Immobilized NNN Pd-complex on magnetic nanoparticles: Efficient and reusable catalyst for Heck and Sonogashira coupling reactions

Article abstract of DOI:10.1039/c6ra22535g

A highly efficient and easily recyclable magnetic nanoparticle supported palladium catalyst was developed and applied in the Heck and Sonogashira reactions in order to show its catalytic applicability in Pd-catalyzed C-C coupling protocols. The catalyst was prepared using a simple chemical process. First, the prepared Fe₃O₄@SiO₂ nanoparticles were reacted with (3-chloropropyl)-trimethoxysilane (3-CPTMS) in order to synthesis chloro-functionalized magnetic nanoparticles (CPS-MNPs). The substitution reaction of synthetic NNN ligand with CPS-MNPs yields the production of CPS-MNPs-NNN ligand. Finally, immobilization of palladium species on CPS-MNPs-NNN ligand surface afforded CPS-MNPs-NNN-Pd catalyst. The structure and morphology of the prepared nanocatalyst was characterized using various methods such as SEM, TEM, XPS, EDX, CHN, ICP, XRD, FT-IR and VSM techniques. The TEM images show that the sizes of the palladium catalyst are in the range of 8-15 nm. The Heck and Sonogashira coupling reactions were performed in the presence of a catalytic amount of this catalyst system (0.5 mol%) and good yields of products were obtained. Due to the magnetic nature of the catalyst it can be separated from the reaction mixture easily by applying an external magnetic field. Heterogeneity tests affirmed that the catalytic activity stayed indefectible during multiple reaction cycles.

Full text of DOI:10.1039/c6ra22535g

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Immobilized NNN Pd-complex on magnetic nanoparticles: Efficient and
reusable catalyst for Heck and Sonogashira coupling reactions
a
a
b
Fahimeh Dehghani Firuzabadi , Zahra Asadi* and Farhad Panahi
a
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
b
Department of Polymer Engineering and Color Technology, Amirkabir University of Technology,
Tehran, Iran
E-mail: zasadi@shirazu.ac.ir
Abstract
A highly efficient and easily recyclable magnetic nanoparticles supported palladium catalyst was
developed and applied in the Heck and Sonogashira reactions in order to show its catalytic
applicability in Pd-catalyzed C-C coupling protocols. The catalyst was prepared using a simple
chemical process. First, the prepared Fe
3
O
4
@SiO nanoparticles were reacted with (3-
2
chloropropyl)-trimethoxysilane (3-CPTMS) in order to synthesis chloro-functionalized magnetic
nanoparticles (CPS-MNPs). The substitution reaction of synthetic NNN ligand with CPS-MNPs yields
the production of CPS-MNPs-NNN ligand. Finally, immobilization of palladium species on CPS-
MNPs-NNN ligand surface afforded CPS-MNPs-NNN-Pd catalyst. The structure and morphology of
the prepared nanocatalyst was characterized using various methods such as SEM, TEM, XPS, EDX,
CHN, ICP, XRD, FT-IR and VSM techniques. The TEM images show that the sizes of the palladium
catalyst are in the range of 8-15 nm. The Heck and Sonogashira coupling reactions were performed
in the presence of a catalytic amount of this catalyst system (0.5 mol%) and good yields of products
were obtained. Due to the magnetic nature of the catalyst it can be separated from the reaction
1

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mixture easily by applying an external magnetic field. Heterogeneity tests affirmed that the catalytic
activity stayed indefectible during multiple reaction cycles.
