Polymer anchored 3-benzoyl-1-(1-benzylpiperidin-4-yl)-2-thiopseudourea-Pd(II) complex: An efficient catalyst for the copper and solvent free Sonogashira cross-coupling reaction

Article abstract of DOI:10.1016/j.jorganchem.2016.08.029

An efficient copper and solvent free Sonogashira cross-coupling reaction catalyzed by polymer supported 3-benzoyl-1-(1-benzylpiperidin-4-yl)-2-thiopseudourea-Pd(II) complex 4 (PS-bpt-Pd) was reported. This inexpensive and simply synthesized heterogeneous catalyst 4 afforded excellent yields of the cross-coupling products under mild reaction conditions. We have achieved high yields for aryl iodide coupling reactions with terminal alkynes at room temperature under copper and solvent-free conditions. In addition, we also observed moderate to good yields for aryl bromides as coupling partner. This insoluble PS-bpt-Pd(II) catalyst 4 can be easily recovered by simple filtration and reused up to five times with stable catalytic activity. In addition to this, heterogeneous Pd (II) complex is even applicable on a gram scale for cross-coupling with high catalytic efficiency.

Full text of DOI:10.1016/j.jorganchem.2016.08.029

Journal of Organometallic Chemistry 822 (2016) 165e172
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Polymer anchored 3-benzoyl-1-(1-benzylpiperidin-4-yl)-2-
thiopseudourea-Pd(II) complex: An efficient catalyst for the copper
and solvent free Sonogashira cross-coupling reaction
,
*
Suresh Thogiti c, Sai Prathima Parvathaneni b, Srinivas Keesara a
a School of Chemistry, University of Hyderabad, Hyderabad, 500046, India
b Inorganic & Physical Chemistry Division, CSIR, Indian Institute of Chemical Technology (IICT), India
c Department of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, 712-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient copper and solvent free Sonogashira cross-coupling reaction catalyzed by polymer supported
3-benzoyl-1-(1-benzylpiperidin-4-yl)-2-thiopseudourea-Pd(II) complex 4 (PS-bpt-Pd) was reported. This
inexpensive and simply synthesized heterogeneous catalyst 4 afforded excellent yields of the cross-
coupling products under mild reaction conditions. We have achieved high yields for aryl iodide
coupling reactions with terminal alkynes at room temperature under copper and solvent-free conditions.
In addition, we also observed moderate to good yields for aryl bromides as coupling partner. This
insoluble PS-bpt-Pd(II) catalyst 4 can be easily recovered by simple filtration and reused up to five times
with stable catalytic activity. In addition to this, heterogeneous Pd (II) complex is even applicable on a
gram scale for cross-coupling with high catalytic efficiency.
Received 6 May 2016
Received in revised form
23 August 2016
Accepted 26 August 2016
Available online 28 August 2016
Keywords:
Thiopseudourea
Polymer
Heterogeneous Pd catalyst
Sonogashira
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
nanoparticles [9] and carbon nanofiber [10].
Polymer supported Pd catalysts are very attractive due to facile
Sonogashira cross-coupling reaction is one of the most popular
palladium-catalyzed CeC bond forming reaction [1]. This involves
the cross-coupling reaction of aryl halides with terminal alkynes,
playing a vital role in the production of many industrially important
chemicals and substituted alkyne compounds with high synthetic
value in the organic chemistry. These moieties are abundant in
biologically active natural products, pharmaceutical products and
materials [2]. Over the past decade, notable achievements have
been made in the development of Pd catalysts for the Sonogashira
transformations in both homogeneous and heterogeneous condi-
tions. Homogeneous palladium catalysts associated with some of
the difficulties, like unwanted palladium contamination, separation
and recovery of the catalysts from the products. Heterogeneous
metal catalysts are emerging as an alternative to the existing ho-
mogeneous ones. A number of heterogeneous palladium catalysts
have been reported for the CeC bond formation reactions with
different solid supports such as polymers [3], metal oxides [4],
zeolites [5], silica-starch [6], clay [7], montmorillonite [8], magnetic
synthesis, stability and easy separation from the reaction mixture
makes growing their applications in heterogeneous catalysis.
