Macroporous resin impregnated palladium nanoparticles: Catalyst for a microwave-assisted green Hiyama reaction

Article abstract of DOI:10.1016/j.molcata.2012.03.022

A non-functional macroporous commercial resin, Amberlite XAD-4, was impregnated with palladium nanoparticles of size 5-10 nm. The supported PdNPs, thus prepared, were used to catalyze the sodium hydroxide activated Hiyama cross-coupling reaction of phenyltrimethoxysilane with a variety of bromo and chloro arenes under microwave heating. They were found to have very high efficiency (TOF ≈ 3 × 10⁴) and excellent recyclability. The procedure, which was carried out in the absence of any additional ligands, surfactants or toxic organic solvents, can lead to the development of a sustainable and green protocol for the production of biaryls.

Full text of DOI:10.1016/j.molcata.2012.03.022

Journal of Molecular Catalysis A: Chemical 359 (2012) 69–73
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Macroporous resin impregnated palladium nanoparticles:
Catalyst for a microwave-assisted green Hiyama reaction
Dipen Shah, Harjinder Kaur∗
Chemistry Department, School of Science, Gujarat University, Ahmedabad, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A non-functional macroporous commercial resin, Amberlite XAD-4, was impregnated with palladium
nanoparticles of size 5–10 nm. The supported PdNPs, thus prepared, were used to catalyze the sodium
hydroxide activated Hiyama cross-coupling reaction of phenyltrimethoxysilane with a variety of bromo
and chloro arenes under microwave heating. They were found to have very high efficiency (TOF ≈ 3 × 104)
and excellent recyclability. The procedure, which was carried out in the absence of any additional ligands,
surfactants or toxic organic solvents, can lead to the development of a sustainable and green protocol for
the production of biaryls.
Received 19 November 2011
Received in revised form 6 March 2012
Accepted 23 March 2012
Available online 31 March 2012
Keywords:
Palladium
Nanoparticle catalysis
Amberlite XAD-4
Hiyama cross-coupling
Microwave heating
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
ligands or expensive palladocycles were still required. Of late,
nanoparticles have emerged as efficient catalysts for coupling reac-
Palladium catalyzed cross-coupling reactions have emerged as
the most efficient methodologies for the production of unsym-
metrical biaryls and are extensively utilized in the synthesis of
polymers, agrochemicals, pharmaceutical intermediates, etc. [1].
Among the coupling reactions, three most frequently employed
are Stille [2], Suzuki-Miyaura [3], and Hiyama [4,5] reactions. In
spite of the excellent yields, high stereoselectivity and superior
functional group tolerance, the use of toxic tin reagents in Stille
couplings, and the difficulties in the preparation and purification
of boron reagents required for the Suzuki reactions, are major
disadvantages. Therefore, the Hiyama coupling reaction that uses
environmentally benign, cheaper and easy to prepare organosili-
con reagents has many advantages in comparison to others. Hiyama
et al. [6–8] have developed several methods for this transformation,
using various Pd catalysts in the presence of phosphine ligands. Ear-
lier, corrosive fluoride anions were used for the activation of C Si
bond of alkylsilanes but later on, the activity of C Si bond was
enhanced by using alkenylfluorosilanes [9], alkenylalkoxysilanes
[10–12], and organosilanols [13–15]. Subsequently, with the devel-
opment of sodium hydroxide as an effective promoter [16,17] for
alkoxysilanes in aqueous media, the potential of Hiyama reaction as
a viable synthetic process was demonstrated. Though the aqueous
Hiyama reaction made significant progress, either toxic phosphine
tions in the absence of ligands. Rothenberg et al. [18], first reported
the usage of bimetallic core–shell Ni–Pd nanoclusters to catalyze
Hiyama cross-coupling reaction between phenyltrimethoxysilane
and various halo arenes. High product yields were obtained with
a variety of iodo- and bromo-aryls in tetrahydrofuran using tetra-
butylammonium fluoride (TBAF) as the activator. Sarkar et al. [19]
reported efficient catalysis of NaOH activated ligandless Hiyama
cross-coupling reaction using polyethylene glycol (PEG) capped
palladium nanoparticles, in aqueous media. Ranu et al. [20] demon-
strated that in situ generated Pd (0) nanoparticles can catalyze
Hiyama cross-coupling reaction in a very short time. Sodium dode-
cyl sulphate (SDS) was used as surfactant in this reaction. These
methods, which used colloidal nanoparticles, lacked recyclability.
Hence, there is a growing need to develop stable and recyclable
catalyst systems.
