Biaryl formation via Suzuki and Stille coupling reactions using palladium nanoparticle/polymeric N-heterocyclic carbene grafted silica as recyclable and efficient catalyst

Article abstract of DOI:10.1002/aoc.3508

Palladium nanoparticles supported on polymeric N-heterocyclic carbene grafted silica as an efficient organic–inorganic hybrid catalyst is introduced. Pd⁰ nanoparticle formation, which is stabilized by the polymeric N-heterocyclic carbene ligands and ionic liquid units, was confirmed using X-ray photoelectron spectroscopy. Scanning electron microscopy images showed microparticles of modified silica while transmission electron microscopy images displayed a fine distribution of Pd nanoparticles. The modified structure was applied successfully in biaryl formation via Suzuki and Stille coupling reactions. Various biaryls were generated through the reaction of phenylboronic acid or tetraphenyltin with a variety of haloarenes via cross-coupling reactions. This catalyst showed promising activity after being recycled several times. Copyright

Full text of DOI:10.1002/aoc.3508

Full paper
Received: 6 February 2016
Revised: 13 April 2016
Accepted: 14 April 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3508
Biaryl formation via Suzuki and Stille coupling
reactions using palladium
nanoparticle/polymeric N-heterocyclic carbene
grafted silica as recyclable and efficient catalyst
a
a,b
a
Soheila Ghasemi *, Fatemeh Farjadian * and Bahman Tamami
Palladium nanoparticles supported on polymeric N-heterocyclic carbene grafted silica as an efficient organic–inorganic hybrid cat-
0
alyst is introduced. Pd nanoparticle formation, which is stabilized by the polymeric N-heterocyclic carbene ligands and ionic liq-
uid units, was confirmed using X-ray photoelectron spectroscopy. Scanning electron microscopy images showed microparticles of
modified silica while transmission electron microscopy images displayed a fine distribution of Pd nanoparticles. The modified
structure was applied successfully in biaryl formation via Suzuki and Stille coupling reactions. Various biaryls were generated
through the reaction of phenylboronic acid or tetraphenyltin with a variety of haloarenes via cross-coupling reactions. This cata-
lyst showed promising activity after being recycled several times. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: N-heterocyclic carbenes; palladium nanoparticles; polymer grafted silica; Suzuki reaction; Stille reaction
removing organotin residues from target products. In addition,
Introduction
organostannane compounds do not react with all types of palladium
[16]
The construction of aryl–aryl bonds for the synthesis of compounds
catalysts in coupling reactions, especially heterogeneous types.
containing a biaryl subunit is important due to their potential utili-
zation in providing natural bioactive products, advanced mate-
Palladium complexation with N-heterocyclic carbenes (NHCs) is a
[1]
rapidly expanding field and has attracted wide attention
[
2,3]
[4]
[17,18]
rials with helical structures
and pharmaceuticals and in
recently.
Several review articles have discussed NHCs from
[
5]
supramolecular chemistry.
Nickel- or palladium-catalysed cross-coupling of aryl halides with
various aspects such as their variety of structures, metal complexes,
reactivity, synthetic routes, stabilization and catalytic and medical
[
6,7]
[8]
[9]
[10]
[17,19–21]
arylated precursors via Suzuki, Stille, Negishi, Kumada and
applications.
A NHC/Pd complex was first reported by Herr-
[11]
[22]
Hiyama reactions are the most common techniques for the prep-
aration of biarylated or heterobiarylated compounds.
mann and Koecher in Suzuki and Heck reactions and today are
applied widely in all kinds of C&bond;C bond forming
[
21,23,24]
Of these methods, the Suzuki–Miyaura reaction that utilizes an
organoboron compound as aryl species is the most useful and
promising organic transformation for the establishment of
reactions.
From this aspect, the preparation and utilization
of NHC/Pd complexes on organic or inorganic supports have been
[
25–27]
investigated.
