Hydrophilic role of deep eutectic solvents for clean synthesis of biphenyls over a magnetically separable Pd-catalyzed Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1016/j.molliq.2020.115078

The development of an efficient and sustainable catalytic system for the preparation of biphenyls through the Suzuki-Miyaura coupling reaction is still a great challenge to green chemistry. Encouraging the prevailing challenge, in the present work, a heterogeneous Pd catalyst was synthesized through a green method and used for the production of biphenyls in deep eutectic solvents (DESs) as green reaction media. In order to prepare the catalyst, magnetite-graphene oxide nanocomposite was modified with cellulose via the click reaction and applied as support for Pd nanoparticles. Cellulose acted as both reducing and stabilizing agent for Pd nanoparticles and eliminated the requirement of a reducing agent. The prepared catalyst was characterized by different methods such as FT-IR, EDX, EDX-mapping, XPS, SEM, TEM, XRD, VSM, and ICP-OES analyses. Catalytic properties of the obtained catalyst was explored in the coupling reaction of aryl halides with aryl boronic acids in different hydrophilic and hydrophobic DESs. The presence of cellulose with hydrophilic character on the structure of catalyst offered well dispersion of the catalyst in hydrophilic DESs, which led to enhancement of its catalytic activity. Among various hydrophilic DESs, the DES composed of dimethylammonium chloride and glycerol was verified as the most effective solvent for the preparation of biphenyls. The catalyst was compatible with a variety of substrates, with which all the Suzuki coupling products were achieved in high to excellent yields. Thanks to the low solubility of catalyst and DES in organic solvents, the separated aqueous phase containing both of the catalyst and DES could be readily recovered by evaporating water and reused up to five successive runs with a stable activity. This simple and new separation strategy provided a clean and highly efficient synthetic methodology for the synthesis of various biphenyls. Moreover, hot filtration test efficiently confirmed that the catalyst is heterogeneous and completely stable under reaction conditions.

Full text of DOI:10.1016/j.molliq.2020.115078

Journal Pre-proof
Hydrophilic role of deep eutectic solvents for clean synthesis
of biphenyls over a magnetically separable Pd-catalyzed Suzuki-
Miyaura coupling reaction
Mahsa Niakan, Majid Masteri-Farahani, Sabah Karimi, Hemayat
Shekaari
PII:
S0167-7322(20)37320-7
DOI:
https://doi.org/10.1016/j.molliq.2020.115078
MOLLIQ 115078
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
15 September 2020
25 November 2020
12 December 2020
Please cite this article as: M. Niakan, M. Masteri-Farahani, S. Karimi, et al., Hydrophilic
role of deep eutectic solvents for clean synthesis of biphenyls over a magnetically
separable Pd-catalyzed Suzuki-Miyaura coupling reaction, Journal of Molecular Liquids
(2020), https://doi.org/10.1016/j.molliq.2020.115078
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2020 Published by Elsevier.

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Hydrophilic role of deep eutectic solvents for clean synthesis of biphenyls over
a magnetically separable Pd-catalyzed Suzuki-Miyaura coupling reaction
a,b
a,b,*
c
c
Mahsa Niakan , Majid Masteri-Farahani
, Sabah Karimi , Hemayat Shekaari
a
Faculty of Chemistry, Kharazmi University, Tehran, Iran
b
Research Institute of Green Chemistry, Kharazmi University, Tehran, Iran
c
Department of Physical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
E-mail address: mfarahani@khu.ac.ir
Abstract
The development of an efficient and sustainable catalytic system for the preparation of biphenyls
through the Suzuki-Miyaura coupling reaction is still a great challenge to green chemistry.
Encouraging the prevailing challenge, in the present work, a heterogeneous Pd catalyst was
synthesized through a green method and used for the production of biphenyls in deep eutectic
solvents (DESs) as green reaction media. In order to prepare the catalyst, magnetite-graphene
oxide nanocomposite was modified with cellulose via the click reaction and applied as support
for Pd nanoparticles. Cellulose acted as both reducing and stabilizing agent for Pd nanoparticles
and eliminated the requirement of a reducing agent. The prepared catalyst was characterized by
different methods such as FT-IR, EDX, EDX-mapping, XPS, SEM, TEM, XRD, VSM, and ICP-
OES analyses. Catalytic properties of the obtained catalyst was explored in the coupling reaction
of aryl halides with aryl boronic acids in different hydrophilic and hydrophobic DESs. The
presence of cellulose with hydrophilic character on the structure of catalyst offered well
dispersion of the catalyst in hydrophilic DESs, which led to enhancement of its catalytic activity.
