A simple and greener approach for the synthesis of PVC supported Pd (0): Application to Heck and Sonogashira reactions in water

Article abstract of DOI:10.1016/j.tetlet.2014.02.083

Preparation of PVC-supported Pd nanoparticles through the reduction of PdCl₂ by a non-toxic and eco-friendly route, employing sodium formate and NaOH in ethanol-water system has been described. The prepared PVC supported Pd nanoparticles were employed as catalyst in the cross coupling reactions, that is, Heck and Sonogashira reactions in water medium to afford the respective products in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2014.02.083

Tetrahedron Letters 55 (2014) 2256–2260
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Tetrahedron Letters
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A simple and greener approach for the synthesis of PVC supported
Pd (0): application to Heck and Sonogashira reactions in water
⇑
Basavaprabhu, M. Samarasimhareddy, Girish Prabhu, Vommina V. Sureshbabu
#109, Peptide Research Laboratory, Department of Studies in Chemistry, Central College Campus, Dr. B.R. Ambedkar Veedhi, Bangalore University, Bangalore 560 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Preparation of PVC-supported Pd nanoparticles through the reduction of PdCl2 by a non-toxic and
eco-friendly route, employing sodium formate and NaOH in ethanol–water system has been described.
The prepared PVC supported Pd nanoparticles were employed as catalyst in the cross coupling reactions,
that is, Heck and Sonogashira reactions in water medium to afford the respective products in good to
excellent yields.
Received 23 November 2013
Revised 18 February 2014
Accepted 23 February 2014
Available online 28 February 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
PVC-Pd
0
Heck reaction
Sonogashira reaction
Nanoparticles
The use of transition metal nanoparticles of Pd, Pt, Ru, Au, Cu,
and Ag as catalysts in CAC cross coupling reactions has placed
nano-metal catalysis as an important frontier of study in recent
stabilized Pd nanoparticles as efficient catalysts for the Suzuki
reaction in an aqueous medium (40% EtOH or CH CN/H O). Hyeon
3
2
et al. reported the use of triblock copolymer in the synthesis and
1
6
years. Palladium-catalyzed reactions, in particular, carbonAcar-
stabilization of Pd nanoparticles. Polymer, dendrimer, organic li-
bon bond forming reactions including Suzuki, Heck, Sonogashira,
and Tsuji–Trost reactions either through homogeneous or hetero-
geneous catalysis have gained a predominant place in the arsenal
gands, or surfactant stabilized Pd nanoparticles are being widely
used for various cross coupling reactions. Most of them use organ-
ic or ionic liquids as solvents and high temperature or microwave
irradiation conditions.
7
2
of organic chemists. The inherent problem associated with homo-
geneous catalysis is the separation of expensive catalyst from the
product for re-use. For instance, the homogeneous Pd catalysts
tend to lose their catalytic activity because of Pd metal aggregation
and precipitation. Thus, heterogenization of the homogeneous Pd
catalyst is an alternative feasibility to overcome such limitations
by employing Pd nanoparticles on solid support.
Several natural biopolymers such as chitosan, cellulose, and
wool, have been used as polymer supports in Pd catalyzed transfor-
mations. Pd nanoparticles supported over polyvinyl chloride
8
(PVC)-amino ethanol matrix and PVC-Schiff’s base have been pre-
pared and employed as efficient catalysts in Suzuki and Heck reac-
9
tions. In these studies PdCl
2
could be reduced to Pd (0) in situ,
For many technological applications the prevention of agglom-
eration of the nanoparticles during the use and storage is a major
objective in order to provide sufficient and reliable material perfor-
mance. Thus the use of polymeric carrier components could be an
interesting approach. Pd nanoparticles dispersed within a poly-
meric matrix are receiving intense attention due to the novel and
highly versatile material properties offered by these systems.
Therefore, the catalytic properties of transition metal–polymer
systems in general and Pd-polymer systems in particular are under
without the assistance of an external reducing agent. However,
the prior functionalization of the polymer requires an extra step.
The application of water as a solvent in organic syntheses involving
CAC cross coupling reactions and in transition-metal catalyzed
reactions has been gradually developing. Water is cheap, non-
toxic, and not flammable which could be considered as an alterna-
tive to expensive organic solvents and an attractive media for the
development of environmentally harmless chemical processes.
