PEDOT nanofiber/Pd(0) composite-mediated aqueous Mizoroki–Heck reactions under ultrasonic irradiation: an efficient and green method for the C–C cross-coupling reactions

Article abstract of DOI:10.1007/s13738-016-1007-7

Abstract: An environmentally benign and highly efficient procedure for the C–C cross-coupling reactions of aryl halides with olefinic compounds has been developed under ultrasonic irradiation in water in the presence of PEDOT nanofibers/Pd(0) composites (PEDOT-NFs@Pd) as a new recyclable catalyst. PEDOT-NFs@Pd catalyzed the reaction efficiently without using any other harmful organic ligands. Cleaner reaction profile, easy workup procedure, high yields and short reaction time duration are some remarkable features of this method. The utilization of ultrasound irradiation offers a fast, clean and versatile synthetic tool that is often unavailable by conventional methods. Moreover, the catalyst can be recovered by simple filtration and reused several times without significant loss of activity. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13738-016-1007-7

J IRAN CHEM SOC
DOI 10.1007/s13738-016-1007-7
ORIGINAL PAPER
PEDOT nanofiber/Pd(0) composite‑mediated aqueous
Mizoroki–Heck reactions under ultrasonic irradiation: an
efficient and green method for the C–C cross‑coupling reactions
Seyed Jamal Tabatabaei Rezaei1
Received: 2 June 2016 / Accepted: 2 November 2016
© Iranian Chemical Society 2016
Abstract An environmentally benign and highly efficient
procedure for the C–C cross-coupling reactions of aryl hal-
ides with olefinic compounds has been developed under
ultrasonic irradiation in water in the presence of PEDOT
nanofibers/Pd(0) composites (PEDOT-NFs@Pd) as a new
recyclable catalyst. PEDOT-NFs@Pd catalyzed the reac-
tion efficiently without using any other harmful organic
ligands. Cleaner reaction profile, easy workup procedure,
high yields and short reaction time duration are some
remarkable features of this method. The utilization of ultra-
sound irradiation offers a fast, clean and versatile synthetic
tool that is often unavailable by conventional methods.
Moreover, the catalyst can be recovered by simple filtration
and reused several times without significant loss of activity.
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-016-1007-7) contains supplementary
material, which is available to authorized users.
* Seyed Jamal Tabatabaei Rezaei
sjt.rezaei@znu.ac.ir
1
Department of Chemistry, Faculty of Science, University
of Zanjan, P.O. Box 45195-313, Zanjan, Iran
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J IRAN CHEM SOC
Graphical Abstract
Keywords PEDOT nanofibers · Palladium · Recyclable
catalyst · Aqueous Heck reaction · Ultrasonic irradiation
The palladium-catalyzed Heck cross-coupling reaction
between aryl halides and alkenes is a versatile method for
carbon–carbon bond formation in synthetic organic chem-
istry [13, 14]. Palladium has been known as an indispen-
sable catalyst for these cross-coupling reactions, and there
is a great deal of literature on its properties in many reac-
tions. Recently, supported palladium nanoparticles on
insoluble solids have captured the attention of researchers
worldwide as new generation of heterogeneous catalysts in
various scientific fields due to their superior catalytic per-
formance, good stability, ease of separation and satisfac-
tory reusability compared to the traditional homogeneous
Pd(OAc)2, PdCl2 catalysts [15–25]. The most commonly
used supports are silica-based materials, magnetic nanopar-
ticles, self-assembled monolayers, carbon nanomaterials,
dendrimers and functional polymers [26–35].
Of the major conducting polymers, PEDOT has been
widely studied because of its simple synthesis, high electro-
chemical stability and charge mobility, moderate band gap,
low redox potential and wide range of applications [36–38].
Generally, supported metal nanoparticles are produced by
chemical reduction of metal ions in the presence of a sup-
port. Commonly used reductants are borohydride, citrate,
ascorbate and elemental hydrogen [39]. PEDOT, however,
provides both stabilization and reducing properties. Due to
the unique features of PEDOT, metal/PEDOT nanocompos-
ites have been the subject of interest of several groups [40–
43]. In general, metal nanoparticles are produced by adding
a metal salt to an aqueous dispersion of PEDOT [44]. This
Introduction
With the increasing environmental awareness in chemi-
cal research, the challenge of designing sustainable envi-
ronmentally friendly procedures has become the funda-
mental goal of green chemistry [1]. Ultrasound-assisted
organic transformation is a promisingly useful approach
to a green technique in organic chemistry. In the last few
years, the use of ultrasound irradiation in organic syn-
thesis became more and more interesting as ultrasonic
waves in liquids are known to cause chemical reactions
either in homogeneous or in heterogeneous systems
[2–4]. In general, organic reactions under the ultrasound
irradiation have many advantages compared to the con-
ventional reactions which need very high temperatures.