Among various hydrophilic DESs, the DES composed of dimethylammonium chloride and
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glycerol was verified as the most effective solvent for the preparation of biphenyls. The catalyst
was compatible with a variety of substrates, with which all the Suzuki coupling products were
achieved in high to excellent yields. Thanks to the low solubility of catalyst and DES in organic
solvents, the separated aqueous phase containing both of the catalyst and DES could be readily
recovered by evaporating water and reused up to five successive runs with a stable activity. This
simple and new separation strategy provided a clean and highly efficient synthetic methodology
for the synthesis of various biphenyls. Moreover, hot filtration test efficiently confirmed that the
catalyst is heterogeneous and completely stable under reaction conditions.
Keywords: Cellulose; Magnetite-graphene oxide nanocomposite; Pd catalyst; Click reaction;
Suzuki-Miyaura reaction; Deep eutectic solvent.
1
. Introduction
The catalytic cross-coupling reaction between organoboronic acids and aryl halides in the
presence of palladium (Pd) compounds, is a powerful tool for the synthesis of biphenyl structures
as essential feedstocks for the production of pharmaceuticals, natural products, polymers, and
agrochemicals [1-3]. Compared to homogeneous Pd catalysts, heterogeneous counterparts have
been accepted as environmentally friendly alternatives owing to their better recoverability and
easier separation [4,5]. In this regard, the homogeneous Pd catalysts have been immobilized on
various solid supports making them heterogeneous in nature [6-8]. Recently, cellulose has
received significant attention as support for developing heterogeneous Pd catalyst systems owing
to its outstanding properties such as abundant availability, non-toxicity, biodegradability,
environmental friendly, and low cost [9-15]. Also, abundant hydroxyl groups in cellulose
backbone can act as both reducing and stabilizing agents for many metal nanoparticles and
therefore eliminate the requirement of a reducing agent or stabilizer [16,17]. However, the
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separation and recycling of cellulose-based Pd catalysts is challenging due to the dispersion
stability of cellulose in different solvents [18]. Consequently, the development of novel routes
for the synthesis of cellulose-based Pd catalysts with good recyclability is still necessary.
Modification of easily separable magnetic graphene oxide nanocomposite with cellulose is an
effective approach to prepare a versatile and promising support for immobilization of Pd
catalysts [19,20]. Among the numerous methods for the modification of magnetic graphene
oxide nanocomposite, the azide-alkyne cycloaddition in the presence of copper as catalyst is an
attractive modification method and offers several advantages including high yields and
selectivity under mild conditions [21,22].
Over the past few decades, researchers were encourage to develop cleaner processes as
alternatives to traditional chemical syntheses and transformations by the application of
environmentally friendly solvents [23-25]. Currently, the utilization of deep eutectic solvents
(DESs) as reaction media has attracted a growing attention in organic syntheses. They are eco-
friendly liquids obtained by reacting a hydrogen bond acceptor and a hydrogen bond donor,
which self-associate via reciprocal hydrogen-bonding to form a new eutectic phase with a
melting point below those of the initial components [26]. Compared to common organic
solvents, DESs are cheap, renewable, easy to prepare and recycle, biodegradable, and non-toxic
[
27,28]. Therefore, they have made a remarkable impact on clean synthesis in recent years.
Although DESs are favorable green solvents, the application of DESs in Suzuki- Miyaura
coupling reaction is in its infancy and only a few works have been reported to date [29-34].
From the foregoing, we decided to prepare a novel heterogeneous catalyst with immobilizing Pd
nanoparticles on clicked cellulose-modified magnetite-graphene oxide nanocomposite and
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investigate its catalytic properties in the Suzuki-Miyaura coupling reaction in DESs as
sustainable and environmentally benign reaction media for the clean synthesis of biphenyls.
2
2
2
. Experimental
.1. Catalyst preparation
.1.1. Preparation of azide-functionalized Fe O /GO (Fe O /GO@N )
3
4
3
4
3
GO was synthesized from graphite by a modified Hummers method as described elsewhere [35].
