PdCl2 on modified poly(styrene-co-maleic anhydride): A highly active and recyclable catalyst for the Suzuki-Miyaura and Sonogashira reactions

Article abstract of DOI:10.1016/j.molcata.2014.07.001

The Suzuki-Miyaura and Sonogashira coupling were performed using a new, efficient, reusable and recyclable polymer-supported palladium catalyst. Poly(styrene-co-maleic anhydride) as polymer-support is modified with 2-aminothiazole. PdCl₂ was immobilized on this prepared support and its appearance was confirmed by scanning electron microscopy (SEM), energy dispersive spectroscopy analysis of X-rays (EDAX), inductively coupled plasma (ICP) analysis, and FT-IR techniques. In addition, a quantitative description for metal-ligand interactions in the aforementioned polymer-supported palladium complex is investigated by performing theoretical calculations using density functional theory techniques.

Full text of DOI:10.1016/j.molcata.2014.07.001

Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
PdCl on modified poly(styrene-co-maleic anhydride): A highly active
2
and recyclable catalyst for the Suzuki–Miyaura and Sonogashira
reactions
a,∗
a
a
b
Majid M. Heravi , Elaheh Hashemi , Yahya Shirazi Beheshtiha , Shervin Ahmadi ,
Tayebeh Hosseinnejada
a
Department of Chemistry, School of Science, Alzahra University, Vanak, Tehran, Iran
Iran Polymer and Petrochemical Institute, Tehran, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2014
Received in revised form 30 June 2014
Accepted 1 July 2014
Available online 9 July 2014
The Suzuki–Miyaura and Sonogashira coupling were performed using a new, efficient, reusable and recy-
clable polymer-supported palladium catalyst. Poly(styrene-co-maleic anhydride) as polymer-support is
modified with 2-aminothiazole. PdCl2 was immobilized on this prepared support and its appearance
was confirmed by scanning electron microscopy (SEM), energy dispersive spectroscopy analysis of X-
rays (EDAX), inductively coupled plasma (ICP) analysis, and FT-IR techniques. In addition, a quantitative
description for metal–ligand interactions in the aforementioned polymer-supported palladium complex
is investigated by performing theoretical calculations using density functional theory techniques.
Keywords:
Polymer-supported catalyst
Poly(styrene-co-maleic anhydride)
Palladium catalyst
©
2014 Elsevier B.V. All rights reserved.
Cross-coupling reaction
1
. Introduction
susceptible being modified using different nucleophilic substances.
These modified polymers can be used as polymer-support in the
syntheses of different reagents [8–13] and preparation of various
catalysts [14]. Reaction of SMA with di(2-pyridyl)methylamine for
the syntheses of an effective and recoverable palladium catalyst
being efficiently used in Heck, Suzuki and Sonogashira cross-
coupling reactions, in neat water, was reported by Nàjera and
co-workers in 2005 [14]. However, further investigation on this
polymer as a support for the syntheses of new structures, via
its reaction with other susceptible coordinating agents, has been
largely overlooked.
In continuationof our studies on Pd-catalyzedreactions [15–24],
our interest in the discovery of new heterogeneous catalysts
[25–32] and encouraged with our very recent fascinating results
from using modified SMA in click [33] and some multi-component
reactions [34], herein, we wish to reveal our results on the syn-
theses of modified SMA using a simple coordinating agent such as
2-aminothiazole and examine the activity of this new synthesized
support for the syntheses of an effective Pd-supported catalyst.
Remarkably, in addition, the efficiency of this new Pd catalyst is
examined in Suzuki and Sonogashira cross-coupling reactions, pur-
posely under ‘green’ conditions. The reusability and recyclability
of this heterogeneous catalyst is also examined and found being
positive.
The syntheses of many important and useful products (drugs,
natural products, optical devices, etc.) require carbon–carbon
bonds formation as central consequence in organic chemistry, via
assembling the palladium (Pd) catalyzed cross-coupling reactions
[
1]. Among Pd-catalyzed reactions, Suzuki–Miyaura [2] and Sono-
gashira coupling [3] are effective for transformations of highly
functionalized molecules in the syntheses of large and complex
molecules from simpler precursors. They are practical in laboratory
and operative in industry [1,4]. Since Pd-catalysts are expensive
and sometimes remain slight contamination in the products, the
application of these catalysts supported on insoluble polymers or
in different common inorganic supports are much in demands and
desirable. The known excellent coordination properties of Pd with
a variety of organic ligands, containing P, N, O, and S, make it a very
good candidate being easily converted to a typically heterogeneous
and recyclable catalysts [5–7].
