Silica-acetylacetone-supported palladium nanoparticles as an efficient and reusable catalyst in the Heck-Mizoroki C-C cross-coupling reaction

Article abstract of DOI:10.1002/aoc.3259

The preparation of palladium nanoparticles supported on acetylacetone-modified silica gel and their catalytic application for Heck olefination of aryl halides were investigated. The catalyst was characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and transmission and scanning electron microscopies. The supported palladium nanoparticles are demonstrated to be a highly active and reusable catalyst for the Heck reaction. Several reaction parameters, including type and amount of solvent and base, were evaluated. The heterogeneity of the catalytic system was investigated with results indicating that there is a slight palladium leaching into the reaction solution under the applied reaction conditions. Despite this metal leaching, the catalyst can be reused nine times without significant loss of catalytic activity.

Full text of DOI:10.1002/aoc.3259

Full Paper
Received: 25 August 2014
Revised: 3 November 2014
Accepted: 14 November 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3259
Silica–acetylacetone-supported palladium
nanoparticles as an efficient and reusable
catalyst in the Heck–Mizoroki C–C cross-
coupling reaction
Abdolreza Hajipoura,b*, Zohre Shirdashtzadea and Ghobad Azizia
The preparation of palladium nanoparticles supported on acetylacetone-modified silica gel and their catalytic application for Heck
olefination of aryl halides were investigated. The catalyst was characterized using X-ray diffraction, X-ray photoelectron spectros-
copy, and transmission and scanning electron microscopies. The supported palladium nanoparticles are demonstrated to be a
highly active and reusable catalyst for the Heck reaction. Several reaction parameters, including type and amount of solvent
and base, were evaluated. The heterogeneity of the catalytic system was investigated with results indicating that there is a slight
palladium leaching into the reaction solution under the applied reaction conditions. Despite this metal leaching, the catalyst can
be reused nine times without significant loss of catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: supported palladium nanoparticles; semi-heterogeneous catalyst; Heck coupling; C–C cross-coupling reaction; C–C bond
formation; arylation of olefins; Heck olefination; immobilized metallic nanoparticles
surface of acetylacetone (acac)-functionalized silica particles (SiO2-
Introduction
acac-PdNPs).
Many organic reactions can be catalyzed effectively by metallic
nanoparticles, and therefore recently the application of nanoparti-
cles has been extensively studied.[1] The catalytic activity of metallic
nanoparticles can be decreased due to their tendency to aggregate
Results and Discussion
In continuation of our recent investigations of the synthesis and ap-
and form larger metal particles. A helpful way to prevent this aggre-
plication of SiO2-acac-PdNPs in carbon–carbon cross-coupling
gation is to immobilize the metallic nanoparticles on the surface of
reactions,[36,37] we now report the extension of the use of this het-
erogeneous nano-catalyst for the cross-coupling reaction of olefins
with various aryl halides (Scheme 1).
suitable solid supports.[2]
The Heck reaction is one of the most important types of
palladium-catalyzed reactions,[3–7] and still is one of the vital C–C
bond formation reactions in modern organic synthesis.[8–10] Gener-
ally homogeneous catalytic systems for the Heck reactions are pal-
ladium salts or organopalladium complexes in the presence of
additional ligands, such as phosphines, or an additive which stabi-
lize the palladium species during the reaction and prevent the ag-
gregation and precipitation of catalytically inactive ‘palladium
black’.[11–14] However, products are commonly contaminated by
the residual metal and ligands,[12,13] which not only can be difficult
to separate from the final products but also they may pose environ-
mental concerns. In addition to these problems, palladium is an ex-
pensive metal, and thus separation and recovery of the palladium
complexes are an important goal in palladium catalysis.
In order to explore the catalytic properties of SiO2-acac-PdNPs,
this catalyst was employed in the Heck cross-coupling reaction. Ini-
tially, the reaction conditions for testing SiO2-acac-PdNPs were op-
timized using 4-bromoacetophenone and methyl acrylate (MA) in
the presence of 10 mg (40 × 10À5 mmol Pd) of catalyst and various
bases and solvents at 140°C in an oil bath. As evident from Table 1
(entries 2 and 8), 1-methyl-2-pyrrolidone (NMP) as solvent and
NaHCO3 as base give the best results.
Furthermore, the amounts of catalyst and NaHCO3 as the base
were optimized for the reaction of 4-bromoacetophenone and
MA in NMP as solvent. The data clearly show that 5 mg
(2× 10À4 mmol Pd or 0.08 mol% Pd) of catalyst and 2 mmol of base
In view of economic and environmental concerns, continuous ef-
forts are now being made to overcome these problems by using
heterogeneous catalysts because these types of catalysts can be
easily separated from the reaction mixture.[12,15,16] Therefore, many
efforts have been made to anchor palladium complexes[17–20] or
metallic palladium nanoparticles[21–35] on various supports. A better
metal dispersion can be obtained in the presence of ligands on the
surface of a solid support. In the study reported here, we prepared
and employed a metallic palladium catalyst stabilized on the
*
Correspondence to: Abdolreza Hajipour, Pharmaceutical Research Laboratory,
College of Chemistry, Isfahan University of Technology, Isfahan 84156, Islamic
Republic of Iran. E-mail: haji@cc.iut.ac.ir
a Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of
Technology, Isfahan 84156, Islamic Republic of Iran
b Department of Neuroscience, University of Wisconsin Medical School, 1300
University Avenue, Madison, WI 53706-1532, USA
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

