Fe3O4 MNPs-DETA/Benzyl-Br3: A new magnetically reusable catalyst for the oxidative coupling of thiols

Article abstract of DOI:10.1080/10426507.2017.1347654

We report a new strategy to immobilize a bromine source on the surface of magnetic Fe₃O₄ nanoparticles (Fe₃O₄ MNPs-DETA/Benzyl-Br₃) leading to amagnetically recoverable catalyst, which exhibits high catalytic efficiency in oxidative coupling of thiols to the disulfides (89–98%). The Fe₃O₄ MNPs-DETA/Benzyl-Br₃ catalyst was fabricated by anchoring 3-chloropropyltrimethoxysilane (CPTMS) onmagnetic Fe3O4 nanoparticles, followed with N-benzylation and reaction with bromine in tetrachloridecarbon. The resulting nanocomposite was analyzed by a series of characterization techniques such as FT-IR, SEM, TGA, VSM and XRD. The catalyst could be recovered viamagnetic attraction and could be recycled at least 5 times without appreciable decrease in activity.

Full text of DOI:10.1080/10426507.2017.1347654

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Fe3O4 MNPs-DETA/Benzyl-Br3: A new magnetically
reusable catalyst for the oxidative coupling of
thiols
Lotfi Shiri & Mosstafa Kazemi
To cite this article: Lotfi Shiri & Mosstafa Kazemi (2017): Fe3O4 MNPs-DETA/Benzyl-Br3: A new
magnetically reusable catalyst for the oxidative coupling of thiols, Phosphorus, Sulfur, and Silicon
and the Related Elements, DOI: 10.1080/10426507.2017.1347654
To link to this article: http://dx.doi.org/10.1080/10426507.2017.1347654
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Fe3O4 MNPs-DETA/Benzyl-Br3: A new magnetically reusable catalyst for the oxidative coupling
of thiols
Lotfi Shiri* and Mosstafa Kazemi
Department of Chemistry, Faculty of Basic Sciences, Ilam University, P.O. Box 69315-516, Ilam,
Iran
Corresponding author Tel&Fax number: +98 (843) 2228216
E-mail: Lshiri47@gmail.com and Mosstafakazemi@gmail.com
Abstract
We report a new strategy to immobilize a bromine source on the surface of magnetic Fe3O4
nanoparticles (Fe3O4 MNPs-DETA/Benzyl-Br3) leading to a magnetically recoverable catalyst,
which exhibits high catalytic efficiency in oxidative coupling of thiols to the disulfides (89-
98%). The Fe3O4 MNPs-DETA/Benzyl-Br3 catalyst was fabricated by anchoring 3-
chloropropyltrimethoxysilane (CPTMS) on magnetic Fe3O4 nanoparticles, followed with N-
benzylation and reaction with bromine in tetrachloridecarbon. The resulting nanocomposite was
analyzed by a series of characterization techniques such as FT-IR, SEM, TGA, VSM and XRD.
The catalyst could be recovered via magnetic attraction and could be recycled at least 5 times
without appreciable decrease in activity.
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Key words
Fe3O4 MNPs-DETA/Benzyl-Br3, Bromine source, Oxidative coupling of thiols, Magnetically
separation, Disulfides
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Introduction
Oxidative coupling of thiols to the disulfides is a key and valuable transformation in organic
synthesis, because compounds containing S-S bond (disulfides) are prevalent in nature and
present in the structural backbones of a broad spectrum of biological and pharmaceutical active
molecules [1-4]. From the medicinal point of view, interest in disulfides has amplified when
researchers found that compounds containing the S-S bond play an important role in the
stabilization of the folded form of proteins [5-6]. The biological and pharmaceutical activities of
disulfide derivatives have encouraged organic chemists to develop new, efficient and green
catalytic systems for their preparation.
During the last decade, a diverse range of reagents, catalysts and oxidants have been reported for
the oxidative coupling of thiols to the disulfides [1-2].