Ultrasound-assisted reactions are cleaner, last only very
few minutes, have high yield and produce minimum
waste [2, 4–7].
On the other hand, water has aroused considerable atten-
tion in synthetic community and proved to be a promising
solvent in organic synthesis due to its economic, environ-
mentally friendly and polar nature. In relation to this, sig-
nificant efforts have been dedicated to developing organic
reactions in water with many inherent advantages over
reactions in conventional organic solvents [8–12].
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J IRAN CHEM SOC
Scheme 1 Mizoroki–Heck
cross-coupling reactions
approach eliminated the need for a separate reduction reac-
tion, which can reduce the spread of toxic reagents during
extra chemical reactions. When rapidly initiated polym-
erization is used for the synthesis of polymer, PEDOT with
nanofibrous morphology is formed [45, 46]. The high surface
area and porosity of the PEDOT nanofibers serve as an ideal
support to make metal/PEDOT nanocomposites [47–50].
To take advantage of these characteristics, in this work
we have developed new methodology for the Heck cross-
coupling reactions of aryl halides with olefinic compounds
in the presence of PEDOT nanofibers/Pd(0) composites as
a green and recyclable catalyst in a green solvent under
ultrasound irradiation (Scheme 1).
spectrometer (Thermo VG scientific) in an ultra-high vac-
uum. The sonication was performed in a UP 400S ultra-
sonic processor equipped with a 3-mm-wide and 140-mm-
long probe, which was immersed directly into the reaction
mixture. The operating frequency was 24 kHz, and the out-
put power was 0–400 Watts through manual adjustment.
Ultrasonic bath (EUROSONIC® 4D ultrasound cleaner
with a frequency of 50 kHz and an output power of 350 W)
was used to disperse materials in solvents.
Preparation of PEDOT nanofibers/Pd composites
(PEDOT‑NFs@Pd)
PEDOT nanofibers/Pd composite was prepared by first syn-
thesizing PEDOT nanofibers in a rapidly initiated polymeri-
zation reaction [43, 44]. Briefly, 1.5 mmol 3,4-ethylenedioxy-
thiophene (EDOT) was dispersed in 90 mL HCl (1.0 M) under
ultrasonic stirring for 10 min. Then, 10 mL acid solution (HCl,
1.0 M) of FeCl3 (0.75 mmol) was added into the above mix-
ture quickly and immediately shaking to ensure sufficient mix-
ing before polymerization begins. The polymerization reaction
was carried out at 20 °C for 22 h without any disturbance. The
synthesized PEDOT-NFs precipitate was obtained through
centrifugation and then washed with deionized water several
times to remove the redundant dopant and oxidant. Finally, the
dark-blue product was obtained after being dried in vacuum
at 60 °C for 24 h. Palladium nanoparticles were grown by
combining a aqueous solution of PEDOT nanofiber (10 mL,
0.002 g mL−1) with a 10 mM solution of palladium(II) nitrate
(2.15 mL) and incubation for one day. The resulting solution
was then dialyzed to remove any remaining palladium salt in
fresh deionized water every 4 h for 2 days. AAS analyses of
the filtrates after dialysis showed no palladium.
Experimental
Apparatus and reagents
All solvents and reagents were purchased from Aldrich or
Merck and used without further purification unless other-
wise stated. Scanning electron microscopy (SEM) is per-
formed by field emission scanning electron microscopy
of Philips XL-30 instrument with the operation of 17 kV.
Transmission electron microscopy (TEM) analyses were
performed by LEO 912AB electron microscope. The con-
tent of Pd in the PEDOT nanofibers/Pd composite was
determined by AA-680 Shimadzu (Kyoto, Japan) flame
atomic absorption spectrometer (AAS) and thermogravi-
metric analysis (TGA) (STA 1500 instrument at a heating
rate of 10 °C min−1 in air). IR spectra were recorded on
a Bomem MB-Series FT-IR spectrophotometer. Cataly-
sis products were analyzed using a Varian 3900 GC (GC
conversions obtained using n-decane as an internal stand-
ard and are based on the amount of aryl halide employed
in relation to authentic standard product). 1H and 13C NMR
spectra (CDCl3) were recorded on a BRUKER DRX-250
AVANCE spectrometer at 250.0 and 62.5 MHz, respec-
tively. X-ray powder diffraction (XRD) data were collected
on an XD-3A diffractometer using Cu Kα radiation. The
average size of crystallites was calculated from the peak
broad (111) by using the Debye–Scherrer equation [51].
