Palladium supported on natural phosphate: Catalyst for Suzuki coupling reactions in water

Article abstract of DOI:10.1016/j.apcata.2012.09.046

The Suzuki-Miyaura coupling reaction is one of the most important synthetic catalytic reactions developed in the 20th century. However, the use of toxic organic solvents for this reaction still poses a scientific challenge and is an aspect of economical and ecological relevance. The use of water as a reaction medium overcomes this issue. In the present work, we described efficient Suzuki coupling reactions in water, without any phase transfer reagents and it is possible to couple challenging substrates like aryl chlorides. Notably, this protocol also works with ultra-low loading of catalyst with high turnover numbers.

Full text of DOI:10.1016/j.apcata.2012.09.046

Applied Catalysis A: General 450 (2013) 13–18
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Palladium supported on natural phosphate: Catalyst for Suzuki
coupling reactions in water
Ayoub Hassinea, Saïd Sebtia, Abderrahim Solhyb, Mohamed Zahouilyb,f
,
Christophe Lenc,∗, Mohamed Nejib Hedhilid, Aziz Fihrib,e,∗∗
a Laboratoire de Chimie Organique Catalyse et Environnement (URAC 17), Université Hassan II Mohammedia-Casablanca, Faculté des Sciences Ben M’Sik,
B.P. 7955, 20702 Casablanca, Morocco
b MAScIR Foundation, VARENA Center, Rabat Design, Rue Mohamed El Jazouli, Madinat Al Irfane 10100 Rabat, Morocco
c Transformations Intégrées de la Matière Renouvelable, UTC/ESCOM, Centre de Recherche Royallieu, B.P. 20529, 60205 Compiègne, France
d Imaging and Characterization Laboratory, King Abdullah University of Science and Technology, KAUST, Thuwal 23955-6900, Saudi Arabia
e KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology, Thuwal 23955, KSA, Kingdom of Saudi Arabia
f
Laboratoire de Catalyse, Chimiométrie et Environnement, URAC 24, Faculté des Sciences et Techniques, Université Hassan II, Mohammedia B. P. 146,
20650, Morocco
a r t i c l e i n f o
a b s t r a c t
Article history:
The Suzuki–Miyaura coupling reaction is one of the most important synthetic catalytic reactions devel-
oped in the 20th century. However, the use of toxic organic solvents for this reaction still poses a scientific
challenge and is an aspect of economical and ecological relevance. The use of water as a reaction medium
overcomes this issue. In the present work, we described efficient Suzuki coupling reactions in water,
without any phase transfer reagents and it is possible to couple challenging substrates like aryl chlorides.
Notably, this protocol also works with ultra-low loading of catalyst with high turnover numbers.
© 2012 Elsevier B.V. All rights reserved.
