Graphene oxide/carbon nanotubes-Fe3O4 supported Pd nanoparticles for hydrogenation of nitroarenes and C-H activation

Article abstract of DOI:10.1039/c5ra25842a

Fe₃O₄ magnetic nanocomposites combined with graphene oxide (GO) and carbon nanotubes (CNTs) are synthesized by a hydrothermal method, followed by synthesis of GO/CNT-Fe₃O₄ supported Pd nanoparticles using a gas-liquid interfacial plasma (GLIP) method with Pd(OAc)₂ as a precursor, and the Pd nanoparticles show a uniform particle size distribution. The catalysts exhibit remarkable catalytic activity during the hydrogenation of nitroarenes under H₂ atmosphere at 60°C in water using a small amount of catalyst, and the catalyst can be magnetically separated from the reaction mixture. The addition of the Fe₃O₄ component to the GO/CNTs can be used to effectively prevent aggregation and restacking of GO and the CNTs, and the GO/CNT composite can adjust the hydrophilic-hydrophobic property of the catalysts. Furthermore, the Pd catalyst can be readily recovered and reused several times without a significant decrease in activity. It is worth mentioning that the Pd-3 catalysts show remarkable activity during C-H functionalization.

Full text of DOI:10.1039/c5ra25842a

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Graphene oxide/carbon nanotubes–Fe3O4
supported Pd nanoparticles for hydrogenation of
Cite this: RSC Adv., 2016, 6, 16911
nitroarenes and C–H activation†
Fan Yang,‡a Andong Feng,‡a Chunxia Wang,b Sen Dong,a Cheng Chi,a Xilai Jia,a
a
a
*
Liqiang Zhang and Yongfeng Li
Fe3O4 magnetic nanocomposites combined with graphene oxide (GO) and carbon nanotubes (CNTs) are
synthesized by hydrothermal method, followed by synthesis of GO/CNT–Fe3O4 supported Pd
a
nanoparticles using a gas–liquid interfacial plasma (GLIP) method with Pd(OAc)2 as a precursor, and the
Pd nanoparticles show a uniform particle size distribution. The catalysts exhibit remarkable catalytic
activity during the hydrogenation of nitroarenes under H2 atmosphere at 60 ꢀC in water using a small
amount of catalyst, and the catalyst can be magnetically separated from the reaction mixture. The
addition of the Fe3O4 component to the GO/CNTs can be used to effectively prevent aggregation and
restacking of GO and the CNTs, and the GO/CNT composite can adjust the hydrophilic–hydrophobic
property of the catalysts. Furthermore, the Pd catalyst can be readily recovered and reused several times
without a significant decrease in activity. It is worth mentioning that the Pd-3 catalysts show remarkable
activity during C–H functionalization.
Received 4th December 2015
Accepted 27th January 2016
DOI: 10.1039/c5ra25842a
www.rsc.org/advances
restacking-induced surface area loss, great efforts have been
made to design and construct innovative composites. Most
Introduction
recently, three-dimensional (3D) interconnected network
carbon nanomaterial compositions of graphene and CNTs have
been disclosed to enhance the availability of the surface area,
and the combined structure can effectively avoid their aggre-
gation and restacking.10–12
Catalysis is becoming a strategic eld of science because it
represents a new way of meeting the challenges of energy and
sustainability. Sustainable chemistry, also called green chem-
istry, is a philosophy of chemical research and engineering that
encourages the design of products and processes that minimize
the use and generation of hazardous substances.1–3 From the
viewpoint of green chemistry, it is highly desirable to develop
reusable heterogeneous catalysts which can be suitable to
catalyze reactions in aqueous media.4,5
As representatives of novel carbon nanomaterials, graphene
and carbon nanotubes (CNTs) have been used as an excellent
catalyst support due to their large surface area and high
thermal, chemical and mechanical stability.6,7 However, two-
dimensional graphene and one-dimensional CNTs usually
tend to aggregate and restack, which leads to the obvious
reduction of available surface area.8,9 In order to minimize the
In the past several years, our group has focused on the