Therefore, a number of polymer anchored palladium complexes
have been synthesized with diverse donor functionalities and their
catalytic activity have been studied for the CeC bond forming re-
actions [11]. It is well known that the nature of ligands in polymer
supported Pd complexes could have a substantial effect on the
catalytic behaviour [12].
Most of the Pd catalysts have been reported for Sonogashira
coupling reaction, along with copper salts as co-catalysts under
different organic solvents [13]. Copper co-catalyst is responsible for
the formation of undesired homo-coupling products through the
Glaser coupling in palladium catalyzed Sonogashira reaction [14].
The use of organic solvents as medium shows severe health and
environmental problems from their associated waste. The main
target of research in this field is to avoid the use of copper co-
catalyst and organic solvents to achieve economical and environ-
mental safety [15]. In this point of view, a small number of efficient
reusable palladium catalytic systems have been reported for
Sonogashira cross-coupling reactions under copper and solvent
free conditions. M. Bakherad and co-workers reported different
* Corresponding author.
E-mail address: drsrinivaskeesara@yahoo.com (S. Keesara).
http://dx.doi.org/10.1016/j.jorganchem.2016.08.029
0022-328X/© 2016 Elsevier B.V. All rights reserved.

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S. Thogiti et al. / Journal of Organometallic Chemistry 822 (2016) 165e172
polystyrene-supported palladium catalysts for copper and solvent
free Sonogashira coupling reactions [16]. R. S. Salunkhe and co-
workers have published Sonogashira coupling reaction with Mer-
rifield resin supported Pd-NHC complex under copper- and
solvent-free conditions [17].
polystyrene (1 g, 1.25 mmol/g of Cl) and 3-benzoyl-1-(1-
benzylpiperidin-4-yl)-2-thiourea (1.5 mmol). The reaction
mixture was stirred for 24 h at 110 ꢀC and was subsequently filtered
and washed thoroughly with DMF, dried in oven at 80 ꢀC for 24 h to
form the PS-bpt (Scheme 1).
Herein, we report the synthesis of polystyrene supported 3-
benzoyl-1-(1-benzylpiperidin-4-yl)-2-thiopseudourea-Pd(II) com-
plex 4 (PS-bpt-Pd(II) complex 4) and its application to the Sono-
gashira cross-coupling reaction under copper and solvent free
conditions. Formerly, we have reported homogeneous and het-
erogeneous thiopseudourea Pd(II) complexes as catalysts for
different CeC cross-coupling reactions [18].
2.2.3. Synthesis of PS-bpt-Pd(II) complex (4)
To a 250-mL round bottom flask containing CH2Cl2 (50 mL),
equipped with magnetic stirrer bar is added with PS-bpt ligand 3
(1.3 g) and Pd(OAc)2 (1 mmol). The resulting mixture was allowed
to stir at room temperature for 24 h. This mixture was filtered and
washed with acetonitrile to obtain PS-bpt-Pd(II) complex
(Scheme 1).
4
2. Experimental
2.3. General procedure for the Sonogashira cross-coupling reaction
of aryl iodides
2.1. General information
All materials were commercial reagent grade. Chloromethylated
polystyrene (1% cross-linked, 200e400 mesh with 1.0e1.3 mmol/g)
was a product of Alfa-Aesar. Aryl halides and terminal alkynes were
obtained from Aldrich. Laboratory grade acetone was purchased
from S.D Fine. The amount of catalyst 4 we used according to mo-
lecular weight of palladium. Thermo gravimetric analysis was
recorded on a TG/DTA-7300, HITACHI, Japan. Samples were heated
from 30 ꢀC to 800 ꢀC ascent at 20 ꢀC/min under N2 atmosphere. IR
spectra were obtained in KBr wafers on Thermo Nicolet Nexus 670
spectrophotometer respectively. Field emission scanning electron
microscope (FESEM) images were obtained from a Carl Zeiss model
Merlin compact microscope using a 30 keV electron beam. Energy
dispersive x-ray (EDX) spectra were recorded using Oxford In-
struments X-MaxN SDD (50 mm2) system and INCA analysis soft-
ware. The 1H NMR and 13C NMR spectra were recorded at 400 MHz
and 100 MHz, respectively. The chemical shifts are reported in ppm
A mixture of aryl iodide (1.0 mmol), phenylacetylene
(1.2 mmol), Et3N (2.0 mmol) and 0.005 mmol (10 mg) of Pd(II)
complex 4 was stirred at room temperature for a desired reaction
time. Further, the reaction mixture was extracted with ethyl acetate
and dried over MgSO4. The solvent was removed under reduced
pressure and the residue was purified by column chromatography
on silica gel using hexane as eluent to give the corresponding
coupling products.