A. Hajipour et al.
The leaching of palladium was examined for the coupling reac-
tion of 4-bromobenzaldehyde with MA under optimized reaction
conditions using the catalyst poisoning method. Pyridine was used
as poison because this ligand is known to bind strongly to Pd(II).[18]
The results presented in Fig. 1 show that pyridine has a significant
effect on the reaction rate in comparison to the reaction under nor-
mal conditions. However, the reaction proceeds slowly and stops at
80% conversion after 200 min. So, the role of supported palladium
nanoparticles is crucial. These findings indicate that part of activity
of the catalyst can result from leached, homogeneous palladium
species. The amount of leached palladium in the reaction solution
is 1.4 × 10À3 mg or 0.13 × 10À4 mmol of palladium after completion
of the reaction based on atomic absorption spectroscopy. There-
fore, the reaction mechanism could include both leached and me-
tallic palladium nanoparticles.
The recyclability of SiO2-acac-PdNPs was also investigated. As
shown in Fig. 2, the catalyst was recycled in the Heck coupling of
iodobenzene with MA. Apart from a little leaching of palladium
from catalyst, it is worth noting that the catalyst has shown a signif-
icant recycling performance in the Heck coupling, the catalyst can
be reused nine times without significant loosing of catalytic activity.
Furthermore, the catalyst can be easily separated from the reaction
mixture, washed three times with water and acetone and finally
dried for the next run.
Scheme 1. Synthetic procedure for preparation of Heck products using
SiO2-acac-PdNPs catalyst.
Table 1. Optimization of base and solvent for the Heck reaction of 4-
bromoacetophenone with methyl acrylatea
Entry
Solvent
Base
Temperature (°C) GC conversion (%)
1
H2O
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaHCO3
NaOAC
NaOH
Et3N
Reflux
140
0
90
80
0
2
NMP
3
DMAc
EtOH
MeOH
DMF
140
4
Reflux
Reflux
140
5
0
6
80
0
7
Dioxane
NMP
Reflux
140
8
95
70
50
40
9
NMP
140C
140C
140C
10
11
NMP
NMP
aReaction conditions: 4-bromoacetophenone (1 mmol), methylacrylate
(1.2 mmol), base (1 mmol), 10 mg (4 × 10À4 mmol Pd) catalyst and
1.5 ml solvent.
Experimental
give the optimal results (Table 2, entry 3). As the catalyst is not sen-
sitive to oxygen, the reactions were carried out under air
atmosphere.
General
All chemicals were purchased from Merck or Aldrich and were used
as received. 1H NMR spectra were recorded using 300 MHz and 13
C
The mentioned conditions were applied for the Heck cross-
coupling reaction of various types of aryl halides with MA,
methyl methacrylate, acrylic acid and styrene as olefin, at
140°C (Table 3). The chemoselectivity of the reaction was exam-
ined using 1-bromo-3-chlorobenzene as a dihalide substrate
(Table 3, entry 10). Although this procedure is also applicable
to C–Cl bonds, by employing a stoichiometric amount of olefin,
only the Br is substituted, which may be due to the higher activ-
ity of Br in comparison to less reactive Cl. Interestingly, in all re-
actions, only the trans isomers are produced. In addition, as
evident from Table 3 (entries 12–14), this method can be used
for the Heck reactions of even less reactive aryl chloride deriva-
tives with lower yields and longer reaction times at a higher tem-
perature (160°C) and CsCO3 as the base.
NMR spectra were recorded using 75 MHz in CDCl3 solutions at
room temperature with a Varian 300 NMR (tetramethylsilane was
used as an internal standard). X-ray photoelectron spectra were ob-
tained using a Twin Anode X-Ray Source System (X-Ray 8025-
Bestes). FT-IR spectra were recorded with a spectrophotometer
(Jasco-680, Japan). The sizes of the synthesized particles were ob-
served using transmission electron microscopy (Hitachi H-700H)
at an accelerating voltage of 150 kV. Samples were prepared by
placing drops of diluted ethanol-dispersed catalyst particles on
the surface of copper grids followed by evaporation.
Palladium content of the catalyst was measured using a
PerkinElmer 2380 atomic absorption spectrophotometer. Also we
used GC (Beifin 3420 gas chromatograph equipped with a Varian
CP SIL 5CB column: 30 m × 0.32 mm × 0.25 μm) for determination
of reaction conversions. Powder X-ray diffraction patterns were ob-
tained with a Philips X’Pert PRO over the 2θ range 10–100° at rate of
2.58° minÀ1, using Cu Kα radiation (λ = 1.54060 Å).
Table 2. Optimization of concentration of base and catalyst for the
Heck reaction of 4-bromoacetophenone with methyl acrylate at 140°Ca
Entry
Amount of catalyst
Base (mmol)
Conversion (%)
(mg) (× 105 mmol Pd)
Synthesis of Silica–acac-Supported Palladium Nanoparticles
1
2
3
4
5
6
7
8
1 (4)
2 (8)
2
2
2
2
1
3
4
5
50
80
This catalyst has been previously synthesized, fully characterized
and utilized for Suzuki and Sonogashira reactions.[30,31] Briefly,
5 (20)
10 (40)
5 (20)
5 (20)
5 (20)
5 (20)
100
100
65
silica gel 60 (5g) was suspended in
a solution of 3-
iodopropyltrimethoxysilane (5mmol) in dry toluene. The suspen-
sion was refluxed for 24 h. After cooling, the particles were collected
by filtration and washed with CH2Cl2, and dried under reduced
pressure to give iodopropyl–silica gel. Iodopropyl–silica gel was
re-suspended in 30 ml of acetonitrile, and 5 mmol (0.50g) of acetyl
acetone followed by 5 mmol (0.69 g) of K2CO3 were added and
refluxed for 24 h. The solids were washed with water and extracted
with ethanol in a Soxhlet apparatus for 5 h and dried under reduced
95
100
100
aReaction conditions: 4-bromoacetophenone (1 mmol), methyl
acrylate (1.2 mmol) and 1.5 ml NMP.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)