Conventional methods of heating reactions need more time,
elaborate and tedious apparatus. This results in higher costs and
environmental pollution too. In today’s world, microwave induced
heating is emerging as a simple, clean, fast, efficient and econom-
ical method for the synthesis of organic molecules. This technique
can reduce the duration of chemical reaction from hours to min-
utes. Also, the ability to heat solvents far above their boiling
points in closed vessels has reduced the usage of high boil-
ing organic solvents. In other words, microwave heating offers
energy efficient and environment-friendly processes [21]. How-
ever, there is need to synthesize catalytic systems which have
greater stability under microwave heating. Polymer ligands such
∗
Corresponding author. Tel.: +91 79 26300969; fax: +91 79 26308545.
E-mail address: hk ss in@yahoo.com (H. Kaur).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.03.022

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D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 359 (2012) 69–73
as PVP and PEG [19,22] that can efficiently stabilize nanoparti-
cles under conventional heating, may not be suitable for under
microwave heating as nanoparticles tend to coagulate. Support-
ing nanoparticles on a neutral polymer resin is a viable alternative
for stabilizing nanoparticles. Microporous resins are reported to
disperse palladium nanoparticles with excellent size control as
they have a narrow pore size distribution [23]. However, such
resins need to be swollen in appropriate solvents (non polar sol-
vents in case of crosslinked polystyrene) to promote transport
of reactants. Polar solvents on the other hand tend to close up
the pores and decrease accessibility to the embedded catalyst.
In comparison to microporous supports, catalysts supported on
macroporous resins remain accessible in a variety of solvents
including water and ethanol [24]. In this study, we investigated the
behavior of palladium nanoparticles, supported on an inert non-
functional macroporous polymer resin for the microwave induced
Hiyama reaction.
added drop wise. Then aryl halide (1 mmol) was added and the
vial heated in CEM microwave (50 W, 110 ◦C) for different intervals
of time. The reaction was quenched by filtering the hot solution
in 10 mL cold water. The resulting solution was extracted with
ET2O/DCM (2× 5 mL). The combined extract was dried over anhy-
drous MgSO4 and the solvent was removed using a rotaevaporator.
The crude product, thus isolated, was recrystallised from an appro-
priate solvent.
2.4. Effect of various parameters on catalyst activity
Effect of various parameters such as time, catalyst concentra-
tion, base, etc. was studied for the coupling reaction between 4-
bromoacetophenone and phenyltrimethoxysilane. Into three 10 mL
vials, 4-bromoacetophenone (2.0 mmol), 4-bromoacetophenone
(1 mmol), sodium hydroxide (1 mL, 3 M), ethanol (1.5 mL) and cata-
lyst (150 mg wet resin) were taken and heated in a CEM microwave
reactor for a hold time of 2, 4, and 6 min, respectively. The reac-
tions were quenched by filtering the hot solution in 10 mL cold
water and the aqueous solution for each run was extracted with
ET2O (2× 5 mL). The combined extracts were dried over anhydrous
MgSO4, and products analyzed by GC–MS.
2. Experimental
2.1. Materials and instruments
All chemicals used were of analytical grade or of the highest
purity available. All glassware was thoroughly cleaned with freshly
prepared 3:1 HCl/HNO3 (aqua regia) and rinsed with Millipore-Q
water. Phenyltrimethoxysilane was purchased from Aldrich. The
Aryl bromides were obtained from BDH and Merck. All aryl bro-
mides and chlorides were of 98–99% purity. Dichloromethane
(DCM), diethyl ether (ET2O), and NaOH were purchased from Finar
Chemicals. Water used in all experiments was purified by the Mil-
lipore system.
GC–MS measurements were carried on Perkin Elmer USA Auto
system XL. High resolution transmission electron microscopy (HR-
TEM) pictures were taken using a Hitachi (H-7500) Instrument.
The swollen resin beads were milled and a drop of alcoholic sus-
pension was placed on a 200 mesh carbon coated copper grid. It
was then dried to evaporate the solvent and used for microscopy.
1H NMR spectra were recorded on a Bruker Advance II 400 NMR
spectrometer. Inductively coupled plasma atomic emission spec-
troscopy (ICP-AES) measurements were carried out on a HJY
Ultima-2 instrument: power 1000 W, nebulizer flow 1.29, nebulizer
pressure 2.96 min−1, wave length 340.458 nm. CEM benchmate
microwave reactor was used for microwave heating. The gen-
eral parameters used were: closed vessel synthesis, stirring on,
power 50 W, temperature 110 ◦C, hold time 6–12 min, external
cooling on.
The effect of catalyst concentration was also studied by taking
different amounts of catalyst while keeping the other parameters
constant.
