Copper nanoparticles supported on polyaniline-functionalized multiwall carbon nanotubes: An efficient and recyclable catalyst for synthesis of unsymmetric sulfides using potassium ethyl xanthogenate in water

Article abstract of DOI:10.1002/aoc.3697

A new and powerful polyaniline-functionalized carbon nanotube-supported copper(II) nanoparticle catalyst was successfully prepared and evaluated as a heterogeneous catalyst for the one-pot synthesis of unsymmetric thioethers by coupling of aryl, alkyl and benzyl halides using potassium ethyl xanthogenate as source of sulfur in water. All of these reactions gave the desired products in good to excellent yields. The catalyst is available, air-stable and can be reused several times without significant loss in its catalytic activity.

Full text of DOI:10.1002/aoc.3697

Received: 1 October 2016
DOI 10.1002/aoc.3697
Revised: 29 October 2016
Accepted: 3 November 2016
F U L L PA P E R
Copper nanoparticles supported on polyaniline‐functionalized
multiwall carbon nanotubes: An efficient and recyclable catalyst
for synthesis of unsymmetric sulfides using potassium ethyl
xanthogenate in water
1,2
1
1
Abdol R. Hajipour | Saeideh Jajarmi | Zahra Khorsandi
1
Pharmaceutical Research Laboratory, Department
A new and powerful polyaniline‐functionalized carbon nanotube‐supported
copper(II) nanoparticle catalyst was successfully prepared and evaluated as a hetero-
geneous catalyst for the one‐pot synthesis of unsymmetric thioethers by coupling of
aryl, alkyl and benzyl halides using potassium ethyl xanthogenate as source of sulfur
in water. All of these reactions gave the desired products in good to excellent yields.
The catalyst is available, air‐stable and can be reused several times without signifi-
cant loss in its catalytic activity.
of Chemistry, Isfahan University of Technology,
Isfahan 84156, Islamic Republic of Iran
2
Department of Pharmacology, University of
Wisconsin, Medical School, 1300 University
Avenue, Madison, Wisconsin 53706‐1532, U.S.A.
Correspondence
Abdol R. Hajipour, Pharmaceutical Research
Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan 84156, Islamic
Republic of Iran.
KEYWORDS
Email: haji@cc.iut.ac.ir
copper(II), potassium ethyl xanthogenate, nanocatalyst, polyaniline, unsymmetric
thioethers
[16]
[17,18]
1
| INTRODUCTION
thiosulfate and ethyl xanthogenate
have been reported as
sulfur sources in C─S bond formation. However, these reactions
are often conducted in dimethylformamide (DMF) at a high tem-
perature. Moreover, the scope of the substrate is also limited in
some cases.
Transition metal‐catalyzed cross‐coupling is one of the most
widely employed reactions in the formation of C─S bonds,
in which the construction of symmetric and unsymmetric sul-
fides is particularly noteworthy due to their importance in
pharmaceutical and biological molecules and chemical mate-
In recent years, nanocatalysis has appeared as a stable,
efficient and attractive alternative to usual catalysis because
the nanoparticles have a high surface‐to‐volume ratio, which
increases their activity and selectivity, though at the same
time maintaining the inherent characteristics of a heteroge-
[
1–3]
rials.
The cross‐coupling reactions of aryl halides with thiols
[
4]
using transition metal catalysts including palladium,
[
5]
[6]
[7]
[8]
[9]
[19–21]
cobalt, nickel, indium, copper and iron salts are
some of the most common methods for the synthesis of these
important materials. However, these reactions require unsta-
ble and toxic thiols, high temperatures, expensive and toxic
solvents, long time for completion the reactions and the use
of homogeneous catalysts that can not be separated and
recovered simply. Therefore finding safe sources instead of
odorous thiols, new support materials for heterogeneous cat-
alysts, green solvents, inexpensive ligands and mild condi-
tions is a necessary issue for the development of catalytic
methods in scale‐up operations.
neous catalyst.
Among nanomaterials, carbon nanotubes, the elegant
empty tubular carbon design of nanometric range, are note-
worthy options for the synthesis of nanocatalysts due to their
appealing mechanical properties and structural specifica-
[
22]
tions.
It has been reported that many heterogeneous cata-
lysts employ surface‐modified carbon nanotubes as
[
23–28]
supports.
Attaching of polymers like polyaniline to carbon nano-
[
22]
tubes is an interesting and developing area of research.
Lately the preparation of identical tubular composite of
[
10]
[11]
[12]
Recently, thiourea, potassium thiocyanate, thiolates,
carbon disulfide,
polyaniline and multiwall carbon nanotubes (MWCNTs) has
[13]
[14]
[15]
[29]
sulfur powder,
thioimide,
sodium
been reported.
This functionalization with polyaniline
Appl Organometal Chem 2017; e3697;
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 6
DOI 10.1002/aoc.3697