Received 29 May 2012
Received in revised form
12 September 2012
Accepted 14 September 2012
Available online 9 October 2012
Keywords:
Natural phosphate
Palladium
Supported catalysts
Suzuki coupling reactions
1. Introduction
and the potential to couple with widely available and inexpen-
sive aryl chlorides [11–15]. However, traditional homogeneous
Palladium-catalyzed reactions are of utmost importance in
synthetic organic chemistry [1,2]. The industrial synthesis of
many aryl-containing pharmaceuticals such as anti-inflammatory
drugs and blood-pressure regulators include one or several steps
of palladium-catalyzed carbon–carbon cross coupling [3–8]. The
Suzuki reaction is one of the most important organic transfor-
mations developed in modern synthetic organic chemistry in the
20th century [9,10]. The popularity of this reaction is attributable
to a variety of factors, such as the commercial availability of a
large number of boronic acids, as well as their nontoxic nature
and stability to heat, air, and moisture. These reactions are usu-
ally catalyzed with palladium in presence of auxiliary ligands using
homogeneous catalytic systems. This process is favorable due to its
high reactivity, high turnover numbers, milder reaction conditions,
catalysis poses significant issues related to product purification and
toxic waste produced after separation of palladium catalyst. These
problems are a significant environmental and economic concern
in large scale-synthesis. One of the most favorable ways to over-
come these problems is the use of heterogeneous catalysis because
of its easy recovery and reusability by simple filtration, decanta-
tion, and recently by magnetic altercations [16]. Palladium can
be immobilized on several solid supports such as activated car-
bon silica and organic polymers [14–28]. In an efforts to develop
a new heterogeneous system, we have initiated a program aimed
at introducing natural phosphate (NP) as it is or conveniently
modified phosphate that can catalyze several organic transfor-
mations. Some examples include Michael-addition, Knoevenagel,
Claisen–Schmidt condensation, Friedel–Crafts alkylation, alkene
epoxidation, nitrile hydration, and transesterification reactions
[29–35]. The use of toxic organic solvents remains a scientific chal-
lenge of economic and environmental relevance. It is now widely
accepted that their use is considered to be one of the major sources
of waste in fine chemical industries and poses serious health as well
as environmental hazards. Hence, the need for alternative reaction
∗
Corresponding author. Tel.: +33 344238828; fax: +33 344971591.
Corresponding author. Tel.: +966 28084570; fax: +966 28080302.
E-mail addresses: christophe.len@utc.fr (C. Len), aziz.fihri@kaust.edu.sa
∗∗
(A. Fihri).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.09.046

14
A. Hassine et al. / Applied Catalysis A: General 450 (2013) 13–18
Fig. 1. XRD diffraction patterns of (a) NP and (b) PdNP.
media is now widely acknowledged and has become a major focus
of research in industry and academia. Using water as a benign reac-
tion medium is highly effective in overcoming some of these issues
[36–38]. Herein, we report a convenient method for the preparation
of a recyclable and heterogeneous natural phosphate-supported
palladium catalyst (PdNP) by treatment of natural phosphate (NP)
with PdCl2(PhCN)2 in acetone and its application for the synthesis
of biaryls via Suzuki couplings using water as solvent. The as syn-
thesized catalyst is well characterized by XRD, FTIR, XPS, ICP-OES,
SEM and CHN elemental analysis.
2. Results and discussion
Fig. 2. FTIR spectra of (a) NP and (b) PdNP.
The natural phosphate used in this work was prepared as
described in the earlier paper [39]. The treatment of the NP with
PdCl2(PhCN)2 in acetone for 8 h at room temperature yielded the
PdNP. The characterization of PdNP was carried out by XRD, FTIR,
XPS, ICP-AES, SEM and CHN elemental analysis. The structure of
NP is similar to that of fluorapatite (Ca10(PO4)6F2) as shown by
the X-ray diffraction pattern and chemical analysis [40]. The palla-
dium content was measured to 0.023 ␮mol g−1 using ICP-OES. The
XRD peaks for PdNP were similar to those of the natural phosphate
(Fig. 1). The NP possesses a very low surface area (BET) of 2 m2 g−1
which further decreased to 1.7 m2 g−1 after the immobilization. No
signals assignable to ꢀ(CN) bands were observed in IR spectroscopy,
suggesting that a PhCN moiety arising from the Pd precursor was
absent in the catalyst (Fig. 2). The absence of carbon and nitrogen
atoms in PdNP was also evidenced by the CHN elemental analy-
sis. Inductively coupled plasma analysis revealed the absence of Ca
in the filtrate after palladium loading which clearly demonstrated
that substitution of Ca with Pd did not occur in the organic solvent.
Fig. 3 illustrates typical microstructures of NP, fresh PdNP and
used PdNP materials. A homogeneous microstructure was observed
for the NP powder where the particles are distributed in the entire
observed surface, while for the PdNP powder, only particle clusters
were observed. The morphology of PdNP does not undergo a major
change except that the agglomerates become more compact. This is
a sign of high heterogeneity, which is evident in this material. Addi-
tionally, it is clear from the images that the size of PdNP particles
is larger as compared to the NP.