Following our previous reports on heterogeneous Pd catalytic
[
6]
C&bond;C bonds in biaryl synthesis. The advantage of the Suzuki
reaction over other well-known coupling processes derives from
availability, high stability and low toxicity of arylboronic acid com-
pounds which results in high tolerance in reacting with precursors
[
28,29]
systems such as Pd nanoparticles supported on polymers
or
[
30,31]
silica and a specific report on the synthesis of Pd nanoparticles
supported on polymeric NHC grafted silica (Si-PNHC-Pd) catalyst,
[
32]
[
6,7]
containing various functional groups
The Stille reaction that was discovered in the late 1970s
herein, we present a new application of Si-PNHC-Pd as a stable and
catalytic system for the preparation of biaryls via Suzuki and Stille
coupling reactions.
[8,12]
has
emerged as another useful and efficient protocol for C&bond;C bond
forming reactions that was has not been reported as much as other
coupling methods. However, in numerous total syntheses, Stille
[
12]
coupling reaction appears as an emerging step
organostannane compounds with organic halides or triflates
catalysed by palladium complexes.
which uses
*
Correspondence to: Soheila Ghasemi or Fatemeh Farjadian, Department of
Chemistry, College of Sciences, Shiraz University, PO Box 7194684795, Shiraz,
Iran. E-mail: ghasemis@shirazu.ac.ir; or farjadian_f@sums.ac.ir
[13,14]
The importance of this reac-
tion is due to the fact that tin reagents are available or easily
prepared, purified and stored and tolerate various functional groups
a Department of Chemistry, College of Sciences, Shiraz University, PO Box
7194684795, Shiraz, Iran
[15]
and are air- and moisture-stable.
Despite such impressive
synthetic potential, the main difficulties of this reaction are the low
polarity of organotin precursors, toxicity and the obstacle of
b Pharmaceutical Sciences Research Center, Faculty of Pharmacy, Shiraz University
of Medical Sciences, PO Box 71345-1583, Shiraz, Iran
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

Ghasemi S., Farjadian F. and Tamami B.
Results and discussion
Synthetic plan and characterization of Si-PNHC-Pd catalyst
The overall synthetic plan for the preparation of Si-PNHC-Pd is
outlined by the sequence of reactions given in Scheme 1.
Acrylamidopropylsilica that was prepared according to a
[
33]
previously reported procedure
was copolymerized with
N-vinylimidazole using a free radical method (Scheme 1). The
resulting hybrid structure (Si-PVI) was quaternized with
iodomethane followed by treatment with sodium chloride to
obtain (Si-QPVI). Further, covalent binding with palladium chloride
and Si-QPVI occurred in the presence of Cs CO as a base through
2
3
Scheme
polyvinylimidazole (Si-QPVI) salt.
2. Argentimetric
titration
method
of
quaternized
NHC formation (Si-PNHC-Pd). The product of each step was charac-
terized using Fourier transform infrared (FT-IR) spectroscopy. The
Si-PVI spectrum showed the characteristic bands of imidazole ring
À1
that showed the crystallographic planes of (111), (200), (220) and
(N&bond;C&dbond;N and C&dbond;N) at 1647 and 1504 cm
[
32]
(311) which are consistent with metallic Pd(0) literature data. Ad-
while the Si-QPVI spectrum exhibited two characteristic aromatic
ditionally, X-ray photoelectron spectroscopy (XPS) was utilized in
order to ascertain the structure, chemical oxidation state and com-
plete reduction of Pd present in the structure. Figure 1(a) shows the
0–1200 eV scanned survey XPS spectrum. The peaks at 102.4, 153.2,
284.8, 399.9 and 531.5 eV are attributed to the binding energies of
Si2p, Si2s, C1s, N1s and O1s atoms, respectively. The two spin–orbit
pairs located at binding energies of 335 and 341.2 eV were charac-
teristic, respectively, of 3d5/2 and 3d3/2 of Pd in the zero oxidation
bands of C&bond;H bending related to imidazolium group at
À1
1677 and 1522 cm , respectively. The FT-IR spectrum of
Si-PNHC-Pd showed the characteristic peak of C&dbond;C bond
À1
of the imidazole ring at 1642 cm and the band of C&bond;Pd
À1
absorption at 792 cm while C&bond;H bending bands disap-
peared. Thermogravimetric analysis (TGA) was performed to deter-
mine the amount of PVI grafted silica which showed a loading of
À1
1
.3 mmol g . The amount of quaternized imidazoline rings in
0
state (Fig. 1(b)). This doublet absorption seen in Pd -containing cat-
Si-QPVI was determined using the argentimetric titration method
2
+
0
À1
alysts proves that almost all of the Pd ions have reduced to Pd . It
and was found to be 1.1 mmol g (Scheme 2).