XPS analysis was performed using a VG Multilab 2000
General procedure for the catalytic tests
A mixture of an aryl halide (1.0 mmol), an alkene (n-butyl
acrylate or styrene) (1 mmol), K2CO3 (1.2 mmol) and
PEDOT-NFs@Pd (96 mg, 0.05 mol% of Pd with respect
to aryl halide) in water (3 mL) was exposed to ultrasonic
irradiation at 170 W at the room temperature for a speci-
fied length of time. The reaction was monitored by TLC
(or GC if necessary). After the completion of the reaction,
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J IRAN CHEM SOC
the catalyst was collected by simple filtration, washed sev-
eral times with absolute ethanol, air-dried and used directly
for the next round of reactions without further purifica-
tion. After separation of the catalyst, the collected solu-
tion was poured into water and extracted with CH2Cl2. The
combined organic phases were dried over Na2SO4, filtered
and evaporated in vacuum. The mixture was then purified
by column chromatography over silica gel (eluent: ethyl
acetate/petroleum ether, 1/4) or recrystallization to afford a
product with high purity. Characterizations of the products
previously reported results [44]. The vibrations at 1340 and
1515 cm−1 are due to the C–C or C=C stretching of the
quinoidal structure of thiophene ring and the ring stretch-
ing of thiophene ring, respectively. The peaks originating
from the stretching in the alkylenedioxy group are observed
at 1210 and 1088 cm−1. The peaks assigned to C–S bond
stretching in the thiophene ring are also observed at 980,
815 and 682 cm−1. The presence of the peak at 1650 cm−1
in the FT-IR spectrum indicates the existence of an over-
oxidized carbonyl group [44].
Figure 2a shows the SEM image of the PEDOT nanofib-
ers. The PEDOT nanofibers are clearly evident in the SEM
image with a length of more than 10 µm and diameters in
the range of 250–500 nm. TEM experiments demonstrate
that palladium nanoparticles were uniformly dispersed on
the surface of PEDOT-NFs (Fig. 2b). This is due to the
existence of a large number of sulfur and oxygen heter-
oatoms, as anchoring sites, on the surface of PEDOT-NFs
(Fig. 3) [37].
The XRD patterns for the PEDOT-NFs@Pd show the
expected crystallinity of Pd(0) nanoparticles and fibrous
PEDOT (Fig. 4). Three broad peaks centered at 2θ = 6.5°,
12.8° and 26.8° and an intense sharp peak at 2θ = 8.9° are
observed in the PEDOT-NFs oxidized by FeCl3 [44]. The
other three peaks of PEDOT-NFs@Pd are characteristics of
face-centered cubic (fcc) crystalline Pd, corresponding to
the planes (1 1 1), (2 0 0) and (2 2 0) at 2θ values of about
40.02, 46.59 and 68.03, respectively. The average particle
size has been calculated using Scherrer’s formula accord-
ing to the broadening of the (1 1 1) characteristic peak of
the XRD pattern. Accordingly, the average particle size of
Pd is 19.6 nm.
1
were performed by comparison with their H-NMR, 13C-
NMR and physical data with those of authentic samples.
Results and discussion
Characterization of the supported catalyst
The chemical structure of the PEDOT nanofibers was char-
acterized by FT-IR spectroscopy. A typical FT-IR spec-
trum is shown in Fig. 1; it is in good agreement with the
The surface composition and chemical oxidation states
of the prepared PEDOT-NFs@Pd catalyst were charac-
terized by XPS. Wide XPS scan from PEDOT-NFs@Pd
(Fig. 5a) shows the presence of Pd 3d signal together with
S 2p, C 1s and O 1s signals derived from PEDOT-NFs.
Thus, the XPS data also confirm the presence of Pd in the
Fig. 1 FT-IR spectrum of PEDOT-NFs
Fig. 2 SEM image of PEDOT-
NFs (a) and TEM image of
PEDOT-NFs@Pd (b)
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J IRAN CHEM SOC
Fig. 3 The molecular structure
of the PEDOT chains forming
PEDOT-NFs@Pd
Fig. 4 XRD pattern of PEDOT-NFs@Pd
prepared PEDOT-NFs@Pd catalyst. The high-resolution
Pd 3d XPS spectrum of PEDOT-NFs@Pd is illustrated in
Fig. 5b which reveals the presence of Pd 3d5/2 and 3d3/2
peaks at binding energies of 335.4 and 340.85 eV, respec-
tively. These results are consistent with those reported
for metallic Pd(0) oxidation state [48–50, 52]. Also, the
1 3

J IRAN CHEM SOC
Fig. 5 XPS survey spectrum of
PEDOT-NFs@Pd (a). High-
resolution Pd 3d (b) and S 2p
(c) XPS spectra of PEDOT-
NFs@Pd
Fig. 6 TGA curves of PEDOT-
NFs (A-a) and PEDOT-NFs@
Pd (A-b). EDS spectrum of
PEDOT-NFs@Pd (B)
high-resolution S 2p spectrum of PEDOT-NFs@Pd is
illustrated in Fig. 5c which correspond to the single sulfur
bonding environment in PEDOT.