High resolution XPS spectrum of Pd 3d core level was also
obtained and is shown in Fig. 4a. The Pd 3d core level was fitted
with three doublets (Pd 3d5/2–Pd 3d3/2) with fixed area ratio equal
Fig. 3. Scanning electron micrograph images of NP: (left most), PdNP (centered) and reused PdNP (right most).

A. Hassine et al. / Applied Catalysis A: General 450 (2013) 13–18
15
phosphate supported palladium was investigated and representa-
tive results are summarized in Table 2. Aryl bromides, consisting
of various functional groups, efficiently reacted with arylboronic
acids (entries 1–10) and yielded Suzuki products with good to
excellent yields. 2-Bromopyridine underwent a smooth reaction
with phenylboronic acid (entry 11), providing a useful way for the
synthesis of aryl-substituted nitrogen heterocycles. However, the
coupling reaction of 2-bromothiophene and phenylboronic acid
failed to provide the target product even with increased cata-
lyst loading or reaction time. This may be due to its instability
in water or when a ␣-substituted heteroaryl bromides are used,
a possible interaction between the hetero-element and the pal-
ladium complex occurred and had a deactivation effect on the
rate of the reaction (entry 12). To resolve this ambiguity we per-
formed the reaction of 4-bromoacetophenone with phenylboronic
acid under same reaction conditions using 3.75 mmol of thiophene.
We observed that the yield drastically reduced to 27% from orig-
inal 95% (entry 13). This clearly indicated the deactivating effect
of 2-bromothiophene on catalytic activity of palladium. Finally, the
coupling reaction of the sterically hindered 2-bromo-m-xylene and
phenylboronic acid leads to the desired product with poor yield
(entry 14).
The utilization of readily available and affordable aryl chlorides
in palladium-catalyzed cross-coupling reactions continues to pose
difficulties [45]. Several research groups have described electron-
rich and bulky ligands for homogeneous palladium complexes, in
particular PBu(adamantyl), PtBu3, PCy3, or dialkylarylphosphines,
that exhibit high activities toward the activation of aryl chlo-
rides [46–50]. Despite impressive progress, a number of challenges
remain unsolved. The cross coupling reactions of aryl chlorides
using heterogeneous palladium catalysts are relatively unexplored
despite offering significant advantages from an environmental
point of view. Although our catalyst was active for various aryl bro-
mides, our main objective was to design the catalyst for the Suzuki
coupling of aryl chlorides [51,52]. Therefore, a further investiga-
tion was conducted to develop an effective heterogeneous catalyst
system for the coupling reaction using of 1-nitro-4-chlorobenzene
with phenylboronic acid as a model reaction. In our initial attempts,
we used the same optimal conditions as for the aryl bromides,
but obtained lower conversions. Therefore, we optimized sev-
eral reaction parameters, including the solvent, base, and reaction
temperature. Finally, we found that the use of additive such as
tetra-n-butylammonium bromide (TBAB) as well as changing the
base from K3CO3 to NaOH was necessary to achieve a good yield
(Table 3). The effect of water might be explained by its role in
increasing the concentration of both TBAB and inorganic bases on
the hydrophilic surface of the PdNP. Water exists as a supported
thin film at the interface between the catalyst and the organic phase
to enhance the accessibility of these reagents [53]. Under identical
conditions the aryl chlorides bearing electron-withdrawing groups,
4-chlorobenzonitrile and 4-chloroacetophenone, the Suzuki reac-
tions with phenylboronic acid led to good yield (81% or 87%,
respectively). Unfortunately, no product was obtained in the cou-
pling of aryl chlorides bearing electron-donating groups.
