An effective and environment-friendly system for Cu NPs@RGO-catalyzed C-C homocoupling of aryl halides or arylboronic acids in ionic liquids under microwave irradiation

Article abstract of DOI:10.1039/c6ra04683e

As an outstanding mesh catalyst support, reduced graphene oxide (RGO) has attracted enormous attention in recent years. Cu nanoparticles-RGO (Cu NPs@RGO) as a green catalyst was prepared through a green reduction method by ascorbic acid in N-methyl-2-pyrrolidone. The structure of prepared Cu NPs@RGO was characterized. The catalytic activity of Cu NPs@RGO was estimated. A green and efficient method for synthesizing symmetrical biaryl compounds was developed by Cu NPs@RGO-catalyzed Ullmann homocoupling of aryl halides or arylboronic acids in ionic liquids under microwave (MW) irradiation. The catalytic system could be recycled five times with slight loss of activity. Through this method, nine kinds of biaryls were prepared by homocoupling reaction of the corresponding aryl iodides, aryl bromides, aryl chlorides and aryl boronic acids in moderate to good yields.

Full text of DOI:10.1039/c6ra04683e

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An effective and environment-friendly system for Cu NPs@RGO-
catalyzed C-C homocoupling of aryl halides or arylboronic acids in
ionic liquids under microwave irradiation
Received 00th January 20xx,
Accepted 00th January 20xx
Hongyan Zhaoa, b, Guijie Maoa, Huatao Hana, b, Jinyi Songa, b, Yang Liua, b, Wenyi Chu* a, b and
Zhizhong Sun* a, b
DOI: 10.1039/x0xx00000x
www.rsc.org/
As an outstanding mesh catalyst support, reduced graphene oxide (RGO) has attracted enormous
attention in recent years. Cu nanoparticles-RGO (Cu NPs@RGO) as a green catalyst was prepared through
a green reduction method by ascorbic acid in N-methyl-2-pyrrolidone. The structure of prepared Cu
NPs@RGO was characterized. The catalytic activity of Cu NPs@RGO was estimated. A green and efficient
method for synthesizing symmetrical biaryl compounds was developed by Cu NPs@RGO-catalyzed
Ullmann homocoupling of aryl halides or arylboronic acids in ionic liquids under microwave (MW)
irradiation. The catalytic system could be recycled five times with slight loss of activity. Through this
method, nine kinds of biaryls were prepared by homocoupling reaction of the corresponding aryl iodides,
aryl bromides, aryl chlorides and aryl boronic acids in moderate to good yields.
Biphenyl compounds are important organic intermediates
in the field of organic synthesis. Suzuki reaction and Ullman
coupling are the main methods for the synthesis of this
Introduction
Reduced graphene oxide (RGO) is a kind of two-dimensional
planar structure consisting of carbon atoms that using sp2
hybrid orbitals.1-3 Graphene as a novel nanocomposite has
attracted a high concern in the field of catalysts due to
powerful specific surface area. Recently, a large number of
papers reported that graphene supported metals
nanoparticles (NPs) has been used4-7 as catalysts. Cu NPs have
received much attention because copper is cheap8 and
abundantly available. Simultaneously, they can substitute
other precious metal NPs such as Ag, Pd and Au. However, Cu
NPs are easily oxidized to CuO in air.9-10 In recent reports,11-13 it
is found that the RGO can effectively avoid the oxidation of
Cu(I) and Cu(0) NPs in air because of its high delocalized
electron characteristic.Obviously, stable Cu NPs@RGO and
Cu2O NPs@RGO catalysts have great development prospects.
The usual methods for preparing Cu NPs@RGO are to reduce
Cu(II) by using hydrazine hydrate or NaBH4.14-16 However, there
are some problems with these methods that need to be
improved such as long reaction time, environmental pollution
and so on. It is necessary to develop a green and efficient
method for preparing the catalyst.
compounds.17-22The reactions often need some expensive
ligands20 and harmful solvents for good yield. Heck and
Sonogashira coupling reactions are also important methods for
the synthesis of biaryls. Recently, the Pd NPs@RGO catalysts
were applied to these reactions.23-25 However, Palladium as
catalyst is relatively expensive. Therefore, it is necessary to
develop an economical, green and efficient method for
preparing these compounds, and to seek or prepare highly
efficient and stable catalyst. Copper as a low-cost metal has a
good performance in organic reactions. As a heterogeneous
catalyst, copper nanocatalysts have good prospects. Some
methods for the synthesis of biphenyl compounds were
reported about reaction solvents and catalysts such as alloy
nanoclusters and reduced graphene oxide supported metal
nanoparticles. In recent years, ionic liquids as a green solvent
were reported to be used in the reactions.26-28 Raghu Nath
Dhital et al.29 described the catalytic activity of Au/Pd alloy
nanoclusters for Ullmann coupling of aryl chloride compounds
at low temperature. Minoo Dabiri et al.30 reported the
synthesis and application of Au/Fe3O4/s-G nanoparticles to
prepare symmetrical biaryls in the water. Najrul Hussain et al.7
reported the synthesis of symmetrical biaryls from arylboronic
acids using Cu NPs@RGO as catalysts under microwave
conditions. However, these methods have certain degree of
limitation and defect in the cost and environmental protection.