Poly(styrene-co-maleic anhydride) (SMA) as a commercially
available co-polymer carrying reactive anhydride groups are
∗ Corresponding author. Tel.: +98 91 21329147; fax: +98 21 88041344.
E-mail address: mmh1331@yahoo.com (M.M. Heravi).
http://dx.doi.org/10.1016/j.molcata.2014.07.001
1
381-1169/© 2014 Elsevier B.V. All rights reserved.

M.M. Heravi et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
75
Scheme 1. Syntheses of SMI–PdCl2 catalyst.
2
. Experimental
poured slowly into 300 ml of vigorously stirring methanol. A fiber-
like precipitated polymer was repeatedly washed with methanol,
collected by filtration and dried under reduced pressure at 70 C to
◦
2.1. Materials
constant weight. For further purification, the SMI polymer (3) was
re-precipitated twice. The amine content of SMI was estimated by
back titration using NaOH (0.2 N) [36,37]. For that, 10 ml of HCl
(0.2 N) was added to 0.05 g of the SMI and the resulted mixture
was stirred for 30 min. The SMI was removed and washed succes-
sively with deionized water. After that, the excess amount of HCl
was titrated with NaOH (0.2 N) in the presence of phenolphthalein
as an indicator. Amine sites content of the synthesized SMI found
N,N-Dimethylformamide (DMF) and triethylamine (TEA) were
distilled and kept over 4 A˚ molecular sieve before use. The other
reagents were purchased from Aldrich and Merck with high-grade
quality (except SMA) and used as they were received. The gen-
eral formula of SMA (KARABOND SAM), used in this study, is
[
(C H ) (C H O )0.4]n with Mn (g/mol) = 86,666, Mw = 182,000,
8 8 0.6 4 2 3
and Mw/Mn = 2.1.
−
1
to be 3.82 mmol g
.
2.2. Equipments
2
.3.2. Preparation of Pd-supported catalyst
The SMI polymer 3 (1.0 g, 3.82 mmol amine site) was treated
The H NMR and 13C NMR spectra were recorded, using a Bruker
1
Ultrashield 400 and 100 MHz respectively Advance instrument,
with DMSO-d6 or CDCl3 used as solvent. Proton resonances are
designated as singlet (s), and multiplet (m). FTIR spectra were
recorded using KBr disks on FT-IR Bruker Tensor 27 instrument in
with PdCl (PhCN) (1.0 g, 3 mmol) in DMF (10 ml), and the resulting
2 2
◦
mixture was heated at 80 C for 15 h. The resulted brown poly-
mer which impregnated with the metal complex was filtered and
washed with acetonitrile to give the catalyst 4 (Scheme 1). After
the filtration of the mixture, the SMI–PdCl2 (4) washed with ace-
−1
the 500–4000 cm region. The vibrational transition frequencies
−
1
◦
are reported in wave numbers (cm ). Band intensities are assigned
as weak (w), medium (m), and strong (s). Palladium content was
measured by inductively coupled plasma (ICP) on a Varian Vistapro
analyzer. The scanning electron micrographs of the catalyst surface
were recorded using a Lecia Cambridge S 360 SEM instrument. All
yields refer to isolated products.
tonitrile (2 × 20 ml) and dried under vacuum at 60 C overnight
(Scheme 1). For evaluation of the palladium content, the catalyst
(30 mg) was treated with a mixture (5 ml) of hydrochloric acid and
◦
nitric acid (3:1, v/v) at 100 C for 4 h. The resulting solution was then
filtered, and the recovered resin beads were washed with distilled
water (2.5 ml × 6). The filtrate was diluted to 50 ml with distilled
water and analyzed by ICP-atomic emission spectrometry (AES).
−
1
The palladium content was determined being 0.934 mmol g (cat-
alyst Pd loading was 9.94% w/w).
2.3. The syntheses of catalyst
2.3.1. Preparation of SMI
The chemical modification of SMA (1) was performed in two
2.4. General procedure for the Suzuki coupling reaction
steps according to Lee et al. optimal reactive conditions [35]. SMA
1.00 g), 2-aminothiazole 2 (1.64 g, 16 mmol) and 15 ml dry DMF
(
A mixture of an appropriate aryl halide (1.0 mmol), phenyl-
were added into a 100-ml glass reactor and then N gas was charged
boronic acid (1.5 mmol, 0.18 g), K CO3 (5.0 mmol, 0.69 g), H O
2
2
2
into the reactor and then it was sealed. The reactor was placed into
a thermostatic oil-bath on the oscillator and the reactive mixture
was oscillated until the reagents were dissolved in DMF and kept
(5 ml), and 0.02 g of catalyst was stirred under reflux (Scheme 2).
