Palladium nanoparticles supported on SiO2 by chemical vapor deposition (CVD) technique as efficient catalyst for Suzuki-Miyaura coupling of aryl bromides and iodides: Selective coupling of halophenols

Article abstract of DOI:10.1002/aoc.2868

Nanoparticles of palladium were supported on SiO₂ by chemical vapor deposition technique. The obtained Pd nanocatalyst was characterized by various techniques. This catalyst was found to be very efficient for the selective cross-coupling of hydroxyl-substituted aryl iodides and bromide with arylboronic acids in water at room temperature to produce the corresponding hydroxyl-substituted biaryls. Coupling of phenylboronic acid with aryl iodides and bromides carrying substituents other than hydroxy group was also performed efficiently in refluxing ethanol. Copyright 2012 John Wiley & Sons, Ltd. Palladium nanoparticles supported on SiO₂ as an efficient heterogeneous catalyst was prepared by CVD technique and applied in Suzuki coupling reaction mild conditions. The Suzuki reaction of halophenols was conducted at room temperature in the presence of this catalyst. Copyright

Full text of DOI:10.1002/aoc.2868

Full Paper
Received: 28 January 2012
Revised: 31 March 2012
Accepted: 31 March 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2868
Palladium nanoparticles supported on SiO by
2
chemical vapor deposition (CVD) technique as
efficient catalyst for Suzuki–Miyaura coupling
of aryl bromides and iodides: selective
coupling of halophenols
a
a
a,b
Nasser Iranpoor *, Habib Firouzabadi , Afsaneh Safavi ,
a
a,b
Somayeh Motevalli and Mohammad M. Doroodmand
Nanoparticles of palladium were supported on SiO by chemical vapor deposition technique. The obtained Pd nanocatalyst
2
was characterized by various techniques. This catalyst was found to be very efficient for the selective cross-coupling of
hydroxyl-substituted aryl iodides and bromide with arylboronic acids in water at room temperature to produce the corresponding
hydroxyl-substituted biaryls. Coupling of phenylboronic acid with aryl iodides and bromides carrying substituents other
than hydroxy group was also performed efficiently in refluxing ethanol. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: silica support; palladium nanoparticle; biphenyls; Suzuki–Miyaura reaction
These biaryls can be obtained by the Suzuki–Miyaura coupling of
hydroxyl-substituted aryl halides and phenylboronic acid, catalyzed
Introduction
Cross-coupling reaction of aryl halides with arylboronic acids
through the palladium-catalyzed Suzuki–Miyaura reaction is one
of the most valuable methods for the synthesis of biaryl deriva-
by palladium. The reaction represents an attractive alternative over
other methods using organometallics because organoboranes are
[
20,21]
air- and moisture-stable with relatively low toxicity.
Since
[1]
tives. This type of reaction is extensively applied in many syn-
thetic transformations, such as natural products, pharmaceutical
halophenols show relatively low reactivity in the Suzuki–Miyaura
reaction, there are only a few methods reported in the literature
[2]
[22–31]
targets and conjugated materials, as well as in conducting poly-
for synthesis of hydroxyl-substituted biaryls.
10% Pd/C has
[3]
mers. Palladium complexes with different phosphine ligands are
been used for this transformation, but the method can only be
successfully applied for iodophenols at room temperature. The yield
of the coupled product for bromophenol is low and the reaction time
usually employed as catalysts in the traditional Suzuki–Miyaura
[4–7]
reaction.
However, some of these complexes have encountered
ꢀ
[22]
problems such as sensitivity to both air and moisture and the pos-
sibility of P&bond;C bond degradation at elevated temperatures,
and generally an inert atmosphere is required for their efficient
manipulation and use. Moreover, their cost and toxicity limit their
is long (12h) and occurs at 50 C.
The use of other catalysts
[
23]
2.2+
such as Pd(II)/azetidine,
Pd/Napht
(naphthidine di(radical
[
24]
[25]
[26]
cation), Pd/NaY zeolite, Pd/PS-co-PAEMA-co-PMAA, Pd(II)/
[
27]
[28]
[29]
[22]
cationic 2,2-bipyridyl, Pd(OAc) /DBU, Pd/Sepiolit, Pd/C,
2
[
8]
[30]
[31]
large-scale industrial application. As phosphine-free catalysis,
Pd(II)/graphite oxide
and Pd/CNT
suffers from use of high
ꢀ
[24–28,30,31]
Suzuki–Miyaura reaction using ligands such as N-heterocyclic
temperatures (80–150 C)
and usually long reaction times
In the present work we wish to report the use
[
9]
[10]
[11]
[14]
[22,24,29–31]
carbenes, palladacycle species,
palladium nanoparticles,
(10–15 h).