Introduction
Carbon-carbon cross-coupling reactions are among the most useful and powerful tools in
advanced organic synthesis.1-3 Heck-Mizoroki and Sonogashira reactions are the most noteworthy
and highly effective protocols for the construction of carbon–carbon bonds in synthetic organic
chemistry.4-11 The resulting products of these two reactions have found extensive applications as
intermediates in the preparation of natural products,12-14 pharmaceuticals,15,16 non-linear optical
materials and molecular electronics,17-19 dendrimeric and polymeric materials.20-22 There are many
homogeneous catalyst systems that have been used in the Heck and Sonogashira reactions.23
Although homogeneous palladium catalysis possesses many merits, such as high reaction rate, high
turnover number, and efficient selectivity, but contaminating by metal leaching, tedious and time-
2
4
consuming work-up procedures are their main drawbacks. For catalytic process, in viewpoint of
“
green chemistry”, the recovery and reusability of catalysts becomes an important factor because of
stringent ecological and economical demands for sustainability.25 Therefore, the development of
heterogeneous catalytic systems has been considered in both academic and industrial research due
to many advantages in comparison with homogeneous systems especially in terms of sustainability
on green chemistry points of view.26,27
Overall, in order to synthesis heterogeneous Pd catalyst systems two different strategies could
be used. One way is synthesis of a supported ligand and then immobilization or complexation of Pd
metal on its surface. Another way is the immobilization of a Pd complex on a solid surface through
immobilization or chemical grafting. Each of these methods is suitable for synthesis of special type
of heterogeneous Pd catalyst system. Usually, heterogeneous Pd nano catalysts have been
synthesized using the stabilization of Pd nanoparticle on ligand modified substrates.28-31
2

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Immobilization of Pd species on different type of supports including silica, polymers, porous
materials, magnetic nanoparticles and etc. were used to prepare heterogeneous Pd catalyst
systems. Compared with other supports, magnetic nanoparticles (MNPs) show obvious superiority
3
2
and have recently received considerable attentions as excellent and smart supports. This class of
supports are paramagnetic in nature and enables facile and effective separation of the catalyst from
the reaction medium by employing an external magnetic field.33-38 This kind of separation hampers
wasting of the catalyst and saves time and energy when compared with filtration or centrifugation
methods.39-42
Generally, MNPs could have most of the specific features of a sustainable and green catalyst
including low cost, high activity, great selectivity, high thermal stability, low toxicity, efficient
recovery, and good recyclability.43-46 From this point of view, MNP supported catalysts obey most
of the principles of green chemistry and sustainability. Even though, MNPs have different
advantages but unfortunately, they sometimes show strong tendency for aggregation and
47
decomposition due to the self interactions that reducing the catalytic efficiency of the catalysts. In
order to overcome the aggregation problem and protect magnetic core, the surface of MNPs is
usually coated with suitable inert shell such as silica layer.48,49 The outer shell of silica not only
improves the dispersibility, water solubility and biocompatibility of MNPs but also provides
suitable sites (Si-OH groups) for potential derivatization with various functional groups allowing
introduction of catalytic sites to MNPs.50,51 In addition to all the above, silica layers prevent metal
leaching from the core of MNPs under harsh shaking conditions .
In continuation of our previous study on synthesis of heterogeneous Pd catalysts52,53 we wish to
report the synthesis of a highly efficient, air- and moisture-stable Pd complex supported on
magnetic nanoparticles for application in Heck and Sonogashira reactions .
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Experimental
All chemicals and solvents were purchased from commercial suppliers and were used without
further purification. FT-IR spectra were run on a Shimadzu FTIR-8300 spectrophotometer, and the
1
13
sample and KBr were pressed to form a tablet. H (250 MHz) and C NMR (62.5 MHz) spectra were
recorded on a Bruker Advance DPX 250 MHz spectrometer in CDCl or DMSO-d solvents using
3
6
tetramethylsilane (TMS) as an internal standard. X-ray powder diffraction (XRD) spectra were
recorded on a Bruker AXS D8-Advance X-ray diffractometer with CuKα radiation (λ = 1.5418). X-ray
photoelectron spectroscopy (XPS) measurements were conducted with an XR3E2 (VG Microtech)
twin anode X-ray source using AlKα = 1486.6 eV. Transmission electron microscopy (TEM) images
were obtained on a Zeiss EM10C transmission electron microscope using Acc voltage of 80 kV.
Magnetic parameters of the prepared MNPs was measured by a vibrating sample magnetometer
(
VSM) (AGFM/VSM 3886 Kashan, Iran) at room temperature in a magnetic field strength of 1 Tesla..
The scanning electron micrography (SEM) images for the catalyst were obtained by SEM
instrumentation (SEM, XL-30 FEG SEM, Philips, at 20 kV). The amount of palladium supported on
MNPs was measured using an ICP analyzer (Varian, Vista-pro) and atomic absorption spectroscopy.
The progress of the reactions was followed by TLC using silica gel polygrams SIL G/UV 254 plates
and GC analysis by using an Aligent Gas chromatograph model 7890A equipped with an HP-1
methyl siloxane column (30m in length × 320 µm × 0.25 µm) equipped with a Flame Ionization
Detector (FID). All yields refer to the isolated products .