In recent times, magnetic separation have been received profound attention in modern research in
catalysis, because catalysts immobilized on magnetic nanoparticles can be readily separated from
reaction medium by using an external magnet, without the need for filtration, centrifugation or
other tedious workup processes [7-10]. The magnetic separation not only prevents the loss of the
catalyst in this process and is not time-consuming but also it improves product purity and
optimizes operational costs [11]. Recent literature publications clearly have shown that magnetic
nanoparticles possess a series of admirable advantages such as high surface area to bulk ratios,
low toxicity, high activity and thermal stability and the capability of surface modifications and
easy dispersion [12-15]. In fact, magnetic separation is a nice strategy to link the gap between
heterogeneous and homogenous catalysis [16]. Among the various employed magnetic NPs as
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the core magnetic support, Fe3O4 nanoparticles have been widely studied both for their scientific
interests and technological applications [17].
Catalysis research using bromine is a well-known topic in organic synthesis [18]. However, the
hazardous and toxic nature of bromine and its negative and deleterious effects on human health
has caused organic chemists to cautiously work with bromine [18-20]. Furthermore, the
separation of bromine source catalysts from the desired products or reaction media is a difficult,
tedious and time consuming task and needs a series of costly and specific techniques [21-22].
The development of heterogenous bromine reagents on magnetic nanoparticles can be considered
as an efficient and fascinating catalytic system to overcome this drawback, because the catalyst
can be readily separated from the reaction mixture by an external magnet [4, 7]. Inspired by this
catalytic strategy, in this paper, we report that tribromide ion immobilized on the surface of
magnetic Fe3O4 nanoparticles (Fe3O4 MNPs-DETA/Benzyl-Br3) is a new magnetically
recoverable catalyst for the oxidative coupling of thiols to the disulfides.
2. Result and discussion
2.1. Preparation and characterization of Fe3O4 MNPs-DETA/Benzyl-Br3:
Tribromide ion immobilized on the surface of magnetic Fe3O4 nanoparticles (Fe3O4 MNPs-
DETA/Benzyl-Br3) was successfully fabricated by using the surface modification strategy as
depicted in Scheme 1. Initially, naked Fe3O4 MNPs were prepared and functionalized with (3-
chloropropyl)-trimehoxysilane (CPTMS) by a previously procedure according to the literature
method [22]. The reaction of Fe3O4 MNPs-propyl chloride with diethylenetriamine (DETA) in
refluxing toluene led to the formation of the Fe3O4 MNPs-DETA [17]. Subsequent, the N-
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benzylation of Fe3O4 MNPs-DETA was performed in the presence of K2CO3 in DMF to obtain
Fe3O4 MNPs-DETA/Benzyl-Br. Finally, the Fe3O4 MNPs-DETA/Benzyl-Br3 was prepared
through the reaction of Fe3O4 MNPs-DETA/Benzyl-Br with Br2 in chloroform at room
temperature.
The structure of Fe3O4 MNPs-DETA/Benzyl-Br3 was analyzed by a series of characterization
techniques such as FT-IR, SEM, VSM, TGA and XRD.
The FT-IR spectra for the Fe3O4 MNPs (a), Fe3O4 MNPs-CPTMS (b), Fe3O4 MNPs-DETA (c),
Fe3O4 MNPs-DETA/Benzyl-Br (d) and Fe3O4 MNPs-DETA/Benzyl-Br3 (e) are displayed in Fig
1. The FT-IR spectrum for the Fe3O4 MNPs shows a stretching vibration at 3440 cm−1 which
incorporates the contributions from both symmetrical and asymmetrical modes of the O-H bonds
which are attached to the surface iron atoms. The presence of Fe3O4 was confirmed by strong
absorption band at around 560 cm1, in which is attributed to the Fe--O bond of Fe3O4 (Fig. 1a)
[15]. The formation of Fe3O4 MNPs-CPTMS is confirmed by C--H stretching vibrations that
appear at 2855-2956 cm−1 (Fig. 1b). Also the absorption around at 1034 cm−1 corresponds to Si--
O stretching vibration (Fig. 1b) [16]. As shown in Fig. 1c, the replacement of DETA instead of
chlorine in Fe3O4 MNPs-CPTMS is confirmed by N--H stretching vibrations that appear at 3400
cm1. In the curve of Fe3O4 MNPs-DETA/Benzyl-Br (Fig. 1d), the presence of the anchored
benzyl groups are confirmed by C--H stretching vibrations that appear at 2925 cm−1 and also C =
C stretching vibration modes as a broad band that appear at 1600 cm1. Also the broad peak at
nearly 1345-1400 cm1 in the Fe3O4 MNPs-DETA/Benzyl-Br is attributed to C-N+ stretching
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vibration [23]. But, Br3 exhibits a signal between 180 and 200 cm1 [22], which is not
recognizable with the FT-IR spectrum.