2
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HAJIPOUR ET AL.
energy‐dispersive X‐ray spectroscopy (EDX; Figure 1d) and
thermogravimetric analysis (TGA).
Also TEM micrograph of the catalyst taken after sixth
recycle (Figure 2) shows no significant change in the morphol-
ogy of the nanocatalyst, and therefore it can be realized that
the catalytic activity is preserved after recycling. As shown in
SCHEME 1 Preparation of heterogeneous catalyst Cu(II)‐PANi‐MWCNT
30]
[
help to disperse metallic particles in the carbon nanotubes.
Especially, the immobilization of copper nanoparticles on
polyaniline‐functionalized carbon nanotubes permits a higher
dispersity and stability of the particles and further use of the
[31]
particular activity of the catalyst.
However, to the best of our knowledge, there is no report
in the literature describing polyaniline‐functionalized
MWCNT‐supported copper(II) nanoparticles (Cu(II)‐PANi‐
MWCNT) as a nanocatalyst for the synthesis of unsymmetric
sulfides from potassium thioacetate with aryl, alkyl and ben-
zyl halides in water. In continuation of our interest in the
development of new catalysts for C─S bond formation reac-
[
11,32]
tions,
we report synthesis of Cu(II)‐PANi‐MWCNT
(Scheme 1) as a new, effective, recyclable and heterogeneous
nanocatalyst for the synthesis of unsymmetric thioethers in
water. This catalyst can be recovered and recycled for up to
six consecutive runs without important loss in its efficiency.
2
| RESULTS AND DISCUSSION
The Cu(II)‐PANi‐MWCNT catalyst employed for C─S cou-
pling was synthesized according to a procedure in the litera-
[
29]
ture.
PANi‐MWCNT composites were first synthesized
by the in situ polymerization of aniline in the presence of a
carbon nanotube suspension. Cu nanoparticles dispersed on
PANi‐MWCNT support were prepared by adding
Cu(OAc) ⋅H O salt to PANi‐MWCNT composites in a sus-
2
2
pension of tetrahydrofuran–water (1:1) (Scheme 1). PANi‐
[
29]
MWCNT composites were characterized by Lee et al.
The Cu(II)‐PANi‐MWCNT catalyst was characterized using
transmission electron microscopy (TEM; Figure 1a,b), induc-
tively coupled plasma (ICP) atomic emission spectrometry,
FIGURE 1 (a, b) TEM images of Cu(II)‐PANi‐MWCNT catalyst. (c)
Particle size distribution of catalyst. (d) EDX spectrum of catalyst