2.4. General procedure for the Sonogashira cross-coupling reaction
of aryl bromides
A
mixture of aryl iodide (1.0 mmol), phenylacetylene
(1.2 mmol), TMG (2.0 mmol), water (2 mL) and 0.01 mmol (20 mg)
of Pd complex 4 was stirred at 80 ꢀC temperature for a desired
reaction time. Further, the reaction mixture was extracted with
ethyl acetate and dried over MgSO4. The solvent was removed
under reduced pressure and the residue was purified by column
chromatography on silica gel using hexane as the eluent to give the
corresponding coupling products.
downfield to TMS (
d
¼ 0) for 1H NMR and CDCl3
(
d
¼ 77.0) for 13C
NMR. High-resolution mass spectra (HRMS) were recorded on ESI-
TOF maXis.
2.2. Preparation of the catalyst
2.5. Procedure for CS2 poisoning test
2.2.1. General procedure for the synthesis of ligand 2
To a solution of 4-amino-1-benzylpiperidine (5 mmol) in 20 mL
of acetone, benzoyl isothiocyanate (5 mmol) was added dropwise at
0 ꢀC and allowed to stir for 3 h at room temperature. The reaction
mixture was concentrated to a solid; water (50 mL) was added, and
extracted with ethyl acetate (50 mL). The organic layer was washed
with brine (20 mL), dried over MgSO4, and filtered, after which the
solvent was removed under reduced pressure. The crude product
was purified by column chromatography on silica gel using ethyl
acetate/hexane as the eluent to give the corresponding coupling
products (Scheme 1).
A 25 mL round bottom flask was charged with iodobenzene
(1.0 mmol), phenylacetylene (1.2 mmol), 0.005 mmol (10 mg) of Pd
complex 4 and 0.2 mL of CS2 (0.8 equiv comparative to palladium)
from a freshly prepared solution in Et3N. This reaction mixture was
stirred at room temperature for 10 h and no conversion takes place
as indicated by TLC analysis.
3. Results and discussion
The new thiourea ligand 2 was prepared by following one step
procedure as shown in Scheme 1. This involves the reaction be-
tween benzoyl isothiocyanate and 4-amino-1-benzylpiperidine in
acetone under nitrogen atmosphere at 0 ꢀC to afford the thiourea
compound 2. The light yellow solid was obtained in 95% yield after
purification by column chromatography. Compound 2 was char-
acterized by NMR, IR and Mass spectroscopic techniques.
2.2.1.1. 3-benzoyl-1-(1-benzylpiperidin-4-yl)-2-thiourea
NMR (400 MHz, CDCl3, TMS) 10.7 (s, 1H), 8.89 (s, 1H), 7.76 (d,
(2). 1H
d
J ¼ 7.57 Hz, 2H), 7.54 (t, J ¼ 7.07 Hz, 1H), 7.43 (t, J ¼ 7.07 Hz, 2H),
7.25e7.18 (m, 5H), 4.26 (s, 1H), 3.45 (s, 2H), 2.72 (s, 2H), 2.18 (t,
J ¼ 10.61 Hz, 2H), 2.06 (d, J ¼ 11.87 Hz, 2H), 1.63 (dd, J ¼ 11.36 Hz,
2H);13C NMR (100 MHz, CDCl3)
d
178.9, 167.1, 136.9, 133.5, 131.7,
In Scheme 1, we have represented the steps involved in the
preparation of new polystyrene supported thiopseudourea Pd(II)
129.4, 129.0, 128.4, 127.5, 127.1, 62.7, 60.4, 51.5, 30.3; IR (cmꢁ1):
ṽ(CONHCSNH) 3417, 3239, ṽ(C]O)1670, ṽ(C]S) 796. HRMS: exact
mass calculated for C20H23N3OS [MþH]þ ¼ 354.1640, found m/
z ¼ 354.1644.
complex
4.
