An Efficient and Recyclable Mesostructured Polymer-Supported N-Heterocyclic Carbene-Palladium Catalyst for Sonogashira Reactions

Article abstract of DOI:10.1016/S1872-2067(10)60284-7

Palladium catalyst systems are the most widely used for the formation of C-C and C-N bonds. Environmentally friendly heterogeneous palladium catalysts have strongly stimulated growth in this research area. Herein, a FDU (Fudan University)-15 mesopolymer w

Full text of DOI:10.1016/S1872-2067(10)60284-7

CHINESE JOURNAL OF CATALYSIS
Volume 32, Issue 11, 2011
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2011, 32: 1712–1718.
RESEARCH PAPER
An Efficient and Recyclable Mesostructured
Polymer-Supported N-Heterocyclic Carbene-Palladium
Catalyst for Sonogashira Reactions
YU Tao, LI Ying, YAO Chengfu, WU Haihong*, LIU Yueming, WU Peng
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University,
Shanghai 200062, China
Abstract: Palladium catalyst systems are the most widely used for the formation of C–C and C–N bonds. Environmentally friendly het-
erogeneous palladium catalysts have strongly stimulated growth in this research area. Herein, a FDU (Fudan University)-15 mesopolymer
with p6mm mesostructures that combine the advantages of organic polymers and mesoporous materials was investigated as a new solid
support for Pd complexes. The FDU-15 mesopolymer-supported N-heterocyclic carbene (NHC) precursor was developed by a simple
chemical functionalization approach. It has been used as a phosphine-free ligand to prepare the novel heterogeneous palladium catalyst
FDU-NHC/Pd(II), which efficiently catalyzes Sonogashira reactions between aryl halides and terminal alkynes to afford the correspond-
2
ing products in good yields. Various techniques such as X-ray diffraction, N adsorption-desorption isotherms, high resolution transmis-
sion electron microscopy, and X-ray photoelectron spectroscopy have been used to characterize the catalyst. The results indicate that the
mesoporous structure of FDU-15 was not affected by immobilization, however, the pore diameter, pore volume, and BET surface area
decrease. The stability of the heterogeneous catalyst was also studied. The FDU-NHC/Pd(II) complex catalyst was easily separated from
the reaction system by a simple filtration of the reaction mixture and it has been shown to be robust without a significant loss of catalytic
activity after recycling and reuse in 10 times.
Key words: mesostructured polymer; N-heterocyclic carbene; Sonogashira reaction; palladium catalyst; immobilization; heterogeneous
catalyst
Palladium-catalyzed coupling reactions between terminal
alkynes and aryl or alkenyl halides, known as the Sonogashira
reaction, are recognized as one of the most powerful and ver-
satile methods to construct C–C bonds in organic synthesis.
These reactions have been widely used in the synthesis of fine
chemicals, agrochemicals, pharmaceutical intermediates, and
precursors of natural products [1–3]. The reactions are mostly
carried out using a homogeneous Pd complex catalyst in con-
junction with a copper salt CuX (X = Cl, Br, I) as a
co-catalyst. Although homogeneous palladium catalysts often
offer higher activities for these transformations, they suffer
from drawbacks such as difficulties in handling sensitive
ligands, product separation and catalyst recovery, reuse of
expensive ligands and Pd complexes, and the contamination of
the desirable product by heavy metals. An alternative method
to overcome these problems is to develop environmentally
friendly heterogeneous catalyst systems.
Heterogeneous catalysts possess significant advantages
from a practical point of view and in terms of sustainable
green chemistry [4–6]. The catalysts can be easily recovered
from the reaction mixture by simple filtration and they can
then be reused. Therefore, an increasing number of heteroge-
neously catalyzed Sonogashira reactions have been developed.
Pd catalysts have been immobilized on various support mate-
rials such as silica [7–16], polymers [17–27], chitosan [28,29],
metal oxide nanoparticles [30,31], and ionic liquids [32] etc.
Received 4 July 2011. Accepted 10 September 2011.
*
Corresponding author. Tel/Fax: +86-21-62238510; E-mail: hhwu@chem.ecnu.edu.cn
This work was supported by the National Natural Science Foundation of China (20925310), the National High Technology Research and Development Program of
China (863 Program, 2009AA064201), the Science and Technology Commission of Shanghai Municipality (09XD1401500), and the Shanghai Leading Academic
Discipline Project (B409).
Copyright © 2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60284-7