[
12]
[13]
and also techniques using microwave or ionic liquids, etc.
of supported palladium nanoparticles on plate SiO by a chemical
2
in various solvents have been reported. From the standpoint of
environmentally benign organic synthesis and easy recovery of
the catalyst, development of immobilized palladium nanoparticles
for generation of heterogeneous catalysts is challenging and
vapor deposition (CVD) technique as an effective and reusable
catalyst for the selective Suzuki–Miyaura coupling reactions of
iodo- and bromophenols with phenyl and 4-fluorophenylboronic
acids in water. This coupling reaction can also be performed
efficiently for other substituted aryl halides in refluxing ethanol.
[15–17]
important.
In this line, a great deal of effort has been made
to immobilize nano metals on different beds to increase the activity
of heterogeneous catalytic systems owing to their higher surface
area compared to bulk systems. Because of environmental and
economic concerns, recently much effort has been directed
towards using water as solvent for organic and organometallic
* Correspondence to: Nasser Iranpoor, Chemistry Department, College of
Sciences, Shiraz University, Shiraz, Iran. E-mail: iranpoor@susc.ac.ir
[18]
reactions. Among functionalized biaryls, those substituted with
hydroxyl groups are a class of important intermediates in the
a Chemistry Department, College of Sciences, Shiraz University, Shiraz, Iran
b Nano Technology Research Institute, University of Shiraz, Shiraz, Iran
[19]
synthesis of natural products possessing biological activities.
Appl. Organometal. Chem. 2012, 26, 417–424
Copyright © 2012 John Wiley & Sons, Ltd.

N. Iranpoor et al.
by XRD, SEM, atomic force microscopy (AFM) and TEM. Inductively
coupled plasma (ICP) analysis of the catalyst showed that the
Experimental
1
ꢁ1
FT-IR spectra were run on a Shimadzu FTIR-8300 spectrometer. H
capacity of Pd was 0.471 mmol g of SiO , which is equivalent
2
1
3
[32]
and C NMR spectra were recorded on a Brucker Avance DPX-
50 MHz spectrometer using tetramethylsilane as internal standard
and CDCl as solvent. Scanning electron microscopy (SEM) was per-
to 5.0 weight percentage.
2
3
Preparation of Nano-Pd/SiO
2
by Solution Technique
formed using a Philips XL-30 FEG instrument at 20 kV. Transmission
electron microscopy (TEM) analyses were performed on a Philips
model CM 10 instrument. X-ray diffraction (XRD) data were
obtained with a Bruker AXS D8 Advance diffractometer. Reaction
monitoring was carried out on silica gel analytical sheets or by
gas chromatographic analysis using a 3 m length column packed
with DC-200 stationary phase.
[
33]
Pd/SiO catalyst (5%) was prepared according to the literature,
2
with some modification as follows: 15.8 mmol, 0.95 g of plate
SiO ; 160–430 mesh, pore size diameter (type 60 GF254) was
added to an aqueous solution (100 ml) of Pd(NO (125 mg,
0.469 mmol) and magnetically stirred at room temperature. Then,
an aqueous solution N (48.0 ml, 1.0 mmol) was added with
2
)
3 2
2 4
H
stirring at room temperature. The black Pd(0) catalyst was formed
immediately upon addition of hydrazine, which was filtered and
washed thoroughly with water. The catalyst was characterized
by SEM and TEM techniques. ICP analysis of the obtained catalyst
[
32]
Preparation of Nano-Pd/SiO by CVD Process
2
Plate SiO (42.0 mmol, 2.50g); 160–430 mesh, pore size diameter
type 60 GF254) was placed inside the cooled collector (Fig. 1). A
2
showed that the Pd weight percentage on SiO was 5%.
2
(
solution of palladium acetate (0.265 mg, 1.181mmol) in 1-propanol
Typical Procedure for the Coupling of 4-Iodophenol and
(
20 ml) was used as palladium source as shown in Fig. 1. The palla-
Phenylboronic Acid Catalyzed with 5% Pd/SiO
2
in Water
ꢀ
dium acetate solution cell was heated to 70 C with a temperature
ꢀ
ꢁ1
ramp of 10.0 C min , while argon was bubbled through the ves-
sel containing the palladium acetate solution. This was then mixed
with hydrogen and finally introduced to the CVD production line.