In our past work, we found that ionic liquid28 and
Pd@PdO–NDG31 exhibited good catalytic activities for Suzuki
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cross-coupling of aryl bromides with arylboronic acid to form
According to percentage mass, different contents of
asymmetric biphenyl compounds. Here, the aim of our study is copper catalysts (5wt%, 10wt%, 15wt%Cu NPs@RGO) were
to synthesize an efficient catalyst using a low-cost copper and prepared. The weight content of copper metal element of the
to catalyze coupling reaction in ionic liquids. Cu NPs@RGO catalysts could be tested by EDXRF. The test results were
nanocomposites were efficiently synthesized by using ascorbic consistent with the theoretical ratio.
acid as reducing agent in N-methyl-2-pyrrolidone and used as Cu NPs@RGO catalyze the Ullmann homocoupling reaction
catalyst to synthesize symmetrical biphenyls by Ullmann
According to the experimental comparison, we found that Cu
homocoupling reaction of aryl halides and arylboronic acids in
NPs@RGO have
a good catalytic activity for Ullmann
ionic liquid under microwave irradiation. Ionic liquids as green
solvents not only meet the requirement of green chemistry,
but also can effectively enhance the reaction yields. The
catalytic system can be suitable for wide substrates including
aryl halides and arylboronic acids. Furthermore the catalytic
system has the advantages of high efficiency, economization,
green environmental protection and recyclability and so on.
Moreover, the catalyst is cheap, recyclable and easy to
prepare.
homocoupling reaction in ionic liquid. In a typical procedure,
aryl halides or arylboronic acids (1 mmol) and Cu NPs@RGO
(10% mmol Cu content) were dispersed in
3 mL of
[BMPy][Tf2N]/H2O(2:1) under sonication for 30 minutes. Then,
the reaction system was irradiated under microwave at 70-90
W for 30 minutes. After adding 10 mL water into the reaction
mixture, the mixture was extracted by ethyl acetate three
times. The organic phase was dried over Na2SO4 and
concentrated by a rotary evaporator. The crude products were
purified by chromatography using silica gel with petroleum
ether/ethyl acetate as the eluent to get the biphenyl
compounds. In addition, after extraction residue mixture was
distilled to remove 10 mL water, the residue mixture as the
catalytic system was recycled.
Experimental section
General Information
Ionic liquids including 1-Butyl-3-methylpyridinium bis(trifluor-
methylsulfonyl)imide ([BMPy][Tf2N]), 1-Butyl-3-Methylpyridi-
nium hexafluorophosphate ([BMPy][PF6]) and 1-butyl-3-
methylimidazolium bis(trifluoromethyl)sulfonylimide ([BMIM]
[Tf2N]) were purchased from Nowe Wuhu Chemical
Technology Co., Ltd. Graphite powder was purchased from
Qingdao Tianyuan Company. All of the chemicals were
purchased from Aladdin, Alfa, Kermel Chemical Company and
used without purification.
The structure of catalyst was observed by high-resolution
transmission electron microscopy (HRTEM, JEM-2100) with an
acceleration voltage of 200 kV. X-ray diffraction (XRD) were
characterized by D/MAX-III-B-40 KV X-ray power diffraction
(Cu-Kα radiation,
λ = 1.5406 Å). X-ray photoelectron
spectroscopy (XPS) was performed on a VG ESCALAB MK II
with an Mg Kα (1253.6 eV) achromatic X-ray source. Infrared
spectroscopy (IR) was measured by SP-100 Fourier transform
infrared spectroscopy. Raman spectra were recorded using a
Renishaw inVia spectrometer with an excitation laser of 532
nm in wave length. Melting points were measured on a digital
melting point apparatus without correction.
Scheme 1 Synthesis of Cu NPs@RGO and catalytic process.
Results and discussion
Characterization of the Cu NPs@RGO catalyst
Cu NPs@RGO catalysts were characterized by fourier
transform infrared spectroscopy (FTIR), powder X-ray
diffraction (XRD), transmission electron microscopy (TEM), X-
ray photoelectron spectroscopy (XPS) and Raman
spectroscopy.