YU Tao et al. / Chinese Journal of Catalysis, 2011, 32: 1712–1718
However, alternative immobilization strategies are still
Herein, we report the preparation of this kind of phosphine-free,
stable and active catalyst, and its catalytic activity as well as
recyclability in the Sonogashira reaction.
needed. It has been demonstrated that the supporting matrix
crucially influences the performance of the Pd catalysts [33].
Thus, research into the development of more efficient catalyst
systems with high activity and high reusability is extremely
important.
1 Experimental
Mesoporous polymers that comprise pure organic frame-
works or pore walls possess the characteristics of both
mesoporous materials (high surface area, large pore volume,
and tunable mesostructure) and organic polymers (high hy-
drophobicity and availability for multi-functionalization).
Hence, they are considered to be potential supports for the
preparation of efficient Pd-based catalysts for Sonogashira
reactions. Among them, FDU-type mesoporous phenolic res-
ins appear to be amenable to either acidic or basic reaction
conditions and they also exhibit a unique hydrophobic property
toward organic reactants or solvents, which is quite different to
silica-based inorganic analogues [34,35]. The abundant aro-
matic rings in their polymeric networks provide
FDU-mesopolymers with abilities useful for functionalization.
Continuous research efforts in our laboratory have been de-
voted to the modification of these materials. A series of
FDU-type mesoporous phenolic resin-based catalysts have
been developed by the chemical modification of organic
framework walls, i.e. sulfonic acid groups, amino groups or
ligand functionalized mesopolymers [36–40]. The resultant
materials exhibit good catalytic activity and good reuse prop-
erties. On the other hand, N-heterocyclic carbene groups are
highly effective ligands for transition metals and have attracted
great attention in homogeneous catalysis. These ligands pro-
vide much stronger binding to transition metals and are
chemically and thermally more inert than their corresponding
phospines [41,42].
1.1 Preparation of the FDU-NHC/Pd(II) catalyst
All reagents were commercially available (Aldrich, Alfa
Aesar) and directly used without further purification. The
FDU-15 mesopolymer was prepared according to previously
reported procedures [35]. FDU-15 was chloromethylated with
chloromethyl methyl ether to obtain FDU-CH Cl [40]. The
2
FDU-CH Cl
was
further
functionalized
with
2
N-methylimidazole under mild conditions. The reaction was
carried out by stirring 1 g of FDU-CH Cl with 1 g of
2
N-methylimidazole (3.3 mmol) in 5 ml anhydrous toluene at
353 K for 24 h. The obtained material was washed with acetone
to remove physisorbed N-methylimidazole and finally dried
under vacuum at 323 K for 8 h. The N-heterocyclic carbene
ligand precursor functionalized mesopolymer was thus ob-
tained, which is denoted FDU-NHC. FDU-NHC (1 g) was
subsequently mixed with 0.044 g (0.2 mmol) Pd(OAc) in 5 ml
2
dimethyl sulfoxide (DMSO). The mixture was stirred at 323 K
for 6 h and then at 373 K for 3 h to bring about the deprotona-
tion of imidazolium groups to form the Pd-NHC complexes
[19]. The resulting suspension was filtered. The residue was
washed with acetone and finally dried under vacuum to give
FDU-NHC/Pd(II).
1.2 Characterization methods
X-ray diffraction (XRD) patterns were collected on a Bruker
In this work, we used the FDU-15 mesoporous phenolic
resin with p6mm symmetry as a new solid support for the Pd
complexes. Based on previous studies, we introduced com-
mercially available and cheap N-methylimidazole as a
N-heterocyclic carbene (NHC) precursor into the mesopolymer
and then developed a new recyclable supported heterogeneous
Pd catalyst (Scheme 1). This was carried out by the chloro-
methylation of the mesopolymer, the quaternary and then the
immobilization of the NHC precursor and finally simple ligand
D8 ADVANCE instrument using Cu K radiation (Ȝ = 0.15418
Į
nm) at 40 kV and 40 mA. Using CDCl as solvent and TMS as
3
1
13
the internal standard, H and C NMR spectra were acquired
on a Bruker DRX500 spectrometer at 500 MHz and 125 MHz,
respectively. Flash column chromatography was performed
using silica gel 60 (230–400 mesh). Nitrogen adsorp-
tion-desorption isotherms were recorded on a Quantachrome
Autosorb-3B instrument after the samples were evacuated at
573 K for 5 h. The specific surface area was evaluated using the
Brunauer-Emmett-Teller (BET) method. The high resolution
exchange with Pd(OAc) to obtain the NHC-Pd complexes.
2
OH
OH
OH
CH2
OH
N
N
N
N
N
CH2
Cl
Cl
Cl
N
OH
OH
CH OCH Cl
Pd(OAc)2
3
2
Cl Pd Cl
CH2
CH2
AlCl3
toluene
Cl
N
N
N
N
CH2
OH
OH
OH
FDU-15
FDU-CH Cl
FDU-NHC
FDU-NHC // Pd(II)
2
Scheme 1. Preparation of the FDU-NHC/Pd(II) catalyst.