Synthesis of silica-coated Fe
3
O
4
magnetic nanoparticles (Fe
3
O
4
@SiO )
2
Fe
3
O
4
magnetic nanoparticles were prepared using co-precipitation method with some
.6H O (6 g) and FeCl .4H O (4.2 g) were
modification.54 In a typical preparation procedure, FeCl
3
2
2
2
added to the solution of cetyltrimethylammonium bromide (CTAB, 0.5 g), as a surfactant, in
deionized water (500 mL) under nitrogen with vigorous stirring at 83°C. Then, NaOH (10 wt. %)
4

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was added dropwise to the solution and the pH value was controlled in the range 12 ≤pH≤ 13. The
dropping rate was controlled with a constant dropper at 0.5 mL/min. Subsequently, the
°
temperature of the mixture was decreased to 63 C and the solution was stirred for 60 minute in
this temperature. Finally, the black precipitate (Fe
3
O ) was separated using a permanent magnet
4
and washed 3 times with deionized water and 3 times with ethanol until the pH value descended to
°
7
.0 and dried in a vacuum at 70 C overnight .
The coating of a layer of silica on the surface of the nano-Fe
procedure. Firstly, Fe nanoparticles (2.0 g) were dispersed in a mixture of ethanol (70 mL) and
deionized H O (10 mL) under ultrasonication for 25 min. Then NH .H O (5 mL) and
tetraethylorthosilicate (TEOS, 4.0 mL) were added successively followed by sonication for another
0 min. The reaction was then allowed to proceed under vigorous mechanical stirring for 24 h.
3
O
4
was achieved by the following
3
O
4
2
3
2
1
Then the coated NPs were collected by an external magnet, washed with ethanol three times and
drying in a vacuum .
Preparation of Fe
3
O
4
@SiO @n-PrCl (CPS-MNPs) using reaction of (3-chloropropyl)-
2
trimethoxysilane with synthetic MNPs
To a mixture of water/ethanol (100 mL, 1:1), Fe
3
O
4
@SiO MNPs (1.0 g) were added and dispersed
2
by sonication for 30 minutes. Then, (3-chloropropyl)-trimethoxysilane (3-CPTMS) (1.5 mL) was
added to this solution and again sonicated for 1 h. After mechanical stirring under N atmosphere at
2
°
3
9 C for 8 h, the resultant products were collected by an external magnet and washed with ethanol
°
three times and then dried under vacuum at 60 C for further use .
Procedure for the preparation of the NNN Schiff base ligand
To a vigorously stirred solution of isovanillin (3.7 mmol, 0.56 g) in 10 mL ethanol, a stoichiometric
amount of diethylenetriamine (1.85 mmol, 0.2 mL) was added dropwise. A red solution, which was
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obtained due to the imine formation, was refluxed for 6 h. The resulting pure precipitate was
extracted from oily liquid by n-hexane and ether and then dried in a vacuum.
Procedure for the preparation of CPS-MNPs-NNN ligand
Excess amount of tributylamine, as a base, was added to 2 mmol of NNN ligand in dry toluene and
stirred for 15 min. Then this mixture was added to 1g of CPS-MNPs which was dispersed in toluene
for 20 min. This mixture was refluxed for 24 h and the resultant solid was separated magnetically
and then washed with ethanol and toluene several times to remove the unreacted residue of the
°
NNN ligand and dried under vacuum at 50 C.
Procedure for the synthesis of CPS-MNPs-NNN-Pd catalyst
1
.0 g of the synthesized CPS-MNPs-NNN ligand was mixed with 0.2 g of palladium acetate in 40 mL
of absolute ethanol. The mixture was refluxed for 24 h under N atmosphere. The solid product was
2
separated by external magnet, washed several times with ethanol and dichloromethane and dried
°
at 50 C under vacuum for 3 h. The palladium content of the catalyst was found to be 0.817 mmol
per gram using ICP analysis.
General procedure for Heck–Mizoroki reaction catalyzed by MNPs-NNN-Pd catalyst
A reaction tube was charged with the appropriate amounts of aryl halide (1.0 mmol), alkene (1.2
mmol), K
2
CO
3
(2.0 mmol), Pd catalyst (0.0062 g, 0.5 mol%). The mixture was vigorously stirred in
°
2
.5 mL DMF/ H
2
O [1/2 (v/v)] and heated to 90 C. After completion of the reaction monitored by GC
or TLC, the catalyst was separated with an external magnet and the organic compound was
extracted with ethyl acetate (3 × 20 mL) from the aqueous layer and the accumulated ethyl acetate
fractions were reduced using a rotary evaporator. Finally, the crude product was purified by
column chromatography on silica gel with n-hexane/EtOAc. The recovered catalyst was thoroughly
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washed with ethyl acetate and ethanol and dried at room temperature for its reuse. All compounds
1
13
were characterized by H and C NMR spectroscopy (See ESI).