Also, separation of water-insoluble organic compounds from the
aqueous phase is easy.
1
0
3
systematic investigation by several groups. Palladium nanoparti-
cle catalyzed coupling was pioneered by Reetz using surfactants
Recently our group demonstrated the PVC supported Pd nano-
particles for the reduction of nitro compounds and alkenes.
as stabilizing agents. El-Sayed et al.5 reported the use of PVP
4
11a
Suzuki cross-coupling of boronic esters with aryl halides using
PVC-Pd0 was also documented by us.
11b
In continuation to these
⇑
Corresponding author. Tel.: +91 08022961339.
E-mail addresses: hariccb@hotmail.com, sureshbabuvommina@rediffmail.com,
studies, we wanted to explore the catalytic activity of PVC sup-
ported Pd nanoparticles to Heck and Sonogashira reactions.
hariccb@gmail.com (V.V. Sureshbabu).
http://dx.doi.org/10.1016/j.tetlet.2014.02.083
0
040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

Basavaprabhu et al. / Tetrahedron Letters 55 (2014) 2256–2260
2257
As a part of this study, we were interested in developing a
greener approach for the preparation of PVC-Pd0 nanoparticles.
Among the preparations of Pd nanoparticles, the chemical reduc-
tion of palladium salt in an aqueous or organic solution by suitable
reducing agents such as hydrazine, hydrogen, and sodium borohy-
dride has been widely adopted, even though these reducing agents
are hazardous in nature.12 However, an efficient and greener ap-
proach for the preparation of Pd nanoparticles is highly desirable
from environmental standpoint. Thus, in the present protocol, the
2
reduction of PdCl was carried out by employing sodium formate
13
and NaOH in the ethanol–water system at reflux condition. The
present method utilizing sodium formate and NaOH serves as a
better alternative for the reduction of PdCl
2
to Pd (0), as it avoids
the use of hazardous H and toxic hydride ion sources. Unlike the
2
reported methods, removal of unreacted residual reagents can be
achieved through simple water wash and thus handling of the
present reagent system is more convenient than the earlier re-
ported reagents.
Preparation of catalyst
2
A previously stirred suspension of PVC (100 mg) and PdCl (4:1
wt/wt) in ethanol (8 mL) was heated to reflux and then treated
with a solution of sodium formate (10%, 3 mL) and 0.1 N NaOH
Figure 2. SEM image of PVC-Pd0 nanoparticles.
(
2 mL) in distilled water for 4–5 min. The resulting homogeneous
solution was maintained at 50–55 °C for 1.5 h. Initial brownish sus-
pension turned colorless upon addition of sodium formate and
NaOH, indicating the conversion of Pd (II) to Pd (0). Overall, the
preparation of catalyst was complete in about two hours.
Thus obtained palladium nanoparticles were filtered, washed
with water, and dried in vacuo. The dried sample was character-
ized by X-ray powder diffraction (XRD), where X-ray diffractogram
confirmed Pd (0) diffraction lines at 2h = 40.039°, 46.581°, and
6
8.047° corresponding to (111), (200), and (220) lattice planes
of Pd (Fig. 1). The Pd nanoparticles were further characterized by
scanning electron microscopy (SEM, Fig. 2) and transmission elec-
tron microscopy (TEM, Fig. 3).
TEM analysis revealed a narrow size distribution of the
palladium nanoparticles. Particle size distribution diagram, that
is, histogram showed Pd nanoparticles with an average size range
of 3–4 nm (Fig. 4) and ICP-AES analysis indicated 13.25% of Pd
loading in the PVC matrix.
Figure 3. TEM image of PVC-Pd0 nanoparticles (bar length 20 nm).
Heck reaction
Heck reaction is widely used in the CAC cross coupling reaction
of alkenes in organic synthesis. It is often used to functionalize
Figure 4. Particle size distribution histogram of PVC supported Pd nanoparticles
particle size 3.2 ± 0.4 nm).
