Tribromide ion immobilized on magnetic nanoparticles as an efficient catalyst for the rapid and chemoselective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1080/10426507.2016.1224878

Tris(tetraalkyl ammonium tribromide) immobilized on Fe3O4 magnetic nanoparticles (Br3-Fe3O4) is a stable, green, and magnetically recoverable source of bromine and used as an efficient nanocatalyst for the clean and rapid oxidation of sulfides to sulfoxides using hydrogen peroxide (H2O2), with short reaction times in good to high yields under solvent-free conditions. After completing reactions, the catalyst was easily separated using an external magnetic field from the reaction mixture and reused for several consecutive runs without significant loss of its catalytic efficiency. The nanomagnetically catalyst was characterized by FT-IR spectroscopy, TGA, VSM, XRD, TEM and SEM.

Full text of DOI:10.1080/10426507.2016.1224878

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Tribromide ion immobilized on magnetic
nanoparticles as an efficient catalyst for the
rapid and chemoselective oxidation of sulfides to
sulfoxides
Lotfi Shiri & Bahman Tahmasbi
To cite this article: Lotfi Shiri & Bahman Tahmasbi (2016): Tribromide ion immobilized on
magnetic nanoparticles as an efficient catalyst for the rapid and chemoselective oxidation
of sulfides to sulfoxides, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
10.1080/10426507.2016.1224878
To link to this article: http://dx.doi.org/10.1080/10426507.2016.1224878
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Sep 2016.
Published online: 02 Sep 2016.
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Date: 06 September 2016, At: 02:13

