Chinese Chemical Letters  2014, Vol.25 Issue (01):141-145   PDF    
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Ying-Peng Zhang, Ai-Hong Shi, Yun-Shang Yang, Chun-Lei Li.Impregnated copper on magnetite as catalyst for the O-arylation of phenols with aryl halides[J]. Chin. Chem. Lett., 2014,25(01): 141-145   
Impregnated copper on magnetite as catalyst for the O-arylation of phenols with aryl halides
Ying-Peng Zhang , Ai-Hong Shi, Yun-Shang Yang , Chun-Lei Li    
* Corresponding authors at:School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China
Received:2013-6-3, Revised:2013-9-4, online:2013-10-28.
E-mail addresses:yingpengzhang@126.com;yangyunshang@tom.com
Abstract: Nanoparticle Fe3O4 encapsulated CuO, as a heterogeneous catalyst, is a facile system for the synthesis of diaryl ethers by the cross-coupling reaction of various substituted aryl halides with various substituted phenols, which avoids using any type of expensive ligand and can be recovered from the reaction mixture by using a simple magnet. Moreover, this catalyst can be reused three times with high catalytic activity.
Key words: CuO-Fe3O4     Ullmann cross-coupling reaction     Diaryl ether     O-Arylation     Heterogeneous catalysis    
1. Introduction

Diaryl ethers are very important organic compounds which have played a significant role in the production of fragrances [1, 2, 3, 4], biological materials [5, 6, 7, 8, 9] and pharmaceutical compounds [10, 11, 12]. The synthesis of diaryl ethers, therefore, has attracted enormous attention of synthetic organic chemists with a great number of methods having been developed. The conventional Ullmann cross-coupling reaction to form diary ethers [13, 14, 15] has been reported using aryl halides and phenols with copper or palladium as catalyst [16, 17, 18, 19, 20, 21]. However, this application was limited by harsh reaction conditions, such as high temperature [22, 23], excess quantities of copper reagents [24] and highly expensive ligands [25, 26, 27, 28]. Moreover, palladium is expensive compared to copper or copper complexes.

Although several researchers have investigated CuO as catalyst for the Ullmann cross-coupling reaction [29, 30], little work has been published on the simple method involving the recyclability of the catalyst. In 2011, an important breakthrough was achieved by Cano and co-workers [31, 32]. They reported the catalyst CuOFe3O4 can be used in the C-N coupling reaction and the different domino Sonogashira cyclization processes between 2-iodophenol and different alkynes with the catalyst very easily removed from the reaction mixture just by using a simple magnet. Herein, we report CuO-Fe3O4-catalyzed O-arylation of a wide range of aryl halides with various substituted phenols. To the best of our knowledge, this is the first report on the use of CuO-Fe3O4 in the formation of the C-O bond in the synthesis of diaryl ethers.

2. Experimental

2.1. Materials and methods

All reagents were purchased from Aladdin and were used without further purification unless otherwise noted. Analytical thin layer chromatography (TLC) was performed on silica G 60 F254 (0.25 mm) plates with visualization by UV light (254 nm and 365 nm). The 1H NMR spectra were recorded on Bruker Avance Digital (400 MHz) spectrometers with Me4Si as the internal standard. The catalyst CuO-Fe3O4 was prepared according to the previous research work [32].

2.2. Catalyst characterization

The morphology of the catalyst was characterized by scanning electron microscopy (SEM, JCM-6700, Japan) and transmission electron microscopy (TEM, TECNAI G2 TF20). The surface composition of catalyst was detected by X-ray photoelectron spectra (XPS, PHI-5702).