The Catalytic activity of PEDOT‑NFs@Pd toward the
Heck cross‑coupling reactions
The AAS technique was used to calculate the weight
percent of palladium in catalyst. The data showed that the
amount of palladium immobilized on PEDOT-NFs is about
5.51 wt%. The actual weight loss corresponding to the
combustion of supported hybrid material was determined
by thermogravimetry as the weight loss between 100 and
700 °C, i.e., excluding the weight loss due to the desorption
of water (Fig. 6A). Comparing the two curves demonstrates
that the weight percent of palladium in synthesized catalyst
is about 5.55% which is consistent with the obtained results
by the AAS technique. Also, energy-dispersive X-ray spec-
troscopy (EDS) analysis of the catalyst directly confirmed
the presence of Pd in the catalyst structure (Fig. 6B). The
EDS spectrum of PEDOT-NFs@Pd shows the elements
C, O, S and Pd with wt% of 48.38, 22.01, 21.73 and 5.12,
respectively.
To evaluate the catalytic properties of this catalyst, various
bases, solvents, catalyst amounts and the output power of
the ultrasound apparatus were employed in a model Heck
coupling reaction between 4-methyl-iodobenzene and
n-butyl acrylate and the conversion yields were measured
by GC.
Initially, the reaction was carried out in the presence of
various solvents and bases. As shown in Tables 1 and 2,
water and K2CO3 were the best solvent and base for this
synthesis, respectively. Other solvents and bases gave only
moderate yields of the product (Tables 1, 2).
Since the effect of solvent and base was optimized, the
influence of the amounts of catalyst on the reaction was
studied in the next step (Table 3). At very low catalyst
loading, the reaction required long duration to get com-
pleted even under harsh thermal conditions (Table 3, entry
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J IRAN CHEM SOC
Table 1 Effect of different solvents on the reaction of 4-methyl-iodo-
Table 4 Effect of sonication power on the reaction of 4-methyl-iodo-
benzene and n-butyl acrylate
benzene and n-butyl acrylate
Entry
Solvent
Conversion (%)
Entry
US power (W)
Time (min)
Conversion (%)
1
2
3
4
5
DMF
92
20
45
53
99
1
2
3
4
5
6
7
120
120
140
170
200
–
25
60
68
71
87
99
99
90a
92a
THF
CH3CN
Toluene
H2O
25
25
25
120
240
Reaction conditions: 1 mmol 4-methyl-iodobenzene, 1 mmol n-butyl
acrylate, 1.2 mmol K2CO3, PEDOT-NFs@Pd (0.05 mol% Pd) and US
power: 170 W at room temperature for 25 min
–
Reaction conditions: 1 mmol 4-methyl-iodobenzene, 1 mmol n-butyl
acrylate, 1.2 mmol K2CO3, 3 mL water and PEDOT-NFs@Pd
(0.05 mol% Pd) at room temperature
a
Reaction temperature (90 °C)
Table 2 Effect of different bases on the reaction of 4-methyl-iodo-
benzene and n-butyl acrylate
Entry
Base
Conversion (%)
at 170 W of power. Using higher acoustic power did not
change reaction yield a considerable amount (99% in the
same time), but using lower power sharply decreased the
conversion to approximately 71% even with more reaction
time (Table 4, entry 2). These facts mean that there is an
optimum power for effective Heck reaction. The similar
ultrasound power effect is observed for several other reac-
tions [7, 11, 53–55].
In order to verify the effect of ultrasound irradiation on
this transformation, we also examined this reaction under
thermal conditions (Table 4, entry 6 and 7). Analysis of
the reactions mixture showed that reaction takes place
efficiently at 90 °C for 2 h. It is apparent that the coupling
reaction can be finished in shorter time to get better yields
under ultrasound irradiation.