General procedure for Sonogashira coupling reaction catalyzed by CPS-MNPs-NNN-Pd
catalyst
A mixture of aryl halide (1 mmol), phenylacetylene (1.2 mmol), K
0.0062 g, 0.5 mol%) in 2.5 mL DMF/ H
2
CO
3
(2.0 mmol) and Pd catalyst
°
(
2
O [1/2 (v/v)] was stirred at 90 C for various reaction
times. The reaction progress was monitored by TLC and GC analysis. After completion of the
reaction, the catalyst was separated by a magnetic bar and the reaction mixture was diluted with
ethyl acetate (15 mL). The solvent was removed under reduced pressure to get the crude product.
The crude product was purified by column chromatography on silica gel with n-hexane/EtOAc
provide the desired pure product. The recovered catalyst was thoroughly washed with ethyl acetate
1
and ethanol and dried at room temperature for its reuse. All compounds were characterized by H
1
3
and C NMR spectroscopy (See ESI).
Results and discussion
A pictorial representation for the preparation of CPS-MNPs-NNN-Pd catalyst is shown in Scheme 1.
First, Fe
aggregation of MNPs they were coated with a silica layer using tetra ethyl ortho silicate (TEOS). In
this way, Fe @SiO nanoparticles were obtained using a sol–gel process. Then, the synthesized
Fe @SiO particles was functionalized with (3-chloropropyl)-trimethoxysilane (3-CPTMS) in
3
O
4
particles were prepared using co-precipitation method. In order to avoid from
3
O
4
2
3
O
4
2
order to functionalized MNPs surface with alkyl halide group for substitution reaction. For the
synthesis of the NNN ligand we used: 1) diethylenetriamine because tridentate nitrogen ligand
formed two five member metallacycles that provided additional stabilization of the nitrogen–metal
5
5
bond. 2) isovaniline due to its –OCH group which makes the catalyst to be more electron rich and
3
leads the Sonogashira and Heck reactions to be done faster. The substitution reaction on C-Cl bond
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of 3-CPTMS with amine groups of NNN Schiff base ligand yields the production of CPS-MNPs-NNN
substrate. Finally, palladium was immobilized on the CPS-MNPs-NNN substrate surface, leading to
the formation of CPS-MNPs-NNN-Pd catalyst.
Scheme 1. A schematic illustration the preparation of CPS-MNPs-NNN-Pd catalyst.
The TEM and SEM images of the CPS-MNPs-NNN-Pd at two different magnifications are depicted in
Figure 1. The TEM image of the fresh catalyst is shown in Figure 1a. The core-shell structure of
particle are shown clearly in Figure 1b and the gray silica shell with thickness of about 4 nm is
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observable in this image. According to the TEM image the size of catalyst particles is estimated to be
in the range of 8-15 nm. The surface morphology of the catalyst is evaluated by SEM image (Figure
1
c) and it shows that the nanoparticles are formed in nearly spherical shape. The elemental
composition of the CPS-MNPs-NNN-Pd catalyst was determined by EDX analysis.
The results shown in Figure 1d reveal that the Pd catalyst contains Fe, Si, Pd, N, C, Cl and O. Based
on the amount of 3.45% for N, it is possible to calculate the amount of the grafted NNN on the
surface of magnetic nanoparticles which is equal to 0.83 mmol per gram of the material. Amount of
Pd was also estimated to be 0.81 mmol/g of catalyst.
Figure 1. TEM pictures of the CPS-MNPs-NNN-Pd catalyst (a,b), SEM image of the CPS-MNPs-NNN-
Pd catalyst (c), EDX pattern of CPS-MNPs-NNN-Pd catalyst.
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In order to prove the successful functionalization of the MNPs, Fourier transform infrared
spectroscopy was used. A comparison between the FT-IR spectra of Fe , Fe @SiO , CPS-MNPs,
CPS-MNPs-NNN and CPS-MNPs-NNN-Pd catalyst are shown in Figure 2. The spectrum of Fe
Figure 2a) exhibits the absorption peaks for the Fe
microspheres at 607 cm−1 due to the
vibration of the Fe–O bond in the tetrahedral site of bare magnetic. The broad band at 3457 cm
is ascribed to the O–H stretching vibration arising from O–H bonds which are attached to the
surface iron atoms. The coating of silica layer on the surface of Fe is affirmed by two new bands
3
O
4
3
O
4
2
3
O
4
(
3
O
4
5
6
−1
3
O
4
-1
-1
at 1118 cm and 832 cm which are attributed to the symmetrical and asymmetrical vibrations of
the Si–O–Si bonds (Figure 2b).57
Figure 2. FT-IR spectra of (a) Fe
NNN-Pd catalyst.
