Palladium on nano-magnetite: A magnetically reusable catalyst in the ligand- and copper-free Sonogashira and Stille cross-coupling reactions

Article abstract of DOI:10.1039/c3ra48003h

This paper reports on the synthesis and use of palladium on nano-magnetite as a magnetically separable catalyst for copper- and ligand-free Songashira and Stille coupling reactions. The catalyst was characterized using powder XRD, SEM, EDS and VSM techniques. This method has the advantages of high yields, simple methodology and easy work-up. Catalytic efficiency remains unaltered even after several repeated cycles.

Full text of DOI:10.1039/c3ra48003h

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Palladium on nano-magnetite: a magnetically
reusable catalyst in the ligand- and copper-free
Cite this: RSC Adv., 2014, 4, 19731
Sonogashira and Stille cross-coupling reactions
a
Mahmoud Nasrollahzadeh, Mehdi Maham,b Ali Ehsania and Mehdi Khalajc
*
This paper reports on the synthesis and use of palladium on nano-magnetite as a magnetically separable
catalyst for copper- and ligand-free Songashira and Stille coupling reactions. The catalyst was
characterized using powder XRD, SEM, EDS and VSM techniques. This method has the advantages of
high yields, simple methodology and easy work-up. Catalytic efficiency remains unaltered even after
several repeated cycles.
Received 27th December 2013
Accepted 3rd March 2014
DOI: 10.1039/c3ra48003h
www.rsc.org/advances
amines, such as triethylamine and piperidine, which are
required in most Sonogashira reactions, have a bad smell and
Introduction
add to the environmental burden.
The Sonogashira coupling reaction of terminal alkynes with aryl
or vinyl halides provides a powerful and straightforward
method for C(sp)–C(sp2) bond formation, which has been
widely applied to diverse areas such as natural product
synthesis, pharmaceuticals, biologically active molecules and
materials science.1–3
Among the various palladium catalysts available for the
coupling reactions, homogeneous catalysts have been widely
investigated, while less expensive heterogeneous catalysts
received less attention.8 The problem with homogeneous
catalysis is the difficulty in separating the catalyst from the
reaction mixture and the impossibility to reuse it in consecutive
reactions. Therefore, the development of a mild, highly efficient
and environmentally benign method for the ligand-, copper-
and amine-free Sonogashira and Stille coupling reactions
remains an active research area. Thus, we decided to concen-
trate on developing an efficient heterogeneous palladium cata-
lyst that is air and moisture stable and highly active.
In recent years, metal nanoparticles have attracted great
attention as heterogeneous catalysts due to their interesting
structure and high catalytic activities.9 In particular, magnetic
nanoparticles have emerged as one of the most useful hetero-
geneous catalysts due to their numerous applications in nano-
catalysis, biotechnology, and medicine.10 Recent reports show
that iron oxide magnetic nanoparticles (Fe3O4-MNPs) are the
most promising catalysts and are excellent supports for cata-
lysts because of their ease of handling, ease of recovery with an
external magnetic eld and high catalytic activities in various
organic transformations.11
Unfortunately, most methods for the immobilization of Pd
on magnetic nanocomposites have two disadvantages: (1) the
ferromagnetic nanoparticles aggregate too easily due to their
magnetic dipole–dipole interactions, which decrease the effi-
ciency of catalysis. (2) Surfactants, which are used to combine
magnetic nanoparticles and Pd nanoparticles, would introduce
other organic groups and as a result reduce the yield of prod-
ucts.12,13 In continuation of our recent studies on applications of
heterogeneous catalysts,14–21 we describe the preparation of
Pd@Fe3O4 nanowires with high magnetic sensitivity, high
The Stille cross-coupling reaction of organohalides with
organotin compounds has been proven to be an important
synthetic tool for C–C bond formation in organic synthesis.4,5
The Sonogashira reaction generally proceeds in an organic