(
aromatic rings and as an alternate to the Friedel–Crafts reaction.14
Heck reaction usually involves the interaction of an aromatic
halide or anhydride with an alkene, in the presence of palladium
Figure 1. XRD pattern of PVC-Pd0 with diffraction lines at 2h = 40.039°, 46.581°,
and 68.047°.

2
258
Basavaprabhu et al. / Tetrahedron Letters 55 (2014) 2256–2260
catalyst. Generally, the Heck reaction entails the use of elevated
temperatures, hazardous organic solvents, and expensive phos-
Table 1
List of cross-coupled products of Heck reaction
1
5
phine ligands. Nowadays, considerable efforts have been devoted
to the improvement of the efficiency and selectivity of the Pd-cat-
alyzed Heck reactions. Key aspects for improving the Heck reaction
include shortening of the reaction duration and employing recycla-
ble catalysts that can be isolated easily from the reaction mixture.
In the first part of the study, the catalytic reactivity of as-
prepared Pd nanoparticles to Heck reaction involving the
cross-coupling of aryl halides with methyl acrylate was studied.
Palladium catalyzed Heck reactions are strongly dependent on a
number of factors such as base, solvent, stabilizing ligand, temper-
ature, and the combined effect of these. Hence, our initial interest
was to optimize the reaction conditions for this protocol. For this
purpose, we systematically evaluated the role of solvent, base,
and catalyst loading for this synthetic protocol by subjecting iodo-
benzene and methyl acrylate to the cross coupling reaction
Entry
1
Reactant (1)
Product (3)
Yield (%)
89
I
O
OMe
O
3
a
I
OMe
2
87
3
b
I
O
OMe
3
4
5
n-Bu
86
92
90
n-Bu
3
O
c
I
(Scheme 1).
OEt
During a sequence of reactions carried out using different sol-
3
d
vents such as toluene, water, acetonitrile, and ethanol, water
proved to be efficient in affording good yields of the product. Con-
sequently, water was chosen as solvent system for the present
I
O
OEt
MeO
study. Among the bases screened, in the presence of K
reaction proceeded well within 2 h at 70 °C compared to other
bases like Cs CO Na CO and triethylamine. In addition,
.69 mol % (20 mg) of the catalyst (PVC-Pd ) loading was found
2 3
CO the
MeO
3e
O
2
3
,
2
3
,
Br
0
1
OMe
6
7
86
85
to be suitable for the reaction to go for completion. Significantly
lower yields were observed when 5 and 10 mg of the catalyst were
used.
To demonstrate the scope of the catalyst system, aryl halides
including iodides and bromides with various substitutions were
cross coupled with ethyl and methyl acrylates to give good yields
of the products irrespective of the nature of substituents like nitro,
methoxy, and n-butyl groups (Table 1). A higher temperature range
of 80–85 °C was essential for the successful cross coupling of the
aryl halides with electron withdrawing groups.
3f
Br
O
OMe
3
g
Br
O
OMe
8
O N
78
2
2
O N
3
h
Sonogashira reaction
drawbacks and consequently to provide access to alkynylation
reactions under aerobic conditions, copper- and phosphine-free
procedure with the use of a polymer supported Pd catalyst has
been developed.
The palladium catalyzed coupling reaction between aryl or
alkenyl halides or triflates and terminal alkynes has become the
most important method to prepare arylalkynes and conjugated
enynes, which are precursors for the assembly of natural products,
pharmaceuticals, and molecular organic materials.16 Development
of a copper-free procedure for the Sonogashira reaction is a partic-
Thus, PVC supported Pd nanoparticles were explored for the
Sonogashira cross coupling reaction employing iodobenzene and
phenyl acetylene as model substrates. In the quest to carry out
the Sonogashira reaction in water, we screened various amine
bases such as triethylamine, N-methylpyrrolidine (NMP), N-meth-
ylmorpholine (NMM), and piperidine. Among these bases, N-meth-
ylpyrrolidine (6.0 equiv) in the presence of nano Pd (20 mg,
6.2 mol %) afforded good yields of the product within 3 h at room
temperature (Scheme 2). The protocol was further extended for
the coupling of iodobenzene substituted with electron withdraw-
ing and donating groups with phenylacetylene (Table 2). The cor-
ular challenge since it is traditionally a dimetallic-mediated pro-
cess (palladium and copper required).1
7a–k
Nevertheless since its
discovery, a great deal of work has been done to improve the ori-
ginal protocol so as to induce an even wider range of reactants as
1
8
well as to circumvent some of major drawbacks of the protocol.