After the completion of the reaction which was monitored by Thin
Layer Chromatography (TLC) using n-hexane as eluent, the solution
was filtered through a Büchner funnel to remove the resin, which
was washed with acetone three times, dried under reduced pres-
◦
oscillating at 35 C for 3.5 h. Subsequently, 0.6 ml acetic anhydride,
0.33 g sodium acetate and 0.3 ml triethylamine were added into
◦
the reactor by syringe (Scheme 1). The temperature was contin-
sured at 70 C for 3 h. and stored for other consecutive reaction runs.
◦
uously raised to 75 C and oscillation continued for another 3.5 h.
Then the reaction product was extracted with CH Cl (3 × 5 ml) and
2
2
The reaction mixture was cooled to room temperature, and was
the solvent was removed on a rotary evaporator under reduced

7
6
M.M. Heravi et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
Scheme 2. Suzuki coupling reaction in the presence of SMI–PdCl2 catalyst.
Fig. 2. FTIR spectrum of SMI and SMI–PdCl2.
completion of the reaction (monitored by GC analysis), the solvent
was removed under vacuum, and the crude product was subjected
to silica gel column chromatography using CHCl –CH OH (97:3)
3
3
as eluent to afford the pure corresponding product (Scheme 3). IR
1
Scheme 3. Sonogashira coupling reaction in the presence of SMI–PdCl2 catalyst.
and H NMR data of a selected product were found to be identical
1
with those of authentic samples [40]. The spectral (IR, Mass and H
NMR) data are as follows:
pressure. The crude was purified by flash column chromatography
Diphenylacetylene (9a): 1H NMR (CDCl3, 400 MHz),
ı = 7.53–7.49 (m, 4H), 7.30–7.21 (m, 6H) MS (EI): m/z (rel%)
(
hexane or hexane/ethyl acetate) to afford the desired correspond-
ing coupling product. The symmetrical and unsymmetrical biaryls
+
−1
1
78 (bp, M ), 152 (24). FT-IR (KBr, cm ): ꢀ = 3045–2930 (C H,
(
7a–h) synthesized in this work have been reported previously and
1
str.), 2050 (C ≡ C, str.). 1634–1477 (C C, str.).
were identified by comparing of their melting points, IR and
H
NMR, of selected product spectra with those of authentic samples,
already reported in the literature and found to be identical [38,39].
3. Results and discussion
1
The spectral (IR and H NMR) data for the selected compound are
as follows:
3.1. Preparation and characterization of the Pd-supported
catalyst
1
4
-Methylbiphenyl (7c): H NMR (400 MHz, CDCl3): ı = 7.60 (2H,
ArH), 7.51 (2H, ArH), 7.46 (1H, ArH), 7.33 (2H, ArH), 7.25 (2H, ArH),
−1
2
.33 (3H, s) ppm; FT-IR (KBr, cm ): ꢀ = 3050–2860 (C H, str.),
Modification of SMA (1) with 2-aminothiazole (2) was success-
1
610–1440 (C C, str.).
fully performed in two steps under Lee et al. conditions (Scheme 1)
[
35]. The 1H NMR spectrum of SMI was recorded in DMSO for
2
.5. General procedure for the Sonogashira coupling reaction
the absolute determination value of conversion which showed a
pattern similar to SMA [35] in addition of the expected aromatic
protons for thiazole of residual 2-aminothiazole as illustrated in
Fig. 1.
A round-bottomed flask was charged with an aryl halide
(
1.0 mmol), an appropriate terminal alkyne (1.0 mmol), pyridine
(
1.0 mmol, 0.079 g), n-dodecane (15–20 mg) as an internal GC
Moreover, the conversion of maleic anhydride moiety in SMA to
maleimide was monitored by FT-IR spectrometer. Fig. 2 shows that
after the reaction of SMA with 2-aminothiazole, a new imide band
appears at 1724 cm and no band is observed at 3300–3500 cm
expected for amino groups. Therefore it seems that the imidiza-
standard and the catalyst (0.02 g) under air atmosphere. The
mixture was stirred at room temperature for 3 h. under aerobic
conditions. After completion of the reaction, CHCl3 was added and
the mixture was filtered in order to recover the catalyst. The cat-
−
1
−1
alyst was washed with acetone, dried under reduced pressured at
tion of SMA was completed and no residual 2-aminothiazole was
◦
−1
7
0 C for 3 h and stored for other consecutive reaction runs. After the
observed in the resulting SMI. The range of 1600–1500 cm
is
Fig. 1. 1H NMR spectrum of SMI.