This process led to the formation of palladium nanoparticles inside
the quartz tube. The amounts of Pd aerosols were controlled by a
piezoelectric vibrator positioned next to the bottom of the palla-
dium acetate solution. The CVD-synthesized Pd nanoparticles were
then passed through UV radiation and finally directed towards the
cooled vessel for deposition on the silica substrate. The palladium
nanoparticle-doped silica support was subsequently collected,
sonicated in a sonication bath (frequency 100 MHz, 60 W) for
A mixture of 5% Pd/SiO2 (6.4 mg, 0.3 mol%), K CO (400 mg,
2
3
3.0 mmol), phenylboronic acid (180 mg, 1.5 mmol), and 4-iodophenol
(220mg, 1.0mmol) 2a in H O (2.0ml) was stirred magnetically at
2
room temperature. TLC of the reaction mixture showed the
completion of the reaction after 1.5h. After completion, the catalyst
was removed by filtration. The aqueous solution was either left for
crystallization of the product or was extracted with CH Cl2
2
(5ꢂ 10 ml) and dried over anhydrous Na SO . 4-Phenylphenol 2b
2
4
was purified by crystallization from CH Cl and petroleum ether
2
2
ꢀ
[34]
(white crystals, 161mg, 95% yield, m.p. 161 C, (literature m.p.
165 C). It was characterized by comparing its H and C NMR
spectra with those reported in the literature.
ꢀ
1
13
~20 min and washed with acetone. The catalyst was characterized
Typical Procedure for the Coupling of 1-Bromo-4-nitrobenzene
2
and Phenylboronic Acid with 5% Pd/SiO in Ethanol
Pd/SiO (5%, 32 mg, 1.5 mol%), NaOH (60 mg, 1.5 mmol), phenyl-
2
boronic acid (180 mg, 1.5 mmol), 1-bromo-4-nitrobenzene 8a
202 mg, 1.0 mmol), and EtOH (2.0 ml) were placed in a 50 ml flask
(
equipped with a magnetic stirring bar and refluxed. TLC of the
reaction mixture showed the completion of the reaction after
15 min. After completion, the catalyst was filtrated off. The filtrate
was concentrated and then purified by column chromatography
over silica gel using n-hexane as eluent to afford highly pure
4
-nitrobiphenyl 8b (169mg, 85% yield) as bright-yellow crystals,
ꢀ
[35]
ꢀ
m.p. 114–114.5 C). The product was
m.p. 116 C, (literature
characterized by comparing its H and C NMR spectra with
the data reported in the literature.
1
13
Results and Discussion
We initially deposited palladium nanoparticles on plate SiO by
2
CVD method according to the procedure which was recently
[
32]
reported by two of us for deposition of Pd on nanotubes.
In
the designed CVD process, mentioned in the Experimental section,
plate SiO was used as a suitable solid substrate for coating by
2
palladium nanoparticles.
To characterize the synthesized nano Pd/SiO catalyst, various
2
analytical techniques were used. Figure 1 shows the SEM, AFM
2
Figure 1. Characterization of Pd/SiO including (A) SEM, (B) AFM and (C)
voltage profile
and voltage profile images of Pd-supported SiO2.
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Appl. Organometal. Chem. 2012, 26, 417–424

Palladium nanoparticles as catalyst for selective Suzuki–Miyaura reaction
The TEM image of CVD-synthesized catalyst is shown in Fig. 2.
Based on the information obtained from AFM and SEM images,
in the presence of a different quantity of this catalyst and 3.0
molar equivalents of K CO3 as a base (Scheme 1). The results
2
the histogram of Pd-doped SiO
2
is presented in Fig. 3. According
are summarized in Table 1.
to this histogram, the average diameter of Pd nanoparticles was
found to be ~35 nm. Also, the data obtained from TEM analysis
confirmed the average size of nanoparticles to be ~30 nm.
The type of silica gel was also found to affect the efficiency of
the supported catalyst. As the results of Table 1 show, the Pd
catalyst supported on plate SiO
diameter 15–40 mm (type 60 GF254) (Table 1, entry 1) is more reac-
tive than a similar one obtained using SiO of 230–400 mesh,
pore size diameter 37–63 mm (Table 1, entry 4). Controlled experi-
ments using Pd(OAc) and Pd(OAc) on bulk SiO were also per-
formed. The results obtained (Table 1, entries 6 and 7) show that
Pd/SiO prepared by CVD is much more reactive.