Copper salts can induce Glasser-type homocoupling of terminal al-
kynes when copper–acetylide intermediates are exposed to oxida-
1
9
tive reagents or air. In addition, use of two metals hinders the
recovery and reutilization of the expensive Pd catalysts. Phos-
phines, which are frequently used in this reaction, are often
air-sensitive and are not commercially available. To avoid these
2
0
responding products were isolated in good yields
and the
2
1
compounds were characterized through NMR analyses.
The reusability of PVC-Pd0 was explored in Heck as well as
0
Sonogashira reactions. The PVC-Pd can be used in 5–6 successive
O
PVC-Pd(0)
CO
X
OR
K
2
3
OR
PVC-Pd(0)
6.0 eq. NMP
H O, 60-70°C
I
2
R'
O
R'
1
.5 - 2 h
R
R''
1
R
R''
H O, r.t.
2
2
3
3
-4h
X = I or Br.
1
4
5
Scheme 1. PVC-Pd0 catalyzed Heck cross coupling reaction.
Scheme 2. PVC-Pd0 catalyzed Sonogashira cross-coupling reaction.

Basavaprabhu et al. / Tetrahedron Letters 55 (2014) 2256–2260
2259
Table 2
nanoparticles within the PVC matrix during the course of the reac-
tion or accumulation of some organic species on the composite that
hinders the interaction between catalyst and reactant molecules.
Herein we describe the preparation of PVC-supported Pd nano-
List of cross coupling products of Sonogashira reaction
Entry
Reactant (1)
Product (5)
Yield (%)
93
I
2
particles through the reduction of PdCl by a non-toxic and
eco-friendly route, employing sodium formate and NaOH in the
ethanol–water system. The prepared PVC supported Pd nanoparti-
cles were employed as catalyst in the cross coupling reactions, that
is, Heck and Sonogashira reactions to afford the respective prod-
ucts in good to excellent yields. Under the present conditions,
Sonogashira reaction can be carried out at room temperature with-
out the use of CuI or phosphine ligands. The generation and stabil-
ization of catalytically active nanoparticles in water have several
important advantages over both traditional homogeneous and sup-
ported transition-metal catalysts in terms of lower cost, and avoid-
ing the usage of organic solvents. This approach also provides two
additional advantages: it could facilitate the recovery and reuse of
palladium and could also reduce the palladium contamination of
the isolated product, a significant problem for the pharmaceutical
industry.
1
5
a
I
MeO
2
3
90
89
MeO
5
b
I
5
c
I
O2N
4
5
6
78
81
69
O2N
Cl
Acknowledgements
5
d
Cl
I
We thank the DST-Nanomission (Grant No. SR/NM/NS-13/2007)
New Delhi, India for financial support and B.P. thanks CSIR for SRF.
5
e
References and notes
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I
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1
cycle
st
2nd
cycle
3rd
cycle
4th
cycle
5th
cycle
Heck (yield): reaction
time—1.5 h
Sonogashira (yield):
reaction time—3.0 h
89
89
92
87
92
86
90
84
88
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93
5
6
.
.
runs with no loss in its activity in both Heck and Sonogashira reac-
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acetylene–iodobenzene and methylacrylate–iodobenzene as
model substrates for Sonogashira and Heck reactions, respectively.
The yields obtained in each recycling process are listed below
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half conversion of the reaction and the reaction was continued.
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the catalyst (as monitored through TLC). This experiment revealed
that there was no significant leaching of the Pd nanoparticles in
solution under the present experimental conditions. In addition,
the recycling experimental results confirm the stability of Pd-PVC
composite. The slight decrease observed in the yield during the
recycling experiment can be accounted to agglomeration of metal
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0
typical procedure, iodobenzene (0.30 g, 1.47 mmol), methyl acrylate (0.14 g,
1.67 mmol), and PVC-Pd (20 mg, 1.69 mol %) were taken in a round-bottomed
flask containing 10 ml of water. To this solution was added 2.0 mmol of K CO .
