Pd doped SiO2 nanoparticles: An efficient recyclable catalyst for Suzuki, Heck and Sonogashira reactions

Article abstract of DOI:10.1039/c4ra01813c

Palladium doped silica (Pd/SiO₂) mesoporous material was synthesized via the sol-gel route using the P123 triblock copolymer as a structure directing agent. Pd/SiO₂ was efficiently used as a catalyst for Suzuki, Heck and Sonogashira reactions under microwave irradiation. The catalyst exhibited high activity for all the coupling reactions and can be recycled nine times without a significant loss in its catalytic activity. This journal is the Partner Organisations 2014.

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Graphical Abstract
Heck
Pd doped SiO2
Green
MWs
process
Coupling products
Palladium doped silica (Pd/SiO2) mesoporous material was synthesized via sol-gel route
using the P123 triblock copolymer as structure directing agent. Pd/SiO2 was efficiently used
as catalyst for Suzuki, Heck and Sonogashira reactions under microwave irradiation. The
catalyst exhibited high activity for all the coupling reactions and can be recycled nine times
without a significant loss in its catalytic activity.

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DOI: 10.1039/C4RA01813C
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ARTICLE TYPE
Pd-doped SiO2 nanoparticles: An efficient recyclable catalyst for Suzuki,
Heck and Sonogashira reactions
Deepali A. Kotadia,a Urmila H. Patel,b Sahaj Gandhib and Saurabh S. Soni*,a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Palladium doped silica (Pd/SiO2) mesoporous material was synthesized via solꢀgel route using the P123
triblock copolymer as structure directing agent. Pd/SiO2 was efficiently used as catalyst for Suzuki, Heck
and Sonogashira reactions under microwave irradiation. The catalyst exhibited high activity for all the
coupling reactions and can be recycled nine times without a significant loss in its catalytic activity.
because of their large pore size, high surface area and tuneable
pore structure. Mesoporous silica nanoparticles, thus have
10 Introduction
received considerable attention for their use as catalyst,
50 adsorption, polymer filter, optical devices, bioꢀimaging, drug
delivery and biomedical applications due to as stated good
morphology, particle size, uniformity and dispersity.8 Since the
discovery of mesoporous silica synthesized using cationic
surfactant as templates, the templating method has been widely
55 applied to prepare mesoporous silica with high surface areas,
tuneable pore sizes, large pore volumes and rich morphology.9,10
These properties make mesoporous silica as an excellent material
for supported catalyst system. Palladium doped silica can be
synthesized through various other routes which were reported
60 earlier for hydrogenation, oxidation reactions, etc.11ꢀ13 Apart
from these applications, there are very few reports of Pd/SiO2 or
modified Pd/SiO2 used as catalyst for coupling reactions like
Suzuki, Heck, Sonogashira and Stille coupling.14ꢀ18 Recently,
Speziali et al. reported Pd and AuꢀPd supported on large surface
65 area containing commercial MCMꢀ41 and silica both and used as
catalyst for Suzuki cross coupling reactions.19 Majority of the
reports are based on the Pd supported metal oxide systems, mixed
metal oxide systems or ligand based Pd catalyst either supported
or complexed, for these reactions. To our knowledge till date no
70 results have yet been reported for Suzuki, Heck and Sonogashira
reaction over Palladium doped silica (Pd/SiO2) synthesized
through Evaporation Induced Self Assembly (EISA) method
using P123 surfactant as a templating agent. In continuation of
our efforts for to developing economical and ecoꢀfriendly
75 synthetic pathways for organic transformations from the
viewpoint of green chemistry,20ꢀ23 here by we wish to report the
synthesis of mesoporous Pd/SiO2 nanoparticles using solꢀgel
process which is used as reusable catalyst for Suzuki, Heck and
Sonogashira coupling reactions. The catalyst shows a high
80 catalytic activity in the above mentioned classic carbonꢀcarbon
bond formation reactions under Microwave (MW) irradiation. In
particular, short reaction time, easy catalyst recovery and
excellent recycling efficiency of the catalyst make it an ideal
system for the reaction under mild conditions.
Aromatic carbonꢀcarbon cross coupling reactions are emerged as
an important methodology for the preparation of complex organic
molecules. Among all the coupling reactions, Heck, Suzuki and
Sonogashira are the most important ones, as these reactions lead
15 to key steps in the synthesis of natural products and a great
variety of complex organic molecules used in many fields like
drugs, pharmaceuticals and agrochemicals, etc.1ꢀ5 Nowadays, the
palladiumꢀcatalyzed crossꢀcouplings are essential tool for carbonꢀ
carbon bond formation which demonstrates that carbon atoms in
20 all hybridization state (mainly sp2 carbon) undergoes CꢀC bond
formation during the reactions.1ꢀ6 It is very well reported that
when these coupling reactions were performed using
homogeneous palladium catalysts; high reaction rates, excellent
activity and selectivity of coupling products were obtained.
25 Practically all form of homogeneous palladium catalyst (either Pd
salts or Pd catalyst in presence of phosphine ligands) can be used
for activation of bulky and electronically unactivated substrates.