The SEM images of Fe3O4 MNPs-DETA/Benzyl-Br3 at different magnification are presented in
Fig. 2. The SEM images confirmed that the catalyst has made up of uniform nanometer-sized
particles with the average size of 10 nm.
The magnetic property of the catalyst was investigated by a vibrating sample magnetometer
(VSM) at ambient temperature. The curves of magnetization for the Fe3O4 MNPs (red line) and
Fe3O4 MNPs-DETA/Benzyl-Br3 (green line) are depicted in Figure S 1 (Supplemental Materials).
According to the magnetization curves, the saturation of the nanoparticles decreased from 75.21
emu/g (in the Fe3O4 MNPs to 45.89 emu/g in the final catalyst. The decrease of the saturation
magnetization can be related to the presence of DETA/Benzyl-Br3 on the surface of the magnetic
Fe3O4 nanoparticles.
Thermogravimetric analysis (TGA) was used to determine the percent of functional groups
chemisorbed onto the surface of magnetic nanoparticles. Figure S 2 (Supplemental Materials)
shows the TGA curves for bare Fe3O4 MNPs (black line), Fe3O4 MNPs-CPTMS (blue line),
Fe3O4 MNPs-DETA (green line) and Fe3O4 MNPs-DETA/Benzyl-Br3 (red line). The TGA curve
of the all samples shows the small amount of weight loss below 200°C is due to desorption of
physically adsorbed solvents and surface hydroxyl groups [3]. Organic groups have been
reported to desorb at temperatures above 260°C [3]. The organic groups on the catalyst are found
to have a mass percentage loss about of 15%, while the CPTMS-Fe3O4 and Fe3O4 MNPs-DETA
have the mass loss, at 5% and 9%, respectively. As shown in Figure S 2, the catalyst was stable
even at 200 C.
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The XRD-diffraction pattern of the prepared catalyst (Fe3O4 MNPs-DETA/Benzyl-Br3) is shown
in Figure S 3. In the XRD pattern of Fe3O4 MNPs-DETA/Benzyl-Br3, several characteristic peaks
at 2θ = 35.1, 41.5, 50.6, 63.1, 67.5 and 74.6 were observed, which are assigned to the
(220), (311), (400), (422), (511) and (440) crystallographic faces of magnetite (in good
agreement with the standard Fe3O4 XRD spectrum reported in the literature) [17]. This analysis
clearly affirmed this theme that the surface modification and conjugation of the Fe3O4
nanoparticles did not lead to phase change.
2.2. Catalytic study
After characterization of catalyst, the catalytic activity of Fe3O4 MNPs-DETA/Benzyl-Br3 in the
oxidative coupling of thiols to the disulfides at ambient temperature was studied. To find the
optimum reaction conditions, the effect of various parameters (such as solvent type, catalyst and
hydrogen peroxide amounts) on the room temperature oxidative coupling of benzo[d]thiazole-2-
thiol (as a model substrate) was studied. The results are summarized in Table 2. First, the effect
of catalyst loading on the model reaction was studied by varying the amount of Fe3O4 MNPs-
DETA/Benzyl-Br3 in EtOH at room temperature (Table 2, Entries 1-4). The results show that
12.5 × 103 mol% of Fe3O4 MNPs-DETA/Benzyl-Br3 is the optimal amount of catalyst for all
subsequent condensations. To clarify the role of catalyst in the process, the reaction was carried
out at in the absence of catalyst. But, only 5% of product 2d was observed after 120 min (Table
1, Entry 5). Next, to evaluate the solvent effect on product yield and reaction time, the model
reaction was carried out with 10 mg of catalyst in several solvent such as EtOH, CH3CN,
acetone, CH2Cl2 and THF (Table 3, Entries 3 and 6-9). As shown in Table 1, the best results
(highest yield and shortest time) were observed in EtOH. Finally, the effect of oxidant loading on
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the reaction was then studied. The reaction was carried out in the presence of different amounts
of hydrogen peroxide (Table 1, Entries 3 and 10-12). As shown in Table 1, the highest yield and
shortest reaction time of product 2d was obtained in the presence of 12.5 × 103 mol% of Fe3O4
MNPs-DETA/Benzyl-Br3 and 0.5 mL of hydrogen peroxide in EtOH at ambient temperature
(Table 1, Entry 3).