In order to have a comparison between reactivity of the sup-
ported nanopalladium on SiO by this technique (CVD method)
and the conventional chemical method, we also prepared 5 wt
nano-Pd on the plate SiO by solution technique according
2
with 160–430 mesh, pore size
Additional characterization was also applied to Pd-doped SiO
by patterned XRD technique. According to the XRD spectrum
2
2
ꢀ
ꢀ
ꢀ
(
Fig. 4), the strong peaks positioned at 2θ = 41 , 48 , and 73 were
related to the Pd(0) nanoparticles doped on the SiO support.
We then studied the reaction of phenylboronic acid with
-iodophenol in H O as a model reaction at room temperature
2
2
2
2
4
2
2
2
%
2
[
33]
to the literature. SEM and TEM of the obtained Pd/SiO
technique are shown in Fig. 5.
2
by this
We then used this catalyst for the coupling reaction of 4-iodophenol
and phenylboronic acid under the same reaction conditions as we
used for the Pd/SiO obtained by CVD technique (Table 1, entry 5).
2
The obtained result shows that the catalyst obtained by CVD
method was much more reactive. The lower reactivity of the nano-
palladium catalyst obtained by solution technique compared to
that obtained by CVD could be due to aggregation of the palladium
nanoparticles, which is clearly demonstrated in its TEM image
(Fig. 5). In comparison, the TEM image of the catalyst obtained by
2
Figure 2. TEM of Pd-supported SiO obtained by CVD technique
Scheme 1. Suzuki–Miyaura cross-coupling reaction of 4-iodophenol
with phenylboronic acid
Table 1. Reaction of 4-iodophenol with phenylboronic acid in water at
a
room temperature using different supported palladium nanoparticles
b
Entry
1
Nanocatalyst/method
of preparation
mol% Time (h) Yield (%)
2
Figure 3. Histogram of Pd-doped SiO based on AFM and SEM images
5% Pd/SiO
160–430 mesh, pore size
diameter: 15–40 mm) / CVD
2
(60 GF254
)
0.3
1.5
95
(
2
3
4
5% Pd/SiO
5% Pd/SiO
5% Pd/SiO
2
2
2
(60 GF254) / CVD
(60 GF254) / CVD
(230–400 mesh,
0.2
0.1
0.3
1.7
2
93
93
88
3.7
pore size diameter:
7–63 mm) / CVD
5% Pd/SiO type 60
GF254 /solution technique
3
5
2
0.3
8
85
[33]
6
7
Bulk Pd(OAc)
Bulk Pd(OAc)
2
0.3
0.3
4
4
60
65
c
2
/ bulk SiO
2
a
Reaction conditions: 4-iodophenol (1.0 mmol), phenylboronic acid
1.5 mmol), K CO (3.0 mmol), catalyst in 2.0 ml H O.
Isolated yield.
(
2
3
2
b
c
Reaction conditions are as mentioned in note ‘a’ in the presence of
.86 mg SiO
2
2
.
Figure 4. XRD patterns
Appl. Organometal. Chem. 2012, 26, 417–424
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N. Iranpoor et al.
Table 2. Effect of different solvents on the coupling of 4-iodophenol
a
with phenylboronic acid at room temperature
Entry Solvent
Time (h)
Isolated yield (%)
Room temperature Reflux Room temperature Reflux
1
2
3
4
5
6
7
THF
12
12
12
48
12
12
1.5
24
5
50
40
45
32
65
60
95
81
77
60
50
90
83
—
Toluene
CH
CH
3
CN
Cl
24
24
12
12
—
2
2
DMF
EtOH
2
H O
a
Reaction conditions: 4-iodophenol (1.0 mmol), phenylboronic acid
1.5 mmol), K CO (3.0 mmol), 5 wt% Pd/SiO (0.3 mol%) in 2.0 ml solvent
(
2
3
2
Table 3. Effect of different bases on the reaction of 4-iodophenol at
a
room temperature
Entry
Base
mmol of base Time (h) Isolated yield (%)
1
2
3
4
5
6
7
8
K
2
CO
NaOH
Na PO
KF.2H
Cs CO
Et
3
3
3
3
3
3
3
2
1
1.5
2.5
6.8
20
1.3
4
95
86
85
50
96
82
91
94
3
4 2
.12H O
2
O
2
3
3
N
K
K
2
CO
CO
3
5
2
3
11
a
Reaction conditions: 4-iodophenol (1.0 mmol), phenylboronic acid
1.5 mmol), base, 5 wt% Pd/SiO (0.3 mol%) in 2.0 ml H O.