2.3. General procedure for CuO-Fe3O4-catalyzed O-arylation of phenols with aryl halides

To a stirred solution of phenol (2.2 mmol) in DMF (15.0 mL) under an argon atmosphere were added CuO-Fe3O4 (0.2 mmol), TBAB (0.2 mmol), Cs2CO3 (326 mg, 1.0 mmol) and aryl halide (2.0 mmol). The reaction mixture was stirred at the required temperature (145 ℃) for 24 h. At the end of reaction, the catalyst was removed by a magnet and the resulting mixture was quenched with water and extracted with EtOAc. The organic phases were dried over MgSO4, followed by evaporation under reduced pressure to remove the solvent. The residue was purified by column chromatography on silica gel to afford the desired product.

3. Results and discussion

The microstructure and elemental composition of the catalyst are characterized by SEM and EDX microanalysis. Fig. 1(a) and (b) shows the SEM and EDX of CuO-Fe3O4. Fig. 1(a) shows that the catalyst is dense, implying the magnetic material exhibited poor dispersion. Fig. 1(b) indicates that the Cu, Fe, and O components are detected by EDX, and no impurities are observed. The inset table in Fig. 1(b) shows the measured elemental composition of the sample surface.

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Fig. 1.SEM (a) image and EDX (b) spectrum of CuO-Fe3O4.
Fig. 2 shows the TEM image and the distribution of CuO-Fe3O4. From Fig. 2(a), the synthesized CuO-Fe3O4 catalyst is nearly spherical shape with an average particle size of 15 nm. Meanwhile, the magnetite particles are uniform in size and poorly dispersed. Moreover, the morphology and size of the particles do not change considerably even after three cycles. The result again verifies the unchanged efficiency of the catalyst (Fig. 2(b)).

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Fig. 2.TEM image of (a) CuO-Fe3O4 and (b) after third reuse of catalyst CuO-Fe3O4.
XPS spectroscopy, one of the most important techniques to determine the chemical state and surface properties of materials, was used to characterize the CuO-Fe3O4 composites. Fig. 3 shows the survey spectrum of the CuO-Fe3O4 composites. Cu2p (935.2 eV) can be found in Fig. 3(a), there are some other peaks mainly attributed to C1s (284.2 eV) (the external environment), O1s (532.0 eV) and Fe2p (708.2 eV) [33]. Since the peaks of Fe2p are lower than the peaks of Cu2p, it can be concluded that nanoparticle Fe3O4 successfully encapsulated CuO. The XPS spectrum of Cu2p can be flitted into a main doublet peak, as shown in Fig. 3(b). The binding energy of the doublet peak at 953.8 eV (assigned to Cu2p1/2) and 933.8 eV (assigned to Cu2p3/2) can be attributed to the CuO state (Cu2+) [34].

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Fig. 3.XPS spectra of sample CuO-Fe3O4: (a) survey spectrum, (b) Cu2p spectrum in CuO-Fe3O4.
In the preliminary studies, phenol and bromobenzene were chosen as the model substrates to optimize the reaction conditions, the results are summarized in Table 1. Firstly, we would like to emphasize that TBAB, as a phase transfer catalyst, was necessary due to a small amount of water present in the commercially available inorganic bases. Under our experimental conditions, when TBAB was removed from the reaction with the catalyst of CuO-Fe3O4 and K3PO4 in DMF at reflux for 24 h, the diaryl ether was afforded only in 2% yield (Table 1, entry 2). However, when the reaction was run with TBAB, the yield was 23% (Table 1, entry 1). Under the condition with CuO-Fe3O4 as catalyst, DMF as solvent, and TBAB as phase transfer catalyst and several bases for the Oarylation reaction at 145 ℃, we were pleased to find that with Cs2CO3 as base, we obtained the best result (compare entries 1, 3, 4, 6 and 7), furnishing the product in 94% yield (Table 1, entry 5). Subsequently, when the reaction was carried out in other solvents, such as CH3CN and NMP, very poor yields of the diaryl ether were achieved (Table 1, entries 8-12). However, DMF as solvent gave the best yield (Table 1, entry 5). When Fe3O4 was used as catalyst, the reaction failed (Table 1, entry 13). In addition, when the reaction was carried out with 1.0 mmol Cs2CO3, we also obtained 89% yield (Table 1, entry 14). Since the base Cs2CO3 was much more expensive, the amount of base was 1.0 mmol in the following work.