3. Results and discussion
We have synthesized palladium nanoparticles supported on
Amberlite XAD-4, a neutral, non-functional, hydrophobic, macrop-
orous, commercial and cross-linked polystyrene resin. The resin is
chemically and mechanically stable under microwave conditions
and due to its inert nature, there is no interference with reac-
tion conditions. While the presence of ligands/functional groups
on a resin can control the size and stability of nanoparticles, they
have a negative influence from the catalytic activity point of view,
reducing the interaction of catalytic sites with the substrates [26].
This also prompted us to choose a non-functional resin in which
nanoparticles are stabilized solely by the steric factor or electro-
static interaction of benzene rings. This probably increases the
accessibility of nanoparticles which is reflected by their high turn
over number (TON). The hydophobic nature of the resin can also
create a favorable mass transfer for the less polar alkylhalides
reactants through the resin and increases interaction with the
embedded catalyst. Swelling of such resin in organic solvent can
be a problem during the reaction. However, the impregnated resin
shows little swelling in ethanol water mixture.
The method used for nanoparticles impregnation was very
simple. It involved sorption of palladium acetate in the resin fol-
lowed by reduction with sodium borohydride. TEM images showed
embedded palladium nanoparticles with size ranging from 5 to
10 nm (Fig. 1). The method gave a metal loading of 0.235 mg of palla-
dium per g of resin which was determined spectrophotometrically
[25] after incinerating the resin. This is a comparatively low value
which makes it desirable for microwave heating. This is because
high content of metal may lead to generation of hot spots and
degradation of polymer support [21].
2.2. Preparation of resin supported palladium nanoparticles
(resin-PdNPs)
The resin supported palladium nanoparticles were synthesized
by a method developed in our lab [25]. Amberlite XAD-4 beads
(5.0 g) were washed repeatedly with hot water to remove salts,
swollen in ethanol and then equilibrated with 25 mL of 1.0 mmol
solution of palladium acetate in ethanol at 10 ◦C. After 1 h, excess
solution was drained and the metal was reduced by passing cold
aqueous NaBH4 (0.1 mol dm−3) solution. The resin particles were
further washed with water to remove excess reagent and stored in
ethanol.
3.1. Catalytic activity of resin supported PdNPs for Hiyama
reaction
2.3. Typical protocol for the Hiyama reaction under microwave
heating
Our initial investigation on the evaluation of the catalytic activ-
ity started with the cross-coupling of phenyltrimethoxysilane and
4-bromoacetophenone under microwave heating (Scheme 1) using
NaOH as the activator. The reaction was carried out in a closed
Into a 10 mL vial phenyltrimethoxysilane (1.5 mmol), resin-
PdNPs (150 mg), and ethanol (1.5 mL) were taken. The reaction
mixture was stirred vigorously and 3 M NaOH (1 mL, 3 mmol) was

D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 359 (2012) 69–73
71
Fig. 3. Time course of the Hiyama reaction between phenyltrimethoxysilane and
4-bromoacetophenone (standard run) and after hot filtration of the resin.
Table 1
Conditions for Hiyama reaction.
Fig. 1. HRTEM image of the resin encapsulated palladium nanoparticles.
No
Conditions
Yielda (%)
TOF (h−1
)
1
2
3
4
5
6
150 mg resin, 3 M NaOH, EtOH, 6 min.
0
58
80
99
90
35
–
17,060
23,530
29,120
26,500
10,290
150 mg resin-PdNPs, 1 M NaOH, EtOH, 6 min.
150 mg resin-PdNPs, 2 M NaOH, EtOH, 6 min.
150 mg resin-PdNPs, 3 M NaOH, EtOH, 6 min
150 mg resin-PdNPs, 3 M NaOH, MeOH, 6 min.
150 mg resin-PdNPs, 3 M Na2CO3, EtOH, 6 min.
Aryl halide (1.0 mmol), phenyltrimethoxysilane (1.5 mmol), resin-PdNPs (150 mg)
Scheme 1. Hiyama reaction between phenyltrimethoxysilane and 4-
bromoacetophenone.
closed vessel, P = 50 W, 110 ◦C.
a
GC yields.
vial. Hiyama reaction has been reported in a variety of organic sol-
vents and water. Water alone cannot solubalize hydrophobic alkyl
halides. Surfactants or phase transfer agents such as TBAB are gen-
erally used to increase solubility. Ethanol/water mixture [27] in
which the reactants dissolve on heating was chosen as the reac-
tion medium. The heating in ethanol/water mixture is also very
efficient under microwave heating and the required temperature
can be achieved in seconds (Fig. 2).