HAJIPOUR ET AL.
3 of 6
carbonodithioate was hydrolyzed in the presence of KOH.
Finally the generated thiolate reacted with iodobenzene to
form second C(aryl)─S bond to give phenyl(p‐tolyl)
[
33]
sulfane
(Scheme 2).
To optimize reaction conditions, we examined the effects
of bases and additives on this reaction. Among the studied
bases, KOH was the best and the corresponding product is
obtained in 95% yield (Table 1, entry 1). Also, among the
studied additives CTAB was the best (Table1, entries 1–4).
Then other solvents were investigated for the reaction
(Table1, entries 5–8). As evident from Table 1, water was
the best solvent. The amount of copper(II) complex was also
studied. A 15 mol% loading of copper(II) was found to be
optimal, since a lower yield was obtained when the amount
of complex is decreased (Table 1, entry 14). Finally, the
time of reaction was investigated (Table 1, entries 15 and
1
6). Time of 12 h for first step and 15 h for second step
were best (Table 1, entry 1).
After determining the optimized conditions, the Cu(II)‐
PANi‐MWCNT complex was used as a catalyst for this meth-
odology with aryl halides containing electron‐donating or
electron‐withdrawing substituents, alkyl halides and benzyl
bromide. The obtained results are summarized in Table 2.
On the whole, all the reactions gave corresponding products
(Table 2, pa–pq) in good to excellent yields ranging from
68 to 96%. Aryl iodides with electron‐withdrawing and elec-
tron‐donating substituents gave the corresponding products in
high yields (Table 2, entries 1–7). One‐pot thioetherification
of various aryl bromides bearing either electron‐withdrawing
groups or electron‐donating groups was also carried out
using potassium ethyl xanthogenate in some hours longer
and produces various aryl sulfides in good yields (Table 2,
entries 8–12). The yields of the coupling between Ar─I and
Ar─Br are good (Table 2, entries 13–17). Also, we investi-
gated the synthesis of various sulfides using alkyl halides.
These reactions proceed well to produce the corresponding
sulfides in good yields (Table 2, entries 18–23). The coupling
reactions of phenyl iodide and phenyl bromide with benzyl
bromide in the presence of potassium ethyl xanthogenate pro-
ceed well to produce the corresponding unsymmetric sulfides
in excellent yields (Table 2, entries 24 and 25).
FIGURE 2 TEM images of catalyst (a) after first run and (b) after sixth run
Figure 3, the TGA curve of catalyst shows three different
mass loss stages. The first mass loss, at around 25–150 °C,
is due to water and organic solvent residues and moisture
absorbed onto the catalyst. The second stage of weight loss
occurs between 200 and 550 °C and then there is a smooth
mass loss until 800 °C These TGA results indicate that the
thermal stability of the catalyst is considerable.
After the characterization of the composite, the catalytic
activity of the Cu(II)‐PANi‐MWCNT catalyst was examined
in the coupling reaction of 4‐iodotoluene with potassium
ethyl xanthogenate in the presence of 15 mol% copper
nanocatalyst and 30 mol% cetyltrimethylammonium bromide
(CTAB) at 80 °C in water. After 12 h, KOH and phenyl
iodide were added to the reaction mixture and the resulting
mixture was further heated for 15 h which afforded 95% of
an unsymmetric diaryl thioether, phenyl(p‐tolyl)sulfane.
Initially, a C(aryl)─S coupling between 4‐iodotoluene
and potassium ethyl xanthogenate was conducted to give
O‐ethyl S‐4‐methoxyphenylcarbonodithioate. Then, this
We also studied the recyclability and reusability of this
catalyst utilizing the reaction of 4‐iodotoluene with
potassium ethyl xanthogenate and iodobenzene under opti-
mum conditions as a model reaction (Table 3). After each
FIGURE 3 TGA curve of Cu(II)‐PANi‐MWCNT catalyst
SCHEME 2 Formation of phenyl(p‐tolyl)sulfane using copper nanocatalyst