Synthesis of the Cu NPs@RGO composites
Graphene oxide was synthesized from flake graphite through
improved Hummer’s method.32 Cu NPs@RGO hybrids with
10wt% Cu contents were prepared through a reduction
method. A mixture of GO 100 mg and copper sulfate 25 mg in
75 mL of N-methyl-2-pyrrolidone was treated under sonication
for 1 hour. The mixture’s pH was adjusted to 10 by NaOH
solution. After stirring for 30 minutes, ascorbic acid 120 mg
TEM images and XRD profile of Cu NPs@RGO catalyst
were shown in Fig. 1. The characteristics of graphene fold
could be clearly seen in Fig. 1a and copper nanoparticles were
evenly dispersed on the surface of graphene (Fig. 1b). The
particle size distribution of Cu NPs@RGO catalyst was counted
(Fig. 1c). The average size of the copper particles was about 8
nm, which might be beneficial to enhance the catalyst activity
because of the large specific surface area. The phase structure
and purity of Cu NPs@RGO were characterized by XRD in the
range of 15–80° (Fig. 1d). The diffraction peaks at 43.2°, 50.3°
0
was slowly added into the mixture and heated to 90 C for 2
hours. Finally, the mixture became dark suspension and was
filtered, and the obtained solids were washed with deionized
water and absolute ethanol, and dried in a vacuum oven at 50
0C for 16 hours.
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and 74.1° of Cu NPs@RGO nanocomposites could be indexed deformation of the C–O band respectively. Compared with the
to (111), (200) and (220) crystal planes of cubic Cu spectrum of GO, the disappearance of C=O, C–OH and C–O in
crystal(JCPDS 04-0836),8.33respectively. Meanwhile appearance the spectrum of Cu NPs@RGO showed that the GO was
of a weak peak at around 25° indicated that GO were reduced during preparing the catalyst.
successfully reduced to RGO during the preparation of Cu
NPs@RGO catalyst. In addition, we found that the catalyst Fig. 2d. The D-bands and G-bands were clearly found in 1368
could be stored in the air for at least one month.
and 1600 cm−1. The D band arised from edges, other structural
The average size of Cu particles can be estimated by the defects in the hexagonal sp2 carbon network or the finite
The Raman spectra of Cu NPs@RGO and GO are shown in
Scherrer equation:
particle-size effect, while the G band arised from E2g phonon of
sp2 carbon pairs in both rings and chains.15 Compared to the
d=0.9λ/β1/2cosθ
d is the average particle size (nm), λ is the wavelength of the X- spectrum of graphene oxide, the D to G peak intensity ratio
ray used (0.15406 nm), β1/2 is the width of the diffraction peak (ID/IG) of Cu NPs@RGO was increased from 0.82 to 1.01.34-35
at half height in radians and θ is the angle at the position of The relative changes of the ID/IG affirmed the reduction of
the peak maximum.4 The calculated average particle size of graphene oxide.
copper supported on graphene is 18 nm, which is similar with
the TEM results.
Fig. 2 (a, b) XPS profile of the Cu NPs@RGO catalyst, (c) FTIR spectra of
GO and the Cu NPs@RGO composite. (d) Raman spectra of Cu
Fig. 1 (a,b) TEM images of the Cu nanoparticles on RGO nanosheets. (c) NPs@RGO and GO.
Particle size distribution of Cu NPs@RGO catalyst. (d) XRD profile of Optimization of the conditions of Cu NPs@RGO-catalyzed Ullmann
the Cu NPs@RGO catalyst.
homocoupling
XPS profile, FTIR and Raman spectra of the Cu NPs@RGO
catalyst were shown in Fig. 2. The binding energies of Cu 2p1/2
at 952.7 eV and Cu 2p3/2 eV at 932.7 eV indicated the presence
The Ullmann reaction is
a convenient method for the
symmetrical biaryl compounds by C–C bond formation from
aryl halides and arylboronic acids. Initially, iodobenzene was
of Cu0 state, and confirmed that copper was supported on used as the starting material to optimize the reaction
graphene sheet as the metallic phase11,12 in Fig. 2a. The
conditions of the Ullmann homocoupling by screening
catalysts, solvents and reaction time. The results were
summarized in Table 1. Firstly, the reaction solvents were
screened. According to the experimental results, we found
that the ionic liquids were better for the reaction. And then we
found that the mass ratio 2:1 of [BMPy][Tf2N]/H2O as the
solvent was able to improve the Ullmann homocoupling
reaction to give a 97% yield by contrast with other solvent
systems (Entries 1-9 in Table 1). Compared with the different
reaction time, 30 minutes under the microwave irradiate was
found to be the optimum reaction time (Entries 8 and 10-11 in
existence of metallic Cu was also supported by XPS profile. The
deconvoluted C1s XPS peaks at 284.6, 285.5, 286.5 and 287.6
eV corresponding to C–C/C–H, C–C, C–O and C=O species,
respectively (Fig. 2b). This comparison indicated the reduction
of GO nanosheets to RGO nanosheets.7
GO and Cu NPs@RGO were characterized by FTIR analysis
as shown in Fig. 2c. In the spectrum of GO, the strong peaks
around 3401 cm−1 owing to O-H stretching vibrations of the
absorbed water molecules.15 The C=C and C=O stretching
vibrations were clear at near 1623 and 1738 cm-1 in the GO Table 1). Meanwhile we found that the yield of 80% was
materials. Moreover, the bands at 1206 and 1094 cm-1 were
obtained without microwave irradiation for 48 hours under
130 0C (Entry 8). Compared with the different molar ratio of
associated with the O–H vibration in carboxyl acid and the
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