Magnetite-graphene oxide nanocomposite (Fe O /GO) was synthesized by using a co-
3
4
precipitation method according to the literature procedure [36]. Fe O /GO (1.0 g) was uniformly
3
4
dispersed into 40 mL of dry toluene with the help of ultrasound for 30 min. Afterwards, 3-
chloropropyltriethoxysilane (3-CPTES, 3 mL) was added and the reaction mixture was refluxed
for 24 h under nitrogen gas. The obtained Fe O /GO@Cl was isolated from the reaction mixture
3
4
with applying the magnet and rinsed repeatedly with ethanol before being vacuum dried at 80 °C
for 8 h. Then, 1.0 g of Fe O /GO@Cl was ultrasonically dispersed in a solution containing 2.0 g
3
4
of NaN in 50 mL of dimethylformamide (DMF) and the mixture was stirred at 80 °C for 10 h.
3
The obtained precipitate was collected magnetically, rinsed with DMF, and vacuum dried at 80
°
C for 12 h to give Fe O /GO@N .
3 4 3
2
.1.2. Preparation of alkyne-functionalized cellulose
Alkyne-functionalized cellulose was synthesized based on the previously reported procedure
37]. 1.0 g of cellulose was suspended in 100 mL of basic aqueous solution (1.1 wt % NaOH) at
[
room temperature for 1 h to allow it swell sufficiently and increase the accessibility of hydroxyl
groups to chemical reagents. The reaction medium was progressively heated up to 60 °C over 1 h
and then the required quantity of propargyl bromide was rapidly added. After stirring at 60 °C
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for 24 h, the mixture was filtered and washed with ethanol and water. After drying at 60 °C,
alkyne-functionalized cellulose was obtained and characterized by FT-IR spectroscopy (Figure
S1).
2
.1.3. Preparation of cellulose-modified Fe O /GO via click reaction (Fe O /GO@CL)
3 4 3 4
Click reaction was applied to conjugate the alkyne-functionalized cellulose with Fe O /GO@N .
3
4
3
Briefly, a mixture of well dispersed Fe O /GO@N (1.0 g), alkyne-functionalized cellulose (2.0
3
4
3
g), copper (II) sulfate (0.1 g), and sodium ascorbate (2.0 g) in 60 mL of dry DMF was stirred at
0 °C for 24 h. After cooling the reaction mixture to ambient temperature, the product was
5
isolated with the help of a magnet. It was rinsed with DMF and deionized water before vacuum
drying at 80 °C for 12 h.
2
.1.4. Immobilization of Pd nanoparticles on Fe O /GO@CL
3 4
Palladium acetate (0.5 mmol, 0.11 g) was added into a mixture of Fe O /GO@CL (1.0 g) in 50
3
4
mL of absolute ethanol and ultrasonicated for 30 min. After stirring for 24 h at room
temperature, the black product was isolated by an external magnet, rinsed with ethanol, and
vacuum dried at 80 °C for 8 h to afford Fe O /GO@CL-Pd.
3
4
2
.2. Preparation of DESs
DESs were synthesized according to the previously described procedure [38-40]. In brief, a
mixture of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) with the molar ratio
given in Table 1 was stirred at given temperature until a colorless solution was achieved.
Table 1. Synthesis parameters for DESs.
DES
HBA
HBD
Molar ratio
T (°C)
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ChCl:Urea
ChCl:CA
Choline chloride
Urea
1:2
1:1
1:1
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
70
80
80
80
80
80
80
80
90
90
90
90
Choline chloride
Choline chloride
Citric acid
Malonic acid
Glycerol
ChCl:MA
ChCl:Gly
Choline chloride
DMAC:Urea
DMAC:CA
DMAC:MA
DMAC:Gly
TBAB:OA
TBAB:DA
TOMAC:OA
TOMAC:DA
Dimethylammonium chloride
Dimethylammonium chloride
Dimethylammonium chloride
Dimethylammonium chloride
Tetrabutylammonium bromide
Tetrabutylammonium bromide
Trioctylmethylammonium chloride
Trioctylmethylammonium chloride
Urea
Citric acid
Malonic acid
Glycerol
Octanoic acid
Decanoic acid
Octanoic acid
Decanoic acid
2
.3. Suzuki-Miyaura cross-coupling reaction in the presence of Fe O /GO@CL-Pd
3 4
A mixture of aryl boronic acid (1.1 mmol), aryl halide (1.0 mmol), K CO (2.0 mmol), and
2
3
Fe O /GO@CL-Pd (0.005 g, 0.2 mol%) in 3 mL of DES (DMAC:Gly) was stirred at 80 °C for
3
4
given time. The extent of reaction was checked using thin layer chromatography. At the end of
reaction, the mixture diluted with 15 mL of water and extracted with ethyl acetate (3 × 10 mL).