(
2
2
dramatically increased and the reactions were not completed
Table 4, entries 10 and 11). As far as we know, there are no
reports in the literature on the selective coupling of halophenols;
we therefore decided to take this advantage and apply it to the
selective coupling of halophenols.
Figure 5. Microscopic images: (A) TEM and (B) SEM of palladium nano-
particles doped on SiO by solution technique
(
2
CVD technique shows the well-separated nano-Pd supported on
SiO (Fig. 2).
Among different loading values of Pd/SiO
When we performed competitive reactions using binary mix-
tures of halophenols and aryl halides carrying substituents other
2
2
(Table 1, entries 1–3),
0.3mol% was selected as the most suitable loading for this reaction.
than hydroxyl group with both C H B(OH) and 4-F- C H B(OH)
6 5 2 6 4 2
The effect of solvent on the reaction of 4-iodophenol was also
studied (Table 2). Among different solvents, H O was found to be
2
the best choice for the reaction (Table 2, entry 7).
The effect of various bases on the coupling of 4-iodophenol
under our optimized reaction conditions in water, high selectivity
was observed and halophenols reacted preferentially (Table 5).
In order to gain some insight into the origin of the selectivity and
discover the effect of solvent, we also performed the above-
mentioned competitive reactions in refluxing ethanol. In compari-
son with the reactions in water which occur at room temperature,
the reactions in ethanol required refluxing conditions. However,
again high selectivity was observed. On the basis of this observation,
it can be concluded that the reaction is selective for halophenols
independent of the nature of the solvent, reaction temperature,
and the solubility effect. The observed selectivity may be explained
based on the possibility of H-bonding between the hydroxyl groups
of the silica and the phenolic ones. This interaction can bring closer
the phenolic substrate and the complex obtained from the oxidative
was also studied. The results obtained showed that K
Cs CO acted as the most effective bases for this reaction (Table 3,
entries 1 and 5).
Under our optimized reaction conditions (1.0 mmol of halo-
phenol, 6.4 mg (0.3 mol%) of Pd/SiO , 3.0 mmol of K CO and
.0 ml of H O), the desired products were obtained in excellent
2 3
CO and
2
3
2
2
3
2
2
yields from iodo- and bromophenols and phenylboronic acid at
room temperature (Table 4, entries 3–6). The coupling reactions
of 4-iodo- and 4-bromophenols were also successfully performed
ꢀ
with 4-fluorophenylboronic acid at room temperature and 50 C
respectively (Table 4, entries 7–9).
2
addition of aryl halide to Pd(0)/SiO catalyst. The stability and recycl-
When this catalyst was applied to aryl halides with substituents
other than hydroxy group, such as &bond;CH or &bond;OCH , it
was observed that the reaction times at room temperature were
ability of the catalyst were determined by using it in successive reac-
tions and also by its thermal analysis. After reusing the catalyst four
times in the coupling reaction of 4-iodophenol and phenylboronic
3
3
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Appl. Organometal. Chem. 2012, 26, 417–424

Palladium nanoparticles as catalyst for selective Suzuki–Miyaura reaction
a
2 2
Table 4. Nano Pd/SiO catalyzes the coupling of halophenols and ArB(OH) at room temperature
Entry
1
Halophenol
ArB(OH)
2
(mmol)
Product
Time (h)
Isolated yield (%)
2-Iodophenol 1a
2-Iodophenol 1a
4-Iodophenol 2a
4-Iodophenol 2a
4-Iodophenol 2a
4-Bromophenol 3a
4-Iodophenol 2a
4-Bromophenol 3a
4-Bromophenol 3a
4-Iodoanisol 4a
PhB(OH)
PhB(OH)
PhB(OH)
PhB(OH)
PhB(OH)
PhB(OH)
2
2
2
2
2
2
(1.5)
(1.5)
(1.0)
(1.2)
(1.5)
(1.5)
2-Phenylphenol 1b
2-Phenylphenol 1b
4-Phenylphenol 2b
4-Phenylphenol 2b
4-Phenylphenol 2b
12
1
64
85
82
95
95
89
88
86
80
48
33
b
2
3
4
5
6
7
3.5
3
1.5
2
4-Phenylphenol 2b
0
0
0
4-FC
4-FC
4-FC
6
6
6
H
4
H
4
H
4
B(OH)
B(OH)
B(OH)
2
2
2
(1.5)
(1.5)
(1.5)
4-(4 -Fluoro)phenylphenol 3b
8
c
8
4-(4 -Fluoro)phenylphenol 3b
5
b
9
4-(4 -Fluoro)phenylphenol 3b
12.5
8.5
8.5
d
1
0
1
PhB(OH)
PhB(OH)
2
(1.5)
(1.5)
4-Methoxybiphenyl 4b
4-Methylbiphenyl 5b
d
1
4-Iodotoluene 5a
2
a
2 3 2 2
Reaction conditions: halophenol (1.0 mmol), arylboronic acid, K CO (3.0 mmol), 5 wt% Pd/ SiO (0.3 mol%) in 2.0 ml H O.