Despite remarkable usefulness of these Pd catalysts, possibility of
reuse in successive reaction is limited which leads to loss of
30 expensive metal. As in many cases excess amount of Pd salts are
used while in actual extremely small amounts of Pd (ppm or ppb
level) are sufficient to give higher conversion. Another drawback
is contamination of advanced chemical intermediates by
palladium residues which is a serious issue for largeꢀscale
35 synthesis particularly for pharmaceutical industries.
A
heterogeneous palladium catalyst provides solution of these
problems, where the palladium metal is immobilized on a solid
support either in the metal complex form or as metal
nanoparticles which makes it easy to remove by filtration leaving
40 products virtually free of palladium residues.1,2,7 The supports
usually used for palladium heterogenization are organic
polymers, carbon materials, silica, zeolites, various metal oxides,
hybrid organicꢀinorganic supports like grafted silica and
molecular complexes bound to a ligand anchored on a support.1,2
45 Among these different supports, ordered mesoporous silica
materials are very attractive for immobilizing/doping Pd catalysts
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Experimental Section
in MW flask. The mixture was irradiated at 245W MW power for
desired time. After completion of reaction (confirmed by TLC
analysis), the mixture was filtered through short pad of celite to
60 remove catalyst. To the filtrate, ether and water were added and
the layers were separated. The aqueous layer was extracted twice
with ether and the combined organic layers were washed with
brine, dried over Na2SO4, filtered and concentrated in vacuo to
get the desired product.
The block copolymer surfactant P123 [(EO)20ꢀ(PO)70ꢀ(EO)20,
MWꢀ5750 g/mol], tetraethoxy silane (TEOS), Pd(OAc)2,
Tetrabutylammonium hexaflourophosphate (TBAPF6) and the
starting coupling substrates were purchased from SigmaꢀAldrich,
India. HCl and Ethanol used were of Analytical grade laboratory
reagent. The Synthesized Pd doped nanoparticles were
characterized for their structure and texture by XRD, BET, TEM,
SEM, EDS and DLS. The XRD measurements were performed
5
65 General Procedure for the Heck Reactions
10 on a Rigaku D/max 2500 Xꢀray Diffractometer. Diffraction
patterns were recorded with Cu Kα radiation (40kV, 30mA) over
a 2θ range of 3ꢀ80oC at a scan rate of 2o/min. BET surface area,
average pore volume, and average pore diameter were measured
by physisorption of N2. The adsorption isotherms were elaborated
15 according to the BET (Brunauer, Emmett and Teller) method for
surface area calculation, and pore size distribution curves derived
from desorption branches for the porous materials with BJH
(BarrettꢀJoynerꢀHalenda) methods on Micromeritics Gemini 2375
Surface area analyzer. The TEM measurements were conducted
20 using a Philips Tecnai 20 (Holland) transmission electron
microscope at an acceleration voltage of 200kV with a W emitter
and LaB6 electron source. A single drop of the sample was
deposited on a carbonꢀcoated copper grid and dried at room
temperature under atmospheric pressure. The SEM observations
25 were carried out using JSMꢀ7600F Field Emission GunꢀScanning
Electron Microscopes (FEGꢀSEM) at an acceleration voltage of
0.1 to 30 kV. The DLS measurements were carried out on
Malvern instrument, Zetasizer ZSꢀ90 (for nanosizer) Particle size
analyzer. The ICP analysis was carried out on Atomic Emission
30 Spectrometer (Perkin Elmer Optima 3300 RL) with SCD
detector; detection limit: up to ppb level.
Aryl halides (1.28mmol), alkenes (1.66mmol), triethylamine
(2.56mmol), dodecane (40mg, as internal standard), Pd/SiO2
(60mg, 0.75 wt%) and DMF (5ml) were taken in MW flask. The
mixture was irradiated at 280W MW power for desired time.
70 After completion of reaction (confirmed by TLC analysis), the
mixture was filtered through short pad of celite to remove
catalyst. To the filtrate, ether and water were added and the layers
were separated. The aqueous layer was extracted twice with ether
and the combined organic layers were washed with brine, dried
75 over Na2SO4, filtered and concentrated in vacuo to get the desired
product.
General Procedure for the Sonogashira Reactions
Aryl halides (1mmol), phenyl acetylene (1.1mmol), triethylamine
(1.5mmol), dodecane (40mg, as internal standard), Pd/SiO2
80 (35mg, 0.47 wt%) and DMF (5ml) were taken in MW flask. The
mixture was irradiated at 280W MW power for desired time.
After completion of reaction (confirmed by TLC analysis), the
mixture was filtered through short pad of celite to remove
catalyst. To the filtrate, ether and water were added and the layers
85 were separated. The aqueous layer was extracted twice with ether
and the combined organic layers were washed with brine, dried
over Na2SO4, filtered and concentrated in vacuo to get the desired
product.
All the coupling reactions were performed in ovenꢀdried
glassware. Reactions were monitored by thin layer
chromatography using 0.25 mm E. Merck silica gel coated glass
35 plates (60Fꢀ254) with UV light to visualize the course of reaction.