M.M. Heravi et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
77
Table 1
H C
2
CH2
O
The calculated values of some selected bond orders in SMI ligand 3 and SMI–PdCl2
complex 4 obtained at M06/631G* level of theory. Note that the numbering of atoms
for ligand and complex is in accordance with Fig. 1.
O
N
PdCl2
PdCl2
Bonded atoms
SMI ligand
SMI–PdCl2 complex
S
N
C6 O7
C5 O8
N4 C6
N4 C5
C1 N3
C1 S2
C1 N4
Pd1 S2
Pd1 O7
Pd2 N3
Pd2 O8
1.792
1.924
0.980
0.912
1.567
1.120
0.902
–
1.683
1.657
0.997
0.985
1.513
0.987
0.932
0.553
0.136
0.251
0.240
Scheme 4. A simple model for polymer-supported palladium catalyst.
attributed to the ꢀ(C C), ꢀ(C N), and ꢀ(C S) stretch of aromatic
rings. In the polymer-supported palladium complexes, ꢀ(C C),
(C N), and ꢀ(C S) show a slight positive shift, indicating the
palladium is chelated with both nitrogen and sulfur atoms of
heterocyclic ring. Therefore, the polymer support can act as a multi-
dentate chelate resulting in the increase of the amount of palladium
in the structure of catalyst. High calculated palladium content from
ICP-AES analysis, 9.94% w/w, can justify this prediction and view-
point.
ꢀ
–
–
–
Recently, we used computational study to confirm success-
fully the regio-selectivity of Sonogashira reaction [41]. Armed
with this experience, we focused on obtaining a quantitative
description for metal–ligand interactions in the aforementioned
polymer-supported palladium complex by performing theoretical
calculations using density functional theory techniques [42,43]. In
this respect, we have employed an effective model for SMI ligand
3
and palladium–SMI complex 4 (as was illustrated in Scheme 4)
considering that this model has a reliable comprise between accu-
racy and time saving efficiency of computational procedure. Thus,
we have first determined the optimized structure of compounds
3
and 4 via DFT calculations at M06/6-31G* level of theory [44].
It should be mentioned that M06 functional has been introduced
recently as a hybrid meta-GGA (generalized gradient approxi-
mation) exchange-correlation functional that was parametrized
including both transition metals and nonmetals and was recom-
mended for being applied in organometallic and inorganometallic
thermochemistry, kinetic studies and noncovalent interactions. In
the case of palladium, the effective core potential (ECP), LANL2TZ(f),
was used together with the accompanying basis set to describe the
valence electron density [45]. The stationary points were character-
ized as minima after verifying the presence of all real frequencies.
All DFT computations have been performed using GAMESS suite
of programs [46]. In Fig. 3 the theoretical optimized structures of
*
compounds 3 and 4 obtained at M06/6-31G level in the gas phase
together with the atomic numbering, were presented.
We have also evaluated comparatively the bond order of some
selected key bonds in SMI ligand 3 with its correspondent pal-
ladium complex 4 to characterize the variation of bond orders
via complexation. In Table 1, the calculated bond orders of some
selected chemical bonds in the coordination sphere have been
listed. As we expected, and the results in Table 1 show that the
bond order of carbonyl groups and also C N and C S bonds of
aminothiazole ring shifted toward lower values due to the dona-
tion of shared electrons from these chemical bond to palladium
through the complexation which is in agreement with our FT-IR
spectroscopic observations.
In the next step, we concentrated on topological study of
electron density via Quantum Theory of Atoms in Molecule
(QTAIM) [47] to interpret the nature of metal–ligand interactions
in SMI–PdCl2 complex 4. In this respect, we focused on the QTAIM
analysis of M06/6-31G* calculated wave function of electron den-
sity using AIM2000 program package [48]. In Table 2, we have
listed QTAIM calculated values of electron density, ꢁ , its Laplacian,
b
2
ꢀ
ꢁ , electronic kinetic energy density, G , electronic potential
b
b
energy density, V , total electronic energy density, H and |V |/G
b
b
b
b
ratio at some selected bond critical points (BCPs) for SMI ligand 3
and SMI–PdCl2 complex 4. It is important to state that the elec-
tron density at BCPs usually correlates with the strength of the
bond between two atoms. Values of ꢁb < 0.1 au are indicative of a
Fig. 3. Atomic numbering and energy-minimized structures obtained at M06/6-
*
3
1G level of theory for SMI ligand 3 and SMI–PdCl2 complex 4.