4
of 6
HAJIPOUR ET AL.
a
TABLE 1 Optimization of conditions for synthesis of phenyl(p‐tolyl)sulfane
b
Entry
Base
KOH
Solvent
Additive
CTAB
Catalyst (mol%)
Time (h)
12/15
12/15
12/15
12/15
12/15
12/15
12/15
12/15
12/15
12/15
12/15
12/15
12/15
12/15
15/18
10/12
Yield (%)
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
15
15
15
15
15
15
15
15
15
15
15
15
20
10
15
15
95
89
73
78
91
90
82
76
52
45
48
59
95
86
95
84
KOH
KOH
KOH
KOH
KOH
KOH
KOH
TBAB
TBAI
TBAF
CTAB
CTAB
CTAB
CTAB
CTAB
CTAB
CTAB
CTAB
CTAB
CTAB
CTAB
CTAB
O–DMSO (1:1)
O–DMF (1:1)
3
O–CH CN (1:1)
O–EtOH (1:1)
K
2
CO
3
O
O
O
O
O
O
O
O
1
1
1
1
1
1
1
0
1
2
3
4
5
6
Na CO
2 3
t‐BuOK
PO
K
3
4
KOH
KOH
KOH
KOH
a
Reaction conditions: 4‐iodotoluene (1.0 mmol), potassium ethyl xanthogenate (1.0 mmol), idobenzene (1.0 mmol), base (3.0 mmol), CTAB (30 mol%), solvent (3 ml),
8
0 °C and aerobic conditions.
b
Isolated yield.
a
TABLE 2 Scope of coupling reactions of aryl halides with potassium ethyl xanthogenate, and aryl, alkyl or benzyl halides
b
Entry
Ar
X
I
Ar′/alkyl/benzyl
X′
I
Product
pa
Time (h)
12/15
8/15
Yield (%)
95
1
2
3
4
5
7
8
9
p‐CH
p‐NO
3
OC
6
H
4
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
6
C H
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
2
C
6
H
4
I
I
pb
pc
92
p‐CH
p‐CH
o‐CH
3
COC
6
H
4
I
I
8/15
90
3
C
C
6
H
H
4
I
I
pd
pe
12/15
12/15
9/15
95
3
6
4
I
I
94
o‐NO
p‐CH
p‐NO
p‐CH
m‐CH
Naphthalene
p‐CH OC
p‐NO
p‐CH
p‐NO
p‐CH
2
C
6
H
4
I
I
pf
91
3
OC
6
H
4
Br
Br
Br
Br
1‐Br
Br
Br
I
Br
Br
Br
Br
Br
I
pa
20/24
18/24
18/24
20/24
20/24
20/15
18/15
12/24
8/24
74
2
C
6
H
4
pb
pc
70
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
3
COC
6
H
4
71
3
OC
6
H
4
pg
ph
pa
70
73
3
6
H
4
81
2
C
6
H
4
I
pb
pa
79
3
OC
6
H
4
Br
Br
1‐Br
I
75
2
C
6
H
4
I
pb
pi
69
3
COC
6
H
4
I
Naphthalene
8/24
68
6
C H
5
Br
Br
I
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
pj
20/1
85
p‐CH
3
C
6
H
4
I
pk
pl
20/1
88
p‐BrC
6
H
4
I
12/1
92
6
C H
5
Br
Br
Br
I
CH
CH
CH
2
2
2
Br
Br
Br
Br
Br
pm
pn
po
pp
pp
20/2
89
p‐NO
p‐CH
2
C
6
H
4
18/2
87
3
COC
6
H
4
20/2
86
C
6
H
H
5
Benzyl
Benzyl
12/2
96
C
6
5
Br
20/2
89
a
Reaction conditions: aryl halide (1.0 mmol), potassium ethyl xanthogenate (1.0 mmol), aryl, alkyl or benzyl halide (1.0 mmol), copper nanocatalyst (15 mol%), KOH
3.0 mmol), CTAB (30 mol%), solvent (3 ml), 80 °C and aerobic conditions.
(
b
Isolated yield.