With the optimized reaction conditions in hand, to further study the scope and generality of this
catalytic system, the applicability of a series of thiols was then subjected to the oxidation
reaction (Scheme 2). The results of this study are listed in Table 2. The obtained results shown
well that a diverse range of thiols could be successfully oxidized to the corresponding disulfides
in high to excellent yields. It is noteworthy that aromatic and heteroaromatic thiols with electron-
donating as well as electron-withdrawing substituents at ortho and para position were well
tolerated and did not exert any tangible effect on the yield of reactions.
The efficiency of Fe3O4 MNPs-DETA/Benzyl-Br3 is shown by comparison our results on the
oxidative coupling of 4-methylbenzenethiol with the previously reported protocols in the
literature (Table 3). The results showed that Fe3O4 MNPs-DETA/Benzyl-Br3 is a better catalyst
with respect to reaction times and yields of the products.
The reusability of the Fe3O4 MNPs-DETA/Benzyl-Br3 catalyst was studied in the oxidative
coupling of 4-methylbenzenethiol as a model reaction. After completion of the reaction, the
Fe3O4 MNPs-DETA/Benzyl-Br3 catalyst was readily separated from the reaction mixture by an
external magnet, washed several times with ethanol and dried to remove residual product. It is
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noteworthy that the recovered catalyst was reused at least five times without significant loss of
its catalytic efficiency (Figure S 4).
3. Conclusion
In summary, we developed a novel magnetically recoverable catalyst based on immobilization of
tribromide ion on the surface of silica-coated magnetic Fe3O4 nanoparticles functionalized with
DETA/Benzyl. The structure of Fe3O4 MNPs-DETA/Benzyl-Br3 was characterized by FT-IR,
SEM, VSM, TGA and XRD analysis techniques. The Fe3O4 MNPs-DETA/Benzyl-Br3 showed
high catalytic activity in the oxidative coupling of thiols to disulfides. It is noteworthy that this
nanosolid catalyst can be readily synthesized by a very simple strategy using inexpensive
precursors, and the recoverable catalyst can be reused at least five times without significant loss
of its catalytic activity. Therefore, this procedure can act as an effective alternative in the
oxidative coupling of thiols to disulfides.
4. Experimental
Materials
Chemicals were purchased from Fisher and Merck. The reagents and solvents used in this work
were obtained from Sigma-Aldrich, Fluka or Merck and used without further purification. The
infrared spectra (IR) of samples were recorded in KBr disks using a NICOLET impact 410
spectrometer. 1HNMR and 13CNMR spectra were recorded with a Bruker DRX-400 spectrometer
at 400 and 100 MHz respectively. Nanostructures were characterized using a Holland Philips X,
pert X-ray powder diffraction (XRD) diffractometer (Co Kα, radiation = 0.154056 nm), at a
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scanning speed of 2 min−1 from 10 to 80. Scanning electron microscope (SEM) was
performed on a FEI Quanta 200 SEM operated at a 20 kV accelerating voltage. The
thermogravimetric analysis (TGA) curves are recorded using a PL-STA 1500 device
manufactured by Thermal Sciences. The magnetic measurements were carried out in a vibrating
sample magnetometer (VSM, BHV-55, Riken, Japan) at room temperature. The Supplemental
Materials contains additional characterization data for the catalyst (Figures S 1 -- S 4).
