Tetrahedron Letters 55 (2014) 5973–5975
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Electrospun copper oxide nanoparticles as an efficient heterogeneous catalyst for N-arylation of indole Abdullah Khalil a, Aziz Fihri b, Mustapha Jouiad c,⇑, Raed Hashaikeh a,⇑ a Institute Center for Water and Environment (iWATER), Department of Mechanical and Materials Engineering, Masdar Institute of Science and Technology, Abu Dhabi 54224, United Arab Emirates b MAScIR Foundation, VARENA Center, Rabat Design, Rue Mohamed El Jazouli, Madinat Al Irfane, Rabat 10100, Morocco c Institute Center for Energy (iENERGY), Department of Mechanical and Materials Engineering, Masdar Institute of Science and Technology, Abu Dhabi 54224, United Arab Emirates
a r t i c l e
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Article history: Received 6 July 2014 Revised 16 August 2014 Accepted 28 August 2014 Available online 6 September 2014 Keywords: Copper oxide nanoparticles Electrospinning Scalable catalyst N-arylation Reusable catalyst
a b s t r a c t The N-arylation of indoles with a variety of aryl bromides is reported using copper oxide nanoparticles as a heterogeneous catalyst. These copper oxide nanoparticles, which were produced in a novel, facile, and scalable fashion via an electrospinning technique, resulted in an excellent product yield under mild conditions. Moreover, the catalyst was easily recovered and reused several times without significant loss of activity. Ó 2014 Elsevier Ltd. All rights reserved.
Metal oxide nanoparticles possess significant technological potential in several commercial applications such as cosmetics,1 energy management,2 and catalysis.3,4 According to an international survey, a conservative market estimate of metal oxide nanoparticles in 2012 was around 270,041 tons with a projected growth of 1,663,168 tons by 2020.5 Among various metal oxides, copper oxide (CuO) nanoparticles are of major importance because of their applications which include sensing,6 heat transfer,7 antibacterial uses,8 and catalysis.9 Based on these reports, there is a need to develop facile, economic, and industrially scalable techniques for the mass-production of CuO nanoparticles. Several chemical methods such as alcohothermal10 and quick-precipitation,11 and physical methods such as laser ablation12 and microwave irradiation,13 have been employed for synthesizing CuO nanoparticles. We have recently reported electrospinning, for the first time, as a novel and alternative route for the continuous and large-scale synthesis of CuO nanoparticles.14 This method is not only simple and low-cost, but can also be easily scaled at the industrial level based on the desired product yield.15 Moreover, through a suitable choice of precursor chemicals and adjustment of electrospinning parameters, nanoparticles of any metal oxide can be easily synthesized. Herein, we report the potential of electrospun CuO ⇑ Corresponding authors. Tel.: +971 8109235 (M.J.), +971 28109152 (R.H.). E-mail addresses:
[email protected] (M. Jouiad),
[email protected] (R. Hashaikeh). http://dx.doi.org/10.1016/j.tetlet.2014.08.120 0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
nanoparticles as highly efficient heterogeneous catalytic agents for the N-arylation of indole, an organic compound commonly used as a fragrance in various cosmetic products and also as a precursor of many pharmaceuticals.16 The arylation of indole and similar compounds is usually carried out by Ullmann-type coupling reactions.17 However, these reactions have limitations such as high-temperature reaction conditions, moderate yields, use of highly polar solvents as well as the use of large amounts of copper-based catalysts; and these aspects greatly limit their applications.18 To overcome these drawbacks, a breakthrough was achieved by Buchwald et al. who reported the combination of copper iodide with N,N-bidentate ligands as effective catalyst systems for the N-arylation of nitrogen-containing heterocycles with aryl halides. Under mild conditions, they obtained good product yields using these catalyst systems.19–21 However, an efficient separation and subsequent recycling of these homogeneous transition metal catalysts remain a challenge and an economic concern. One of the best ways to overcome these difficulties is to employ heterogeneous catalysis, as it enables a convenient recovery and reuse of the catalyst from the reaction mixture through simple filtration or decantation.22–30 Therefore, in this context, we have developed CuO nanoparticles through a simple fabrication route based on an electrospinning technique, and analyzed their potential as a simple and efficient reusable heterogeneous catalyst for the N-arylation of indole under mild conditions.