solvent, such as an amine, benzene, THF, DMF or DMAC, with a
complexed palladium catalyst in conjunction with CuX (X ¼ Cl,
Br, I) as a co-catalyst and in the presence of toxic phosphine
ligands and a stoichiometric amount of base under inert
conditions, which are economically and environmentally
malignant.6 Earlier reported methods for the Sonogashira
coupling reaction suffer from certain disadvantages such as the
use of expensive and air or moisture sensitive phosphine
ligands, the environmental pollution caused by the utilization
of homogeneous catalysts, low yields, long reaction times,
tedious work-up, the need for waste control and the formation
of side products.6,7 In the Sonogashira coupling reaction, the
copper salt facilitates the coupling reaction by the in situ
generation of a copper acetylide. However, the copper salts used
as the co-catalysts would induce a Glaser-type oxidative dimer-
ization of the terminal acetylene as a side reaction.7 In addition,
aDepartment of Chemistry, Faculty of Science, University of Qom, PO Box 37185-369,
Qom, Iran. E-mail: mahmoudnasr81@gmail.com; Fax: +98 25 32103595; Tel: +98 25
32850953
bDepartment of Chemistry, Aliabad Katoul Branch, Islamic Azad University, Aliabad
Katoul, Iran
cDepartment of Chemistry, Buinzahra Branch, Islamic Azad University, Buinzahra,
Qazvin, Iran
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stability and applications in Sonogashira and Stille coupling
reactions (Scheme 1). Pd@Fe3O4 nanowires were synthesized
through the arc discharge of Fe in deionized (DI) water and
electroless deposition of palladium. The main advantage of the
present method is the direct formation of Fe nanowires from
the discharge of iron electrodes in water.
To the best of our knowledge, there are not any reports on
the preparation of iron nanowires by the electrical arc discharge
method. In general, electrical arc discharge in water has the
advantage in this regard as it produces self-crystallized nano-
particles due to the high temperature caused by joule heating.
Moreover, compared with other techniques, electrical arc
discharge in water is an attractive method because of the
simplicity of the experimental set up, the lack of complicated
equipment, the low level of impurities and fewer production
steps, leading to a high-throughput and cost-effective procedure
to generate high yields of nanoparticles. Furthermore, it was
found that the synthesized Pd@Fe3O4 nanowires exhibited
efficient catalytic activities for Sonogashira and Stille coupling
reactions. Fe3O4 was used as a support for Pd nanoparticles, as
well as enhancing the dispersion of catalytically active sites and
improving its catalytic activity. More importantly, the synthe-
sized Pd@Fe3O4 nanowires presented good magnetic proper-
ties, and could be easily separated from the reaction mixture
using an external magnet and reused many times with no loss of
activity, indicative of a potential application in industry.
Fig. 1 The scanning electron microscope image of a Pd@Fe3O4
sample after deposition of Pd.
with sizes ranging from 1 mm to less than 20 mm in length and
about 100 nm in diameter. In addition, Pd particles appear as
bright dots over the surface of the Fe3O4 nanowires with average
sizes of less than 30 nm in diameter.
Phase investigation of the crystallized product was per-
formed by XRD and the powder diffraction pattern of the
Pd@Fe3O4 nanowires is presented in Fig. 2. The presence of
iron and palladium was conrmed with powder XRD
measurements. The results showed that the products consisted
of a face-centered cubic (fcc) structure.
The elemental composition of the catalyst was also analyzed
by Energy Dispersive X-ray Spectroscopy (EDS). This further
conrmed that the Pd@Fe3O4 nanowires were composed of
palladium, iron and oxygen (Fig. 3). The EDS results show that
Fe, Pd and oxygen concentrations are about 9%, 1.2% and 51%,
respectively. In the EDS analysis, Fe is the major element which
is derived from Fe3O4 and small amounts of Pd can be identied
Result and discussion
The Pd@Fe3O4 nanowires were characterized using powder
XRD, SEM, VSM and EDS.