7
8
M.M. Heravi et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
Table 2
Mathematical properties of some selected BCPs in SMI ligand 3 and SMI–PdCl2 complex 4. The properties have been obtained via QTAIM analysis on the M06/6-31G* calculated
wave function of electron density. All values in the parenthesis are corresponded to the BCPs of SMI ligand. Note that numbering of atoms is in accordance with Fig. 1.
2
Bonded atoms of BCPs
ꢁb
ꢀ ꢁb
Gb
Vb
Hb
|Vb|/Gb
C1 S2
0.193 (0.197)
0.357 (0.372)
0.293 (0.290)
0.296 (0.278)
0.274 (0.298)
0.400 (0.422)
0.417 (0.431)
0.090
−0.337 (−0.342)
−0.622 (−1.031)
−0.787 (−0.871)
−0.906 (−0.799)
−0.777 (−0.913)
0.239 (0.269)
0.262 (0.160)
0.204
0.060 (0.067)
0.444 (0.364)
0.231 (0.188)
0.170 (0.156)
0.146 (0.192)
0.730 (0.789)
0.777 (0.740)
0.070
−0.205 (−0.219)
−1.044 (−0.986)
−0.659 (−0.595)
−0.567 (−0.513)
−0.487 (−0.612)
−1.400 (−1.510)
−1.489 (−1.441)
−0.090
−0.145 (−0.152)
−0.600 (−0.622)
−0.428 (−0.407)
−0.397 (−0.357)
−0.341 (−0.420)
−0.670 (−0.721)
−0.712 (−0.701)
−0.020
3.416 (3.268)
2.357 (2.708)
2.852 (3.164)
3.335 (3.288)
3.335 (3.187)
1.917 (1.913)
1.916 (1.947)
1.285
C1 N3
C1 N4
N4 C5
N4 C6
C5 O8
C6 O7
Pd1 S2
Pd1 O7
Pd2 N3
Pd2 O8
0.026
0.096
0.036
0.106
0.364
0.355
0.023
0.119
0.090
−0.019
0.004
0.826
1.235
1.022
−0.147
−0.028
−0.092
−0.002
Fig. 4. SEM image of SMI support (a) and SMI–PdCl2 (b and c).
closed-shell (i.e. predominantly electrostatic) interaction [49]; it
size and roughness of the surface. Energy dispersive spectroscopy
analysis of X-rays (EDAX) data for the Pd-catalyst is given in Fig. 5.
The EDAX data also proved the attachment of Pd on the surface of
the polymer matrix.
is usually accompanied by a relatively small and positive value of
2
ꢀ
ꢁb [50]. By contrast, for a shared (i.e. predominantly covalent)
2
interaction, ꢁ is usually >0.1 au [38] and ꢀ ꢁ is usually negative
51] and typically of the same order as ꢁ . Furthermore, a good reli-
b
b
[
b
able indicator for classifying interatomic interactions is the total
3.2. Optimization of the reaction conditions
electronic energy density, that is defined as H = G + V at BCPs. In
b
b
b
the closed-shell interactions, Hb has the positive value (viz., Gb is
dominating) and for shared interactions, it is negative (viz., Vb is
dominating).
According to the reported results of Table 2, the following facts
can be discussed: (i) decrease in the calculated values of ꢁb at
3.2.1. Suzuki coupling reaction
To examine the potency of the Pd-supported catalyst, it was
used in the Suzuki coupling reaction between various aryl halides
(6a–j) and phenylboronic acid 5. The effects of base, solvent and
reaction temperature were screened to optimize reaction condi-
tions for coupling of iodobenzene (1 mmol) and phenylboronic acid
C
O, C N and C S BCPs via complexation, confirms the dona-
tion of shared electrons to the metal center and is in agreement
with diminution in the stretching frequency of IR spectrum; (ii)
(1.5 mmol) as the model reaction (Table 3). In the Suzuki–Miyaura
coupling reaction, choice of solvent has a crucial role in the rate
the large values of electron density together with the negative val-
2
ues of ꢀ ꢁ and Hb approve the covalent character of C O, C
N
b
and C S chemical bonds in SMI ligand 3 and Pd complex 4 and
(
iii) the small values of electron density with the positive values
2
of ꢀ ꢁ and Hb at Pd1 S2, Pd1 O7, Pd2 N3 and Pd2 O8 BCPs
disclose that the metal–ligand interactions comprise no covalent
b
character. Moreover, the corresponding 1 < |V |/G < 2 values con-
b
b
firms the presence of partially covalent-electrostatic interactions
at these Pd1 S2, Pd1 O7, Pd2 N3 and Pd2 O8 BCPs.