development of carbon material supported Pd and Au nano-
particles for organic molecule transformations in water.13–17
Although the GO/CNT support can be efficiently used as a cata-
lyst support for catalyzing organic molecular transformation
reactions in water due to its good hydrophilic–hydrophobic
property,15,18–21 the isolation and recovery of these tiny nano-
catalysts from the reaction mixture is not easy. Therefore, the
limitation of such protocols is the difficulty in recovering these
nanocatalysts from the reaction mixture.22 To overcome this
issue, the use of magnetic nanoparticles has emerged as a viable
solution;23 their insoluble and paramagnetic nature enables
easy and efficient separation of the catalysts from the reaction
mixture with an external magnet. They offer a promising option
that can meet the requirements of high accessibility with
improved reusability.24,25
aState Key Laboratory of Heavy Oil Processing, China University of Petroleum,
Changping 102249, Beijing, China. E-mail: yi@cup.edu.cn; Fax: +86-10-89739028;
Tel: +86-10-89739028
bBeijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical
Chemistry for Living Biosystems, Institute of Chemistry, The Chinese Academy of
Science, Beijing 100190, China
In continuation of our previous work, we introduce an Fe3O4
nanocomposite into the GO/CNT supports for further avoiding
their aggregation and restacking.26,27 Moreover, the insoluble
and paramagnetic natures of magnetic nanoparticles enable
easy and efficient separation of the catalysts from the reaction
mixture with an external magnet.28–32 In this work, we report
† Electronic supplementary information (ESI) available: Materials, illustration of
plasma system, SEM and TEM images, Pd nanoparticle size, high resolution XPS
spectra, contact angle, XRD of reused catalyst, procedure for C–H
functionalization, and 1H NMR spectra data. See DOI: 10.1039/c5ra25842a
‡ These authors contributed equally.
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a facile GLIP one-pot synthesis of GO/CNT–Fe3O4 supported Pd
nanoparticles (Scheme 1). The Pd catalysts exhibit high catalytic
activity for hydrogenation of nitroarenes and C–H
functionalization.
Preparation of GO/CNT–Fe3O4 composites
The GO/CNT–Fe3O4 composites were prepared by a hydro-
thermal synthesis method.34–36 GO (30 mg) and CNTs (15 mg)
were dispersed in water (10 mL) by ultrasonication for 30 min,
respectively. The above two solutions were mixed together, and
the mixture was sonicated for another 30 min. Then, 45 mg
iron(II) oxalate dihydrate dispersed in water (10 mL) was added
and stirred for 10 min. The solution was transferred into an
Experimental
General information
ꢀ
autoclave and aged at 180 C for 12 h. Aer that, the obtained
composites were freeze-dried and heated to 600 ꢀC for 2 h.
Several kinds of GO/CNT–Fe3O4 with different GO/CNT ratios
(2 : 1, 1 : 1, 1 : 2, only CNTs, and only GO) were prepared, and
the weight ratio of GO/CNT and Fe3O4 for all samples was 7 : 3.
All chemicals and solvents were purchased from commercial
suppliers. The morphology of the catalysts was characterized by
transmission electron microscopy (TEM, Tecnai G2, F20)
combined with selected area electron diffraction (SAED) at an
acceleration voltage of 200 kV. The crystalline phase of the
catalysts was identied by X-ray diffraction (XRD) on a Rigaku
D/Max 2500 X-ray diffractometer with Cu Ka radiation (18 kW).
The elemental composition of the samples was obtained by X-
ray photoelectron spectroscopy (XPS) with an ESCALab220i-XL
electron spectrometer from VG Scientic using 300 W Al Ka
radiation. The water contact angle measurements were con-
ducted on the contact angle meter (JC2000D3, Shanghai
Zhongchen Digital Technology Apparatus Co., Ltd.). The
leaching of Pd in four consecutive cycles was determined by
inductively coupled plasma optical emission spectrometry (ICP-
OES). 1H NMR spectra were recorded on JNM-LA300FT-NMR for
checking the nal product from the hydrogenation of nitro-
aromatics and C–H functionalization reactions.