b
ꢀ
The reaction was performed at 50 C.
c
The reaction was performed in refluxing water.
d
Yields were determined by gas chromatographic analysis.
a
Table 5. Competitive reactions using binary mixtures of halophenols and other substituted aryl halides with PhB(OH)
2
6 4 2
and 4-F-C H B(OH)
Entry
ArBr
Product
Isolated yield (%) Isolated yield (%)
in water / Time (h) in refluxing EtOH
[
[
[
47,51]
[36,47–50]
1
2
3
4-Bromophenol 3a4-Bromoanisole 6a 4-Phenylphenol
4-Bromophenol 3a4-Bromotoluene 7a 4-Phenylphenol
2b4-Methoxybiphenyl
4b
5b
8b
80 / 99
81 / 95
83 / 93
85 / 124
80 / 126
39,47,51]
34,47,51]
[36,47,48]
2b4-Methylbiphenyl
[35,46–48,50]
b
b
4-Bromophenol 3a1-Bromo-4-
nitrobenzene 8a
4-Phenylphenol
2b4-Nitrobiphenyl
— —
0
c
c
4
4-Bromophenol 3a4-Bromotoluene 7a 4-(4 -Fluoro)phenylphenol 3b4-
83 / 9 4
78 / 125
87 / 123
0
[37]
(
4 -Fluoro)phenyltoluene 7b
[
34,47,51]
[36,47,48]
5
4-Iodophenol 2a4-Bromotoluene 7a
4-Phenylphenol
2b4-Methylbiphenyl
5b
88 / 97
a
Reaction conditions: halophenol (1.0 mmol), other substituted aryl halide (1.0 mmol), phenylboronic acid and 4-fluorophenylboronic acid
2.0 mmol), K CO (3.0 mmol), 5 wt% Pd/SiO (0.5 mol%, 10.6 mg) in 2.0 ml of solvent at room temperature.
(
2
3
2
b
No reaction occurs in refluxing ethanol for both substrates.
The reaction was performed in refluxing water.
c
acid, the coupled product was obtained in 95–89% yield but with
some reaction time elongation from the first to the second run.
In order to determine the leaching of the catalyst, the reaction
of 4-iodophenol and phenylboronic acid in the first run was
stopped every 30 min, and the filtered solution was analyzed by
ICP analysis. The results of ICP analysis did not show any appre-
ciable amounts of Pd leaching. Analysis of the reaction after every
run by ICP and TG showed only 0.81% of leached Pd. In support
of the stability of the catalyst, in another set of experiments we
also took the thermograms of the catalyst after every 30 min
reaction time at the first run and after each run. The thermo-
grams of the Pd-doped silica are shown in Fig. 6. According to
thermograms B and C, the decrease to ~0.4% in the weight
to the air oxidation of palladium nanoparticles, revealing the air
stability and lack of significant leaching of Pd from the surface
of silica. Also, a significant decrease (~3.6%) in the weight
ꢀ
percentage at a temperature of ~280 C is related to desorption
of the previously adsorbed organic molecules. The data
obtained from thermal analysis can also be considered as
further evidence for the lack of significant Pd leaching from
the surface of the catalyst.
In order to show the efficiency of this nano-catalyst for the Suzuki–
Miyaura coupling reaction of halophenols, we compared some of
our results for the coupling of 4-bromophenol with those reported
in the literature in Table 6. This comparison shows that our catalyst
provides the highest isolated yield of the coupled product at the
shortest reaction time and the lowest reaction temperature.