The scanning electron micrographs (Fig. 4) of the polymer
supported ligand and supported Pd-catalyst clearly showed the
morphological change which occurred on the surface of SMI after
being loaded with palladium. As vividly observed in Fig. 4, the poly-
mer supported ligand have different size and roughness and the
presence of Pd has caused obvious changes, in the polymer particle
Fig. 5. EDAX data of SMI–PdCl2.

M.M. Heravi et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
79
Table 3
Screening reaction conditions of iodobenzene with phenylboronic acid under Suzuki protocol.
a
◦
Yieldb (%)
Entry
Solvent
Temperature ( C)
Base
Cat. (g)
Time (h)
1
2
3
6
7
8
9
MeOH/H2O (3:1 v/v)
H2O
CH3OH
CH3CN
1,4-Dioxane
H2O
H2O
H2O
H2O
H2O
RT
RT
RT
RT
RT
50
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
Na3PO4
NaOAc
NaOH
pyridine
NEt3
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.01
0.02
0.02
0.02
0.02
0.02
0.02
0.02 g of PdCl2
3
4
4
5
5
3
2
4
2
5
5
5
5
5
5
98
90
89
89
50
96
97
93
5
65
55
50
41
39
4
10
11
12
13
14
15
16
17
H2O
H2O
H2O
H2O
H2O
K2CO3
a
Iodobenzene (1 mmol) with phenylboronic acid (1.5 mmol), and base (5 mmol).
Refers to the isolated yield.
b
and the product distribution. It has already been found that water
profoundly increases the activity of the Suzuki–Miyaura catalyst
It is absolutely clear that the reactivity of aryl chlorides was
lower than aryl iodides and bromides in the Suzuki coupling reac-
tions [36,56]. Aryl chlorides required longer reaction times and
affording lower yields (Table 4, Entry 10).
[
52,53]. Thus, we used, H O or MeOH/H O (3:1, v/v), as green and
2 2
relatively green solvents respectively. Although, the experimental
results showed that in the cases that MeOH/H O (3:1 v/v) system
used as a solvent the time of the reaction for completion is shorter
The turn-over frequency (TOF) value defined as the mol prod-
uct/(mol catalyst. hour); was calculated from the isolated yield
of product, the amount of palladium used and the reaction
time. In all reactions, TOF parameter was determined and sug-
gested that SMI–PdCl2 as a good catalyst for this kind of reaction
(Table 4).
2
(
Table 3, Entry 1), we chose neat H O as solvent (Table 3, Entries
2
8
–16). Water as an inexpensive, eco-friendly, green, readily avail-
able, non-inflammable, and non-toxic solvent provides remarkable
advantages over common organic solvents both from economic and
environmental aspects and point of views [36,54,55] and is cur-
rently considered as one of the most suitable targets of sustainable
chemistry [54]. In addition, using water as solvent not only mini-
mized the homo-coupled product of phenylboronic acid, but also
providing the facile separation of the desired product from homo-
coupled product due to their different solubility, in water [52,53]. In
addition, using of 0.02 g of catalyst at reflux conditions were found
to be the optimal conditions (Table 3, Entries 8–10). As we expected,
in the absence of any base, the reaction has a negligible progress
3.2.2. Sonogashira coupling reaction
The solvent and copper-free Sonogashira coupling reaction
was also examined in order to evaluate the catalyst efficiency.
The optimized conditions for the coupling between iodoben-
zene and phenylacetylene as the model reaction are shown in
Table 5. From of the bases examined, pyridine gave the best
results: 98% GC yield of the corresponding coupled product 9a
(Table 5, Entry 4). It was observed that using inorganic bases
(Table 3, Entry 11). For selection of the appropriate base, the effect
such as Na PO , NaOAc, KOH, and K CO , and also low palla-
3
4
2
3
of bases such as K CO , Na PO , NaOAc, NaOH, pyridine and NEt
2
3
3
4
3
dium concentration, decreases the yield of the product 9a (Entries
–10). As it was mentioned from the experimental section, the
was examined on the aforementioned coupling reaction. As a result,
we found that the inorganic bases used were more effective than
6
products were purified trough the column chromatography using
CHCl –CH OH (97:3) as eluent. The application of hexane or hex-
organic bases tested. K CO3 as an easily available and inexpensive
2
3
3
chemical was selected as a base of choice (Table 3, Entries 12–16). In
order to discriminate the effect of the polymer supported catalyst
ane/ethyl acetate as eluent which is usually used in the purification
of the Suzuki coupling products is not worthy since these eluent
are not able to separate the by-products of this reaction, 1,3-diyne
derivatives usually obtained from the hemo-coupling of terminal
alkynes.