Fabrication of GO/CNT–Fe3O4 supported Pd nanoparticles
The GO/CNT–Fe3O4 supported Pd nanoparticles were synthe-
sized by the gas–liquid interfacial plasma method. The glow
discharge plasma (Fig. S1†) was generated between the top at
stainless steel (SUS) and bottom ionic liquid electrode by
Glassman High Voltage Inc. with the model as FJ1P120. Ar gas
was introduced and used as the plasma-forming gas, the
chamber was stainless steel with an inner diameter of 70 mm
and four glass windows, and the gap between the electrodes was
4 mm. A direct current (DC) power source with voltage VDC
¼
220–250 V is applied to a stainless steel electrode in the gas
phase for the generation of an Ar plasma, where the discharge
current I ¼ 2 mA is xed, and the Ar gas is introduced up to
a pressure Pgas ¼ 290 Pa. 4.9 mg Pd(OAc)2 was dispersed in 1-
butyl-3-methylimidazolium-tetrauoro-borate ([BMIM]BF4) by
sonicating for 30 min. Aer that, 30 mg composites were added
to the reactor, then the palladium solution was added to the
reactor incubating for 15 min. Electrons were irradiated toward
the ionic liquid for 15 min, then the mixture was washed with
ethanol several times to remove the excess impurities and
extracted from the ionic liquid by a magnet. The Pd nano-
particle decorated GO/CNT–Fe3O4 was prepared with the
different GO/CNT ratios (2 : 1, 1 : 1, 1 : 2, only CNTs, and
only GO), and the respective materials were called Pd-n (n ¼ 1, 2,
3, 4, 5).
Preparation of graphene oxide
GO was synthesized according to a modied Hummer’s
method.33 A mixture of concentrated H2SO4–H3PO4 (60 mL/3.3
mL) was added to a ask containing a mixture of graphite
powder (500 mg) and KMnO4 (3 g). Then the mixture was heated
to 50 ꢀC and stirred for 12 h. Aer that, the ask was cooled to
room temperature and the contents were poured into an ice
bath (40 mL). Aer that, 30% H2O2 (2.5 mL) was added dropwise
with stirring. The mixture was centrifuged and cleaned with 100
mL HCl (0.04 g mLꢁ1) and then wet-cleaned multiple times with
deionized water to remove oxidizing agents. The remaining
solution was dialyzed for one week to eliminate excess ions.
Then, the obtained graphite oxide was dispersed in water and
ultrasonicated for 3 h to form GO. Eventually, the nal product
was freeze-dried.
The typical procedure for the hydrogenation of nitrobenzene
in water
In a typical procedure for the hydrogenation of nitrobenzene in
water, all hydrogenation reactions were carried out under
ꢀ
standard conditions (60 C, 1 atm of H2). A micro-reaction vial
charged with Pd-n (7.3 mg), corresponding to a percentage of Pd
of 1 mmol% with respect to nitrobenzene, was used, and 0.5
mmol nitrobenzene (51 mL) in 2 mL water with a magnetic
stirrer was connected to a H2 balloon. Aer consumption of
nitrobenzene, which was monitored by thin layer chromatog-
raphy (TLC), the crude product was isolated by extracting with
ethyl acetate four times. The Pd-n catalysts in the aqueous phase
were recovered by a magnet and washed by both water and
alcohol twice, respectively. Then, the catalysts were dried at 50
ꢀC for the next cycles. To the organic phase, the saturated
Scheme 1 The synthesis process of GO/CNT–Fe3O4 supported Pd
nanoparticles.
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Table 1 Pd nanoparticle size (nm)a, and Pd and Fe3O4 loading content
of Pd-n (wt%)b
hydrochloric ethyl acetate solution was added to induce the
precipitation of the amine hydrochloride salt. The hydrochlo-
ride was removed by the ltration process, and washed with
ether to give the yield.
Sample
Pd-1
Pd-2
Pd-3
Pd-4
Pd-5
Pd-n size (nm)
Pd loading (wt%)
Fe3O4 loading (wt%)
4.1
11.8
27.9
4.1
10.4
24.6
4.7
10.1
28.9
4.5
11.4
25.9
3.6
10.0
26.3
Results and discussion
a
The GO/CNT–Fe2O3 hybrid was rst synthesized by the hydro-
thermal reaction with ferrous oxalate dehydrate, GO and
CNTs.35 Then, the GO/CNT–Fe3O4 composites were synthesized
by calcination of the synthesized GO/CNT–Fe2O3.36 Finally, the
GO/CNT–Fe3O4 supported Pd nanoparticles (Pd-n catalysts)
were synthesized by a GLIP method. Fig. 1a displays an optical
photo of the GO/CNT–Fe3O4 porous aerogel, and the post-
synthesized Pd-n catalysts can be attracted by a magnet easily
as shown in Fig. 1b. The morphologies of the different GO/CNT–
Fe3O4 supported Pd nanoparticles are characterized by SEM and
TEM images, as shown in Fig. 1c–f and S2.† It can be observed
that GO and the CNTs are uniformly recombined in nanoscale
dimensions.