YU Tao et al. / Chinese Journal of Catalysis, 2011, 32: 1712–1718
transmission electron microscopy (HRTEM) images were
mesopolymer, FDU-CH Cl. Elemental analysis by the Vol-
2
recorded using a JEOL JEM-2100 microscope. Cl elemental
analyses were performed by the Volhard method. The nitrogen
content was quantified on an Elementar VarioEL III CHN
elemental analyzer. The bulk loading amount of Pd was de-
termined by inductively coupled plasma (ICP, Thermo Elec-
tron Corporation IRIS Intrepid II XSP). X-ray photoelectron
spectroscopy (XPS) was performed on a Kratos Axis Ultra
DLD spectrometer with a single anode (Al) apparatus using
hard method indicated that FDU-CH Cl contained 1.2 mmol/g
2
of –CH Cl groups. FDU-CH Cl was reacted with excess
2
2
N-methylimidazole to afford FDU-NHC. FDU-NHC/Pd(II)
was obtained by the chemical coordination of the
N-heterocyclic carbene ligand with Pd(OAc) . The loading of
2
the NHC ligand was determined from the nitrogen content by
elemental analysis. A nitrogen content of 0.84 mmol/g im-
plied the presence of 0.42 mmol/g of NHC ligand in
FDU-NHC/Pd(II). In addition, the amount of Pd was 0.18
mmol/g as determined by ICP. To coordinate one Pd ion re-
quires a pair of carbene ligands that are in its neighborhood
and as close as possible. Some NHC ligands may be present
on the mesopolymer walls in a random or an isolated manner.
Thus, it is reasonable that the maximum Pd loading is less
than that determined by the NHC ligand method.
Al K radiation.
Į
1
.3 General procedures for the Sonogashira reaction
The reactions were carried out in an Argonaut advantage
TM
series
2410 personal screening synthesizer. In a typical
experiment aryl halide (0.5 mmol), substituted phenylacetylene
0.75 mmol), K CO (1.0 mmol), direct memory access (4.0
(
XPS was applied to trace the above-mentioned functionali-
zation procedures qualitatively. Chloromethylation was re-
sponsible for the Cl 2p peaks centered at 200.3 eV (Fig. 1(a)
curves (1) and (2)). The quaternary with N-methylimidazole
caused the Cl 2p peaks to shift to the lower binding energy
region (Fig. 1(a) curve (3)), during which chlorine became
negatively charged from a covalent state. This eases electron
constraints in the outer orbitals, which lowers the binding
energy. The N atoms in FDU-NHC became positively charged
ions and donated electrons to Pd(II) in FDU-NHC/Pd(II). As a
result, the N 1s peaks in the catalyst are present at 401.8 and
401.6 eV, respectively (Fig. 1(b) curves (3) and (4)). The car-
bene ligands captured the Pd species successfully. The XPS
spectrum shows peaks at 337.5 and 342.8 eV (Fig. 1(c) curve
(4)), which are assigned to Pd 3d5/2 and Pd 3d3/2, respectively.
2
3
ml), and FDU-NHC/Pd(II) (1 mol %) were mixed in a reaction
tube, which was heated to 373 K. The reaction progress was
monitored by thin-layer chromatography (TLC). After reaction
completion the catalyst was separated by simple filtration and
washed with acetone for further reaction. The filtrate was
diluted with water and the product was extracted with ethyl
acetate (3 × 10 ml). The combined extracts were dried over
anhydrous Na SO and concentrated under vacuum. The resi-
2
4
due was purified by column chromatography on silica gel
petroleum ether) affording the product. The structures of the
(
1
known products were characterized by comparison with H
1
3
NMR and C NMR data from the literature [43].
2
Results and discussion
0
By comparison with the Pd 3d5/2 peak at 335 eV for Pd , the
2
.1 Catalyst characterization
higher binding energy value found for FDU-NHC/Pd(II) in-
dicated that the incorporated Pd species is in the +2 oxidation
state.
The Pd catalyst was synthesized according to the procedures
described in Scheme 1. The chloromethylation of the benzene
rings in FDU-15 was realized effectively by catalysis with
anhydrous AlCl , which resulted in the –CH Cl group-grafted
Figure 2 shows the powder XRD patterns of the FDU-15
mesopolymers before and after functionalization. FDU-15 was
a
typical mesostructured material that possessed
a
3
2
(a)
(b)
(c)
(4)
(
4)
(
4)
3)
2)
(1)
(
3)
(
(
2)
(
3)
(
(1)
(
2)
1)
04
(
2
202
200
198
196
408
404
400
396
348
344
340
336
332
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
2
Fig. 1. XPS spectra of Cl 2p (a), N 1s (b), and Pd 3d (c) for FDU-15 (1), FDU-CH Cl (2), FDU-NHC (3), and FDU-NHC/Pd(II)(4).