HAJIPOUR ET AL.
5 of 6
a
TABLE 3 Recyclability of catalyst under optimum reaction conditions
4 | EXPERIMENTAL
.1 | General
b
Entry
Cycle
Yield (%)
4
1
2
3
4
5
6
1
2
3
4
5
6
95
95
94
94
93
93
MWCNTs were purchased from Neutrino Co. (www.
neunano.com; diameter of 10–20 nm, length of 30 μm, purity
>
95%). All reagents and solvents were purchased from
Merck or Aldrich and used without further purification.
TEM images were obtained with an EM208S at an accelerat-
ing voltage of 100 kV. The metal content of the complex was
measured using ICP analysis (Varian vista‐mpx). GC was
performed using a BEIFIN 3420 gas chromatograph
equipped with a Varian CP SIL 5CB column (30 m,
a
Reaction conditions: 4‐iodotoluene (1.0 mmol), potassium ethyl xanthogenate
1.0 mmol), idobenzene (1.0 mmol), KOH (3.0 mmol), copper nanocatalyst
(
(
2
15 mol%), CTAB (30 mol%), H O (3 ml), 80 °C and aerobic conditions.
b
Isolated yield.
0
.32 mm, 0.25 mm). TGA of the catalyst was conducted
run, the catalyst was recovered by centrifugation, followed by
washing with water and ethyl acetate and then was dried. The
activity of the catalyst remains almost the same after being
reused sixth times.
using a laboratory‐made TGA instrument. The EDX analysis
(SEM, MIRA3 TESCAN) was engaged for characterization
1
13
of the catalyst. H NMR and C NMR spectra were recorded
with CDCl as solvent. Chemical shifts were measured in
3
It seems the substituent effects in this protocol are not
very important. A plausible pathway for the synthesis of sul-
fides with xanthogenate has been reported by Akkilagunta
ppm using tetramethylsilane as an internal standard. Also,
GC–MS was conducted using an Agilent 7890A‐5975C
instrument. Column chromatography was carried out on
columns of silica gel 60 (70–230 mesh).
[
17]
and Kakulapati.
In order to determine the absolute amount of the copper
species dissolved in solution caused by leaching, the reaction
of 4‐iodotoluene with potassium ethyl xanthogenate and
iodobenzene was carried out under the optimum conditions
and the catalyst was removed from the reaction mixture by
hot filtration after 12/7.5 h (at this time, almost 50% conver-
sion). After elimination of the catalyst, the reaction was con-
tinued and no more product conversion was seen and no
copper metal was identified in the solution using ICP analy-
sis. Also, a TEM image of recycled catalyst after six runs
was taken (Figure 2b), which does not show major changes
in the morphology or the size of the catalyst (2–3 nm;
Figure 1c). This confirms the preservation of the catalytic
activity of the catalyst after recycling.
4
.2 | Preparation of PANi‐MWCNT
PANi‐MWCNT composites were prepared according to a
[
29]
previously reported procedure. A round‐bottom flask con-
taining HCl solution (2 M, 100 ml) and MWCNT (1 g) was
sonicated for half an hour at room temperature. Then were
added a stoichiometric amount of aniline monomer and two
drops of FeSO (5%) in this suspension. An aqueous solution
of ammonium persulfate (1 g of (NH ) S O in 20 ml of
deionized water) was then added. This reaction mixture was
continuously stirred in an ice bath for 1 h and then brought
up to room temperature and stirred for another 3 h. At the
end of this time, the obtained PANi‐MWCNT was separated
by centrifugation and washed with distilled water and ethanol
and dried at 80 °C.
4
4 2
2 8
The catalytic activity of the Cu(II)‐PANi‐MWCNT cat-
alyst was compared with that of other copper nanoparticles
[
17,33]
that have been reported before.
This demonstrates
that the reaction time is shorter and the protocol is green
for the present copper catalyst compared to other copper
nanoparticles.
4
.3 | Preparation of Cu(II)‐PANi‐MWCNT
To a round‐bottom flask equipped with a magnetic stirrer bar
and containing 100 ml of THF–H O (1:1) was added PANi‐
2
MWCNT (100 mg). The reaction mixture was suspended
using ultrasonication for 30 min, and following that was
stirred for another 30 min. Cu(OAc) ⋅H O (0.03 g) was
3
| CONCLUSIONS
2
2
added to the solution and the reaction mixture was stirred
for 12 h at room temperature. Then the obtained black solid
was separated by centrifugation and washed several times
with distilled water, and dried in vacuum overnight at 60 °C.
We successfully prepared polyaniline‐functionalized
MWCNT‐supported copper(II) nanoparticles and developed
an efficient and green procedure for the synthesis of
unsymmetric sulfides using potassium ethyl xanthogenate in
good yields and with a broad range of functional group toler-
ance. This catalyst could be recovered by centrifugation and
reused six times without an important loss in its activity.
The reaction proceeded in a short period of time without
expensive metal and unclean smell of thiols.
4
.4 | General procedure for synthesis of unsymmetric
sulfides
An aryl halide (1.0 mmol) and potassium ethyl xanthogenate
(1.0 mmol) were added to a mixture of water (3 ml),

6
of 6
HAJIPOUR ET AL.
Cu(II)‐PANi‐MWCNT (15 mol%) and CTAB (30 mol%) in a
glass flask under vigorous stirring. The reaction mixture was
stirred at 80 °C until the aryl halide was consumed (12 h), as
determined by GC. Then KOH (3 mmol) and second aryl
halide (1.0 mmol) were added. The mixture was heated at
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[
20] M. Hosseini‐Sarvari, E. Sodagar, M. M. Doroodmand, J. Org. Chem. 2011,
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determined by GC. At the end of the reaction, the catalyst
was separated by centrifugation, washed with ethyl acetate,
ethanol and water, vacuum dried and kept for a subsequent
reaction. Then ethyl acetate was added to the reaction mixture
and the organic layer was extracted using saturated brine
solution. The solvent was removed under vacuum, and the
crude product was subjected to silica gel column
chromatography.
7
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