8000
6000
4000
2000
a
Pd 3d
PdPN
348
346
344
342
340
338
336
334
332
330
Binding energy (eV)
b
Pd 3d
2000
1800
1600
PdNP reused
348
346
344
342
340
338
336
334
332
330
Binding energy (eV)
Fig. 4. XPS spectra of (a) Pd2+ in PdNP and (b) Pd0 in used PdNP catalyst.
to 3:2 and doublet separation at 5.3 eV. The Pd 3d5/2 was composed
of three components located at 335.7 eV, 337.5 eV and 338.4 eV.
The main peak was observed at 337.5 eV binding energy corre-
sponds to Pd (Pd2+) in a complex with two Pd Cl bonds, having
the same binding energy as PdCl2 [41]. The atomic concentrations
of detected elements were determined from the survey spectrum of
PdNP powder. The measured Pd/Cl ratio was about 0.57:1. The the-
oretical Pd/Cl ratio was about 0.5:1 for PdCl2. This indicated that the
measured Pd concentration was greater than the theoretical value.
Pd was not only present in the PdCl2 complex but also existed in
different species or/and in different oxidation states. Furthermore,
the peak located at 335.7 eV was attributed to metallic Pd (Pd0) and
the peak located at 338.4 can be assigned to Pd (Pd2+) complexes
surrounded by four oxygens [41–44].
To explore the potential activity of this heterogeneous cata-
lyst, we initially investigated the reaction of 4-bromoacetophenone
with phenylboronic acid to establish the feasibility of our strategy
and to optimize the reaction conditions (Table 1). Initially, the reac-
tion was conducted without a base and no activity was observed,
despite a prolonged reaction time. After screening several bases, the
relatively weak and less expensive K2CO3 yielded the best results
among those that were surveyed.
We compared our results with reported hydroxyapatite sup-
ported palladium catalyst for Suzuki coupling reaction of aryl
bromides; however the reported catalyst system requires the use
of TBAB as phase transfer agent with high catalytic loading [54]. In
addition, our catalyst system exhibits comparable activity in terms
of catalyst loading and yield for the Suzuki coupling reaction of
activated aryl chlorides reported using activated carbon supported
palladium [55,56]. In case of unreactive aryl chlorides we did not
observed any activity contrary to what is reported by Jin et al. using
silica coated magnetic nanoparticle supported palladium [57].
Encouraged by these overall results, the recyclability and leach-
ing study of the catalytic system for Suzuki coupling reaction
During the course of the reaction, the color of the catalyst
changed. The high resolution XPS spectrum of the used PdNP
showed peaks at 335 eV and 340.6 eV for Pd 3d5/2 and Pd 3d3/2,
respectively, which clearly demonstrated that Pd in the used cat-
alyst is reduced to Pd(0) (Fig. 4b). To further examine the scope
of this protocol, the coupling of various aryl halides by natural

16
A. Hassine et al. / Applied Catalysis A: General 450 (2013) 13–18
Table 1
Optimization of the reaction conditions.
a
Entry
Base
Yield (%)
1
2
3
4
5
6
NaOH
KF
K3PO4
Cs2CO3
K2CO3
K2CO3
91
92
70
72
95
94b
a
Reaction conditions: 4-bromoacetophenone (3.75 mmol), phenylboronic acid (4.5 mmol), base (7 mmol), PdNP (0.005 g, Pd: 0.117 ␮mol), 2 mL of H2O, 100 ◦C, 5 h, yield
determined by using GC.
b
10 mL of water.
Table 2
Heterogeneous Suzuki coupling of aryl bromides with arylboronic acid.
a
Entry
1
R1
R2
H
Product
Yield (%)
95
COMe
NO2
CN
MeCO
2
3
4
5
6
7
NO2
CN
H
96
90
90
97
85
92
H
OMe
OH
MeO
OH
H
H
MeOC
O2N
OMe
MeCO
NO2
OMe
OMe
OMe
NC
OMe
8
9
CN
OMe
OMe
93
80
MeO
OMe
OH
OMe
MeO
10
11
OH
OMe
H
90
70
Br
Br
N
N
S
12
13
H
H
0
S
27b
COMe
MeCO

A. Hassine et al. / Applied Catalysis A: General 450 (2013) 13–18
17
Table 2 (Continued)
Entry
R1
R2
H
Product
Yield (%)
10
Br
14
a
Reaction conditions: aryl bromides (3.75 mmol), arylboronic acids (4.5 mmol), K2CO3 (7 mmol), PdNP (0.005 g, Pd: 0.117 ␮mol), 2 mL of H2O, 100 ◦C, 4 h, yields determined
by using GC.
b
3.75 mmol of thiophene.