A
3-benzoyl-1-(1-benzylpiperidin-4-yl)-2-
thiopseudourea functionalized polystyrene resin (PS-bpt) (2%
DVB) was formed by heating a mixture of chloromethylated poly-
styrene and 3-benzoyl-1-(1-benzylpiperidin-4-yl)-2-thiourea in
DMF solvent at 110 ꢀC for 24 h. This solid polymer-supported thi-
opseudourea 3 is insoluble in common organic solvents. Reaction of
polymer-bound thiopseudourea with DCM and Pd(OAc)2 in 1: 1 M
2.2.2. Synthesis of PS-bpt ligand (3)
To a 250-mL round bottom flask containing DMF (50 mL),
equipped with magnetic stirrer bar is added with chloromethylated

S. Thogiti et al. / Journal of Organometallic Chemistry 822 (2016) 165e172
167
Scheme 1. Synthesis of PS-bpt-Pd(II) complex 4.
molar ratio for 12 h at room temperature resulted in covalent
attachment of palladium to give the functionalized polymer. The
literal complex formation was confirmed by FT-IR analysis. The FT-
IR spectrum of polystyrene, PS-bpt ligand 3 and PS-bpt-Pd(II)
complex 4 is revealed in supplementary information. A sharp
peak at 1263 cmꢁ1 corresponding to CeCl peak completely dis-
appeared in polymer-bound thiopseudourea Pd complex indicates
the introduction of thiourea on the polymer. In addition, the
stretching vibration of the C]N and C]O double bond peaks
observed at 1600 and 1724 cmꢁ1 corresponds to the formation of
polystyrene-supported thiopseudourea palladium complex [19].
Scanning electron micrographs (SEM) were reported for pure
chloromethylated
polystyrene,
and
polymer
anchored
thiopseudourea-Pd(II) complex 4 to study the morphological
changes. Usually the pure polymer bead had a smooth and flat
surface, while the anchored polymeric complex showed rough-
ening of the top layer. The complete morphology change in the
complex 4 indicates the presence of palladium metal on the surface
of polymer (Fig. 1(A) and (B)). Next we studied the Energy Disper-
sive X-ray Spectroscopy (EDX) analysis of PS-bpt-Pd(II) 4, which
showed catalyst was composed with palladium, carbon, nitrogen,
oxygen, and sulphur. This confirms that the thiopseudourea-Pd(II)
complex anchored to the polymer matrix (Fig. 2(a) and (b)).
Thermal stability of the complex 4 was investigated by TGA. The
Fig. 1. SEM images of (A) Chlomethylated polystyrene (B) PS-bpt-Pd(II) complex 4.
Fig. 2. EDX analysis of: (a) PS-bpt ligand 3 (b) PS-bpt-Pd(II) complex 4.

168
S. Thogiti et al. / Journal of Organometallic Chemistry 822 (2016) 165e172
towards aryl halide and alkyne cross-coupling in our reaction
conditions due to fast hydrogen halide formation. As expected no
coupling product 7a was obtained under base free conditions
(Table 1, entry 9). Next we screened various organic solvents with
Et3N base. The reaction proceeded well with all the organic solvents
and gave the coupling product 7a in moderate to excellent yields
(Table 1, entries 10e16). Next we studied the same reaction under
solvent-free conditions with Et3N as the base. Effective coupling
product was observed with in 5 h under these reaction conditions
without any formation of Glaser-type oxidative homo coupling
product (Table 1, entry 17).
Further, we studied the scope of coupling of aryl iodides with
phenylacetylene under the above optimized reaction conditions
(Table 2). We observed high yields of coupling products for both
electron rich and electron deficient aryl iodides reactions under
solvent free conditions. Substituent effect was not prominent for
the coupling reaction of para and meta substituted aryl iodides with
phenylacetylene which resulted the corresponding products 7a-7g
in excellent yields (Table 2, entries 1e7). Due to steric effect low
yield of the coupling product 7h was obtained for the 2-
bromoiodobenzene reaction with phenylacetylene. We observed
slight improvement in yield (from 40% to 65%) for the same
experiment when carried out in water (Table 2, entry 8). Next we
investigated the effect of various aryl iodides using 4-
ethynyltoluene as the substrate led to good yield of the desired
products 7b and 7i-7n (Table 2, entries 9e15).