(
SMI–PdCl ), the reaction was performed over palladium chloride
2
in the same reaction conditions and in this case, a significant lower
activity was observed (Table 3, Entry 17). Therefore, coupling reac-
tions of aryl halides (1 mmol) with phenylboronic acid (1.5 mmol)
This optimized condition were applied for the copper- and
solvent-free Sonogashira coupling of different alkynes (8a and 8b)
with various aryl halides (6a–j) containing electron-withdrawing
or donating substituents in the presence of SMI–PdCl2 as a cata-
lyst. As illustrated in Table 6, the electron-neutral, electron-rich
or electron-poor aryl iodides reacted with phenylacetylene under
the optimized reaction conditions and generate the corresponding
cross-coupling products in high yields (Table 6, Entries 1–7). The
Sonogashira coupling of the less reactive terminal acetylene such
as propargyl alcohol with different aryl iodides provided the corre-
sponding products in high yields (Entries 8 and 9). As expected, the
coupling of various aryl bromides with terminal alkynes produced
the desired coupling product but in lower yield in comparison with
their corresponding aryl iodides (Entries 10 and 11). The yields
of coupling product 9a of the aryl chloride under similar copper-
free conditions were also much lower than their iodo and bromo
counterparts (Entry 12).
in the presence of K CO3 (5 mmol), and 0.02 g of the catalyst in
water under reflux conditions were carried out to obtain compar-
atively better results.
2
These optimized reaction conditions were applied (Table 4)
in the Suzuki cross-coupling reaction of various aryl halides
and phenylboronic acid with 0.02 g of SMI–PdCl2 (Pd/aryl halide
molar ratio: 0.018). We examined the electronic and steric effects
of various aryl halides bearing electron-donating and electron-
withdrawing groups, focusing on the yields and the time required
for conversion. According to the results presented in Table 4,
aryl iodides and bromides with electron-donating and electron-
withdrawing groups (Entries 1–9) both afforded excellent yields
of coupling products in reasonable times. However, the coupling
of 2-iodotoluene and 2-iodo-5-nitrotoluene with phenylboronic
acid resulted in moderate yields (72 and 65% yield respectively)
which may be attributed to the steric effect of 2-iodotoluene and
2
-iodo-5-nitrotoluene.

8
0
M.M. Heravi et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
Table 4
Suzuki reaction of various aryl halides with phenylboronic acid.
a
Yieldb (%)
97
Time (h)
2
TOF (h )c
−1
Mp ( C)/Lit. Mp [ref]
◦
Entry
Substrate
Product
1
26.94
69–71/70–72 [38]
2
3
92
90
3
3
17.04
16.67
90–92/91–92 [38]
49–53/45–50 [38]
4
5
6
90
72
65
3
7
8
16.67
5.71
4.72
30–34/32.5–33.5 [38]
Colorless oil/colorless oil [38]
52–55/55–56 [39]
7
8
9
93
92
2
2
25.83
25.55
54–56/57–59 [38]
117–120/120–122 [38]
95
90
67
29
3
4
5
8
17.59
12.49
7.44
71–74/70–72 [38]
73–75/70–72 [38]
91–93/91–92 [38]
90–93/91–92 [38]
1
1
1
0
1
2
2.01
a
b
c
Aryl halides (1 mmol) with phenylboronic acid (1.5 mmol), K2CO3 (5 mmol), and 0.02 g of catalyst in water under reflux.
Refers to the isolated yield.
Turn-over frequency.
◦
It is also remarkable to note that resin did not suffer from exten-
and dried under reduced pressured at 70 C for 3 h and stored for
another consecutive reaction run. This dried catalyst was reused
for another fresh reaction mixture without any further activation.
The obtained results emphasized that the catalyst could be recov-
ered and was reused up to at least 5 runs without any significant
loss of activity. It is worthwhile to mention that, no noticeable loss
of the weight of the catalyst after its usage in subsequent runs is
observed. Afterwards, leaching of Pd from the SMI support into the
reaction mixture of the Suzuki reaction was tested, using ICP-AES.