3
O
4
, (b) Fe
3
O
4
@SiO , (c) CPS-MNPs-NNN ligand, (d) CPS-MNPs-
2
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In addition, on moving from Fe
3
O
4
to Fe
3
O
4
@SiO the intensity of the Fe–O and O-H vibration bands
2
is decreased significantly. In Figure 3c the presence of vibration bands at 3461 cm−1 (O-H
stretching), 2914-3012 cm−1 (CH stretching), 1657 cm−1 (C=N), 1490 cm−1 (C=C aromatic ring
stretching) confirms that the NNN Schiff base ligand is chemically bonded to the anchored propyl
groups on the surface of silica.58 Furthermore, on metallation with palladium (II) acetate, it is
−
1
observed that the prominent absorption peak of C=N at 1657cm is shifted to a lower frequency
−
1
(
1627 cm ) in Figure 3d, indicating the coordination of the metal with lone electron pair of
nitrogen group of C=N as a sigma donor ligand.
In order to investigate crystallinity, phase composition and purity of the synthesized nanoparticles
(
CPS-MNPs-NNN ligand and CPS-MNPs-NNN-Pd catalyst) and confirm their structural modification,
powder XRD studies were investigated. The XRD pattern of the CPS-MNPs-NNN ligand (Figure 3a)
shows the same characteristic peaks of Fe @SiO , which implies the stability of the crystalline
phase of nanoparticles during the subsequent surface modification. As presented in Figure 3a,3b
3
O
4
2
◦
◦
◦
◦
the reflection planes of (220), (311), (400), (422), (511) and (440) at 2θ = 30.3 , 35.7 , 43.4 , 53.1 ,
◦
◦
5
7.3 and 63.0 respectively, are in good agreement with face centered cubic Fe
3
O (JCPDS Card
4
no.19-0629).59-61 The XRD pattern of CPS-MNPs-NNN-Pd catalyst (Figure 3b) shows that apart from
◦
◦
◦
the original peaks, the appearance of the new peaks at 2θ=40.0 , 46.5 and 67.6 is attributed to the
Pd species indicating that the Pd nanoparticles have been successfully immobilized on the surface
of CPS-MNPs-NNN ligand.62 The average crysallite size, which is calculated using Scherrer’s
equation, is nearly matches with the average particle size seen in the TEM analysis.
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Figure 3. XRD patterns of CPS-MNPs-NNN ligand (a) and CPS-MNPs-NNN-Pd catalyst (b).
The magnetization measurements of CPS-MNPs-NNN ligand and CPS-MNPs-NNN-Pd catalyst were
investigated by a vibrating sample magnetometer (VSM) at room temperature in the applied
magnetic field from −8000 to +8000 Oe (Figure 4). As shown in Figure 4, the values of the
-1
-1
saturation magnetization are 22 emu g for CPS-MNPs-NNN ligand and 28 emug for CPS-MNPs-
NNN-Pd catalyst. The decrease in the saturation magnetization is due to the presence of Pd
immobilized on the surface of the CPS-MNPs-NNN ligand. This Figure also shows that despite this
decline in the saturation magnetization, the catalyst still can be competently and easily removed
from solution media with a permanent magnet .
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Figure 4. Magnetization curves obtained by VSM at room temperature for CPS-MNPs-NNN ligand
a) and CPS-MNPs-NNN-Pd catalyst (b).
(
Another technique that confirms the attachment of Schiff base ligand and Pd metal to the surface of
nano magnetic support is XPS. The peaks due to C, N, O, Si, Fe and Pd atoms are labeled in Figure 5a.
XPS can also determine the oxidation state of surface atoms. As can be seen from the high-
resolution Pd 3d XPS data (Figure 5b), the presence of Pd (0) is indicated by the characteristic peak
at 336.50 and 341.71 eV and for Pd (II) at 338.97 and 343.12 eV. The relative intensities of the Pd
3
d XPS lines show that majority of Pd in catalyst is in the Pd (0) form. The formation of Pd (0)
species is acquired via the reduction of Pd (II) to Pd (0); ethanol acts as a reducing agent during the
synthesis procedure. 63,64
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Figure 5. XPS spectrum of CPS-MNPs-NNN-Pd catalyst (a), high-resolution XPS Pd 3d spectrum (b).
