almost consistent activity.

Key words

heterogeneous catalysis - cross-coupling - magnetic nanoparticle - nickel

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Synthesis of Silica-Coated Magnetic Particles: A two-necked round-bottomed flask (RBF) containing NH4OH (250 mL) and deionized (DI) H2O (250 mL) was vigorously stirred with an overhead stirrer at 500 rpm. A freshly prepared aq solution of iron chloride (100 mL) containing FeCl2˙4H2O (2.57 g, 12.93 mmol) and FeCl3˙6H2O (6.18 g, 22.6 mmol) was dropwise mixed with NH4OH solution for 30 min. The resultant mixture was stirred for additional 1 h at r.t., then the formed MNPs were separated using a magnet (4000 gauss). The MNPs were washed with DI H2O (20 ×) and MeOH (5 ×) by repeating magnet separation and decantation. The MNPs (1.35 g) dried in vacuo were treated with 1% 3-aminopropyltriethoxysilane in CHCl3 (80 mL) at 60 ˚C for 4 h. The aminated MNPs were washed with CHCl3 (10 ×) and MeOH (5 ×) by repeating magnet separation and decantation. The MNPs (1 g) dried in vacuo (elemental analysis, N%: 0.382%, 0.27 mmol/g) were treated with 2% TEOS (tetraethyl orthosilicate) in EtOH and shaken at 30 ˚C for 12 h. The particles were washed with EtOH (6 ×) repeating magnet separation and decantation.

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Preparation of Catalyst 5 [MNP-Si-NHC(Pyr)-Ni] and Catalyst 7 [MNP-Si-NHC(Bz)-Ni]: The aforementioned magnetic nanoparticles (MNPs) were redispersed in toluene and the resulting solution was heated to reflux. Then, 1-(3-triethoxysilylpropyl)-2-imidazoline was added. After 24 h, the solution was cooled to r.t. MNPs were concentrated magnetically by using an external permanent magnet and washed with toluene and EtOH. Imidazolinyl-functionalized MNPs were redispersed and reacted with 2-picolyl chloride or benzyl chloride at 80 ˚C in CHCl3 for 12 h, affording 4 and 6. After washing, the loadings of imidazoles on MNP-Si were determined by their nitrogen contents by elementary analysis (4, 0.47 mmol/g; 6, 0.67 mmol/g). The NHC ligand functionalized MNPs were then redissolved in a mixture
of Ni(acac)2 at 60 ˚C in DMSO under basic conditions (Scheme  [²] ). After 12 h, the mixture was cooled to r.t., and then catalyst 5 [MNP-Si-NHC(Pyr)-Ni] or catalyst 7 [MNP-Si-NHC(Bz)-Ni] were magnetically separated by using the external magnet. The prepared catalysts were washed with CHCl3, EtOH, and H2O subsequently.

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General Procedure for C-S Cross-Coupling Reaction: To a mixture of 10 mol% of catalyst in DMF (1 mL) and Cs2CO3 (2 mmol) were added an aryl halide (1 mmol) and a thiol
(1 mmol). The temperature was raised to 100 ˚C. After 10 h, the mixture was cooled to r.t. The used catalysts were removed magnetically by using an external permanent magnet and dried for reuse in the next round of reactions. The desired products were washed with H2O and extracted with EtOAc. Then, the organic phase was evaporated in vacuo and the residues were subjected to flash column chromatography purification.

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After the 4th run, unfortunately the coupling yields were decreased (ca. 10%) and leaching of nickel species was detected from the filtrates after the 4th run and the 5th run (14% and 18%, by ICP-AES analysis of the filtrate).