All the coupling products have been previously reported and their
1
90 identities were confirmed by comparison of their H, 13C NMR
1
The coupling products were analyzed by H and 13C NMR (400
and GCꢀMS spectral data with the values of earlier reports.
MHz Bruker Scientific, Switzerland) and GCꢀMS (GCꢀMass
Spectrometer, Perkin Elmer, Auto system XL, GC+)
spectroscopic techniques.
Result and Discussion
Characterization of Pd/SiO2
The ordered mesoporous Pd/SiO2 nanoparticles were synthesized
95 based on our earlier report using an EISA route23 where
Pd(OAc)2 was introduced in initial sol for uniform distribution of
Pd in silica matrix. As P123 block copolymer contains
polyethylene oxide (PEO) units which has the capability to
reduce metal salts to nanoparticles,24,25 Pd2+ ions were reduced to
100 Pd0 (Pd black) in-situ. During the solvent evaporation, ~50% RH
was maintained to obtain mesophase formation. In order to obtain
mesoporous Pd/SiO2 the obtained material was washed with hot
ethanol for removal of polymeric surfactant giving very smooth
free flowing powder which was subjected for structural and
105 morphological characterization. Figure 1 portrays the powder Xꢀ
ray diffraction pattern of Pd/SiO2 at low angles, which shows the
characteristic peaks of ordered structured of silica.26 The figure
for powder XRD at wide angles is depicted in Fig. S1 (supporting
information) and confirm amorphous silica and no particular
110 reflections of Pd NPs were found which might be due to low %
Pd loading in the silica matrix.
40 Preparation of Pd/SiO2
Mesoporous doped silica was synthesized using a prehydrolyzed
solution containing TEOS(42mmol), ethanol (5g) and HCl (37%)
(0.055M) stirred for an hour. A second solution was prepared by
dissolving the Pd(OAc)2 in P123 surfactantꢀethanol solution
45 under acidic conditions, was then added to the former one. The
resultant mixture was then kept for ageing for one day under
humid atmosphere and then under atmospheric conditions for self
evaporation of solvent which was then dried, grounded to form
fine greyꢀblack colored powder. The final doped silica was rinsed
50 in ethanol for 2h at 80˚C to remove surfactant and then dried at
80˚C yielded into fine free flowing powder. The synthesized
Pd/SiO2 was then characterized.
General Procedure for the Suzuki Coupling Reactions
Aryl halides (1mmol), aryl boronic acid/ester (1.1mmol), KOH
55 (2mmol), TBAPF6 (0.2mmol), dodecane (40mg, as internal
standard), Pd/SiO2 (40mg, 0.5 wt%) and water (5ml) were taken
2
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25 with a maximum at 4.8nm.
The morphology of Pd/SiO2 was studied using
TEM, SEM, EDS and DLS measurements. The
TEM image (Figure 3) clearly shows highly
dispersed Pd particles on the SiO2 surface. The
30 palladium particles/clusters were found to be in
spherical shape with average diameter 3ꢀ5nm
with particles in the range of
1
2
3
4
0
1
2
3
4
ο
2
θ
Fig. 1 Low angle XRD patterns of Pd/SiO2 mesoporous materials. Inset
Fig. represents the magnification of area marked with square on graph
Nitrogen adsorptionꢀdesorption isotherms and the pore diameter
distribution of the Pd/SiO2 was measured using Brunauerꢀ
EmmettꢀTeller (BET) and BarettꢀJoynerꢀHalenda (BJH) methods.
The isotherm (Figure 2) of the Pd doped SiO2 is classic type IV
curves which indicates that the silica is mesoporous in nature (as
such materials have pore sizes in the range of 2ꢀ50nm and are
characterized by type IV isotherm which have a hysteresis).
5
35 Fig. 3 TEM micrographs of Pd/SiO2 mesoporous materials
15ꢀ20nm confined to the surface of SiO2 particles. The SEM
measurement (Figure 4) shows formation of mesoporous material
with small and uniform particle size.
300
1.0
0.8
250
0.6
0.4
200
0.2
0.0
0
20
40
60
80
100
Porediameter,(A)
150
100
50
40 Fig. 4 SEM micrographs of Pd/SiO2 mesoporous materials
The actual content of dopant was tested by Energy Dispersive
Spectroscopy (EDS) analysis and a typical EDS spectrum is
showed in Figure 5(a). Quantification of the EDS peaks gives the
main elements Pd, Si, O and C in 3.1%, 19.4%, 39.0% and 49.7%
45 by weight respectively. In order to correlate the morphology of
nanoparticles, the material was characterized by Dynamic Light
Scattering (DLS) measurements in aqueous media.
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
10
Fig.