Silica–acac-supported palladium catalyst in Heck reaction
Table 3. Heck reaction of various aryl halides and olefinsa
Entry
R″
X
R
R′
Time (min)
Conversion (%)
Yield (%)
1
H
I
I
I
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOH
COOH
COOH
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
H
15
12
100
100
100
100
100
100
50
90
92
93
87
93
92
41
88
90
89
85
75
80
55
89
85
89
91
87
91
82
72
39
73
76
85
90
92
2
4-OMe
4-NO2
H
3
10
4
Br
Br
Br
Br
Br
Br
Br
40
5
4-CHO
4-COMe
4-OMe
4-CN
70
6
30
7
24 h
20
8
100
100
100
100
80
9
4-NO2
3-Cl
45
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
15
1-Bromonaphthalene
45
H
Cl
11 h
14 h
20 h
45
4-NO2
4-COMe
H
Cl
Cl
I
87
68
CH3
CH3
CH3
CH3
H
100
100
100
100
100
100
95
4-OMe
4-Cl
I
70
Br
Br
I
30
4-CN
H
50
10
4-Cl
Br
I
H
25
4-OMe
H
H
25
I
H
10.30 h
8 h
80
4-OMe
4-COMe
3-Cl
I
Ph
H
50
Br
Br
Br
I
Ph
H
3.30 h
14.30 h
6.30 h
15
85
Ph
H
100
100
100
100
4-CN
H
Ph
H
COOMe
COOMe
H
4-OMe
I
H
12
aReaction conditions: aryl halide (1 mmol), olefin (1.2 mmol), base (2 mmol), 5 mg (2 × 10À4 mmol) catalyst and 1.5 ml NMP.
bGC conversions.
cIsolated yield.
dFor aryl chlorides, the reaction temperature was 160°C.
Figure 2. Effect of catalyst recycling on the conversion of Heck reactions of
iodobenzene and MA with NaHCO3 and NMP at 140°C.
solid was washed with acetone and dried under vacuum at room
temperature to afford the desired catalyst. The amount of palla-
dium was 0.04mmolgÀ1 (4.26mggÀ1) determined using atomic
absorption spectroscopy.
Figure 1. Influence of catalyst poison (pyridine) on the progress of Heck
reactions.
pressure. Finally, the resulting solid was stirred at room temperature
for 2 h with acetonitrile in the presence of 0.06 g of Pd(OAc)2. After
washing the solids with acetonitrile, the Pd(II)–acac supported on
silica was refluxed in acetonitrile to form a black solid. The black
Procedure for Coupling Reaction of Iodobenzene with MA
A round-bottom flask equipped with a magnetic stirrer bar and a
condenser was charged with a mixture of iodobenzene (1mmol),
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.
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A. Hajipour et al.
MA (1.2mmol), NaHCO3 (2mmol), Pd catalyst (5mg, 2 × 10À4 mmol
Pd) and NMP (1.5ml). The mixture was heated at 140°C in an oil
bath and stirred under air atmosphere. The reaction progress was
followed using TLC. After completion of the reaction, the mixture
was cooled to room temperature and the product was extracted
with n-hexane (3× 5 ml) and dried over CaCl2. The extracts were fil-
tered and evaporated to dryness to give methyl trans-cinnamate
(Table 3, entry 1) as a pale yellow solid. All products are known com-
pounds and were characterized by comparing their melting points
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In summary, it was demonstrated that palladium nanoparticles
could be stably immobilized on the surface of acac-modified silica
gel, and thus could act as a highly active and reusable catalyst for
promoting the Heck cross-coupling reaction of various aryl halides
with olefins to produce the corresponding products in good to ex-
cellent yields with high chemoselectivity.
Acknowledgements
We gratefully acknowledge the funding support received for this
project from the Isfahan University of Technology (IUT), IR Iran. Fur-
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Supporting Information
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web-site.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)