After removing the volatiles from the organic fraction, the residue was purified by column
chromatography on silica gel with an eluent consisting of ethyl acetate and n-hexane to achieve
1
the desired product. The products were characterized by H NMR and melting point followed by
comparison with authentic standard samples. The aqueous phase, which contained both of the
catalyst and DES, was used for another run after evaporation of water.
3
. Results and discussion
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3
.1. Preparation and characterization of Fe O /GO@CL-Pd
3
4
The catalyst was prepared according to the synthetic route described in Scheme 1. GO was first
prepared via modified Hummers method and then modified with magnetite nanoparticles to
achieve Fe O /GO. Afterwards, Fe O /GO was organo-functionalized by the condensation
3
4
3
4
reaction of 3-CPTES and hydroxyl groups on the surface of Fe O /GO, followed by the
3
4
substitution reaction of chloro groups with azide ions to produce Fe O /GO@N . The prepared
3
4
3
Fe O /GO@N was subsequently reacted with pre-synthesized alkyne-functionalized cellulose to
3
4
3
give Fe O /GO@CL through click reaction. Finally, treatment of Fe O /GO@CL with Pd(OAc)
2
3
4
3
4
in absolute ethanol provided the desired catalyst (Fe O /GO@CL-Pd). Based on inductively
3
4
coupled plasma optical emission spectroscopy (ICP-OES) analysis, the amount of Pd loaded on
-1
Fe O /GO@CL-Pd was found to be 0.41 mmol.g .
3
4
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Scheme 1. Synthetic route for the preparation of Fe O /GO@CL-Pd.
3
4
Qualitative identification of functional groups present in the products formed in each step was
performed by recording FT-IR spectra (Figure 1). The peaks at 3400, 1715, 1607, 1225, and
-1
1
052 cm in the FT-IR spectrum of GO can be attributed to the stretching vibrations of O-H,
C=O, C=C, C-O-H, and C-O-C epoxy groups, respectively [41]. The formation of Fe O /GO
3
4
-1
nanocomposite was affirmed by the observation of two peaks at 510 and 630 cm due to the Fe-
-1
O stretching vibrations in Fe O nanoparticles [42]. A new peak at 2175 cm in the spectrum of
3
4
Fe O /GO@N , was due to the stretching vibration of azide (-N ) groups, verifying their
3
4
3
3
-1
attachment on the surface of Fe O /GO. Besides, the peaks observed at 2800-3000 cm was
3
4
assigned to the stretching vibrations of the alkyl chains (C-H) in silylating agent. However, the
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-1
characteristic peak of azide groups at 2000-2200 cm was eliminated in the spectrum of
Fe O /GO@CL upon formation of triazole rings during the click reaction of Fe O /GO@N with
3
4
3
4
3
alkyne-functionalized cellulose. Such changes have also been observed by other researchers
-1
[
43,44]. In parallel, the intensity of O-H stretching vibrations at 3420 cm increased significantly
because of the presence of abundant hydroxyl groups in cellulose structure. The results clearly
confirmed the successful attachment of cellulose to Fe O /GO via click reaction. The spectrum
3
4
of Fe O /GO@CL-Pd exhibited almost no change after immobilization of Pd nanoparticles on
3
4
the surface of Fe O /GO@CL.
3
4
Figure 1. FT-IR spectra of GO, Fe O /GO, Fe O /GO@N , Fe O /GO@CL, and Fe O /GO@CL-Pd.
3
4
3
4
3
3
4
3
4
Energy dispersive X-ray (EDX) analysis was done to explore the surface chemical composition
of the catalyst. All the expected elements C, N, O, Si, Fe, and Pd were observed in the EDX
spectrum of Fe O /GO@CL-Pd (Figure 2). Furthermore, the homogeneous dispersion of these
3
4
elements throughout the catalyst surface was demonstrated by the corresponding EDX elemental
mapping images of Fe O /GO@CL-Pd (Figure S2).