2 3
The reaction mixture was stirred for 1.5–2 h at 60–70 °C. The progress of the
reaction was monitored by TLC. After completion of the reaction, the reaction
mixture was filtered and the filtrate was extracted with EtOAc (10 ml  2). The
1
1
3. Lyubimov, S. E.; Vasilev, A. A.; Korlyukov, A. A.; Ilyin, M. M.; Pisarev, S. A.;
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2 4
combined organic layer was dried over anhydrous Na SO and evaporated
2
009, 69, 755.
under reduced pressure. The crude material was purified by column
chromatography (hexane–EtOAc, 100:1) to afford the corresponding product
in high purity.
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Synthesis; Wiley-VCH: Weinheim, Germany, 1998.
0
(b) PVC-Pd catalyzed Sonogashira reaction of iodobenzene with phenylacetylene:
Typical procedure: To a mixture of the PVC-Pd catalyst (20 mg, 6.2 mol %) and
iodobenzene (81.6 mg, 0.40 mmol) in
H
2
O
(3.0 mL) were added
phenylacetylene (81.7 mg, 0.80 mmol) and NMP (6.0 mmol). The reaction
mixture was stirred at room temperature for 3–4 h and filtered. The filtrate
was extracted twice with EtOAc (6 mL). The combined EtOAc extracts were
1
1
5. (a) Reetz, M. T.; de Vries, J. G. Chem. Commun. 2004, 1559; (b) Stephan, M. S.;
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2 4
washed with water (4 mL) and brine (2 mL), dried over anhydrous Na SO , and
37, 662; (c) Gooben, L. J.; Paetzold, J. Angew. Chem., Int. Ed. 2002, 41, 1237.
concentrated in vacuo. The resulting residue was chromatographed on silica
gel (hexane–EtOAc, 100:1) to give quantitative yield of diphenylacetylene.
21. Characterization data for selected compounds:
6. (a)Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J.,
Sonogashira, K., Eds.; Wiley-VCH: Weinheim, 1998; p 203; (b) Negishi, E.-i.;
Anastasia, L. Chem. Rev. 2003, 103, 1979; (c) Chinchilla, R.; Najera, C. Chem. Rev.
Methyl 3-phenyl acrylate (3a): Colorless liquid; 1H NMR (400 MHz, CDCl
) d
3
2
007, 107, 874; (d) Plenio, H. Angew. Chem., Int. Ed. 2008, 47, 6954; (e)
3.80 (s, 3H), 6.42 (d, J = 16.2 Hz, 1H), 7.36–7.39 (m, 3H), 7.50–7.54 (m, 2H), 7.70
Nicolaou, K. C.; Dai, W.-M. Angew. Chem., Int. Ed. Engl. 1991, 30, 1387; (f)
Grissom, J. W.; Gunawardena, G. U.; Klingberg, D.; Huang, D. Tetrahedron 1996,
(d, J = 16.2 Hz, 1H) ppm; 13C NMR (75 MHz, CDCl
3
) d 51.6, 117.8, 128.0, 128.6,
Ethyl 3-p-tolyl acrylate (3b): White solid; 1H NMR (400 MHz, CDCl
) d 1.35 (t,
3
130.2, 134.4, 144.8, 167.5 ppm.
5
2, 6453.
7. (a) Lu, N.; Chen, Y.-C.; Chen, T.-L.; Wu, S.-J. J. Organomet. Chem. 2009, 694, 278;
b) Komaroni, A.; Nov a´ , K. Z. Chem. Commun. 2008, 4968; (c) Komura, K.;
1
J = 7.2 Hz, 3H), 2.38 (s, 3H), 4.27 (q, 7.1 Hz, 2H), 6.38 (d, J = 16.2 Hz, 1H), 7.19 (d,
(
J = 18.2 Hz, 2H), 7.40 (d, J = 8.2 Hz) ppm; 13C NMR (100 MHz, 75 CDCl
3
) d 14.6,
Nakamura, H.; Sugi, Y. J. Mol. Catal. A 2008, 293, 72; (d) De Lima, P. G.; Antunes,
O. A. C. Tetrahedron Lett. 2008, 49, 2506; (e) Kim, J.-H.; Lee, D.-H.; Jun, B.-H.;
Lee, Y.-S. Tetrahedron Lett. 2007, 48, 7079; (f) Cwik, A.; Hell, Z.; Figueras, F.