Table 3
Suzuki coupling reaction of aryl chlorides with phenylboronic acid using PdNP.a
R
Yield (%)
NO2
MeCO
CN
97
87
81
a
Reaction conditions: aryl chlorides (3.75 mmol), phenylboronic acid (4.5 mmol), NaOH (7 mmol), PdNP (0.24 g, Pd: 5.64 ␮mol), 2 mL of water, 100 ◦C, 24 h, yields deter-
mined by using GC.
Fig. 5. Recycling study of PdNP using the Suzuki coupling reaction of 4-bromoacetophenone and phenylboronic acid.
were examined. After recovery of PdNP by simple filtration fol-
lowed by washing with ethanol and drying, we established a set of
experiments using the recycled catalyst for the Suzuki reaction of
4-bromoacteophenone and phenylboronic acid. The reactions were
carried out under similar conditions in water. After the completion
of the first reaction affording the corresponding coupling product
with 95% yield, the catalyst was recovered and a fresh reaction was
then performed under same conditions. The product yield for 2nd
and 3rd cycle was nearly the similar (93%). However during the 4th
cycle; it reduced to 88% (Fig. 5). Thus, this catalyst system could be
used for at least 4 cycles with a negligible change in its activity. The
XRD and FTIR of the used catalyst did not reflect any changes.
To check the leaching of our catalyst in the Suzuki reaction of
4-bromoacetophenone and phenylboronic acid, the catalyst was
removed at 40% conversion by simple filtration and the filtrate
was further reacted with fresh K2CO3 at 100 ◦C for 4 h. The above
treatment of the filtrate gave no yield. It clearly demonstrated that
no leaching of palladium in active form occurred which was also
confirmed by ICP-OES analysis.
recovered and reused without any significant loss of its catalytic
activity. No leaching of the catalyst to the organic layer was
observed. The novelty of the system lies in its low cost and avail-
ability of the solid support. Further studies in the application of this
catalyst to other organic reactions are underway in our laboratory.
4. Experimental
All the reactions and workup procedures were performed under
an inert atmosphere of argon using conventional vacuum-line and
glasswork techniques. Solvents were dried and freshly distilled
under argon. All other materials were obtained from commer-
cial suppliers and were used as received. Water (deionized) was
degassed by sparging with nitrogen. Chromatography was per-
formed on neutral silica gel. NMR spectra were recorded on a Bruker
instrument operating at 400.17 MHz for proton, 100.63 MHz for
carbon. X-ray powder diffraction (XRD) data were collected on a
Simens/D-5000 diffractometer using Cu K␣ radiation. The exter-
nal morphology of the samples was observed on a Philips TECNAI
F12 FEI transmission electron microscope. XPS studies were car-
ried out in a Kratos Axis Ultra DLD spectrometer equipped with
a monochromatic Al K␣ X-ray source (hꢀ = 1486.6 eV) operating
at 150 W, a multi-channel plate and delay line detector under
1.0 × 10−9 Torr vacuum. The survey and high-resolution spectra
were collected at fixed analyzer pass energies of 160 and 20 eV,
respectively. The instrument work function was calibrated to give
an Au4f7/2 metallic gold binding energy of 83.95 eV. The spectrome-
ter dispersion was adjusted to give a binding energy of 932.63 eV for
3. Conclusions
In the present work, we have shown that natural phosphate-
supported palladium can provide a suitable catalyst for Suzuki
coupling reactions. This heterogeneous catalyst displayed high cat-
alytic activity for Suzuki coupling of bromides in neat water without
using any phase transfer reagent and chloroarenes at 100 ◦C in
the presence of TBAB. Furthermore, this catalyst can be simply

18
A. Hassine et al. / Applied Catalysis A: General 450 (2013) 13–18
metallic Cu 2p3/2. Samples were mounted in floating mode in order
to avoid differential charging, charge neutralization was required
for all samples. Binding energies were referenced to the C 1 s bind-
ing energy of adventitious carbon contamination which was taken
to be 284.80 eV [58,59]. The data were analyzed with commercially
available software, CasaXPS. The individual peaks were fitted by a
Gaussian (70%)–Lorentzian (30%) (GL30) function after Shirley type
background subtraction.