0
thus indicates that the polymer in our system stabilized the Pd
Palladium loading was determined using the inductively coupled
plasma (ICP) technique and revealed the presence of 0.26 mmol of
Pd per gram of support. In this catalytic system, Si-PNHC-Pd is a pre-
catalyst that is the origin of the Pd(0) nanoparticles which is the
active catalyst. In fact, the polymer-bearing ionic liquid unit is capa-
ble of reducing Pd(II) to Pd(0) and stabilizing Pd nanoparticles and
preventing their aggregation by an electrostatic stabilization mech-
anism. Steric stabilization of polymer chains is also a main factor for
nanoparticles efficiently and prevented them from oxidation. To
have a better understanding of Pd nanoparticle distribution and
the hybrid structure morphology, scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) were carried
out. The SEM images showed microparticles where gleaming Pd
nanoparticles are dispersed on the surface. The TEM images
showed a fine distribution of black Pd particles with a size of
[
32]
[
34]
10 nm on the surface.
preserving Pd nanoparticles
(Scheme 1). Furthermore, the
formation of Pd(0) is confirmed from X-ray diffraction (XRD) analysis
Suzuki cross-coupling reaction
Suzuki reactions for the production of biarylated compounds were
examined in the presence of Si-PNHC-Pd as a catalyst. Optimized
reaction conditions were N-methyl-2-pyrrolidone (NMP) as a sol-
vent and K CO as a base using 0.5 mol% of catalyst at 120 °C.
2
3
Various types of aryl halides containing electron-donating or
electron-withdrawing substituents were reacted with
phenylboronic acid to construct biarylated products (Table 1).
Chloroarenes with various substituents which are less active but
economical and available precursors successfully coupled under
optimized reaction conditions while the addition of catalytic quan-
tity of tetrabutylammonium bromide (TBAB) as co-catalyst was
necessary (Table 1).
Scheme 1. Synthetic pathway for the preparation of Si-PNHC-Pd catalyst.
Figure 1. XPS spectra of Si-PNHC-Pd catalyst: (a) survey spectrum; (b) Pd 3d.
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Copyright © 2016 John Wiley & Sons, Ltd. Appl. Organometal. Chem. (2016)

Biaryl formation via coupling reactions using pdnps on hybrid support
Table 1. Formation of biarylated compound via Suzuki coupling reac-
tion in the presence of Si-PNHC-Pd
Table 3. Formation of biarylated compounds via Stille coupling in the
presence of Si-PNHC-Pd
a
a
Entry
Ar
X
Time (h)
Yield (%)
Entry
Ar
X
Time (h)
Yield (%)
1
2
Ph
Ph
I
1
1
95
80
90
95
80
90
75
75
80
75
1
2
3
4
5
6
7
8
9
Ph
4-MeOPh
Ph
I
3
3
95
96
90
95
90
100
90
95
90
80
85
75
85
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
I
3
4-NO
2
Ph
2
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
5
4
4-CNPh
4-ClPh
2
4-NO
2
Ph
2.5
4
5
2
4-CNPh
4-COMePh
4-MePh
4-PhPh
4-ClPh
6
4-MePh
3-MePh
4-MePh
3
3
7
10
12
10
12
8
8
8
9
2
4-NO Ph
6
10
4-COMePh
10
11
12
13
Ph
15
10
6
a
All reactions were run using 1.0 eq. ArX, 1.5 eq. tetraphenyltin, 2.0 eq.