3
4
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Figure 2. EDX spectrum of Fe O /GO@CL-Pd.
3
4
To further elucidate the chemical composition of the catalyst, X-ray photoelectron spectroscopy
XPS) was conducted as exhibited in Figure 3. The full XPS spectrum of Fe O /GO@CL-Pd
(
3
4
exhibited peaks for Si 2s, Si 2p, C 1s, Pd 3d, N 1s, O 1s, and Fe 2p, which approved the
successful synthesis of the catalyst. The high-resolution XPS spectrum of Pd 3d showed doublet
peaks at the binding energies of 335.67 and 341.18 eV due to Pd 3d5/2 and Pd 3d , respectively,
3/2
2+
confirming the zero oxidation state of Pd [45]. Additionally, no peaks corresponding to the Pd
2
+
oxidation state were found, indicating the efficient reduction of the Pd ions into metallic Pd
nanoparticles during the catalyst synthesis. From these results, it can be concluded that cellulose
2
+
component not only reduced the Pd ions but also stabilized Pd nanoparticles and prevented
from their oxidation.
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0

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Figure 3. XPS spectrum of Fe O /GO@CL-Pd. Inset: high-resolution XPS spectrum of Pd 3d core-level.
3
4
The morphologies and structures of the prepared materials were studied by scanning electron
microscopy (SEM) method and the images were presented in Figure 4. The pristine GO had a
transparent, wrinkled, and lamellar morphology (Figure 4a). Compared to the pristine GO,
Fe O /GO (Figure 4b) showed the presence of Fe O nanoparticles on the surface of GO
3
4
3
4
nanosheets, which was consistent with previous reports [46,47]. As can be seen in Figure 4c,
Fe O /GO@CL-Pd also exhibited a similar morphology to Fe O /GO.
3
4
3
4
Transition electron microscopy (TEM) technique was employed for further investigation of the
structural properties of the prepared samples and the results were depicted in Figure 4. The
layered and sheet-like structure characteristic of GO structure was seen in the TEM image of the
pristine GO (Figure 4d). After incorporation of magnetite, Fe O nanoparticles with diameter of
3
4
8
-10 nm and spherical morphology were well distributed on the surface of GO nanosheets
without obvious aggregation as illustrated in the TEM image of Fe O /GO (Figure 4e). The TEM
3
4
image of Fe O /GO@CL-Pd (Figure 4f) revealed that the structure and morphology of Fe O /GO
3
4
3
4
nanocomposite was well retained during the synthesis of the catalyst.
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1

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Figure 4. SEM images of (a) GO, (b) Fe O /GO, and (c) Fe O /GO@CL-Pd, and TEM images of (d) GO,
3
4
3
4
(
e) Fe O /GO, and (f) Fe O /GO@CL-Pd.
3
4
3
4
The X-ray diffraction (XRD) patterns of GO and Fe O /GO@CL-Pd were recorded to explore
3
4
their crystalline structures (Figure 5). A relatively strong diffraction peak at 2θ = 11.3°
corresponding to (001) plane was observed for GO which disappeared in Fe O /GO@CL-Pd,
3
4
likely owing to the incorporation of magnetite nanoparticles between the nanosheets of GO.
Similar phenomenon have also been reported by other researchers preparing magnetite-graphene
oxide nanocomposite [48]. The XRD pattern of Fe O /GO@CL-Pd exhibited peaks at 2θ = 30°,
3
4
3
5°, 43°, 53°, 57°, and 63° that were associated with the reflections of the (220), (311), (400),
(
422), (511), and (440) planes of Fe O with face centered cubic structure (JCPDS Card no. 19-
3
4
0
629) [49]. Also, the peaks detected at 40°, 46°, and 67° which corresponded to the (111), (200),
2
+
and (220) planes of metallic Pd, definitely indicated the successful reduction of the Pd ions into
Pd nanoparticles [45]. The XRD peaks of Pd nanoparticles appeared in very low intensity. Such
a feature, which was also reported by other research groups [50,51], can be due to the small size
of Pd nanoparticles and their well-dispersion on the surface of Fe O /GO@CL.
3
4
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Figure 5. XRD patterns of GO and Fe O /GO@CL-Pd.