The results show that the difference between the palladium con-
tent of the fresh catalyst and the reused catalyst, in 5th run, was
sive mechanical degradation after few times of running. For an
ideal heterogeneous catalyst, supported catalyst should not leach
to the reaction mixture and the recyclability of the supported cata-
lyst is highly important. For investigation of these properties in our
new synthesized catalyst, as illustrated in Fig. 6, the reusability of
the catalyst in reactions of iodobenzene with phenylboronic acid
Suzuki reaction), and iodobenzene with phenylacetylene (Sono-
gashira reaction) as representative reactions in the presence of
.02 g of SMI–PdCl2 was examined. After the first run, the cata-
lyst was separated by filtration, washed with acetone three times,
(
0

M.M. Heravi et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
81
Table 6
Sonogashira reaction of various aryl halides and terminal acetylene.a
Entry
Substrate
Product
Yieldb (%)
1
98(93)
97
2
3
6
96
95
7
8
92
95
9
93(87)
10
11
12
80
84
54
a
Aryl halides (1 mmol) with phenylacetylene or propargyl alcohol (1 mmol), pyridine (1 mmol), and 0.02 g of catalyst. at r.t. in 3 h.
Refers to GC yield, isolated yields are given in parentheses.
b
Table 5
Screening reaction conditions for the Sonogashira reaction of iodobenzene and
phenylacetylene.a
Entry
Base
Cat. (g)
Yieldb (%)
1
2
3
4
5
6
7
8
9
Et3N
Et2NH
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.01
80
65
45
98
74
50
52
54
47
65
Butylamine
Pyridine
Piperidine
Na3PO4
NaOAc
K2CO3
KOH
Pyridine
10
Fig. 6. The recyclability of the SMI–PdCl2 in the preparation of 7a and 9a.
a
Iodobenzene (1 mmol) with phenylacetylene (1 mmol), pyridine (1 mmol), and
0
.02 g of cat. at r.t. in 3 h.
Refers to GC yield.
b

8
2
M.M. Heravi et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 74–82
only 4.8% which indicated the low leaching amount of Pd catalyst
into the reaction mixture.
[20] M.M. Heravi, E. Hashemi, N. Nazari, Mol. Divers. 18 (2014) 441–472.
[
[
[
21] M.M. Heravi, S. Sadjadi, Tetrahedron 65 (2009) 7761–7775.
22] M.M. Heravi, A. Fazeli, Heterocycles 81 (2010) 1979–2026.
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Neumuller, Tetrahedron Lett. 46 (2005) 1607–1610.
4
. Conclusions
[
[
[
[
[
[
[
[
[
[
[
24] J.M. Nuss, B.H. Levine, R.A. Rennels, M.M. Heravi, Tetrahedron Lett. 32 (1991)
5
243–5246.
In conclusion, we have synthesized a novel polymer-supported
25] M.M. Heravi, K. Bakhtiari, Z. Daroogheha, F.F. Bamoharram, J. Mol. Catal. A:
Chem. 273 (2007) 99–101.
26] M.M. Heravi, M. Tajbakhsh, A.N. Ahmadi, B. Mohajerani, Monatsh. Chem. 137
palladium complex being efficiently applicable to the Suzuki and
Sonogashira reaction, as a clean and green catalyst. The catalyst
exhibited not only high activity, but also afforded a diverse range
of coupling products in good to excellent yields. In addition, the
catalyst which is stable in the reaction conditions can be used with
no pre-activation and can also be recycled for at least five consec-
utive cycles without an appreciable loss of activity. These benefits
make this supported palladium catalyst as an interesting and use-
ful alternative to other Pd heterogeneous catalytic systems and
recommended, being tried in other Pd-catalyzed reactions.
(
2006) 175–179.
27] M.M. Heravi, K. Bakhtiari, Z. Daroogheha, F.F. Bamoharram, Catal. Commun. 8
(2007) 1991–1994.
28] M.M. Heravi, G. Rajabzadeh, F.F. Bamoharram, N. Seifi, J. Mol. Catal. A: Chem.
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29] M.M. Heravi, M. Khorasani, F. Derikvand, H.A. Oskoole, F.F. Bamoharrarm, Catal.
Commun. 8 (2007) 1886–1890.
30] M.M. Heravi, V. Zadsirjan, K. Bakhtiari, H.A. Oskooie, F.F. Bamoharrarm, Catal.
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31] M.M. Heravi, K. Bakhtiari, V. Zadsirjan, F.F. Bamoharrarm, O. Heravi, Bioorg.
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Acknowledgements
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M.M.H. is thankful to Alzahra University Research Council for
partial receiving of his grant upon this publication.
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