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

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

References and Notes

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  Choi, J.; Yoon, H.; Lee, S.; Kim, J.; Kim, J.; Lee, Y., submitted for publication.

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18

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.

20

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.

21

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.

22

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