NaOH and 0.005 eq. Pd catalyst in 5 ml NMP as solvent. The crude was
mixed at 120 °C for appropriate time. Yields refer to the isolated
products based on the aryl halide used; 5 mol% of TBAB was used as a
co-catalyst for chloroarenes
2
4-NO Ph
4-COMePh
4-CHOPh
8
14
4-MeO-Ph
Cl
24
40
a
All reactions conducted using 1.0 eq. ArX/1.2 eq. phenylboronic acid/
.0 eq. K CO and 0.005 eq. Pd catalyst in NMP (5 ml) as solvent. The
The scope and generality of this coupling system were investi-
gated by utilizing various aryl halides in reactions with
tetraphenyltin under the optimized reaction conditions (Table 3).
Similar to the Suzuki reaction, aryl halides containing sub-
strates without substituent, and with activating or deactivating
groups reacted with tetraphenyltin to generate the correspond-
ing biaryl products (Table 3). Moreover, the Stille reaction under
these conditions could be performed with aryl chlorides contain-
ing various substituents using this catalyst in combination with a
catalytic amount of TBAB as co-catalyst. The reaction is also tol-
erant to heterocyclic substrate. 2-Iodothiophene coupled with
tetraphenyltin successfully to produce the corresponding
heterobiaryl product (Scheme 3).
The mechanism of Stille coupling involves a series of reactions
around the Pd catalyst. These are oxidative addition in which the
active Pd(0) catalyst inserts into the aryl halide bond,
transmetalation with the organotin compound in the presence of
base and reductive elimination to produce the desired coupling
product and regenerate the active Pd(0) species.
Investigation the recyclability of the Pd catalyst reveals that the
catalyst is highly reusable and can be filtered and reused in a new
coupling reaction up to 10 times without substantial loss of its activ-
ity. Figure 2 shows the results of catalyst reusability of a model cou-
pling reaction of p-bromotoluene in Suzuki and Stille coupling
reactions. The crude reaction mixture was analysed after the last
run of recycling using the ICP technique, which indicated negligible
leaching. Additionally, low Pd corruption was observed between
loading of fresh catalysts and recycled ones.
2
2
3
crude was mixed at 120 °C for appropriate time. Yields refer to the
isolated products based on the aryl halide used; 5 mol% of TBAB was
used as a co-catalyst for chloroarenes
Stille cross-coupling reaction
The Stille reaction catalysed by supported palladium catalysts has
[
35,36]
received less attention than other coupling reactions.
In con-
sideration of this, the aforementioned catalyst was examined for
the formation of biaryls through the Stille reaction using
tetraphenyltin with various aryl halides. First, the reaction condi-
tions were optimized in terms of catalyst, base, solvent and temper-
ature for the reaction between tetraphenyltin and p-bromotoluene.
The best results were obtained with 0.5 mol% of catalyst in the
presence of sodium hydroxide as a base and NMP as a solvent at
1
20 °C. The results are summarized in Table 2.
Table 2. Optimization of solvent, base and amount of Pd catalyst for
a
Stille reaction of p-bromotoluene with tetraphenyltin
b
Entry
Base
Solvent Pd (mol%) Time (h) Conversion (%)
1
2
3
4
5
6
7
K
K
K
2
2
2
CO
CO
CO
3
DMF
EtOH
NMP
NMP
NMP
NMP
NMP
0.5
0.5
0.5
0.5
0.5
0.5
0.3
5
10
4
100
100
100
90
3
3
In order to ascertain whether the physical properties of the cata-
lyst such as functional groups, size and chemical oxidation state are
Et
3
N
3
KF
3
90
NaOH
NaOH
3
100
85
5
a
Reactions conducted using 1.0 eq. p-bromotoluene, 1.5 eq.
tetraphenyltin, 2.0 eq. base and 0.003–0.005 eq. Pd catalyst in 5 ml
solvent at 120 °C
b
Conversions based on p-bromotoluene
Scheme 3. Stille coupling of 2-iodothiophene with tetraphenyltin using Si-
PNHC-Pd catalyst.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Ghasemi S., Farjadian F. and Tamami B.
Figure 2. Reusability of the Pd catalyst in Suzuki and Stille coupling
reactions.