In order to prove the catalytic applicability of PEDOT-
NFs@Pd, we studied the Heck coupling reactions of sty-
rene and n-butyl acrylate with different aryl halides under
similar conditions (Scheme 1; Table 5). The results show
that various aryl bromide or iodide bearing either elec-
tron-withdrawing or electron-donating groups (Table 5,
entries 1–10) could be efficiently converted to the cor-
responding coupling products in high yields under opti-
mum condition. Furthermore, as shown in Table 5 (entries
11–16) the catalytic amount of PEDOT-NFs@Pd can
be used in the Heck coupling of even less reactive aryl
chloride derivatives in good yields with an increase in the
reaction time as the electron-donating properties of sub-
stituents increase.
1
2
3
4
5
None
Trace
91
Et3N
K2CO3
NaOAc
CsCO3
99
88
67
Reaction conditions: 1 mmol 4-methyl-iodobenzene, 1 mmol n-butyl
acrylate, 3 mL water, PEDOT-NFs@Pd (0.05 mol% Pd) and US
power: 170 W at room temperature for 25 min
Table 3 Influence of the amount of catalyst on coupling reaction of
4-methyl-iodobenzene with n-butyl acrylate
Entry Amount of catalyst (mol% Pd) Temp. (°C) Conversion (%)
1
2
3
4
5
0.001
0.001
0.01
0.05
0.1
r.t.
80
r.t.
r.t.
r.t.
17
26
75
99
99
Reaction conditions: 1 mmol 4-methyl-iodobenzene, 1 mmol n-butyl
acrylate, 1.2 mmol K2CO3, 3 mL water and US power: 170 W for
25 min
1 and 2). As the catalyst amount was increased (0.05 mol%
Pd with respect to 4-methyl-iodobenzene), the reaction
went to completion more rapidly at room temperature.
It was found that the best system was water as solvent
and K2CO3 as base using 0.05 mol% of catalyst at room
temperature.
We also found that the output power of the ultrasound
apparatus greatly affected this transformation. The obvious
improvement in the conversion (99%) reached a plateau
For practical applications of heterogeneous catalysts,
especially in industry, the heterogeneity and level of reus-
ability are very important. To address these issues, the
stability and reusability of the PEDOT-NFs@Pd cata-
lyst was studied for the Heck cross-coupling reaction of
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J IRAN CHEM SOC
Table 5 Heck
reaction
of
styrene
and
n-butyl
acrylate
with
different
aryl
halides
Entry
1
R1ArX
R2
Time (min)
18
Yield (%)a
93
Ph
I
I
2
3
4
5
CO2Bu
Ph
15
35
25
40
96
91
95
89
H3C
H3C
I
I
CO2Bu
Ph
Br
6
CO2Bu
Ph
35
45
40
20
15
60
60
75
60
45
45
92
90
92
94
95
71
83
68
77
88
91
Br
7
H3C
H3C
O2N
O2N
Br
Br
8
CO2Bu
Ph
9
Br
Br
10
11
12
13
14
15
16
CO2Bu
Ph
Cl
Cl
CO2Bu
Ph
H3C
H3C
O2N
O2N
Cl
CO2Bu
Ph
Cl
Cl
Cl
CO2Bu
Reaction conditions: 1 mmol R1ArX, 1 mmol olefin, 1.2 mmol K2CO3, 3 mL water, PEDOT-NFs@Pd (0.05 mol% Pd) and US power: 170 W at
room temperature
a
Isolated yield of pure product
4-methyl-iodobenzene with n-butyl acrylate in optimum
condition up to six cycles. After each experimental run,
the reaction products were extracted into CH2Cl2 followed
by a simple decantation and subsequently the conversion
of product was determined by means of GC. Also, the fil-
tered catalyst washed several times with absolute ethanol,
air-dried and used directly for the next round of reactions.
The results shown in Fig. 7 demonstrate that the recovered
catalyst had not significantly lost its activity after six
cycles, clearly illustrating the high stability and excellent
reusability of the catalyst. Also, the amount of leached Pd
species in the reaction solution was measured by the AAS
technique. The amount of palladium leaching after the first
and last run was determined by AAS analysis to be only
0.01 and 0.3%, respectively. Therefore, we may conclude
that the observed catalysis is truly heterogeneous in nature.
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J IRAN CHEM SOC
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Conclusion
An environmentally benign and highly efficient procedure
for the C–C cross-coupling reactions of aryl halides with
olefinic compounds has been developed under ultrasonic
irradiation in water in the presence of PEDOT nanofibers/
Pd(0) composites (PEDOT-NFs@Pd) as a new recyclable
catalyst. High stability and reusability of catalyst, short
reaction times, excellent yields, easy purification, very low
palladium leaching and use of green solvent are main char-
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