YU Tao et al. / Chinese Journal of Catalysis, 2011, 32: 1712–1718
(
1)
(100)
300
250
200
150
(2)
(
3)
(110) (200)
(
(
(
1)
2)
3)
100
50
0
1
2
3
)
4
5
o
2T/(
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
Fig. 2. Powder XRD patterns of FDU-15 (1), FDU-NHC (2), and
FDU-NHC/Pd(II) (3).
Fig. 4.
N
2
adsorption-desorption isotherms of FDU-15 (1),
FDU-NHC (2), and FDU-NHC/Pd(II) (3).
two-dimensional (2D) hexagonal structure with a P6mm space
group as the XRD pattern revealed three diffraction peaks
indexed as the (100), (110), and (200) planes (Fig. 2(1)). Al-
though the intensity of the diffraction peaks decreased after
multi-step modification, well-resolved XRD patterns con-
firmed that the p6mm symmetry of the mesostructure was
reserved even after chemical coordination with Pd(II) ions
Table 1 Textural properties of the parent FDU-15 and the function-
alized samples
ample
A
BET
Pore volume
Pore size
(nm)
3.5
2
3
(
m /g)
489
(cm /g)
DU-15
0.38
0.27
0.20
DU-NHC
389
3.0
(
Fig. 2(2) and (3)).
DU-NHC/Pd(II)
271
3.0
HRTEM indicated that the array of mesopores in
FDU-NHC/Pd(II) was typical of a 2D hexagonal mesostruc-
ture even after multi-functionalization (Fig. 3). Thus, in
agreement with above XRD patterns the HRTEM images also
verified that the well-ordered mesostructure remained intact
after chemical modification.
2
the surface area of the parent FDU-15 was 489 m /g while
2
FDU-NHC/Pd(II) had a lower specific surface area of 271 m /g,
and the pore diameter decreased from 3.5 to 3.0 nm. Changes in
the N adsorption-desorption isotherms, the surface area, and
2
the pore volume values can be attributed to the incorporation of
organic groups with relatively large molecular dimensions.
Figure 4 shows the nitrogen adsorption-desorption isotherms
of FDU-NHC/Pd(II) and its precursors. These samples exhib-
ited a typical type IV isotherm, which is characteristic of
mesoporous materials. Furthermore, as summarized in Table 1,
Nevertheless, based on XRD, HRTEM, and N adsorption
2
characterization, we conclude that the Pd(II) supported
mesopolymer catalyst FDU-NHC/Pd(II) with an ordered hex-
agonal structure was prepared successfully.
(
a)
2
.2 Performance of the catalysts
The Sonogashira reaction between iodobenzene and
phenylacetylene over FDU-NHC/Pd(II) was studied as a
model reaction to optimize the reaction conditions. The results
listed in Table 2 show the effect of solvent, reaction tempera-
ture, and catalyst dose. When DMA was used as a solvent, a
high yield of product 1a was obtained (Table 2, entry 1).
DMF and DMSO gave lower yields (Table 2, entries 2 and 3).
Other solvents such as toluene, acetonitrile and ethanol were
also evaluated and the results were not satisfactory (Table 2,
entries 4–6). The effect of reaction temperature and catalyst
dose on the Sonogashira coupling was also investigated. When
the reaction proceeded at a lower temperature of 353 or 363 K,
a prolonged reaction time was needed to obtain a considerably
higher product yield (Table 2, entries 7 and 8). Furthermore,
when the amount of FDU-NHC/Pd(II) catalyst was reduced to
(
b)
Fig. 3. TEM images of FDU-NHC/Pd(II) as viewed in the [11] (a)
and [10] (b) directions.