Fe3O4 nanowires decorated with Pd nanoparticles were
fabricated in two steps using a simple process. Fe3O4 nanowires
in lengths of several microns were synthesized through arc
discharge of iron electrodes in DI water and then they were
dispersed on glass substrates and dried in air. Palladium
nanoparticles were overlaid on the surface of the Fe3O4 nano-
wires via a simple drop-drying process by dropping PdCl2
solution onto the Fe nanowire lms and drying them at room
temperature. Fig. 1 illustrates typical morphologies of the
Pd@Fe3O4 nanowires obtained with 5 A arc currents. In general
Fe tends to form wire-like structures due to its lattice geometry.
The presence of a high temperature region and DI water in the
reactor leads to the formation of a temperature gradient and a
fast condensation process. In fact, rapid cooling of the products
reduces the growth rate of the created nuclei and they do not
have enough time to form wire-shaped structures before stabi-
lization. Hence, there are also shapes other than nanowires in
our samples, which is due to fast condensation. Scanning
electron microscopy images illustrate Pd@Fe3O4 nanowires
Scheme 1 Sonogashira and Stille coupling reactions of different aryl
halides.
Fig. 2 XRD powder pattern of the Pd@Fe3O4 nanowires.
19732 | RSC Adv., 2014, 4, 19731–19736
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Table
1
Optimization of reaction conditions in the Sonogashira
coupling reaction of p-iodoanisole with phenylacetylenea
Pd@Fe3O4
nanowires (mol%)
Entry
Base
Yieldb (%)
1
2
3
4
5
6
7
8
9
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
1.0
DMAP
Pyridine
K3PO4
NaOH
KOH
0
0
42
Trace
Trace
35
84
50
NEt3
Piperidine
Piperidine
Piperidine
84
a
Reaction conditions: 1.0 equiv. of aryl halide, 1.0 equiv. of terminal
b
alkyne, and 1.5 equiv. of base, 110 ꢁC, 24 h. Isolated yield.
Fig. 3 EDS spectrum of the Pd@Fe3O4 nanowires.
reaction, and the effects of the base were examined. As shown in
Table 1, the reaction was inuenced signicantly by the base
employed and the best result was obtained in the case of
piperidine (Table 1, entry 7). The optimum conditions were
found to be 1.0 mmol of aryl halide, 1.0 mmol of phenyl-
acetylene, 0.5 mol% of the Pd@Fe3O4 nanowires and 1.5 mmol
of piperidine in DMF (4 mL) at 110 ꢁC. The use of a lower
catalyst loading resulted in incomplete reaction (entry 8).
Increasing the amount of catalyst gave no substantial
improvement in the yield (entry 9).
at 2.8387 keV of energy. The excess oxygen is due to physical
absorption of oxygen from the environment during sample
preparation.
In addition, the room temperature magnetic properties of
the Pd@Fe3O4 nanowires were also investigated. Fig. 4 shows
the hysteresis loop of the Pd@Fe3O4 nanowires obtained in an
applied magnetic eld at 298 K by using a SQUID magnetom-
eter. The magnetic saturation value of the composite was esti-
mated to be 58.4 emu gꢀ1. The advantage of the synthesized
nanowires is that they can be easily separated from the solution
by the application of an external magnetic eld.
Upon optimization of the reaction conditions, the scope of
the protocol was subsequently extended to
a range of
substituted aryl halides and terminal alkynes as substrates
(Table 2). The electron-neutral, electron-rich and electron-poor
aryl halides reacted with phenylacetylene very well to generate
the corresponding cross-coupling products in good to excellent
yields under the standard reaction conditions. In addition, the
steric hindrance of the substituent did not inuence the
product yield in the copper-free Sonogashira reaction of deac-
tivated aryl halides using the Pd@Fe3O4 nanowires (Table 2,
entries 7 and 15).
Evaluation of the catalytic activity of the Pd@Fe3O4 nanowires
in the Sonogashira coupling reaction
The catalytic behaviour of the Pd@Fe3O4 nanowires was studied
for the Sonogashira coupling reaction.