2 N2 adsorption–desorption isotherms of Pd/SiO2 and the
corresponding pore size distribution derived from adsorption isotherm
(inset)
This type of isotherm indicates that capillary condensation is
100
15 occurring during the adsorption process. Interestingly, on
comparing BET results of Pd/SiO2 and unꢀdoped silica, it was
found that the surface area of the Pd/SiO2 is smaller than that of
pure silica (undoped silica synthesized using this same process
without Pd) without Pd nanoparticles. The average pore diameter
20 and surface area of pure silica mesoporous material are 5.01nm
and 58.63m2/g respectively. Upon doping with palladium the
values of pore diameter and surface area decreased to 3.6nm and
44.5m2/g respectively. The pore size distribution of Pd/SiO2
derived from the isotherm data using BJH model was quite broad
(b)
90
(a)
80
70
60
50
40
30
20
10
0
1
10
Size (d.nm)
Fig. 5 (a) EDS and (b) DLS study or Particle size distribution of Pd/SiO2
50 mesoporous materials
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Figure 5(b) depicts the particle size distribution which shows the
MW powers as the temperature required to occurred the reaction
was not achieved. However, the reaction was remarkably
enhanced at higher MW power (Table 1, Entry1ꢀ4) with
maximum reaction temperature reached around 85˚C. Thus 245W
nanoparticles are polydispersed in shape with two different sizes,
one in the range of 3ꢀ5nm for Pd nanoparticles (might be assign
to nonꢀincorporated Pd NPs in silica matrix) and the other
5
between 13ꢀ15nm belongs to silica (with Pd NPs inside) and the 45 MW power proved to be the best choice for the reaction to
DLS results are in correlation with the previously confirmed
TEM and SEM data.
progress under MW irradiation. The active role of Pd in Pd/SiO2
as catalyst was established by control experiments. In an initial
experiment, the reaction was carried out without catalyst in
presence of water and KOH at 245W MW power where no
50 product formation was observed (Table 1, Entry 9). Next we
examined the catalytic effect of Pd(OAc)2 salt (Table 1, Entry 13)
and SiO2 (Table 1, Entry 14) individually as a catalyst for the
coupling reaction under same reaction conditions and it was
found that Pd(OAc)2 has less prominent effect compared to
55 Pd/SiO2 which could be due to the combined role of Pd and
support for the reaction. The use of Pd/SiO2 as catalyst showed
excellent results for the coupling reaction proving to have better
catalytic sites compared to others. Reaction time of 7ꢀ10 minutes
was required for completion of reaction under MW conditions. It
60 was worthy to note that the reactions could also be completed in
2h at 60oC under conventional heating conditions. We have also
screened the amount of Pd/SiO2 required for the efficient
conversion of reactants and as shown in Table 1, (Entry 3 and 10)
0.5 wt% of Pd/SiO2 was found to be optimal for reaction to
65 proceed.
Catalytic activity of Pd/SiO2
The Pd/SiO2 heterogeneous system was tested as catalyst for
10 Suzuki crossꢀcoupling reactions. For the initial assessment of the
reaction, we have chosen crossꢀcoupling between pꢀ
bromoacetanilide with phenyl boronic acid as a model reaction to
optimize the reaction conditions under MW irradiation. The test
reaction was performed using different bases such as K2CO3,
15 Na2CO3, Et3N, NaOH and KOH in both solvent (water) and
solvent less conditions at different power levels ranging from
140, 210, 245 to 280W.
Table 1 Test experiments for optimization of Suzuki Coupling reactions
of pꢀbromoacetanilide with phenyl boronic acidsa
Entry
Base
Catalyst
Time
(min)
Microwave Yield$
Power (W)
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
KOH
KOH
KOH
KOH
K2CO3
Na2CO3
Et3N
NaOH
KOH
KOH
KOH
K2CO3
KOH
KOH
KOH
KOH
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
ꢀ
20
20
7
7
7
7
7
7
7
140
210
245
280
245
245
245
245
245
245
245
245
245
245
245
ꢀ
52
78
97
94
92
86
89
90
>5
83b
69c
71c
54
85
>5
87d
Table 2 The Suzuki Coupling reactions of aryl bromides with aryl
boronic acidsa
Entry
R1
R2
Yield (%)$
1
2
3
4
5
6
7
8
9
10
11
12
13
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Naphthyl
ꢀNHCOCH3
ꢀCOCH3
ꢀCOC6H5
ꢀOH
ꢀCHO
ꢀCOOH
ꢀOCH3
ꢀCN
ꢀOCH3
ꢀCOCH3
ꢀCN
ꢀNHCOCH3
ꢀOCH3
97
96
96
89
95
92
97
95
93
91
93
90
98
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd(OAc)2
Pd(OAc)2/SiO2
SiO2
7
12
12
7
7
7
Pd/SiO2
120
a
20
Reaction conditions ꢀ aryl halides (1mmol), aryl boronic acid
Naphthyl
Naphthyl
Naphthyl
(1.1mmol), KOH (2mmol), TBAPF6 (0.2mmol), dodecane (40mg, as
internal standard), Pd/SiO2 (0.5 wt%) and water (5ml) under MW
irradiation, Reaction carried out using Pd/SiO2 (0.3 wt% ), c Solvent less
b
Phenyl boronic acid 1,3ꢀ
propanediol ester
Phenyl boronic acid 1,3ꢀ
propanediol ester
Phenyl boronic acid 1,3ꢀ
propanediol ester
conditions, Reaction carried out under conventional conditions at 60oC.
d
25 $ Uncertainty = ± 2%
14
15
ꢀCOCH3
ꢀCN
97
95
Among the different base used, KOH gave better coupling yield
of 97% than other bases as shown in Table 1, Entry 3. For the
reaction to precede well, choice of solvent is a key parameter and
for that the model reaction was carried out in water, which
30 proved to be an excellent solvent giving superior results
compared to reaction which carried out in solvent less condition.