Table 1
Optimization of the reaction conditions.a
Using the above optimized conditions, we explored the scope of this methodology with the CuO-Fe3O4-catalyzed Ullmann-type reaction for a variety of phenols with aryl halides. The results are summarized in Table 2 and shows that electron-rich substituted phenols lead to the desired product in good to excellent yields (Table 2, entries 2 and 5). Also, even in the presence of an ortho substituted phenol or a meta-position phenol (which is capable of providing a steric bias) the reaction proceeded smoothly (Table 2, entries 3, 4,8 and 9).However, the yieldswithelectron-rich phenols were different from electron-deficient ones. Those bearing strong electron-withdrawing groups seemed difficult to undergo the O-arylation. For example, 4-nitrophenol and 4-hydroxybenzaldehyde (Table 2, entries 6 and 7) decreased the yields of the product. Finally, a better quantitative yield was obtained when 4-bromobenzaldehyde reacted with p-cresol (Table 2, entry 10). With aryl halide components, the present catalytic system was also effective for the coupling reaction of aryl halides with phenols. Iodobenzene afforded the corresponding diaryl ethers in good to excellent yields (Table 2, entries 11 and 18); whereas chlorobenzene only produced a 32% yield (Table 2, entry 12). The yields were excellent in the aryl halides with electronwithdrawing groups, such as nitro or formyl (Table 2, entries 13, 14 and 16). However, aryl halides with electron-withdrawing groups decreased the yield (Table 2, entry 15), in comparison with chlorobenzene. With the existence of steric hindrance (Table 2, entry 17), the reaction also proceeded smoothly. In addition, heterocyclic substrates also reacted in this system. For example, the reaction of phenol and 2-bromine pyridine led to good yields (Table 2, entry 19).

Table 2
CuO-Fe3O4-catalyzed O-arylation of phenols with aryl halides.a
Next, we considered the problem of the catalyst recovery. After the completion of the coupling reaction, we used a magnet to remove the catalyst from the reaction medium. The solid was washed with EtOAc, ethanol and water and dried at 80 ℃ for 12 h. The isolated catalyst was used for the next reaction with Cs2CO3 as base in DMF as solvent at 145 ℃, the results are summarized in Table 3. Based on these results, it was shown that the catalyst retained its high catalytic activity in these repeating cycles.

Table 3
Recycle of catalyst.a
According to published research work [26], the possible mechanism for O-arylation proposed in Scheme 1 may involve the CuO-Fe3O4 catalyzed nucleophilic substitution that proceeds via the formation of the complex (a) with phenols and then subsequently to the oxidative addition of aryl halide via the formation of another complex (b) followed by an instantaneous in situ reductive elimination to release the diaryl ether product (c), as well as the CuO-Fe3O4 catalyst in its original form (to recycle again).

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Scheme 1.Plausible mechanism for the CuO-Fe3O4-catalyzed O-arylation of phenols with aryl halides.
4. Conclusion

In conclusion, we have developed a ligand-free, highly efficient, experimentally simple Ullmann cross-coupling reaction of phenols with aryl halides using impregnated copper on magnetite as catalyst. The optimal reaction conditions were determined as follows: phenol and bromobenzene in the presence of CuO-Fe3O4 as catalyst; DMF as solvent; TBAB as phase transfer catalysis; Cs2CO3 as base at 145 ℃. Moreover, the catalyst can be recovered by a simple magnet from the reaction mixture and reused 3 times with high catalytic activities.

Acknowledgments

This work was supported by the Natural Science Foundation of Gansu Province, China (No. 1014RJZA022) and Postgraduate Tutor Fund of Gansu Province Education Department (No. 1101ZTC103).

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