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A. Khalil et al. / Tetrahedron Letters 55 (2014) 5973–5975
The CuO nanoparticles were synthesized via an electrospinning technique in a novel manner. The method is based on producing rough and discontinuous electrospun nanofibers from a precursor based on copper acetate and a poly(vinyl alcohol) (PVA) polymer. Selectively removing the polymeric phase from the fibers produces highly rough and discontinuous CuO nanofibers. The nanofibers consist of nanoparticles that are weakly held together in a onedimensional (1D) manner. Sonication, in a suitable liquid under controlled conditions, completely disintegrated the nanofibers into nearly spherical nanoparticles (diameter 30–70 nm) resulting in the formation of a uniform and relatively well-dispersed nanoparticle suspension. The detailed description of the nanoparticle synthesis procedure can be found in our previous work.14 Highresolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed for examining the morphology and structure of the CuO nanoparticles, whereas X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) were employed for determining the phase purity and composition of the CuO nanoparticles. The morphology of the CuO nanoparticles, as revealed from high-resolution SEM, was found to be nearly spherical with an average size below 100 nm, Figure 1a. The relatively large particles may be representative of several smaller nanoparticle clusters. Further, the high-resolution TEM image of a single nanoparticle, shown in Figure 1b (nanoparticle highlighted in color to differentiate from the amorphous carbon film) reveals a nearly single crystal nature (d-spacing = 2.87 Å), also reflected via the Fast-Fourier Transform (FFT) shown in the inset. The XRD spectrum shown in
Figure 1c contains all the peaks associated with the crystalline planes of pure CuO (JCPDS Card No. 05-0661) confirming the crystallinity and phase purity of the nanoparticles. The EDS spectrum and the corresponding quantitative results of the CuO nanoparticles, (Figure 1d), show a Cu/O ratio of about 78:22 wt/wt which is in a close stoichiometric ratio to that of pure CuO (79.8:20.2 wt/wt). These results confirm the purity and single crystal nature of our electrospun CuO nanoparticles. To explore the potential activity of this heterogeneous catalyst, we evaluated these CuO nanoparticles as a catalyst for the N-arylation of indole. Initial trials were performed to optimize the reaction conditions for the N-arylation of indole with 4-bromotoluene. First, the reaction was conducted without any catalyst and no activity was observed despite a prolonged reaction time. This was expected because the presence of a base is vital for the catalytic cycle as it removes the acidic proton from the indole. After screening a range of inorganic and organic bases and exploring various solvents, this nanocatalyst was found to be most active in DMSO as the solvent with potassium hydroxide as the base and 1,10-phenanthroline as the ligand.31 Table 1 summarizes the scope of the reactions performed in this Letter. The N-arylated product was obtained in good yield under reflux (entry 1). When 2,20 -bipyridine was used as the ligand instead of 1,10-phenanthroline, the product yield was reduced to 70% (entry 2). This difference in yield was attributed to the difference in spatial distribution of each ligand and to electron cloud density, which is more important in 1,10-phenanthroline. Another reason is that rotation of the CAC bond in 2,20 -bipyridine is slow making stabilization of the copper more
Figure 1. (a) SEM image, (b) high-resolution TEM image (inset shows the FFT), (c) XRD spectrum, and (d) EDS spectrum of the CuO nanoparticles obtained from the electrospinning technique.
A. Khalil et al. / Tetrahedron Letters 55 (2014) 5973–5975 Table 1 CuO nanoparticle catalyzed N-arylation of indole with aryl bromidesa
Entry
Amine
X-Br
Yieldb (%)
Product
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electrospun CuO nanoparticles are efficient heterogeneous catalytic agents. The high product yields under mild conditions and easy recovery and reuse reflect the great potential of these electrospun CuO nanoparticles as industrial catalysts. Also, since electrospinning is an economic, facile, and scalable technique, it is possible to produce large amounts of CuO and other oxide nanoparticles for various catalytic applications to achieve better product yields with reduced costs. Acknowledgment
1
80
N N H
Br
N H
Br
2
70c
N
3
References and notes
56d
N N H
Br Br
4
76
N N H
Br
5 N H
6 N H
79, 75e
N Cl
Cl
N
The authors acknowledge the financial support for this work provided by the Masdar Institute of Science and Technology through Grant Number 12NAMD1.
Br
73
N N
a
Reaction conditions: CuO nanoparticles (5 mol %), 4-bromotoluene (1 mmol), indole (0.5 mmol), KOH (1 mmol), 1,10-phen (0.256 mmol), DMSO (2 mL), reflux, 24 h. b Isolated yield. c Reaction using 2,20 -bipyridine as the ligand. d CuO nanoparticles (5 mol %) without any ligand. e Yield after the fourth cycle.
difficult. In the absence of any ligand, however, the product yield remained significantly low (entry 3). Under identical conditions, the N-arylation of indole with bromobenzene and 1-bromo-4-chlorobenzene afforded the corresponding N-arylated products in excellent yields (76% and 79%, respectively, entries 4 and 5). This system also gave a good yield of 73% in the N-arylation of indole with the heteroaryl halide, 2-bromopyridine (entry 6). The recyclability of the catalyst is very important for industrial applications. After recovery of the CuO nanoparticles by simple filtration followed by washing with ethanol and drying, we established a set of experiments using the recycled catalyst for the N-arylation of indole with 4-bromotoluene. The results showed that the catalyst could be used four times with almost consistent activity. Even in the fourth run, the yield of N-arylated product was 75% (entry 5), indicating excellent reusability and chemical stability of the catalyst. Importantly, the absence of any copper species in the filtrate was confirmed by atomic absorption spectroscopy, which clearly demonstrated that no leaching of copper had occurred during the reaction and provided evidence for heterogeneity throughout the reaction. Through the N-arylation of indoles with aryl bromides bearing an electron-donating group, we have demonstrated that
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