Water has clear advantages as a solvent for use in MW mediated
synthesis, as is nonꢀtoxic, readily available, nonꢀflammable and
above all, MW active due to its polar nature. The use of water as
35 a solvent significantly improves the catalyst activity probably
because the bromine atoms of substituted aryl bromides could
efficiently contact the palladium metal on the hydrophilic
support, SiO2, in aqueous media due to their increased relative
lipophilicity/hydrophobicity.27 The Pd/SiO2 (0.5wt%) catalysed
40 Suzuki cross coupling reaction was strongly suppressed at lower
a
Reaction conditions
ꢀ Aryl halides (1mmol), Aryl boronic acid
(1.1mmol), KOH (2mmol), TBAPF6 (0.2mmol), Dodecane (40mg, as
70 internal standard), Pd/SiO2 (0.5 wt%) and Water (5ml) under MW
irradiation. $ Uncertainty = ± 2%
With the optimised reaction conditions (0.5 wt% Pd/SiO2, KOH,
H2O, 245W power, 7min), the catalytic system was further
75 extended to the coupling reaction of different aryl bromides with
varying aryl boronic acids (Table 2). A wide variety of aryl
bromides bearing electron withdrawing and electron donating
groups were coupled efficiently with phenyl and naphthyl boronic
acids generating the corresponding products in excellent yields
80 under MW irradiations. Of the different aryl bromides used, both
4
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electron withdrawing and releasing groups were found to react
Furthermore, the Heck reaction between pꢀiodobenzene and tꢀ
efficiently by giving highest yields. The reaction under MW 45 butylacrylate to give tꢀbutyl cinnamate ester using Pd/SiO2 as
irradiation reached a temperature of 85oC within 3 minutes of
initiation. The yields of coupling products were determined by
GCꢀMS. The result exhibited biphenyl products in good to
excellent yields along with a little by product formed from phenyl
catalyst was performed. In order to adjust the effect of various
parameters on the reaction time and yield, the reaction was
carried out using different bases under MW irradiations as shown
in Table 3. When iodobenzene was stirred in water with tꢀ
5
boronic acid. To obtain maximum higher yields, we replaced aryl 50 butylacrylate , Pd/SiO2 (0.75wt%), Et3N as base at 280W MW
boronic acid with boronates which are useful substitutes of aryl
boronic acids owing to their high stability to heat and solubility in
10 a variety of organic solvents. We carry out the Suzuki coupling
reaction using Phenyl boronic acid 1,3ꢀpropanediol ester (Table
power for 20 min the reaction was incomplete producing the
desired tꢀbutyl cinnamate in only 40% yield (Table 3, Entry 1).
The use of polar aprotic solvent such as DMF (Dimethyl
formamide) significantly improved the progress of reaction and
2, Entry 13ꢀ15) in presence of KOH, TBAPF6, 0.5 wt% Pd/SiO2 55 the same is completing within 12min (Table 3, Entry 4). Presence
and water. All the three substrates, 4ꢀbromoanisole, 4ꢀ
bromoacetopheneone and 4ꢀbromobenzonitrile were smoothly
15 crossꢀcoupled with the phenyl boronic acid propyl ester in water
to afford corresponding biphenyl derivatives in excellent yields.
of base plays a very vital role in Heck reaction, thus test reactions
were performed for screening Et3N, K2CO3 and NaOAc as base
(Table 4, Entry 4, 6, 7) in DMF for the iodobenzene and tꢀ
butylacrylate reaction. However, the use of K2CO3 and NaOAc
The GCꢀMS result shows excellent coupling yield without any by 60 slightly decreased the reaction efficiency, while the use of Et3N
product formation. We also tested the catalytic yields using
conventional heating system in a preheated oil bath at 110oC for
20 30, 60, 90 and 120 minutes of reaction time. Nearly the same
yields were observed when the Suzuki coupling reaction was
leads to better reaction progress with excellent coupling yields.