Since water was found to be a very suitable medium for the
coupling of only halophenols, we decided to study the possibility
ꢀ
percentage at ~120 C is due to the desorption of water vapor
from the nanocatalyst. Also, the slight enhancement in the
ꢀ
weight percentages in all thermograms at around 180 C is due
Appl. Organometal. Chem. 2012, 26, 417–424
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N. Iranpoor et al.
of using this new Pd nano-catalyst for the coupling of other
substituted aryl halides using other solvents. In Table 7 the results
obtained results for the coupling of 1-bromo-4-nitrobenzene in
different solvents are shown.
As demonstrated in Table 7, refluxing ethanol was found to
be very suitable for the coupling of 1-bromo-4-nitrobenzene,
and therefore this condition was extended to other aryl halides
(Table 8). The results shown in Table 8 demonstrate that this
Figure 6. Thermograms of Pd-doped silica after every 30 min and after first, second and third run
Table 6. Comparison of methods for the coupling of 4-bromophenol with phenylboronic acid
ꢀ
Entry
Solvent
Temp. ( C)
Catalyst
Time (h)
Isolated yield (%)
Ref.
a
1
2
3
4
5
6
7
8
9
H
H
2
O
Room temp.
Pd/SiO
2
(0.3)
2
95
91
48
86
99
99
24
99
76
96
99
—
2
O
50
Azetidine-Pd(II) (0.01)
3
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[22]
[30]
[31]
2
.2+
Dioxane
DMA/H
80
Pd/Napht
(0.6)
15
0.25
1
2
O (1:1)
100
Pd/NaY zeolite (0.001)
H
H
2
O
O
90
Pd/ PS-co-PAEMA-co-PMAA (0.2)
Pd(II)/cationic 2,2-bipyridyl (0.001)
2
100
1
EtOH/H
2
O (1:1)
O (1:1)
150
Pd(OAc)
2
/DBU (0.4)
0.16
20
12
17
10
H
H
2
O
O
Room temp.
Pd/sepiolite (0.1)
Pd/C (0.3)
2
50
80
90
2
+
10
1
EtOH/H
2
Pd /GO (0.25)
Pd/CNT (0.3)
1
2
H O
a
Present method.
a
Table 7. Optimization of the different parameters for the coupling reaction of 1-bromo-4-nitrobenzene with phenylboronic acid
ꢀ
b
Entry
Solvent
Base (mmol)
Catalyst (mol%)
Temp. ( C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
H
H
H
2
2
2
O:EtOH (1:2)
O:Dioxane (1:2)
O:EtOH (1:2)
K
K
K
K
K
K
K
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
(3.0)
(3.0)
(3.0)
(3.0)
(3.0)
(3.0)
(3.0)
0.3
0.3
1
90
100
90
4.5
4
20
15
4
90
EtOH
DMF
DMF
DMF
EtOH
DMF
DMF
DMF
DMF
EtOH
1
80
8
50
0.3
1
110
90
5
100
100
100
91
4
1
110
80
1
NaOH (3.0)
NaOH (3.0)
NaOH (3.0)
1
3.5
0.25
7
1
110
110
110
110
80
100
52
1
1
1
1
0
1
2
3
0.6
1
Na
3
PO
4
.12H
2
O (3.0)
4
100
100
100
NaOH (1.5)
NaOH (1.5)
1
7.5
0.25
1.5
a
Reaction conditions: 1-bromo-4-nitrobenzene (1.0 mmol), phenylboronic acid (1.5 mmol), solvent (1–3 ml).
b
Gas chromatographic analysis.