The time course of the reaction (Fig. 3) was studied by quench-
ing the reaction at different time intervals and the reaction mixture
was analyzed by the GC–MS (Fig. S1). It was observed that after
6 min, the peak for 4-bromoacetophenone disappeared completely
and 4-biphenyl-4-yl-ethanone was identified as the sole coupling
product by its mass spectrum. No homocoupling product was
observed. Three moles of NaOH were required for the completion
of the reaction and reaction was found to be incomplete when
Na2CO3 was used as base. Optimization of conditions is sum-
marized in Table 1. It was also observed that 150 mg of resin
(≈0.00035 mmol of Pd, 0.035 mol% with respect to aryl halide) was
sufficient to complete the reaction and no reaction was observed
in the absence of the catalyst. The effect of catalyst concentration
is shown in Fig. 4. We were able to obtain high TON (2912) and
TOF (29,120/h) using microwave heating.
3.2. Recycling of the catalyst
Easy catalyst separation and recycling in successive batch oper-
ations can greatly increase the efficiency of the reaction. Resin
Fig. 2. Temperature and potency profile of the microwave-assisted Hiyama reaction.
Fig. 4. Effect of catalyst concentration on Hiyama coupling.

72
D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 359 (2012) 69–73
Table 2
Hiyama cross-coupling of aryl halides with phenyltrimethoxysilane.
Entry
1
Aryl halide
Product
T min
Yielda (%)
94
6
2
3
6
6
95
96
4
6
6
96
5
6
7
76
89
92
12
12
8
9
12
6
85
92
10
11
6
8
90
90
Aryl halide (1.0 mmol), phenyltrimethoxysilane (1.5 mmol), EtOH–H2O resin-PdNPs (150 mg) closed vessel, P = 50 W, 110 ◦C.
a
Isolated yield.
supported nanoparticles were separated from the hot reaction mix-
ture by simple filtration and were recycled many times. The beads,
after separation, were extracted with hot water, ethanol or DCM
(5 mL) to remove any sorbed products and were reused. Recyclabil-
ity of the resin is shown in Fig. 5. No change in activity was observed
up to five cycles.
be 35 10 ppb. The negative results of the hot filteration test indi-
cate that the leaching of palladium is negligible and most probably,
the reaction is heterogeneous in nature.
Leaching can cause depletion of the metal from the support and
limit the service life and reusability of the catalyst. It can also lead
to metal contamination of the product. Leaching depends on the
solvent used, interaction with reactants, products and the reac-
tion temperature. To probe leaching of the catalyst, hot filteration
test was performed. The reaction between 4-bromoacetophenone
and phenyltrimethoxysilane was stopped after 2 min and the resin
filtered off while the solution was still hot. The filterate was fur-
ther heated and the reaction mixture analyzed by GC–MS after 6,
8 and 10 min. The results shown in Fig. 3 indicate that reaction
stops after the resin is filtered off. Additionally, the reaction mix-
ture was extracted with ether and aqueous layer was analyzed for
palladium by ICP-AES. Total palladium concentration was found to
Fig. 5. Recycling of the catalyst.

D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 359 (2012) 69–73
73
3.3. Hiyama cross-coupling reaction
Acknowledgements
The excellent results obtained in the cross-coupling of 4-
bromoacetophenone with phenyltrimethoxysilane prompted us to
carry out the reaction with other aryl bromides. A series of aryl
bromides with both electron withdrawing and donating groups
were used and the results are summarized in Table 2. The reaction
was uniform irrespective of the nature of the substituents (elec-
tron withdrawing or electron donating) on the aromatic ring. A
wide range of substituents, which included CH3, CHO, OCH3,
NO2, and COCH3, were compatible with this procedure. In case of
aldehyde, chemo selectivity was totally in favor of Hiyama cross-
coupled product. We also carried out the reaction with 2-bromo
thiophene (entry 11) and isolated coupled product in good yield.
In case of NHCOCH3 group (entry 5), 4-aminobiphenyl was also
isolated. CO2H and CN groups were found to be incompatible.
No reaction occurred with 4-bromobenzoic acid and in case of 4-
bromobenzonitrile, only hydrolysis product was isolated. We also
tried reaction with arylchlorides (entry 6–8) and found that under
the present set of conditions reaction with chlorobenzene (entry
6), did not proceed to completion. However, on increasing the reac-
tion time from 6 to 12 min, we could isolate the products in high
yields. All the products were of high purity and were characterized
by matching their 1H NMR spectra with the reported data and the
results are summarized in Supporting Information.