Carbon-carbon-Catalyzed coupling reaction with CPS-MNPs-NNN-Pd catalyst
The catalytic applicability of the CPS-MNPs-NNN-Pd catalyst was investigated in the Heck and
Sonogashira coupling reactions. To explore the optimal reaction conditions, a series of
circumstances were evaluated including time duration, solvent, base, and temperature.
First, the catalytic activity of the CPS-MNPs-NNN-Pd catalyst was evaluated in the Heck reaction
between iodobenzene and n-butyl acrylate. In this situation, K
2
CO was used as base, Pd catalyst
3
and DMF as solvent (Table 1). In the presence of 0.5 mol% of catalyst after 0.4 h at 90 °C about 98%
of coupling product was isolated. Because of the awful environmental impact of DMF, we decided to
decrease the portion of DMF. Therefore, H O/DMF [2/1 (v/v), 2.5 mL] was chosen as a solvent
2
medium and coupling products were obtained in good yields. The yield reaction between
iodobenzene and n-butyl acrylate in water medium was 60% (Table 1, entry 2) and this yield was
improved to 85% by adding TBAB (tetrabutyl ammonium bromide) as a surfactant. (Table 1, entry
3
). To survey the influence of the solvent on the reaction yield, other solvents (e.g. EtOH, DMSO)
were also tested (Table 1, entries 4-8).
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Table 1 Optimization of the Heck–Mizoroki reaction between iodobenzene and n-butyl acrylate
catalyzed by CPS-MNPs-NNN-Pd catalyst.a
°
Yieldb
(%)
Entry
Solvent
DMF
Catalyst
Base
Temp. ( C)
amount (mg)
1
2
3
6
6
5
K
K
K
2
CO
2
CO
2
CO
3
3
3
90
90
90
98
60
H
H
2
O
O
c
2
85
4
5
6
H
2
O/DMF [1/2 (v/v)]
O/DMF [1/1 (v/v)]
6
6
6
K
2
CO
CO
3
3
90
90
90
97
H
2
K
2
95
H
2
O/DMF [2/1 (v/v)]
K
2
CO
3
95d
7
8
9
EtOH
DMSO
6
6
7
K
K
K
2
2
2
CO
CO
CO
3
Reflux
90
90
38
30
95
3
2
H O/DMF [2/1 (v/v)]
3
3
3
1
0
2
H O/DMF [2/1 (v/v)]
5
K
2
CO
CO
90
71
1
1
1
2
none
6
6
K
2
90
90
70
69
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O/DMF [2/1 (v/v)]
O/DMF [2/1 (v/v)]
O/DMF [2/1 (v/v)]
O/DMF [2/1 (v/v)]
O/DMF [2/1 (v/v)]
O/DMF [2/1 (v/v)]
O/DMF [2/1 (v/v)]
NEt
3
1
1
1
1
1
1
3
4
5
6
7
8
6
NaOH
90
100
80
85
95
90
45
95
83
86
94
---
6
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
3
3
3
3
3
6
6
6
------
aReaction conditions: iodobenzene (1 mmol), n-butyl acrylate (1.2 mmol), base (2 mmol), in solvent
b
c
d
(
2.5 mL) for 4 h. Isolated yields. 2 mmol of TBAB was used. The optimum conditions for Heck
reaction.
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Results showed that H O/DMF [2/1 (v/v)] is the superior solvent in comparison with other solvent
2
checked (Table 1, entry 6). In the cases of EtOH and DMSO the reaction yield was decreased
significantly. It seems that this result is referring to the ligation character of DMSO which may be
poison the active site of catalyst. Also, this solvent can coordinate with Pd complex and have many
affects on its catalytic activity.65
Also different bases including NEt
3
and NaOH were used and no improvement in the reaction yield
CO (Table 1, entry 6, 12, 13). The catalyst loading was also
were observed in comparison with K
2
3
investigated and 0.5 mol% of catalyst was chosen as optimum (Table 1, entry 6). It should be noted
that no conversion was observed in the absence of the catalyst. Thus, the optimum conditions for
CPS-MNPs-NNN-Pd catalyzed Heck reaction is entry 6 of Table 1. In order to explore the scope of
the reaction, several differently substituted aryliodides with various alkenes were examined
(
Scheme 2).