3
4
The magnetic properties of Fe O /GO and Fe O /GO@CL-Pd were evaluated by vibrating
3
4
3
4
sample magnetometry (VSM). As depicted in Figure 6, neither coercivity nor remanence was
observed in the magnetization curves of both samples, indicating their superparamagnetic
-1
behavior. The saturation magnetization (M ) value of Fe O /GO@CL-Pd (19.8 emu g ) was
s
3
4
-1
lower than that of Fe O /GO (37.1 emu g ) owing to the less magnetic source component
3
4
(
Fe O ) per gram in the catalyst. Nevertheless, the catalyst still possessed high magnetic
3 4
response and could be efficiently separated from a solution with the help of a magnet (inset
photograph in Figure 6).
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Figure 6. Magnetization curves of Fe O /GO and Fe O /GO@CL-Pd.
3
4
3
4
3
.2. Catalytic performance of Fe O /GO@CL-Pd in the Suzuki-Miyaura cross-coupling
3 4
reaction
After complete characterization of Fe O /GO@CL-Pd, its catalytic efficiency was explored in
3
4
the preparation of biphenyls by the Suzuki-Miyaura cross-coupling reaction. This reaction is
generally performed with a phase transfer catalyst in organic solvents. Hence, any improvement
of biphenyl synthesis, mainly from a view of clean reaction, is desirable.
The coupling reaction of 4-iodoanisole with phenyl boronic acid was adopted to find the
optimized conditions, and the obtained data were concluded in Table 2. Since the main objective
of this work was to find a clean route to synthesize biphenyls, we were interested to use DESs as
reaction media. Consequently, the reactions were first screened with various hydrophilic and
hydrophobic DESs at 90 °C using 0.3 mol% of the catalyst. The catalyst showed higher
efficiency in hydrophilic DESs (Table 2, entries 1-8) than hydrophobic one (Table 2, entries 8-
1
2). This might be attributed to the hydrophilic nature of the catalyst that offered more dispersion
in hydrophilic DESs and thus improved its catalytic activity. Among the various hydrophilic
DESs used in the reaction, DMAC:Gly afforded the highest product yield.
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Under the optimized reaction solvent, the effect of using different bases like K CO , Cs CO ,
2
3
2
3
Et N, NaOH, Na CO , KOH, and K PO on the model reaction was examined (Table 2, entries
3
2
3
3
4
1
2-18). Although K CO and Cs CO revealed comparable results, we adopted K CO as the
2 3 2 3 2 3
optimal base because of its economic material benefit. Then, the reaction temperature in the
range of 70-100 °C was optimized, and the results displayed that 80 °C was the best choice
(Table 2, entries 19-21). Finally, the influence of catalyst loading was studied and it was found
that 0.2 mol% of the catalyst was appropriate for reaction to proceed (Table 2, entries 22-24). It
was found that in the absence of catalyst no reaction took place confirming the essential role of
the catalyst in this reaction (Table 2, entry 25). Consequently, it was decided to use DMAC:Gly
as reaction medium, K CO as base, 0.2 mol% of the catalyst at 80 °C as the optimized reaction
2
3
conditions for further experiments.
Table 2. Optimization conditions for the Suzuki-Miyaura coupling reaction using Fe O /GO@CL-Pd
3
4
a
catalyst.
b
Entry Catalyst (mol%)
DES
Base
T (°C) Time (min) Yield (%)
1
2
3
4
5
6
7
0.3
0.3
0.3
0.3
0.3
0.3
0.3
ChCl:Urea
ChCl:Gly
ChCl:CA
K CO3
90
90
90
90
90
90
90
30
30
30
30
30
30
30
82
88
67
74
86
93
72
2
K CO3
2
K CO3
2
ChCl:MA
DMAC:Urea
DMAC:Gly
DMAC:CA
K CO3
2
K CO3
2
K CO3
2
K CO3
2
1
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8
9
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.4
0.2
0.1
-
DMAC:MA
K CO3
90
90
90
90
90
90
90
90
90
90
90
100
80
70
80
80
80
80
30
90
90
90
90
30
120
120
45
60
30
30
30
45
30
30
45
300
81
52
63
59
71
95
53
47
69
82
86
94
93
91
94
93
89
0
2
TBAB:OA
TBAB:DA
K CO3
2
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
K CO3
2
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
TOMAC:OA
TOMAC:DA
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
DMAC:Gly
K CO3
2
K CO3
2
Cs CO3
2
Et N
3
NaOH
Na CO3
2
KOH
K PO4
3
K CO3
2
K CO3
2
K CO3
2
K CO3
2
K CO3
2
K CO3
2
K CO3
2
a
Reaction conditions: 4-iodoanisole (1.0 mmol), phenyl boronic acid (1.1 mmol), base (2.0 mmol), and
DES (3 mL).
b
Isolated yields.