For optimization of the reaction conditions, we chose the
reaction of p-iodoanisole with phenylacetylene in DMF in the
presence of 0.5 mol% of the Pd@Fe3O4 nanowires as the model
Evaluation of the catalytic activity of the Pd@Fe3O4 nanowires
in the Stille coupling reaction
Due to the importance of the Stille cross-coupling reaction in
synthetic organic chemistry for the manufacture of C–C bonds,
we next turned our attention to applying the Pd@Fe3O4 nano-
wires to the Stille cross-coupling reaction of organohalides with
organotin compounds. The desired products are obtained in
good yields.
We initially selected the coupling of bromobenzene and
tributylphenylstannane as a model reaction. As expected, no
target product could be detected in the absence of catalyst.
Among the various solvents (THF, DMF, benzene, MeCN and
1,4-dioxane) tested in the presence of the Pd@Fe3O4 nanowires,
1,4-dioxane led to signicant conversion. The reactivity of the
Pd@Fe3O4 nanowires in 1,4-dioxane in the presence of various
bases (K2CO3, Na2CO3, Cs2CO3 and CsF) was also investigated.
Among the tested bases, CsF was found to be superior, giving
Fig. 4 Room temperature magnetization curve of the Pd@Fe3O4
nanowires.
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Table 2 Sonogashira coupling reactions of different aryl halides with Table
terminal alkynes
3
Stille coupling reactions of different aryl halides with
organostannanesa
Yielda (%)
Entry
Aryl halide
Organostannane
Time (h)
Yieldb (%)
Entry
Aryl halide
Terminal alkyne
1
2
12
16
91, 88c
85
1
2
3
4
C6H5C^CH
C6H5C^CH
C6H5C^CH
C6H5C^CH
84 (81)b
76
79
3
4
14
20
84
77
70
5
6
C6H5C^CH
C6H5C^CH
84
75
5
6
7
8
9
13
14
13
15
18
90
88
86
87
84
7
C6H5C^CH
82
8
C6H5C^CH
77
68
67
85
80
87
80
9
C6H5C^CH
10
11
12
13
14
C6H5C^CH
10
20
75
p-OCH3C6H4C^CH
p-OCH3C6H4C^CH
p-OCH3C6H4C^CH
p-OCH3C6H4C^CH
11
12
13
14
16
16
16
17
88
86
84
84
15
p-OCH3C6H4C^CH
82
15
20
77
16
17
p-FC6H4C^CH
p-BrC6H4C^CH
84
82
16
17
22
22
83
80
18
p-CH3C6H4C^CH
81
a
Yields are aer work-up. b Yield aer the 6th cycle.
18
22
82
a
Reaction conditions: aryl halide (1.0 mmol), organostannane (1.1
mmol), Pd@Fe3O4 nanowires (0.5 mol%), CsF (2.0 mmol) and 1,4-
b
dioxane (4 mL), aerobic conditions, 100–110 ꢁC. Isolated yield.
the highest yield of product. The best result was obtained with
bromobenzene (1.0 mmol), tributylphenylstanne or 1-(tributyl-
stannyl)-2-phenylethyne (1.1 mmol), Pd@Fe3O4 nanowires (0.5
mol%), CsF (2.0 mmol), and 1,4-dioxane (4 mL) at 100–110 ꢁC,
which gave the product in a good yield (88%). We have exam-
ined the coupling reaction of organostannanes with various aryl
halides containing electron-withdrawing or electron-donating
groups. As shown in Table 3, organostannanes were converted
into the corresponding products in high yields under ligand-
free and aerobic conditions.
c
Yield aer the 6th cycle.
Aer removal of the solvent and purication, the products
were characterized by their melting points, elemental analysis
(CHN), IR, 1H NMR and 13C NMR. All of the products are known
and were identied by comparison of their physical and spectral
data with those of authentic samples. All melting points
compared satisfactorily with those reported in the literature.
19734 | RSC Adv., 2014, 4, 19731–19736
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