Figure 5. TEM image of the recycled catalyst.
nanoparticles on the surface of the polymer without or with
negligible amount of agglomeration which was proved using
TEM. The third improvement was the efficiency of this catalyst
in Stille cross-coupling reactions where heterogeneous cata-
lysts were less successful in this reaction.
Conclusions
Figure 3. High-angle powder XRD spectrum of the recycled catalyst.
An ionic liquid structure of polymeric NHC grafted silica capable of
conjugation and nanoparticle formation with Pd has been intro-
duced and characterized. Characterization techniques including
FT-IR spectroscopy, TGA and ICP have proved the successful trans-
formations and determined the amount of organic layer and Pd
content. XRD showed the crystallinity of amorphous silica and Pd
nanoparticles. SEM and TEM showed Pd nanoparticle formation in
0
the range of 7–10 nm. XPS analysis as a powerful tool showed Pd
formation. Using the high efficiency of the aforementioned struc-
ture in cross-coupling reactions, the formation of biaryls through
Suzuki and Stille transformations was evaluated. A wide variety of
aryl halides including iodo, bromo, chloro and heterocyclic sub-
strates produce the coupling products in short time with high yields.
The catalyst was successfully recycled with simple filtration and
reused in 10 repeat cycles without considerable loss of its activity.
Figure 4. FT-IR spectrum of the recycled catalyst.
changed or not, FT-IR and XRD studies were conducted and TEM
images of the recovered catalyst obtained. The powder XRD pattern
of the recovered catalyst showed nearly identical diffraction peaks
to those of the fresh catalyst and confirmed its stability (Fig. 3). In
addition, the FT-IR spectrum of the recycled catalyst indicated a
negligible change in the structure of the catalyst (Fig. 4). Further-
more, a TEM image reveals that the characteristic shape of the cat-
alyst after recovery from the reaction mixture is retained almost
unchanged in size and no agglomeration of Pd nanoparticles oc-
curred (Fig. 5).
Finally, a comparison of this catalytic system with our previ-
ous reports on heterogeneous Pd catalysts was made to evalu-
ate the efficiency of the reported system. It showed
improvement in some important aspects. The first improve-
ment was apparent in the reusability of the catalyst, which is
due to covalent bonds between Pd and NHC ligand. These co-
valent bonds are responsible for keeping Pd firmly in the struc-
ture and inhibiting its loss during several cycles of recycling.
The second improvement was the fine dispersion of Pd
Experimental
General remarks
Aminopropylsilica with microparticle size of 0.015–0.035 mm (>400
mesh ASTA) and amine content of 0.95 mmol g was purchased
from Fluka. N-vinylimidazole, iodomethane, acryloyl chloride and
palladium chloride (98% pure) were purchased from Sigma-Aldrich.
Solvents were provided by Merck Millipore. Product characterization
À1
1
13
was performed by comparing FT-IR, H NMR and C-NMR spectra
and physical profiles with those of authenticated samples. FT-IR
spectra were obtained with a Shimadzu FT-IR-8300 spectrophotom-
1
13
eter. H NMR and C NMR spectra were recorded with a 250 MHz
Bruker Avance DPX. Polygram SIL/UV 254 sheets of silica-gel were
applied for TLC. GC was performed with a Shimadzu GC 10-A instru-
ment, having a hydrogen flame ionization detector. ICP was con-
ducted to determine Pd loading and leaching (Varian, vista-pro).
XPS spectra were recorded with a Bestec spectrometer (Germany)
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Biaryl formation via coupling reactions using pdnps on hybrid support
system equipped with aluminium anode X-ray source for exciting
Acknowledgements
photoelectrons. The spectrum was observed with Al Kα radiation
The authors gratefully acknowledge the partial support of this study
by Shiraz University Research Council.
(
hν = 1486.6 eV) and the pressure of residual gases in the analytical
À10
chamber was 10 × 10
mbar during data acquisition. The energy
shift due to the static charging of the samples was corrected using
the adventitious C 1 s peak at 285.0 eV as a reference.
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Supporting information
(
95:5). GC analysis was performed to determine the yield of
the reactions. After reaction completion, the above outlined
process was followed.
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web site.
Appl. Organometal. Chem. (2016)
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