1
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To survey the catalytic performance of Fe O /GO@CL-Pd, the reaction of different aryl boronic
3
4
acids and aryl halides was evaluated under the optimized reaction conditions and the data were
illustrated in Scheme 2.
Phenyl boronic acid were efficiently reacted with some aryl iodides and bromides containing
electron-donating or electron-withdrawing substituents in high yields (2a-l). Generally, aryl
iodides revealed higher reactivity compared to aryl bromides. Moreover, the ortho-substituted
aryl halides reacted slightly slower than para-substituted aryl halides owing to the steric effect.
Para-substituted aryl boronic acids also coupled with iodo- and bromobenzene in excellent yields
(2m, n). Besides, a heterocyclic aryl iodide (iodothiophene) was used and the corresponding
coupling product was produced in good yield (2o). Notably, no product arising from the
homocoupling reaction of aryl boronic acids was observed in all of the examined reactions.
Overall, all the reactions were performed in the absence of organic solvents as well as other
additives and high yields of different biphenyls were synthesized with a clean route.
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Scheme 2. Suzuki-Miyaura coupling reaction of various aryl halides and aryl boronic acids
using Fe O /GO@CL-Pd catalyst. Reaction conditions: aryl halide (1.0 mmol), aryl boronic
3
4
acid (1.1 mmol), K CO (2.0 mmol), and DMAC:Gly (3 mL). All yields are isolated.
2
3
Based on the reported literature [52], a probable mechanism for the Suzuki-Miyaura coupling
reaction in the presence of Fe O /GO@CL-Pd was proposed in Scheme 3. At first, the oxidative
3
4
addition of aryl halide to Pd nanoparticles immobilized on Fe O /GO@CL took place. In the
3
4
1
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next step, another aryl ring from aryl boronic acid attached to Pd nanoparticles through a
transmetalation process. Finally, the coupling product was formed in a reductive elimination with
regeneration of the catalyst.
Scheme 3. Plausible reaction mechanism for the Suzuki coupling reaction catalyzed by Fe O /GO@CL-
3
4
Pd.
3
.3. Reusability and stability of Fe O /GO@CL-Pd
3 4
Heterogeneous nature of Fe O /GO@CL-Pd was confirmed by performing a hot filtration test for
3
4
the reaction of phenyl boronic acid and 4-bromoanisole under the optimized conditions. The
catalyst was magnetically isolated after 30 min and the reaction was further continued for 5 h at
the same temperature of 80 °C. After removing the catalyst, no increase in the product yield was
observed indicating that the catalytic process was heterogeneous.
It is essential to explore the reusability of a heterogeneous catalyst and also DES medium. In this
regard, the recoverability and reusability of Fe O /GO@CL-Pd and DES (DMAC:Gly) was
3
4
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evaluated in the Suzuki-Miyaura reaction of phenyl boronic acid and 4-iodoanisole. The
hydrophilic nature of the catalyst and DES provided a simple and clean procedure for their
recovering. At the end of each reaction run, a biphasic system was formed upon addition of water
and ethyl acetate to the mixture. The organic phase was collected for further product purification
and analysis and the aqueous phase possessing both of the catalyst and DES was recovered by
evaporating water under vacuum and recharged with fresh coupling partners and K CO for the
2
3
subsequent run. Both of the catalyst and DES were found to be efficiently reused at least five
times and the product yield remained as 90% (Figure 7).
Figure 7. Recovery of Fe O /GO@CL-Pd and DES (DMAC:Gly) in the Suzuki-Miyaura reaction of 4-
3
4
iodoanisole and phenyl boronic acid under the optimized reaction conditions.