The Fe3O4 nanoparticles are attached onto the surface of the
GO sheets and intercalated between the sheets with the inter-
spersed CNTs which hinder the restacking of the sheets.26,27 The
Pd nanoparticles show a monodisperse morphology on the
surface of GO/CNT–Fe3O4, and the particle sizes of Pd-n are all
around 4.0 nm (Table 1 and Fig. S3†). These results illustrate
Average size obtained from the size distribution histogram.
Calculated by XPS.
b
that the GO/CNT–Fe3O4 porous aerogel structure can be effi-
ciently synthesized by the hydrothermal method, and the GLIP
method can decorate the Pd nanoparticles homogeneously on
the surface of the supports without destruction of the GO/CNT–
Fe3O4 microstructure.
Further insight into the structural properties of the Pd-n
catalysts is obtained from XRD characterization, as shown in
Fig. 2a. Several characteristic diffraction peaks at 30.1ꢀ, 35.5ꢀ,
40.1ꢀ, 43.1ꢀ, 57.0ꢀ, 62.6ꢀ and 68.1ꢀ correspond to the reections
of Fe3O4 (220), Fe3O4 (311), Pd (111), Fe3O4 (400), Fe3O4 (511),
Fe3O4 (440) and Pd (220), respectively.37,38 In addition, the lattice
spacings measured from the diffraction rings are 0.35, 0.30,
0.25, 0.23, 0.21, 0.16, 0.15 and 0.13 nm as indicated in Fig. 2b,
which correspond to the reections C (002), Fe3O4 (220), Fe3O4
(311), Pd (111), Fe3O4 (400), Fe3O4 (511), Fe3O4 (440) and Pd
(220), respectively. The SAED image of Pd-3 is consistent with
the XRD results, which illustrates that Pd-n possesses a face-
centered crystalline structure.
The XPS spectra of different samples are shown in Fig. 3a.
The result indicates that the main chemical components of the
materials are Pd, Fe, C and O. The peaks of the chemical
components C 1s and O 1s are at 284.28 and 533.75 eV,
respectively. The high resolution XPS spectrum for Pd 3d of Pd-3
is shown in Fig. S4.† Two peaks for Pd 3d centered at 335.98 and
341.38 eV can be assigned to Pd 3d5/2 and Pd 3d3/2, respectively.
The Pd-3 catalyst shows two peaks for Pd 3d5/2 and Pd 3d3/2
which are split into two types of Pd electronic states (Pd0 and
PdO) centred around 335.96, 338.54, 341.26 and 343.44 eV. The
part of the Pd0 nanoparticles was oxidized to PdO nanoparticles
by the oxygen-containing functional groups in the graphene
oxide, in agreement with our previous work.39 The Fe 2p XPS
spectrum exhibits two peaks at 712.0 and 725.1 eV (Fig. 3b),
corresponding to the Fe 2p3/2 and Fe 2p1/2 spin–orbit peaks of
Fe3O4.40,41 The Pd and Fe3O4 loading content of Pd-n is calcu-
lated by XPS, which is in agreement with the amount of Pd and
Fe input, as shown in Table 1.
Fig. 1 Optical photo of the GO/CNT–Fe3O4 porous aerogel (a), and
the GO/CNT–Fe3O4 supported Pd nanoparticles being quickly sepa-
rated from a dispersion of them by an external magnetic field (b). SEM
and TEM images of Pd-3 (c–f).
Fig. 2 XRD patterns (a), and SAED pattern (b), of Pd-n.
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prove our prediction, the measurements for the water contact
angles of the Pd-n experiment are taken, as shown in Fig. S5.†
The water contact angles show an increase by decreasing the GO
component with a hydrophilic property. The water contact angle
results are in good agreement with our prediction, which is
caused by the hydrophilic property of GO and hydrophobic
property of the CNTs. Moreover, the reaction is performed at 40
ꢀC, 60 ꢀC, and 80 ꢀC, respectively (entries 3, 6 and 7). The
optimal reaction temperature for the hydrogenation of nitro-
Fig. 3 XPS spectra of Pd-n: wide scan (a), and Fe 2p spectrum of Pd-3
(b).