Encouraged by these results, further we extended the scope of
the catalyst 4 to Sonogashira coupling reaction of aryl bromides
Table 3. As expected, the above optimized reaction conditions were
ineffective for the bromobenzene coupling reaction with phenyl-
acetylene. Aryl bromides are less reactive compared to aryl iodides
in Sonogashira cross-coupling reactions. Efficient coupling was
observed with TMG base and water as solvent at 70 ꢀC (Table 3,
entry 1). All the reactions of aryl bromides with phenylacetylene
and 1-ethynyl-4-methylbenzene furnished good yields of coupled
products 7b-7d and 7i-7k by varying reaction times (Table 3, en-
tries 2e8). No coupling product was observed for chlorobenzene
Fig. 3. TG-DTA analysis of PS-bpt-Pd(II) complex 4.
negligible weight loss below 200 ꢀC is due to the physically
adsorbed solvent molecules. Thermogravimetric analysis shows
stability of the PS-bpt-Pd(II) complex (4) up to 320 ꢀC and further
weight loss at a higher temperature (above 320 ꢀC) was attributed
to the decomposition of complex (Fig. 3). The exact concentration of
palladium incorporated into the polymer was determined by
atomic absorption spectroscopy (AAS), which showed the value of
about 6.46% (0.609 mmol/g).
Initially we tested the catalytic activity of PS-bpt-Pd(II) complex
4 in the solvent and copper-free Sonogashira cross-coupling reac-
tion. We have chosen iodobenzene and phenylacetylene as model
substrates to optimize the reaction conditions (Table 1). To choose a
suitable base, we screened different bases using 0.005 mmol of
catalyst 4 in water at room temperature. No coupling product 7a
was observed in case of inorganic bases and NaOAc.3H2O employed
reactions (Table 1, entries 1e4). High yields were observed with all
the organic amine bases (TMG, DABCO, DBU, and Et3N) (Table 1,
entries 5e8) [20]. The organic amine bases were more favourable
Table 1
Optimization reaction of the of iodobenzene with phenylacetylene.a
Entry
Base
Solvent
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
K2CO3
K3PO4
LiOH$H2O
NaOAc$3H2O
TMG
DABCO
DBU
Et3N
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
12
12
12
12
6
6
6
6
6
8
8
8
8
8
8
8
5
Nrc
Nrc
Nrc
Nrc
98
90
98
97
Nrc
72
80
94
95
95
96
94
99
9
e
H2O
10
11
12
13
14
15
16
17
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
MeOH
EtOH
DMF
DMSO
THF
Dioxane
Toluene
e
a
Reaction conditions: Iodobenzene (1 mmol), Phenylacetylene (1.2 mmol), base (2 mmol), catalyst 4 (0.005 mmol), solvent (1 mL) at room temperature.
Isolated yield.
No reaction.
b
c

S. Thogiti et al. / Journal of Organometallic Chemistry 822 (2016) 165e172
169
Table 2
Sonogashira coupling of different aryl iodides catalyzed by complex 4.a
,
c
Entry
1
R
Ar
Product
Yield (%)b
H (5a)
Ph (6a)
99, 97c
2
3
4
4-Me (5b)
4-OMe (5c)
4-NO2 (5d)
6a
6a
6a
96, 95c
86, 84c
96, 95c
5
6
7
4-Cl (5e)
4-Br (5f)
3-Br (5g)
6a
6a
6a
98, 97c
97, 97c
96, 94c
8
2-Br (5h)
6a
40, 65c
9
5a
5b
5c
4-CH3C6H5 (6b)
98
90
88
10
11
6b
6b
12
13
5d
5e
6b
6b
96
95
14
15
5f
6b
6b
98
95
5g
a
Reaction conditions: Aryl iodide (1 mmol), alkyne (1.2 mmol), catalyst 4 (0.005 mmol %), Et3N (2 mmol) at room temperature for 6 h.