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Pd catalyst (0.5 mol%)
K2CO3
R2
X
+
R2
R = R, Ar, CO R
R1
o
H O/DMF, 90-100 C
2
R1
X = Cl, Br, I
2
2
3a-3r
O
O
O
O
O
O
3a
3b
3c
Br
X = I, 0.4h, 95%
X = Br, 5h, 85%
X = Cl, 12h, 80%
X = I, 2h, 87%
X =Br, 10h, 75%
X = I, 1h, 92%
X = Br, 8h, 70%
O
O
O
O
O
S
3
d
3e
N
3f
X = I, 2h, 93%
CN
X = Br, 10h, 70%
CN
O
X = I, 24h, 88%
CN
CN
3g
O
3i
3j
3h
X = I, 3h, 95%
X = Br, 24h, 89%
X = Cl, 36h, 70%
X = I, 5h, 94%
X = Br, 24h, 80%
X = I, 4h, 93%
X = Br, 24h, 83%
X = I, 5h, 90%
CN
CN
CN
S
3
m
3k
Br
3l
O N
2
X = I, 4h, 95%
X = Br, 24h, 85%
X = I, 3h, 95%
X = I, 7h, 95%
O
3p
3n
3o
X = I, 7h, 85%
X = Br, 24h, 80%
X = I, 10h, 72%
X = Br, 24h,67%
X = I, 11h, 70%
O N
3q
3s
2
3r
Br
X = I, 11h, 73%
X = I ,5h, 85%
X = I, 2h, 91%
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Scheme 2. Heck reaction of different aryl halides with alkenes under the optimized conditions.
Reaction conditions: aryl halide (1 mmol), alkene ( (1.2 mmol), K CO (2.0 mmol) in H O/DMF [2/1
v/v), 2.5 mL]. Yields are isolated.
2
3
2
(
As illustrated in Scheme 2, the presented protocol was demonstrated to be amenable for the
successful conversion of aryl iodides bearing either electron-withdrawing (3c, 3k, 3l, 3q, 3s) or
electron-donating groups (3b, 3f, 3h, 3i, 3j, 3o, 3p, 3r) to the corresponding coupled products in
good to excellent yields. The completing reaction of aryl bromides and chlorides was slower than
aryl iodides. In order to improve the reactions yield of bromo and chloroarenes, higher temperature
°
of 100 C is needed. The sterically hindered 2-iodoanisole reacted with styrene and acrylonitrile to
give the corresponding products in 85-90 % yield (3j, 3r). Since, heteroaromatic compounds are
very important and essential fragments in most of the drugs, the cross-coupling reaction of some
heterocyclic substrate were investigated (3d, 3e, 3m).
The CPS-MNPs-NNN-Pd catalyst was also employed in Sonogashira coupling reaction. Interestingly,
this coupling reaction was accomplished efficiently under the optimized conditions for Heck
reaction. Thus, the coupling of aryl halides (1 mmol) and phenylacetylene (1.2 mmol) was carried
°
out in H
2
O/DMF [2/1 (v/v), 2.5 mL] with the Pd catalyst (0.5 mol%, 0.006 g) at 90 C in the
presence of 2 mmol of K
2
CO . The reaction of phenylacetylene and iodobenzene generated the
3
corresponding product in good to excellent yields. The generality of this methodology was
investigated in other substrates and results demonstrate that high yield of diarylethynes could be
obtained using the CPS-MNPs-NNN-Pd catalyst (Scheme 3).
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Pd catalyst (0.5 mol%)
K CO
X +
2
3
R1
R1
o
4a
H O/DMF, 90 C
X = Cl, Br, I
2
5a-5e
5a
5b
5c
X = I, 7h, 91%
X = Br, 24h, 76%
X =Cl, 24h, 51%
X = I, 9h, 90%
X = Br, 24h, 70%
X = I, 11h, 80%
O N
2
N
5f
5d
5e
X = I, 10h, 76%
X = Br, 24h, 54%
X =Br, 24h, 82%
X = I, 6h, 92%
X = Br, 24h, 80%
Scheme 3. Summary of the Sonogashira coupling reaction of aryl halides with phenylacetylene
under the optimized conditions. Reactions conditions: aryl halides (1.0 mmol), phenylacetylene (1.2
mmol), K
2
CO
3
(2mmol), H O/DMF [2/1 (v/v), 2.5 mL]. Yields are isolated.
2
Heterogeneity tests of CPS-MNPs-NNN-Pd catalyst
Recyclability is a crucial parameter to evaluate the heterogeneity of a supported catalyst in organic
synthesis. Some of the supported catalysts are not reusable and this is not heterogeneous in nature.
