Magnetic nanoparticle-supported Hoveyda–Grubbs catalysts for ring-closing metathesis reactions

Abstract

Magnetically recyclable Hoveyda–Grubbs catalyst can be readily assembled using magnetic nanoparticles as support, and this catalyst combines convenient recyclability and excellent activity on ring-closing metathesis (RCM) reactions.

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The Grubbs-type¹ and Hoveyda–Grubbs-type²catalysts are the most widely used catalysts in ring-closure metathesis (RCM). They are extraordinarily versatile to tolerate a variety of functional groups in the alkenes and are compatible with a wide range of solvents. Despite these advantages, the separation of the soluble catalyst from the product and any reaction solvent remains difficult. Current effort³ toward solving the problem is focused on anchoring catalysts to solid or soluble matrices via ligand exchange.⁴ In this context, immobilized catalysts offer inherent operational and economic advantages over their homogeneous counterparts.

During recent years we have witnessed a significant progress on nanoparticle catalysis because the nanoparticle-based catalysts not only show superior catalytic activities to their corresponding bulk materials,⁵ but also increase their loading capacity. However, one issue associated with the application of nanoparticle-based catalysis is the removal of the catalysts from the reaction system by conventional methods, such as filtration, which frequently leads to the blocking of filters and valves (Fig. 1).

Fig. 1 Illustration of the separation of magnetic nanoparticle catalysis from the reaction system in RCM reaction.

Recently, magnetic nanoparticles (MNPs)⁶ emerged as new catalyst supports because of their facile separation from the reaction mixture with the aid of an external magnet.⁷ In this respect, the development of an MNP catalysts based on ruthenium carbene complexes would find a wide range of applications both in academia and industry. Herein, we report our recent effort in developing a magnetic nanoparticle-supported Hoveyda–Grubbs ruthenium carbene complex for promoting RCM reactions, which lead to a convenient synthesis of five-, six- and seven-membered carbocycles or heterocycles in high yields.

The method utilized for the preparation of magnetic nanoparticles (iron oxide) was reported by Hyeon,⁸ and the procedure allowed us to prepare monodispersed iron oxide nanoparticles (about 10 nm) in a narrow range of particle-size distribution. The formed magnetic nanoparticles were characterized by transmission electron microscopy (TEM), and the TEM image of the nanoparticles (Fig. 2(a)) shows that they are relatively uniform with an average size of 10 nm. The magnetic behavior of the magnetic nanoparticles was investigated using a Magnetic Properties Measurement System-5 (MPMS, Quantum Design) magnetometer. The field-dependent magnetization curve at 300 K reveals that the catalyst is superparamagnetic with a saturation magnetization of 36.5 emu g⁻¹ (Fig. 2(b)). Surface modification of magnetic nanoparticles with amino groups was then carried out using organic silane bifunctional groups (3-aminopropyltriethoxysilane, APTS), which served as robust anchors to immobilize the functional ligands on the iron oxide shells of the magnetic nanoparticles. The resulting amino-functionalized particles had an amino group loading level of 0.28 mmol g⁻¹.

Fig. 2 (a) TEM image of magnetic nanoparticles; (b) magnetization curves of magnetic nanoparticles.

Scheme 1 shows the synthesis of the magnetic nanoparticle supported Hoveyda–Grubbs catalyst. Ligand 1^4o was coupled to the magnetic nanoparticles under standard conditions to form 2, which was then treated with the first-generation Grubbs catalyst in the presence of CuCl to provide the desired magnetic Hoveyda–Grubbs catalyst 3 as a purple powder. The Ru loading level of the obtained magnetic catalyst as determined by ICP-AES is 0.025 mmol g⁻¹.

Scheme 1 Synthesis of magnetic nanoparticle-supported catalyst 3.

To test the efficiency and reactivity of the new magnetic catalyst 3 in the RCM reaction, substrates 4–12 were treated with one single batch of catalyst 3 (2.5 mol%) in consecutive manner in CH₂Cl₂ at 40 °C, and the corresponding products 4p–12p were obtained in excellent yields (Table 1).

Table 1 Activity of magnetic catalyst 3 in RCM reactions^a

a Conditions: substrate (0.5 mmol) and catalyst 3 (2.5 mol%) in CH₂Cl₂ (5 ml) at 40 °C for 2 h (cycles 1–3) and 12 h (cycles 4–9). ^b Determined by ¹H NMR.

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For evaluation of recyclability, substrates 4 and 10 were selected, and the RCM reactions were run with recycled catalyst 3 up to 22 times. The results as illustrated in Table 2 show almost no loss of the catalytic efficiency.

Table 2 Experiments for recycle of magnetic catalyst 3

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It is worthwhile to mention that after the reaction, the catalyst was simply collected using a magnetic bar, and the reaction mixture was then transferred out of the flask. The isolated catalyst was washed twice with CH₂Cl₂ and then employed in the next reaction under nitrogen.

In summary, we have developed a novel type of Hoveyda–Grubbs catalyst by assembling magnetic nanoparticles with Grubbs I catalyst. This type of catalyst was found to be effective in the synthesis of a series of cyclic olefins, and found to be active with no loss of catalytic efficiency after repeated use. We expect this type of magnetic nanoparticle-based catalyst to find broad applications in organic synthesis.

This work is supported by grants of the National Science Foundation of China (20325208, 20672004 and 20521202).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section. See DOI: 10.1039/b911999j

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