Isolated yield.
Water as solvent (1 mL).
b
c
coupling reaction (Table 3, entry 9).
In order to disclose the effectiveness of this catalyst, we
conducted different reactions with iodobenzene and phenyl-
acetylene under solvent free conditions up to 10 mmol millimole

170
S. Thogiti et al. / Journal of Organometallic Chemistry 822 (2016) 165e172
Table 3
Sonogashira coupling of different aryl bromides catalyzed by complex 4.a
Entry
1
R
Ar
Product
Yield (%)b
82
5a
6a
2
3
4
5b
5c
5d
6a
6a
6a
80
72
81
5
6
7
5a
5b
5c
6b
6b
6b
78
70
66
8
9
5d
6b
6a
79
Nr
5ec
a
Reaction conditions: Aryl bromide (1 mmol), alkyne (1.2 mmol), catalyst 4 (0.01 mmol), TMG (2 mmol) at 70 ꢀC temperature for 15 h.
Isolated yield.
Chlorobenzene (1 mmol) used as substrate. Nr ¼ No reaction.
b
c
Table 4
Sonogashira coupling of iodobenzene with phenylacetylene catalyzed by complex 4.a
Entry
Reaction scale
Catalyst 4 (mmol)
Time
Yield (%)b
1
2
3
4
5
6
7
1 mmol
2 mmol
3 mmol
4 mmol
5 mmol
10 mmol
10 mmol
0.005
0.005
0.005
0.005
0.005
0.005
0.01
6
8
99
98
97
94
92
90
96
12
20
32
60
40
a
Iodobenzene (1.0 equiv), phenylacetylene (1.2 equiv), Et3N (2 eq) at room temperature.
Isolated yields.
b

S. Thogiti et al. / Journal of Organometallic Chemistry 822 (2016) 165e172
171
scale (Table 4). Iodobenzene (1e5 and 10 mmol scale) was trans-
heterogeneous conditions.
formed into product 7a with 0.005 mmol of catalyst 4 for all the
reactions and obtained excellent yields with time variation (Table 4,
entries 1e6). Herein we observed that the reaction time was
increased with increasing number of millimoles of substrates. Next
we increased catalyst loading from 0.005 mmol to 0.01 mmol for
10 mmol scale reaction resulted decreasing reaction time from 60 h
to 40 h (Table 4, entries 7).
4. Conclusions
In conclusion, we have reported efficient polymer supported 3-
benzoyl-1-(1-benzylpiperidin-4-yl)-2-thiopseudourea-Pd(II) com-
plex 4 as a heterogeneous catalyst for copper/solvent free Sono-
gashira reaction. The use of environmental benign catalyst 4 for
Sonogashira cross-coupling reaction may considerably decrease the
cost of energy in the chemical industry and serves as useful alter-
native to the reported procedures. In addition to this, it is even
applicable for gram scale reactions with excellent yields of coupling
products.
3.1. Recyclability of the catalyst
Economical and industrial point of view, recyclability of the
catalyst is a significant factor. The recycling efficiency of the catalyst
4 was investigated for the coupling of iodobenzene with phenyl-
acetylene under the above optimized reaction conditions. After
every experiment, this heterogeneous catalyst was easily separated
from the reaction mixture by simple filtration and washed with
acetonitrile. The air-dried catalyst can be reused directly without
any additional treatment. We observed stable catalytic activity of
catalyst 4 for the first five cycles and thereafter decreased gradually
(6e10 cycles) (Fig. 4). After consecutive runs, no perceptible loss of
the weight of the catalyst is observed.
Acknowledgements
KS thanks the University Grants Commission (UGC), New Delhi
for Dr. D.S. Kothari postdoctoral research fellowship. SP thanks the
CSIR, New Delhi for Senior Research Associateship. Special thanks
to professor Samudranil Pal for his support.
Appendix A. Supplementary data
Next we determined the leaching of Pd metal from the polymer
supported catalyst into the Sonogashira reaction mixture by using.
After ten consecutive experiments, we observed 8.7% amount of Pd
loss into the reaction mixture with excellent reusability of the
heterogeneous catalyst.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2016.08.029.
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