General procedure for the oxidative coupling of thiols to the disulfides:
A mixture of thiol (1 mmol), hydrogen peroxide (0.5 mL) and Fe3O4 MNPs-DETA/Benzyl-Br3
(12.5 × 103 mol%) in ethanol (2 mL) was stirred at ambient temperature. Reaction progress was
monitored by TLC (acetone: n-hexane, 2:8). After completion of the reaction, catalyst was
separated by external magnet and washed with ethyl acetate, and next, the product was extracted
with ethyl acetate (4 × 5 mL). The organic layer was dried over anhydrous Na2SO4 (1.5 g).
Finally, the organic solvents were evaporated, and the corresponding disulfides were obtained in
high to excellent yields (89-98%).
All the products are known compounds and were characterized by comparison of their IR and
NMR spectral data and physical properties with those reported in the literature.
Acknowledgements
Authors thank the research facilities of Ilam University, Ilam, Iran, for financial support of this
research project.
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Table 1. Optimization of the reaction conditions a
Entry
1
Catalyst (mol%)
3.75×10-3
6.25×10-3
12.5×10-3
18.75×10-3
--
H2O2 (mL)
0.5
Solvent
EtOH
EtOH
EtOH
EtOH
EtOH
CH3CN
Acetone
THF
CH2Cl2
EtOH
EtOH
EtOH
Time/min
30
Yield (%) b
91
95
98
97
5
92
94
90
91
89
94
98
2
3
4
5
6
7
8
9
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
20
15
15
120
25
35
115
80
50
30
12.5×10-3
12.5×10-3
12.5×10-3
12.5×10-3
12.5×10-3
12.5×10-3
12.5×10-3
10
11
12
0.3
0.4
0.6
15
a Reaction conditions: The oxidative coupling of benzo[d]thiazole-2-thiol (1 mmol) by using
H2O2 in the presence of catalyst and solvent (2 mL). b Isolated yield.
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Table 2. Fe3O4 MNPs-DETA/Benzyl-Br3 catalyzed the oxidative coupling of thiols into
disulfides in ethanol at room temperature.
Entry
1
Thiol
Benzenethiol
4-Methylbenzenethiol
Naphthalene-2-thiol
Pyridine-2-thiol
Benzo[d]thiazole-2-thiol
4-Fluorobenzenethiol
4-Bromobenzenethiol
2-Aminobenzenethiol
2-Mercaptobenzoic acid
Phenylmethanethiol
2-Mercaptoethanol
2-Mercaptoacetic acid
Product
2a
Time (min) Yield (%)a
Mp (c) [Ref.]
60-61 [4]
40-42 [4]
133-134 [4]
54-56 [4]
171-172 [3]
Oil [4]
88-89 [6]
80-82 [4]
285-287 [3]
70-71 [4]
Oil [6]
50
15
20
50
15
15
25
30
30
55
35
45
90
98
91
92
98
93
96
94
93
95
90
89
2
3
4
5
6
7
8
9
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
10
11
12
Oil [6]
a Isolated yield
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Table 3 Comparison of the activity of various catalysts in the oxidative coupling of 4-
methylbenzenethiol.
Entry
Substrate
4-Methylbenzenethiol
4-Methylbenzenethiol
Catalyst
Ni-salen-MCM-41
Fe NPs@SBA-15
Time (min) Yield (%)a
[Ref.]
[24]
[25]
[26]
[27]
1
2
3
4
5
6
25
45
95
94
83
83
82
98
4-Methylbenzenethiol Fe3O4@SiO2--NH2@Mn(III)
120
72
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
Cu(II) Schiff base MCM-41
Mn--ZSM-5
360
15
[28]
This work
This catalyst
a Isolated yield
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Figure 1. FT-IR spectra of Fe3O4 MNPs (a), Fe3O4 MNPs-CPTMS (b), Fe3O4 MNPs-DETA (c),
Fe3O4 MNPs-DETA/Benzyl-Br (d) and Fe3O4 MNPs-DETA/Benzyl-Br3 (e)..
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Fig 2. SEM images of the Fe3O4 MNPs-DETA/Benzyl-Br3 at different magnification.
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Scheme 1. Stepwise preparation of Fe3O4 MNPs-DETA/Benzyl-Br3.
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Scheme 2. Fe3O4 MNPs-DETA/Benzyl-Br3 catalyzed the oxidative coupling of thiols to the
disulfides.
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