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(PdNP)
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from an ore in the region of Khouribga (Morocco). A sample grain
size of 100–400 ␮m was washed with water and calcined at 900 ◦C
for 2 h, washed again, recalcined at 900 ◦C for 0.5 h and ground
(63–125 ␮m). The NP (2 g) was stirred in 100 mL acetone solution
of PdCl2(PhCN)2 (2.8 M) at room temperature for 8 h. The obtained
suspension was filtered, washed with acetone and dried under vac-
uum, yielding PdNP.
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5.1. Typical procedure for Suzuki coupling of aryl bromides using
PdNP as catalyst
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PdNP (0.005 g, Pd: 0.117 ␮mol), K2CO3 (0.967 g, 7 mmol), and 4-
methoxyphenylboronic acid (0.683 g, 4.5 mmol) were placed into
a 10 mL Schlenk vessel under nitrogen atmosphere. Water (2 mL)
and 4-bromoacetophenone (0.746 g, 3.75 mmol) were added and
the resulting mixture was stirred in the oil bath at reflux for 4 h.
The reaction mixture was extracted with dichloromethane and the
organic layers were combined, dried over MgSO4, filtered and evap-
orated under reduced pressure. Purification of the crude product by
flash chromatography on silica gel afforded 0.721 g of 4-acetyl-4ꢀ-
methoxybiphenyl (85%) as a white solid. 1H NMR (400 MHz, CDCl3):
ı 7.95 (d, J = 8.8 Hz, 2H), 7.58 (d, J = 8.8 Hz, 2H), 7.52 (d, J = 8.8 Hz,
2H), 6.94 (d, J = 8.8 Hz, 2H), 3.79 (s, 3H), 2.56 (s, 3H). 13C NMR
(100 MHz, CDCl3): ı 197.7, 159.9, 145.3, 135.3, 132.2, 128.9, 128.3,
126.6, 114.4, 55.4, 26.6.
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chlorides using PdNP as catalyst
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PdNP (0.24 g, Pd: 5.64 ␮mol), NaOH (0.280 g, 7 mmol), phenyl-
boronic acid (0.549, 4.5 mmol), and TBAB (0.129 g, 0.4 mmol) were
placed into a 10 mL Schlenk vessel under nitrogen atmosphere.
Water (2 mL) and 1-nitro-4-chlorobenzene (0.590 g, 3.75 mmol)
were added, and the resulting mixture was stirred at 100 ◦C. The
progress of the reaction was monitored by GC. After 24 h, 97% yield
of 4-nitrobiphenyl was obtained. 1H NMR (400 MHz, CDCl3,): ı 8.30
(d, J = 8.8 Hz, 2H), 7.74 (d, J = 8.8 Hz, 2H), 7.63 (d, J = 6.8 Hz, 2H), 7.50
(t, J = 6.8 Hz, 2H), 7.45 (t, J = 6.8 Hz, 1H). 13C NMR (100 MHz, CDCl3,)
ı 147.6, 138.7, 135.6, 129.1, 128.9, 127.8, 127.4, 124.1.
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