Although the reaction in Et3N and DMF at lower MW power
(Table 3, Entry 2) tends to produce lesser yields of cinnamate
products, while the reaction efficiency significantly increased as
carried out in a preheated oil bath at 110oC for 120min under 65 the MW power were raised to 280W (Table 3, Entry 4). However,
conventional process.
no improvement in efficiency was observed at 350W MW power
(Table 3, Entry 5), thus opting 280W as the optimal MW power
range for the reaction where maximum temperature achieved is
105˚C. For the optimization purpose the coupling reaction was
Further, the alternative way by which the reaction is taking
25 place in this heterogeneous system for Suzuki coupling reaction
was studied. The Pd/SiO2 system in water can be viewed as threeꢀ
phase system: A solid catalyst Pd/SiO2 which is hydrophilic in 70 also individually catalysed by Pd(OAc)2 salt and SiO2 (Table 3,
nature, a lipophilic phase which comprising organic substrates
and an aqueous phase of water. The hydrophilic Pd/SiO2 catalyst
30 would more likely to be confined in aqueous phase with
minimum possible contact with the organic substrates. The
presence of TBAPF6, a phase transfer agent together with MWs
brings in the activity by increasing the contacts of the catalyst and
organic substrates, resulting in an ultimate increase in the activity
35 of catalyst suggesting that the reaction takes place at the phase
boundary near the catalyst surface in truly heterogeneous fashion.
Entry 11, 12), but their catalytic sites are inferior to that of
Pd/SiO2. Therefore, the following Heck reactions were performed
in DMF and Et3N with Pd/SiO2 at 280W MW power.
Table 4 The Heck Coupling reactions of aryl halides with alkenesa
Entry
R1
R2
X
Yield (%)$
1
2
3
4
5
6
7
8
9
ꢀH
ꢀCH3
ꢀCOOH
ꢀCOCH3
ꢀCN
ꢀOCH3
ꢀNHCOCH3
ꢀCHO
ꢀH
ꢀCH3
ꢀCOOH
ꢀOCH3
ꢀCO2C4H9ꢀt
ꢀCO2C4H9ꢀt
ꢀCO2C4H9ꢀt
ꢀCO2C4H9ꢀt
ꢀCO2C4H9ꢀt
ꢀCO2C4H9ꢀt
ꢀCO2C4H9ꢀt
ꢀCO2C4H9ꢀt
ꢀC6H5
I
I
I
Br
Br
Br
Br
Br
I
97
92
96
96
94
97
91
89
90
90
93
95
Table 3 Test experiments for optimization of the Heck Coupling
reactions of iodobenzene and tꢀbutylacrylatea
Microwave Yield (%)$
Power (W)
10
11
12
ꢀC6H5
ꢀC6H5
ꢀC6H5
I
I
Br
Entry
Base
Catalyst
Time
(min)
1
2
3
4
5
6
7
8
9
Et3N
Et3N
Et3N
Et3N
Et3N
K2CO3
NaOAc
Et3N
Et3N
Et3N
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd(OAc)2
SiO2
20
17
12
12
12
12
12
5
10
150
12
12
280
210
245
280
350
280
280
280
280
ꢀ
40b
74
87
96
94
91
85
79
92
94c
63
10
a
75 Reaction conditions ꢀ Aryl halides (1.28mmol), Alkenes (1.66mmol),
Triethylamine (2.56mmol), Dodecane (40mg, as internal standard),
Pd/SiO2 (0.75 wt%) and DMF (5ml) under MW irradiation, giving Transꢀ
isomer as major product. $ Uncertainty = ± 2%
A variety of aryl halides reacts with various alkenes such as
80 acrylates and styrene, to give the required products in good to
excellent yields (Table 4). Interestingly, the ability of Pd/SiO2 to
promote coupling reactions of different alkenes with a variety of
activated and nonꢀactivated aryl bromides is satisfactory. Aryl
bromides possessing either an electron withdrawing (ꢀCOOH, ꢀ
85 COCH3, ꢀCN, ꢀCHO) or electron releasing (ꢀOCH3, ꢀCH3, ꢀ
NHCOCH3) substituent on the benzene ring efficiently crossꢀ
coupled with acrylates, regardless of the substituent position to
afford the products in good to excellent yields. The Pd/SiO2
10
11
12
Et3N
Et3N
280
280
a
Reaction conditions ꢀ Aryl halides (1.28mmol), alkenes (1.66mmol),
40 triethylamine (2.56mmol), dodecane (40mg, as internal standard),
Pd/SiO2 (0.75 wt%) and DMF (5ml) under MW irradiation, bReaction
c
carried out in water, Reaction carried out under conventional conditions
at 80oC, giving Transꢀisomer as the major product. $ Uncertainty = ± 2%
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

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View Article Online
DOI: 10.1039/C4RA01813C
catalyzed crossꢀcoupling reactions were next investigated for
separation of Pd/SiO2 catalyst from the reaction mass after a
reaction time of 3 min under MW conditions was carried out and
the hot reaction mass was allowed to react further under similar
conditions. It was found that no further reaction occurred , thus
styrene (Table 4, Entry 9ꢀ12) which proves to be a suitable
substrate that worked effectively as acrylates showing Pd/SiO2 to
be excellent catalyst for Heck reaction also.