Tetrahedron Lett. 2006, 47, 3023; (g) Tyrrell, E.; Al-Saardi, A.; Millet, J. Synlett
21.8, 60.8, 116.8, 128.2, 129.6, 131.8, 140.7, 144.6, 167.8 ppm.
1
Methyl 3(4-methoxyphenyl)acrylate (3e): White solid;
CDCl
2H), 7.46 (d, J = 8.7 Hz, 2H), 7.66 (d, J = 15.8 Hz, 1H) ppm; C NMR (75 MHz,
CDCl ) d 51.5, 55.3, 114.4, 115.2, 127.1, 129.8, 144.5, 161.2, 167.6 ppm.
H NMR (400 MHz,
3
) d 3.78 (s, 3H), 3.84 (s, 3H), 6.30 (d, J = 15.8 Hz, 1H), 6.90 (d, J = 8.7 Hz,
13
2
005, 487; (h) Djakovitch, L.; Rollet, P. Adv. Synth. Catal. 2004, 346, 1782; (i)
3
1
Djakovitch, L.; Rollet, P. Tetrahedron Lett. 2004, 45, 1367; (j) Fukuyama, T.;
Shinmen, M.; Nishitani, S.; Sato, M.; Ryu, I. Org. Lett. 2002, 4, 1691; (k)
Bakherad, M.; Keivanloo, A.; Bahramian, B.; Mihanparast, S. Tetrahedron Lett.
1,2-Diphenylethyne (5a): White solid; H NMR (400 MHz, CDCl
3
) d 7.31–7.38
(m, 6H), 7.51–7.56 (m, 4H) ppm; 13C NMR (75 MHz, CDCl
128.3, 131.8 ppm.
) d 89.4, 123.3, 128.2,
3
2
009, 50, 6418.
8. (a) Bakherad, M.; Keivanloo, A.; Bahramian, B.; Hashemi, M. Tetrahedron Lett.
009, 50, 1557; (b) Gu, S.; Chen, W. Organometallics 2009, 28, 909; (c) Chandra,
3-Methoxy-4-(2-phenylethynyl)benzene (5b): White solid; 1H NMR (400 MHz,
1
CDCl
7.56 (m, 4H) ppm; C NMR (75 MHz, CDCl
123.5, 127.8, 128.3, 131.4, 131.7, 140.2 ppm.
1-(2-(4-Nitrophenyl)ethynyl)benzene (5d): Yellow solid; 1H NMR (400 MHz,
CDCl ) d 7.34–7.38 (m, 3H), 7.52–7.56 (m, 2H), 7.63 (d, J = 8.6 Hz, 2H), 8.18 (d, J
3
) d 3.82 (s, 3H), 6.86 (d, J = 6.8 Hz, 2.1 Hz, 2H), 7.28–7.39 (m, 3H), 7.41–
13
2
3
) d 55.3, 88.2, 89.1, 114.2, 115.4,
A.; Singh, B.; Upadhyay, S.; Singh, R. M. Tetrahedron 2008, 64, 11680; (d)
Komaromi, A.; Tolnai, G. L.; Nov a´ , K. Z. Tetrahedron 2008, 49, 7294; (e) Ines, B.;
SanMartin, R.; Churruca, F.; Dominguez, E.; Urtiaga, M. K.; Miren, K.; Arriortua,
M. I. Organometallics 2008, 27, 2833; (f) Shi, S.; Zhang, Y. Synlett 2007, 1843; (g)
Kawanami, H.; Matsushima, K.; Sato, M.; Ikushima, Y. Angew. Chem., Int. Ed.
3
13
= 8.6 Hz, 2H) ppm; C NMR (75 MHz, CDCl
3
) d 87.8, 94.6, 122.2, 123.6, 128.4,
129.1, 130.2, 131.8, 132.3, 146.8 ppm.