Also some of the supported catalysts have tedious work up procedure and after recovery a
remarkable decreasing in catalyst efficiency is observable. However, magnetic reusable catalysts
show better reusable capability and simple work up process. In our catalytic system, due to the
magnetic nature of the CPS-MNPs-NNN-Pd catalyst, it could be recovered from the reaction mixture
by magnetic separation. The recycling performance of CPS-MNPs-NNN-Pd catalyst was determined
by its utilization in Heck reaction of iodobenzene with n-butyl acrylate under the optimized
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conditions. After separation of the catalyst from the reaction mixture using an external magnet, it
was washed with ethanol, dried and then directly used in the next run. As shown in Figure 6, our Pd
catalyst system could be recycled at least 5 times without a significant loss in its catalytic activity.
9
5%
95%
9
3%
93%
9
2%
Figure 6. Recycling of the CPS-MNPs-NNN-Pd catalyst in the Heck reaction of iodobenzene and n-
butyl acrylate under optimized conditions.
To show the advantages and the merit of the proposed catalyst in comparison with other reported
Pd catalysts, we summarized some of results for Heck coupling reaction of iodobenzene and n-butyl
acrylate in Table 2. As shown in Table 2, CPS-MNPs-NNN-Pd is more efficient catalyst with respect
to reaction condition, time and yield than some of the previously reported ones.
Table 2. Comparison of the present methodology with other reported catalysts in the Heck
Coupling reaction of iodobenzene and n-butyl acrylate.
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c
-1
Entry [Ref.]
[This work] CPS-MNPs-NNN-Pd
Catalyst
Reaction conditions Time (h) Yield (%)a TONb
TOF (h )
475
235
2
16
4
6.5
9.2
°
1
2
3
4
5
6
7
K
K
Bu
Et
2
CO
CO
3
, H
, H
2
2
O/DMF, 90 C, 0.5 mol% Pd 0.4
95
94
96
97
99
90
92
190
235
48
48.5
19.8
19.5
92
°
[52]
[66]
[67]
[68]
[69]
[70]
Tetraimine Pd(0)
Pd-dendr
Pd-SiO
Pd/Fe
Pd–ZnFe
2
3
O, 90 C, 0.4 mol% Pd
1
°
3
N, THF, 60 C, 2 mol% Pd
N,MeCN,130 C, 2 mol% Pd
24
3
°
2
3
°
3
O
4
K
2
CO
3
,NMP, 130 C, 5 mol% Pd
5
°
2
O
4
MNP
Et
Et
3
N, DMF, 120 C, 4.62 mol% Pd
3
°
TiO
2
@Pd NPs
3
N, DMF, 140 C, 1 mol% Pd
10
8
°
8
9
[71]
[72]
PdCl
2
K
2
CO
3
, H
2
O, 80-100 C, 1.5 mol% Pd
92
97
61.3
37.9
7.7
°
Pd-SMTU -boehmite
Na CO
2
3
, PEG, 120 C, 2.56 mol% Pd
0.3
114.8
a
b
c
-1
Isolated yield, TON = mol product/mol cat, TOF = (mol product/mol cat) h .
The reused catalyst was investigated using ICP analysis and negligible amount of Pd leaching was
observed (0.3 ppm). The morphology of the catalyst after recovery was investigated using TEM
image and it was shown that the morphology and nano structure of the catalyst remained
unchanged after four run, and only some aggregation is occurred (Figure 7). These images reveal
that the size and shape of the Pd catalyst almost do not change.
Figure 7. TEM image of CPS-MNPs-NNN-Pd catalyst after four cycles in the Heck–Mizoroki reaction
of iodobenzene with n-butyl acrylate.
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These experiments proved that our Pd catalyst system is stable and efficient during the catalytic
cycles indicating that the CPS-MNPs-NNN-Pd catalyst is heterogeneous in practice.
Conclusions
To sum up, CPS-MNPs-NNN-Pd complex acts as an efficient, economic and green catalyst that shows
an extraordinary robustness, air or moisture stability and catalytic activity in Heck/Sonogashira C–
C coupling reactions in the presence of only 0.5 mol% of supported Pd catalyst and generated the
corresponding products in good to excellent yields. This catalyst is prominent due to the efficient
recoverability for at least 5 times in consecutive runs with negligible loss of its reactivity. More
significantly, the paramagnetic property provides the possibility of simple separation of the catalyst
by a convenient magnetic separation technique. We believe that the simplicity of the design of this
catalyst, in combination with its great catalytic activity and recyclability, may inspire future
research in the pharmaceutical field and practical applications. Also, further work is underway to
extend more applications of this catalyst to other chemical transformations.
Acknowledgements
The authors thank the financial support of this work by the Research Council of Shiraz University.
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1
337x842mm (144 x 144 DPI)