5
Encouraged by the above satisfactory results for Suzuki and 50 no coupling product formation was observed (GCꢀMS). The ICP
Heck reactions, the synthetic potential of Pd/SiO2 was then
evaluated for Sonogashira reaction (crossꢀcoupling of aryl or
vinyl halides with terminal alkynes). The major drawbacks
associated with this reaction is the use of homogeneous catalysts
test confirmed the presence of about 6 ppm of Pd in the reaction
media. Thus both these analysis confirm that presence of very
few amount of Pd is not affecting the coupling reactions, which
provides evidence of SiO2 acting as a good scaffold for Pd
10 (which are difficult to remove from the reaction mixture), the use 55 catalyst precursors.
of copper salts to promote the reaction,28 and the use of expensive
and specific phosphine ligands.29ꢀ32 The use of Pd/SiO2 as
heterogeneous catalyst for the Sonogashira reaction in absence of
copper salt and phosphine ligand might prove useful in our
For heterogeneous catalyst, the most crucial thing is the
reusability of the catalyst. The major advantage of the
heterogeneous catalyst is its easy recovery by filtration or
centrifugation. The recycling of Pd/SiO2 was investigated for the
15 present studies. Interestingly, the Sonogashira reaction of aryl 60 Suzuki coupling of pꢀbromoacetophenone with phenyl boronic
iodides and activated aryl bromides under the optimized reaction
conditions (0.5wt% Pd/SiO2, DMF, Et3N, MW 280W, 15ꢀ20
minutes) in absence of copper salts were performed.
acid using the optimized conditions under MW irradiations. After
the reaction was over, the catalyst was simply filtered and washed
successively with water, diethyl ether and acetone until TLC
confirmation of the filtrate showed no detectable amount of
65 reactant and products. The catalyst was then dried under vacuum
for overnight before doing reusability test and was found to be
recycled at least nine times without significant loss in its activity
(Figure S1, ESI).
Table 5 The Sonogashira Coupling reaction of aryl halides with phenyl
20 acetylenesa
Yield (%)$
It is not easy to compare the catalytic results with the literature
70 data due to wide range of different reaction conditions and
catalysts. But a tentative comparison is proposed here for Suzuki
coupling reaction using homogeneous and heterogeneous
catalyst. Nonnenmacher et. al. investigated et(impy)2PdBr2
complex as a catalyst precursor in Suzuki reaction using Cs2O3 as
75 base where maximum coupling yield obtained was 80%.35 In
similar manner Srinivas et. al. used Thiopseudourea liganted Pd
complexes for coupling reactions and maximum Suzuki coupling
yield obtained was 99% in 3h of reaction time using H2O/DMF
solvent system at 80oC.36 Li and coꢀworkers synthesized
80 magnetic nanaoparticlesꢀsupported palladium (SiO2@Fe3O4ꢀPd)
for coupling reactions where the coupling yield obtained was
99% in presence of K3PO4 as base and methanol as solvent at
60oC.37 Our findings are in consonance with LI and coꢀworkers
where a maximum of 98% coupling yield was obtained with
85 Pd/SiO2 catalyst using water as solvent within 7min under
microwave irradiations. It can be summarized from above
discussions that in both the earlier cases, homogeneous Pd
catalyst was used where, though the yield obtained was very good
but lack of catalyst recyclability effected the cost and greenness
90 of the reaction, while it was not the case with heterogeneous Pd
catalysts. Among all, our reaction conditions show far more
superiority in terms of reaction time, yield, solvent and also
greenness of the reaction.
Entry
R1
X
1
2
3
4
5
6
7
8
9
ꢀCOCH3
ꢀOCH3
ꢀNHCOCH3
ꢀCHO
ꢀCOC6H5
ꢀCN
ꢀH
ꢀH
ꢀCH3
ꢀCOOH
ꢀCOCH3
Br
Br
Br
Br
Br
Br
Br
I
94
96
91
88
93
92
81
83
89
93
85b
I
I
Br
10
11
a
Reaction conditions
(1.1mmol), triethylamine (1.5mmol), dodecane (40mg, as internal
ꢀ Aryl halides (1mmol), phenyl acetylene
b
standard), Pd/SiO2 (0.5 wt%) and DMF (5ml) under MW irradiation,
reaction carried out under conventional heating at 80 oC for 120 min
$
25 Uncertainty = ± 2%
Most of the coupling reactions proceeded completely and
generated the corresponding products in very good yields. The
reaction goes smoothly with the halobenzenes possessing both
electron withdrawing and electron releasing group as substrates in
30 presence of Pd/SiO2. While the Sonogashira crossꢀcoupling of a
neutral arylbromide or iodide with terminal alkyne, for example,
the coupling of iodobenzene with phenyl acetylene generated the
corresponding products in good to moderate yields (Table 5,
Entry 7, 8). This might be due to the possible homocoupling of
35 the alkynes which in turn decreases the reaction rate of the crossꢀ
coupling using the less reactive neutral aryl halides.33
Conclusions
Hot filtration test and Reusability test for the Suzuki-
Miyaura cross-coupling reaction
95 In summary, Palladium metal doped Silica (Pd/SiO2) catalyst has
been successfully prepared in a stable form through solꢀgel route.