ꢀ
arenes reaction is 60 C. The result illustrates that the H2 can
become active by the Pd-3 catalyst at least at 60 ꢀC, and
a continuous enhanced temperature produced a lower yield of
nitrobenzene due to the hydrogenation reaction being an
exothermic-type reaction.46 We have further identied several
reactions with various solvents including EtOH, THF, and
EtOAc, being carried out under the benchmark conditions. It is
found that the Pd-3 catalyst shows great compatibility for these
three solvents in the current catalytic system (entries 8–10).
In view of sustainable development, we have further explored
the generality of the reduction reaction in water using Pd-3 as
the catalyst (Table 3). To our surprise, in all cases, the hydro-
genation of nitroarenes proceeds completely, yielding aniline
with high selectivity. Both electron-donating and electron-
withdrawing groups on the nitroarenes have little effect on the
reaction yield or selectivity. However, nitroarenes substituted
with –NO2, –OH, –CN, –COOH, –COOCH3 and –COCH3 need
a long time to obtain a high yield (entries 3–10). The reason for
the low reaction rate is due to the fact that these substrates are
solid, and the reaction products are also water-insoluble. The
slower reaction rate is rationalized by the lattice energy of the
solid substrates, which increases the activation energy of the
reaction.45 For compounds with two nitro groups, 1,3-dinitro-
benzene (1j), both nitro groups can be reduced (entry 10). The
heterocycle nitroarene 8-nitroquinoline (1k) is converted to 8-
aminoquinoline in quantitative yields (entry 11).
At the current stage, there are some reports for the hydro-
genation of nitroarenes using Pd catalysts. Sodium borohy-
dride, hydrazine and silanes are commonly used as reducing
agents. Compared with these methods, catalytic hydrogenation
using H2 as a reduction reagent is of particular interest owing to
its environmental friendliness, atomic efficiency and compati-
bility with industrial processes. However, the hydrogenation of
nitroarenes is usually carried out under hydrogen at high
pressure and high temperature, leading to high energy input
and difficulties in H2 handling.42–44 On the other hand, chemical
reactions using water as a solvent are expected to receive much
more attention in organic synthesis from both a practical and
an economic standpoint. However, the hydrogenation of nitro-
arenes carried out by a Pd catalyst at atmospheric pressure in
water is seldom reported.45 The results of the hydrogenation of
nitroarenes catalyzed by our synthesized catalysts are shown in
Table 2. To our delight, when using 1 mol% Pd-n catalyst to
catalyze 0.5 mmol of nitrobenzene with H2 in H2O, the aniline is
obtained in a good yield. The Pd-1, Pd-2, and Pd-3 catalysts
show better catalytic activity than Pd-4 and Pd-5, indicating that
the GO and CNT combined composite supports are indispens-
able for enhancing the hydrogenation of nitroarenes reactivity
in water (entries 1–5). The Pd-3 catalyst with a GO/CNT ratio of
1 : 2 shows the best activity, probably due to the suitable
hydrophilic–hydrophobic property of Pd-3 produced by GO/CNT
for the hydrogenation of nitroarenes in water.15 In order to
Moreover, the recyclability of the catalyst is further examined
in the hydrogenation of nitrobenzene. The reaction is carried
ꢀ
out under atmospheric H2 pressure in water at 60 C for 3 h.
Aer completion of the reaction, the catalyst is separated from
the reaction mixture using a magnet, and washed with EtOH for
the next run. The isolated yield obtained from each run aer the
reaction is shown in Fig. 4a. No leaching of Pd in the reaction
mixture was detected by ICP-OES examination. This result
illustrates that the reaction is fully catalyzed by the heteroge-
neous Pd catalyst. The above results indicate that the synthe-
sized hybrid materials exhibit good reactivity and recyclability
without prolonging the reaction time. Aer the reaction for 4
cycles, the catalyst was examined by TEM, and it is found that
particle size shows a little bit of an increase as shown in Fig. 4b.
Moreover, the structural property of the recovered catalyst is
again detected by XRD, as shown in Fig. S6.† The Fe2O3 crystal
peak is detected with the peak at 33.2ꢀ. The result indicates that
the property of the support is changed aer four consecutive
cycles, which is possibly used to explain the reason why the
catalysts lose their activity.