YU Tao et al. / Chinese Journal of Catalysis, 2011, 32: 1712–1718
fluoro all gave satisfactory yields for the corresponding prod-
Table 2 Influence of solvents, reaction temperature, and catalyst dose
ucts within 6 h (Table 3, entries 1–6). On the other hand, the
catalyst offered moderate yields at a lower reaction rate for
the substrates with electron-donating groups such as methyl
on the model reaction of iodobenzene and phenyl acetylene
FDU-NHC/Pd(II)
I +
1
a
(
Table 3, entries 7–9). The reactions of various substituted
Temperature Catalyst dose
a
b
phenylacetylene and iodobenzene also resulted in moderate
product yields of 70%–84% (Table 3, entries 10–12). Fur-
thermore, the reaction of bromobenzene also gave the product
with a moderate yield (85%) whereas the reactions of
Entry
Solvent
Time (h) Yield (%)
(K)
(mol %)
1
2
3
4
5
6
7
8
9
DMA
DMF
373
373
373
355
373
343
363
353
373
1
3
3
94
90
92
84
28
10
81
61
84
1
DMSO
1
3
4
-bromobenzonitrile and 1-bromo-4-nitrobenzene both gave
good product yields (Table 3, entries 13–15). For
-chloro-4-nitrobenzene, only a trace amount of correspond-
CH
Toluene
CH CH OH
3
CN
1
28
5
1
1
3
2
1
3
ing product was detected (Table 3, entry 16).
DMA
DMA
DMA
1
5
1
6
The reusability of FDU-NHC/Pd(II) in the model reaction
between iodobenzene and phenylacetylene was also investi-
gated. The catalyst was recovered by simple filtration after the
reactions, washed with acetone, dried and reused without any
further activation. As shown in Fig. 5, the catalyst did not
show noticeable deactivation after 10 times. The high reus-
ability of the supported catalyst in the Sonogashira
cross-couplings further demonstrates that the Pd ions are ef-
fectively stabilized by the N-Heterocyclic carbene ligands in
the catalyst. Moreover, TEM characterization indicated that no
Pd nanoparticles were present in the used FDU-NHC/Pd(II)
catalyst and its Pd 3d XPS spectra were essentially the same as
the fresh one.
0.5
3
Reaction conditions: iodobenzene 0.5 mmol, phenyl acetylene 0.75 mmol,
K
2 3
CO 1 mmol, catalyst 1 mol%, CuI 10 mol %, solvent 5.0 ml.
a
Monitored by TLC.
b
Isolated yield.
0
.5 mol% relative to the substrate the product was obtained in
only 84% yield.
After optimizing the reaction conditions, we investigated
the performance of FDU-NHC/Pd(II) for the transformation of
various aryl halides with substituted phenylacetylene. The
results are summarized in Table 3. The reactions between aryl
iodides bearing electron deficient groups such as nitro and
100
Table 3 Sonogashira reactions of various aryl halides and terminal
alkynes catalyzed by FDU-NHC/Pd(II)
7
5
2
5
0
5
0
FDU-NHC/Pd(II)
I +
1
a
a
b
Entry
R
4-NO
2-F
R’
H
X
I
Time (h) Product Yield (%)
1
2
3
4
5
6
7
8
9
2
1
6
1b
1c
1d
1e
1f
92
93
H
I
4-F
H
I
3
90
3-CF
4-COCH
4-COOEt
4-CH
4-OCH
3
H
I
4
91
3
H
I
4
98
1
2
3
4
5
6
7
8
9
10
H
I
2
1g
1h
1i
95
Number of recycling
3
H
I
24
10
16
23
11
3
80
Fig. 5. Recoverability of FDU-NHC/Pd(II) in the model reaction.
Conclusions
3
H
I
51
1,3-CH
3
H
I
1j
45
3
1
0
H
H
3-CH
3
3
I
1k
1h
1l
83
11
4-CH
I
70
We developed a novel phosphine-free NHC-Pd(II) complex
12
13
14
15
16
H
4-CN
H
I
84
supported mesopolymer catalyst, FDU-NHC/Pd(II), which is
efficient and reusable in Sonogashira coupling reactions. The
catalyst can be easily separated from liquid reaction mixtures
and reused more than 10 times without significant loss of ac-
tivity. The optimal coordination modes of the N-heterocyclic
carbene ligand with the palladium complex and the application
of this catalyst to other palladium-catalyzed reactions will be
further investigated.
H
Br
Br
Br
Cl
17
13
24
24
1a
1l
85
4-CN
H
93
4-NO
2
H
1m
1m
87
4-NO
2
H
Trace
Reaction conditions: aryl halides 0.5 mmol, substituted phenyl acetylene
2 3
0.75 mmol, K CO 1 mmol, catalyst 1 mol%, CuI 10 mol %, DMA 5.0 ml,
3
73 K.
a
Monitored by TLC.
b
Isolated yield.

YU Tao et al. / Chinese Journal of Catalysis, 2011, 32: 1712–1718
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