The synthesized Pd/SiO2 was characterized by XRD, BET, TEM,
SEM, EDS and DLS techniques. The size of Pd/SiO2 mesoporous
nanomaterials ranges from 3ꢀ20nm. The formation of metal
100 nanoparticles of Pd on SiO2 is supported by EDS which shows
3.09 wt% of Pd. The Pd doped SiO2 as catalyst exhibited high
activity for Suzuki, Heck and Sonogashira reactions in presence
A strong test for assessing the heterogeneity of the catalyst is the
40 hot filtration test, also known as split test in which solid preꢀ
catalysts are filtered out of the reaction and the filtrate is
monitored for continued activity.34 In order to determine whether
the reaction proceeded in a homogeneous or heterogeneous
fashion, hot filtration test was performed for Suzuki coupling
45 reaction of pꢀbromoacetophenone and phenylboronic acid. Rapid
6
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

RSC Advances
Page 8 of 8
View Article Online
DOI: 10.1039/C4RA01813C
65 19. G. Speziali, A.G. Marques da Silva, D.M. Vaz de Miranda, A.L.
of water and DMF respectively under MW irradiation. The
synthetic reaction conditions maintained here are environmentally
benign giving very high yields for the respective coupling
products, while the catalyst can be recycled more than nine times.
Monteiro and P.A. RoblesꢀDutenhefner. Appl. Catal. A : Gen., 2013,
462-463, 39.
20. D.A. Kotadia and S.S. Soni, J. Mol. Catal. A: Chem., 2012, 353-354,
44.
5
The combination of advantages displayed by the Pd/SiO2 catalyst 70 21. D.A. Kotadia and S.S. Soni, Catal. Sci. Technol., 2013, 3, 469.
22. D.A. Kotadia and S.S. Soni, Monatsh Chem., 2013, 144, 1735.
such as ease of preparation, high catalytic activity, stability,
reusability, versatility (organic or aqueous solvent catalysis) and
no measurable Pd leaching, proves that the catalyst should be
considered as a viable alternative in crossꢀcoupling reactions on
23. S.S. Soni and D.A. Kotadia, Catal. Sci. Technol., 2014, 4, 510.
24. Y. Piao, Y. Jang, M. Shokouhimehr, I. S. Lee and T. Hyeon, Small,
2007, 3, 255.
75 25. S. S. Soni, R. L. Vekariya and V. K. Aswal, RSC Adv. 2013, 3, 8398.
26. S. Dourdain, A. Gibaud Appl. Phys. Lett. 2005, 87, 223105.
27. K. Wang, S. Liang and C. Wang, Adv. Mater. Res., 2010, 92, 207.
28. P. Siemcen, R.C. Livingston and F. Diederich, Angew. Chem., 2000,
112, 2740; Angew. Chem. Int. Ed., 39, 2000, 2632.
10 efficiency, environmental and economical grounds.
Acknowledgments
The authors are thankful to SICART (Vallabh Vidyanagar) for
TEM and GCꢀMS analysis and SAIF (IIT Bombay) for SEM and
15 EDS analysis. Authors are also very thankful to Department of
Material Science (Dr. Mrs. Manocha and Mr. Parth Joshi) for
BET analysis. One of the Author DAK is highly thankful to
CSIR, NewꢀDelhi for providing financial assistance in terms of
CSIRꢀSRF.
80 29. D. Gelman and S.L. Buchwald, Angew. Chem., 2003, 115, 6175;
Angew. Chem. Int. Ed., 2003, 42, 5993.
30. A. Arques, D. Aunon and P. Molina, Tetrahedron lett., 2004, 45,
4337.
31. J. Cheng, Y. Sun, F. Wang, M. Guo, J. Xu, Y. Pan and Z. Zhang, J.
85
90
Org. Chem., 2004, 69, 5428.
32. M. Feuerstein, H. Doucet and M. Santelli, Tetrahedron Lett., 2005,
46, 1717.
33. Y. Monguchi, K. Sakai, K. Endo, Y. Fujita, M. Niimura, M.
Yoshimura, T. Mizusaki, Y. Sawama, H. Sajiki, ChemCatChem,
2012, 4, 546.
20 Notes and references
a Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar
388120, Gujarat, INDIA Fax: +91-2692 236475; Tel: +91-2692-226857-
Ext.216; E-mail: soni_b21@yahoo.co.in
34. R.A. Sheldon, M. Wallau, I.W.C.E. Arends and U. Schuchardt, Acc.
Chem. Res., 1998, 31, 485.
35. M. Nonnenmacher, D. Kunz, F. Rominger and T. Oeser J.
Organometal. Chem., 2007, 692, 2554.
95 36. K. Srinivas, P. Srinivas, P.S. Prathima, K. Balaswamy, B. Sridhar and
M.M. Rao, Catal. Sci. Technol., 2012, 2, 1180.
37. P. Li, L. Wang, L. Zhang, G.ꢀW. Wang, Adv. Synth. Catal., 2012,
354, 1307.
b Department of Physics, Sardar Patel University, Vallabh Vidyanagar
25 388120, Gujarat, INDIA
† Electronic Supplementary Information (ESI) available: [details of
1
catalyst recyclability and H NMR characterization of coupling products
were given in supplementary information]. See DOI: 10.1039/b000000x/
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Journal Name, [year], [vol], 00–00 | 7