Authors are grateful to the UGC, New Delhi for the financial
support to carry out this work and to SAIF, Punjab University,
Chandigarh for the NMR spectra and HRTEM pictures. Sicart, Val-
labh Vidhya Nagar, Gujarat for GC–MS.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2012.03.022.
References
[1] E. Negishi, Handbook of Organopalladium Chemistry for Organic Synthesis,
Wiley, New York, 2002.
[2] A.F. Littke, G.C. Fu, Angew. Chem. Int. Ed. 41 (2002) 4176–4211.
[3] A. Zapt, M. Beller, Top. Catal. 19 (2002) 101–109.
[4] A. Suzuki, J. Organomet. Chem. 576 (1999) 147–168.
[5] K.J. Stille, Angew. Chem. Int. Ed. Engl. 25 (1986) 508–524.
[6] Y. Hatanaka, T. Hiyama, J. Org. Chem. 53 (1988) 918–920.
[7] Y. Hatanaka, K. Goda, T. Hiyama, J. Organomet. Chem. 465 (1994) 97–100.
[8] E. Hagiwara, K. Gouda, Y. Hatanaka, T. Hiyama, Tetrahedron Lett. 38 (1997)
439–442.
[9] Y. Hatanaka, T. Hiyama, J. Org. Chem. 54 (1989) 268–270.
[10] K. Tamao, K. Kobayashi, Y. Ito, Tetrahedron Lett. 30 (1989) 6051–6054.
[11] M.E. Mowery, P. DeShong, J. Org. Chem. 40 (1999) 1673–1676.
[12] T. Jeffery, Tetrahedron Lett. 40 (1999) 1673–1676.
[13] S.E. Denmark, J.Y. Choi, J. Am. Chem. Soc. 121 (1999) 5821–5822.
[14] S.E. Denmark, D. Wehrli, Org. Lett. 2 (2000) 565–568.
[15] S.E. Denmark, D. Wehrli, J.Y. Choi, Org. Lett. 2 (2000) 2491–2494.
[16] M. Murata, R. Shimazaki, S. Watanabe, Y. Masuda, Synthesis (2001) 2231–2233.
[17] C. Wolf, R. Lerebours, Org. Lett. 6 (2004) 1147–1150.
[18] L.D. Pachón, M.B. Thathagar, F. Hartl, G. Rothenberg, Phys. Chem. Chem. Phys.
8 (2006) 151–157.
[19] D. Srimani, S. Sawoo, A. Sarkar, J. Org. Chem. 9 (2007) 3639–3642.
[20] B.C. Ranu, R. Dey, K. Chattopadhyay, Tetrahedron Lett. 49 (2008) 3430–3432.
[21] O.C. Kappe, A. Stadlier, Microwaves in Organic and Medicinal Chemistry, Wiley-
VCH, 2006.
[22] A.L.F. Souza, L.C. Silva, B.L. Oliviera, O.A.C. Antunes, Tetrahedron Lett. 49 (2008)
3895–3898.
[23] T. Hasell, C.D. Wood, R. Clowes, J.T.A. Jones, Y.Z. Khimyak, D.J. Adams, A.I. Cooper,
Chem. Mater. 22 (2010) 557–564.
[24] F.Z. Dorwld, Organic Synthesis in Solid Phase: Supporters Linkers and Reactions,
Wiley-VCH, 2002.
4. Conclusion
The role of microwave synthesis is increasing day by day. To the
best of our knowledge, this is the first study of Hiyama reaction
under microwave heating using supported palladium nanopar-
ticles. Clarke [28] reported first microwave assisted Hiyama
cross-coupling reaction using in situ synthesized palladium phos-
phine ligands as catalyst and TBAF as activator. Alacid and Najera
[29] reported coupling of vinyltrialkoxysilane with arylbromides
and chlorides in the presence of phenone oxime-derived pal-
ladocycles as catalysts. In both the reactions, no isolation of
catalyst was reported. The significant improvements offered by
the present method are: operational simplicity, general applica-
bility, recyclability and the absence of hazardous organic solvents,
fluoride activators, phosphine ligands, surfactants, etc. This can
contribute to the development of green strategy for the synthesis
of biaryls.
[25] H. Kaur, D. Shah, U. Paul, Catal. Commun. 12 (2011) 1384–1388.
[26] D. Astruc, Nanoparticles and Catalysis, Wiley-VCH Verlag Gmbh & Co., 2008.
[27] N.E. Leadbeater, Chem. Commun. (2005) 2881–2902.
[28] M.L. Clarke, Adv. Synth. Catal. 347 (2005) 303–307.
[29] E. Alacid, C. Najera, J. Org. Chem. 73 (2008) 2315–2322.