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 417–424

Palladium nanoparticles as catalyst for selective Suzuki–Miyaura reaction
a
Table 8. Suzuki–Miyaura reaction of phenylboronic acid with aryl halides
Entry
ArX
Ar-Ph
Time
Yield (%)
[
35,46–48,50]
1
2
3
4
5
6
7
8
9
1-Bromo-4-nitrobenzene 8a
4-Bromobenzonitrile 9a
5-Bromopyrimidine 10a
1-Bromo-4-chlorobenzene 11a
3-Bromopyridine 12a
4-Iodoanisole 4a
4-Nitrobiphenyl
8b
15 min
10 min
30 min
60 min
90 min
30 min
20 min
10 min
5 min
30 min
25 min
25 min
1 h
85
90
87
88
80
90
93
97
98
75
90
72
85
88
78
80
[
38,47]
4-Cyanobiphenyl
5-Phenylpyrimidine 10b
9b
[39]
[
40,48]
4-Chlorobiphenyl
11b
[41]
3-Phenylpyridine 12b
[36,47–50]
4-Methoxybiphenyl
4b
[36,47,48]
4-Iodotoluene 5a
4-Methylbiphenyl
5b
[
42,48,50]
Iodobenzene 13a
Biphenyl
4-Nitrobiphenyl
13b
[35,47,48]
1-Iodo-4-nitrobenzene 14a
4-Iodoaniline 15a
8b
15b
[43,48]
10
11
12
13
14
15
16
4-Phenylaniline
[44,48]
2-Iodotoluene 16a
2-Methybiphenyl
16b
43,48]
[
1-Iodonaphthalene 17a
2-Iodo-5-nitrotoluene 18a
Bromobenzene 19a
1-Phenylnaphthalene
17b
[45]
2-Phenyl-5-nitrotoluene 18b
b
b
[42,48,50]
Biphenyl
4-Methylbiphenyl
1-Phenylnaphthalene 17b
13b
24 h
[36,47,48]
4-Bromotoluene 7a
5b
24 h
[43]
1-Bromonaphthalene 20a
4 h
a
Reaction conditions: aryl halide (1.0mmol), phenylboronic acid (1.5mmol), 5wt% Pd/SiO
2
(1.5mol%, 32.0mg), NaOH (1.5mmol), EtOH (2.0ml), reflux.
b
ꢀ
Reaction conditions: aryl halide (1.0mmol), phenylboronic acid (1.5mmol), 5 wt% Pd/SiO
2
(1.5mol%, 32.0mg), NaOH (3.0mmol), DMF (2.0ml), 110 C.
I. Parrot, Synlett 2006, 3185; f) F. Alonso, I. P. Beletskaya, M. Yus,
Tetrahedron 2008, 64, 3047; g) G. A. Molander, B. Canturk, Angew.
Chem. Int. Ed. 2009, 48, 9240.
catalyst can be applied efficiently to the coupling of aryl
bromides and iodides having both electron-withdrawing and -
donating groups as well as heteroaromatic compounds in
refluxing ethanol.
[
2] a) R. F. Heck, Palladium Reagents in Organic Syntheses, Academic Press,
New York, 1985; b) E. Negishi (Ed.), Handbook of Organopalladium
Chemistry for Organic Synthesis, Wiley, Hoboken, NJ, 2002.
3] a) J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev.
We also performed the hot filtration test for the reaction of
[
1
-bromo-4-nitrobenzene with phenylboronic acid in refluxing
2
2
002, 102, 1359; b) S. Kotha, K. Lahiri, D. Kashinath, Tetrahedron
002, 58, 9633; c) A. F. Littke, G. C. Fu, Angew. Chem. Int. Ed. 2002,
ethanol. ICP analysis of the filtrate showed only 1.1% of
palladium leaching. The low leaching of the palladium from
the catalyst shows that the reaction occurs mainly through a
heterogeneous pathway.
4
1, 4176; d) C. Bolm, J. P. Hildebrand, K. Muniz, N. Hermanns, Angew.
Chem. Int. Ed. 2001, 40, 3284.
[
4] a) A. Zapf, A. Ehrentraut, M. Beller, Angew. Chem. Int. Ed. 2000, 39,
4153; b) R. C. Smith, R. A. Woloszynek, W. Chen, T. Ren, J. D. Protasiewicz,
Tetrahedron Lett. 2004, 45, 8327; c) M. Joshaghani, E. Faramarzi,
E. Rafiee, M. Daryanavard, J. Xiao, C. Baillie, J. Mol. Catal. A: Chem.
Conclusion
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J. Xiao, C. Baillie, Tetrahedron Lett. 2007, 48, 2025.
2
In summary, adsorption of nano-palladium on SiO by CVD tech-
[
5] a) Z. Freixa, P. W. N. Van Leeuwen, Dalton Trans. 2003, 1890; b) R. C.
Smith, C. R. Bodner, M. J. Earl, N. C. Sears, N. E. Hill, L. M. Bishop, N.
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nique provides an efficient and heterogeneous catalyst which
works under very mild reaction conditions for the Suzuki–
Miyaura coupling of halophenols ArX (X = Br, I) and phenylboro-
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selective for the coupling of halophenols, which can be consid-
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[
Acknowledgments
We acknowledge the partial support of this work by Shiraz
University Research Council.
[
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