To verify the durability of the catalyst, Fe O /GO@CL-Pd was analyzed by ICP, FT-IR, SEM,
3
4
and VSM techniques after five sequential recoveries. The Pd amount of the used catalyst was
-1
measured to be 0.37 mmol.g by ICP, which was very close to that of the fresh catalyst (0.41
-1
mmol.g ). This revealed that Fe O /GO@CL is an effective support for immobilizing Pd
3
4
nanoparticles and can protect them from leaching during catalytic cycles. No significant change
was found in the FT-IR spectrum, SEM image, and magnetization value of the used catalyst as
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compared to the fresh one (Figure S3a-c). These results clearly confirmed the high stability and
recyclability of Fe O /GO@CL-Pd.
3
4
3
.4. Comparison of the catalytic performance of Fe O /GO@CL-Pd with earlier reports
3 4
To highlight the advantages of Fe O /GO@CL-Pd, its catalytic performance was compared with
3
4
earlier Pd-based catalysts reported for reaction of phenyl boronic acid and 4-bromoanisole. As
shown in Table 3, Fe O /GO@CL-Pd exhibits better results than other listed catalysts including
3
4
high yield in less reaction time, environmental friendly and green conditions. More importantly,
most of the reported catalytic systems suffer from time-consuming workup process and difficulty
of separation, recovery, and reusability of the catalyst, which make them uneconomical and even
unclean systems.
Table 3. Comparison of the catalytic performance of Fe O /GO@CL-Pd with other reported Pd-based
3
4
catalysts in the Suzuki- Miyaura reaction of 4-bromoanisole and phenyl boronic acid.
Catalyst (mol%)
Reaction conditions
DES (ChCl:EG
Time
12 h
Yield (%) Ref
Pd-DNA-Fe O (2.8)
71.4
[30]
[33]
[34]
[29]
[31]
3
4
(
1:2)), K CO , 80 °C
2 3
Fe O @PFC-Pd(0) (0.24)
DES (K CO :glycerol 50 min
2 3
93
3
4
(
1:5)), 70 °C
DES (ChCl:EG
1:2)), K CO , 100 °C
Bipyridine-Palladium (1.0)
4 h
2 h
46
(
2
3
Pyridiniophosphine/PdCl (1.0) DES (ChCl:glycerol
80
2
(1:2)), K CO , 100 °C
2 3
Palladium/Mesoionic/Carbene DES (ChCl:CH OH)2 3 h
90
2
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(0.5)
(1:2)), K CO , rt
2
3
Pd@APGO (0.23)
EtOH:H O, K CO , 6 h
45
99
95
[53]
2
2
3
8
0 °C
Cell-Sb-Pd(II) (0.3)
EtOH:H O, K CO ,
1 h
[12]
2
2
3
7
0 °C
DES
1:2)), K CO , 80 °C
Fe O /GO@CL-Pd (0.2)
(DMAC:Gly 50 min
[This work]
3
4
(
2
3
4
. Conclusions
In conclusion, Pd nanoparticles were supported on clicked cellulose-modified magnetite-
graphene oxide nanocomposite through a facile and green reduction method without any
reducing agent to afford a new hydrophilic/magnetically separable Pd catalyst. The catalyst was
successfully applied for the reaction of various aryl boronic acids with aryl halides in different
hydrophilic and hydrophobic DESs as sustainable reaction media to achieve a clean protocol for
the synthesis of biphenyls. According to the obtained results, the catalyst exhibited superior
activity in hydrophilic DESs compared to hydrophobic one that may be ascribed to the better
dispersion of the catalyst in hydrophilic DESs. Among various hydrophilic DESs, the mixture of
dimethyl ammonium chloride and glycerol was the best one and provided excellent yield of the
coupling product. Because of the very low solubility of the catalyst and DES in organic solvents,
the separated aqueous phase possessing both of the catalyst and DES could be easily isolated by
evaporating water and recycled five times without a discernible decrease in the product yield.
This separation and recovery strategy undoubtedly provides a much cleaner route for the
synthesis of biphenyls compared to other previously reported catalytic systems. Generally, the
following benefits can be remarked in the present catalytic system: (a) the use of natural
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cellulose as both reducing and stabilizing agents for Pd; (b) the utilization of sustainable and
environmentally benign solvent (DES); (c) the recovery of the catalyst and DES in fast and easy
manner; and (d) the easy separation of the catalyst by a magnet.
Declaration of competing interest
The authors declare no conflict of interest
Acknowledgements
The authors gratefully acknowledge financial support (Grant Number H/4/361) from Kharazmi
University.
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