Table 2 Hydrogenation of nitrobenzene under different conditionsa
T
Entry
Cat.
Solvent
(ꢀC)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
Pd-1
Pd-2
Pd-3
Pd-4
Pd-5
Pd-3
Pd-3
Pd-3
Pd-3
Pd-3
H2O
H2O
H2O
H2O
H2O
H2O
H2O
EtOH
THF
EtOAc
60
60
60
60
60
40
80
60
60
60
3
3
3
3
3
3
3
3
3
3
85
87
90
82
76
85
67
98
96
99
a
Eventually, to extend the Pd-3 applications, we further use
the Pd-3 catalyst for C–H bond functionalization reactions, and
Reaction conditions: nitrobenzene (0.5 mmol), cat. Pd (1 mol%), H2 (1
atm) in 2 mL H2O. b Isolated yield.
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Table 3 Pd-3 catalyzed hydrogenation of nitroarenes in watera
Entry
Ar-NO2 (1)
Ar-NH2 (2)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
4-MeC6H4NO2 (1a)
4-MeC6H4NH2 (2a)
3
3
88
99
93
91
87
92
90
90
94
99
99
4-MeOC6H4NO2 (1b)
4-NO2C6H4COOCH3 (1c)
4-NO2C6H4OH (1d)
2-NO2C6H4OH (1e)
4-NO2C6H4COOH (1f)
2-NO2C6H4COOH (1g)
4-NO2C6H4CN (1h)
4-NO2C6H4COCH3 (1i)
3-NO2C6H4NO2 (1j)
8-Nitroquinoline (1k)
4-MeOC6H4NH2 (2b)
4-NH2C6H4COOCH3 (2c)
4-NH2C6H4OH (2d)
2-NH2C6H4OH (2e)
4-NH2C6H4COOH (2f)
2-NH2C6H4COOH (2g)
4-NH2C6H4CN (2h)
4-NH2C6H4COCH3 (2i)
3-NH2C6H4NH2 (2j)
8-Aminoquinoline (2k)
12
24
24
12
12
12
12
12
3
9
10
11
a
Reaction conditions: nitroarenes (0.5 mmol), cat. Pd-3 (1 mol%), H2 (1 atm) in 2 mL H2O. b Isolated yield.
performed using a GLIP method. The Pd nanoparticles on the
surface of GO/CNT–Fe3O4 show uniform size distribution, and
the size distributions of Pd-n are around 4 nm. It is found that
GO and CNT combined composite supports are indispensable
for enhancing the hydrogenation of nitroarenes reactivity in
water. Our results reveal that the catalyst with a GO/CNT ratio of
1 : 2 shows the best reactivity. Moreover, the current catalyst can
be easily separated from the reaction mixture using an external
magnetic eld. The catalyst can be used for four consecutive
runs without signicant decrease in activity. It is worth
mentioning that the Pd-3 catalyst shows remarkable activity
during C–H functionalization. Further work is in progress to
extend other applications by using such kinds of catalyst.
Fig. 4 The yields for 4 cycles of the Pd-3 catalyzed nitrobenzene
hydrogenation reaction (a), and the TEM image of the Pd-3 catalyst
after 4 cycles (b).
Acknowledgements
This work was supported by National High-tech R&D Program of
China (863 Program, No. 2015AA034603), National Natural
Science Foundation of China (No. 21202203, 21576289 and
21322609), Science Foundation Research Funds Provided to
New Recruitments of China University of Petroleum, Beijing
(No. YJRC-2013-31), and Science Foundation of China Univer-
sity of Petroleum, Beijing (No. 2462015YQ0306 and
2462014QZDX01).
Fig. 5 C–H bond functionalization reaction and Glaser coupling
reaction.
the results are shown in Fig. 5. The reaction of benzoxazole (3a)
with iodobenzene (4a) has been carried out with Pd-3, CsF and
CuI in DMF. The result shows that the reaction gives 2-phe-
nylbenzoxazole (5a) with a 76% yield. Moreover, we have
examined the Glaser reaction of phenylacetylene (6a) performed
with Pd-3 and Ag2O in THF, and 1,4-diphenylbutadiyne (7a) is
obtained in a good yield.
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