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Tribromide ion immobilized on magnetic nanoparticles as an efficient catalyst for the rapid
and chemoselective oxidation of sulfides to sulfoxides
Lotfi Shiri* and Bahman Tahmasbi
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran
E-mail address: lshiri47@yahoo.com
Abstract
Tris(tetraalkyl ammonium tribromide) immobilized on Fe O magnetic nanoparticles (Br -Fe O )
3
4
3
3
4
is a stable, green and magnetically recoverable source of bromine and used as an efficient
nanocatalyst for the clean and rapid oxidation of sulfides to sulfoxides using hydrogen peroxide
(
H O ), with short reaction times in good to high yields under solvent free conditions. After
2 2
completing reactions, the catalyst was easily separated using an external magnetic field from the
reaction mixture and reused for several consecutive runs without significant loss of its catalytic
efficiency. The nanomagnetically catalyst was characterized by FT-IR spectroscopy, TGA,
VSM, XRD, TEM and SEM.
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Keywords
Magnetic nanoparticles, Tribromide, oxidation, sulfoxides, sulfides.
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INTRODUCTION
Catalytic efficiency and reusability are two important factors for catalytic processes; however
recycling of homogeneous catalysts is difficult, expensive and time consuming, in the other
hands, immobilization of homogeneous catalysts on heterogeneous supports led to recyclability
of catalyst and decrease in catalytic activity. These drawback can be overcame using
nanoparticles as catalyst support, because nanoparticles are included advantages of both
homogeneous and heterogeneous catalysis [1, 2]. To investigate this issue, magnetic
nanoparticles (MNPs) have been employed as catalyst support for various organic
transformations because magnetic separation is more effective than filtration or centrifugation [3,
4
]. Among various magnetic nanoparticles, Fe O MNPs are widely has been used because of
3 4
their intrinsic properties such as large surface area, low toxicity, facile recovery and recyclability
via external magnetic force [5]. In addition to that, Fe O is included a large number of hydroxyl
3
4
groups on the surface [6]. This characteristic is led to functionalization of Fe O MNPs surface
3
4
using different homogeneous catalyst [7, 8]. For example, many catalyst has been supported on
Fe O MNPs and applied as efficient catalyst for organic reactions, successfully [4-8].
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4
Therefore herein a stable Br -Fe O has been reported as new and heterogeneous catalyst for the
3
3
4
oxidation of sulfides to sulfoxides. Because sulfoxides are useful in various medical, biological,
and nanotechnological applications such as anti-ulcer, antifungal, antibacterial, anti-
atherosclerotic, antihypertensive and anti-atherosclerotic [9-11]. Furthermore, omeprazole,
lansoprazole and pesticide fipronil are several importance application of sulfoxides in the
pharmaceutical and fine chemical industries [12-14].
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For this reason, several procedures including various catalysts (homogeneous and heterogeneous
catalysts) and different oxidants (e.g. metal oxidants, organic oxidants, peroxides, halogens, and
air) have been reported for oxidation of sulfides [10, 15]. Hydrogen peroxide was widely used
for the oxidation of sulfides, because it is environmentally, readily available, high atom
efficiency and produces only water as a side-product [5, 16].
RESULTS AND DISCUSSION
The catalyst was prepared by the concise route outlined in Scheme 1. Initially, naked Fe O
3
4
MNPs were prepared and modified with (3-chloropropyl)-triethoxysilane (CPTES) by a
previously reported procedure [17, 18] and subsequently were functionalized with
diethylenetriamine (DETA). The immobilized ammonium bromide on Fe O (ammonium
3
4
bromide-Fe O ) was performed via reaction of benzyl bromide and supported-diethylenetriamine
3
4
groups. Ultimately, the Br -Fe O was prepared using the reaction of ammonium bromide-Fe O
4
3
3
4
3
with Br in chloroform at room temperature.
2
Br -Fe O have been characterized by Fourier transform infrared spectroscopy (FT-IR),
3
3
4
transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating
sample magnetometer (VSM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA).
The size of the catalyst was evaluated using scanning electron microscopy and transmission
electron microscopy. The SEM image of Br -Fe O and TEM images of DETA-Fe O and Br -
3
3
4
3
4
3
Fe O are shown in Figure S1. The SEM and TEM images of Br -Fe O was shown that the
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4
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4
catalyst was formed of nanometer-sized particles (ESI, Figure S 1 Supplemental Materials).
The FT-IR spectrum of Br -Fe O is shown in Figure 1e, which clearly differs from
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4
unfunctionalized Fe O and CPTES-Fe O magnetic nanoparticles. The FT-IR spectrum for the
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4
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−
1
Fe O MNPs shows a stretching vibration at 3440 cm which incorporates the contributions
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4
from both symmetrical and asymmetrical modes of the O-H bonds which are attached to the
surface of MNPs (Figure 1a). The presence of Fe O was shown in FT-IR by strong absorption
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4
-1
band around 560 cm , which corresponds to the Fe–O bond of Fe O [17]. The anchored CPTES
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4
−
1
on Fe O MNPs is confirmed by C–H stretching vibrations that appear at 2956–2855 cm
3
4
−
1
(
Figure 1b). Also the absorption around at 1000 and 1044 cm corresponds to Si–O stretching
vibration [19, 20]. As shown in Figure 1c, the functionalized DETA instead of Cl is confirmed
-1
by N–H stretching vibrations that appear at 3446 cm . In the FT-IR spectra of ammonium
bromide-Fe O (Figure 1d), The presence of the anchored benzyl groups are confirmed by C–H
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4
−
1
stretching vibrations that appear at 2924 cm and also C=C stretching vibration modes as a
-1
broad band that appear at 1648 cm .
TGA was used to determine the percent of functional groups chemisorbed onto the surface of the
magnetic nanoparticles. Figure S 2 (Supplemental Materials) shows the TGA curves for bare
Fe O MNPs (black line), CPTES-Fe O (blue line), DETA-Fe O (green line) and Br -Fe O
4
3
4
3
4
3
4
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3
(brown line). The TGA curve of the all samples shows the small amount of weight loss below
2
2
00 °C is due to desorption of physically adsorbed solvents and surface hydroxyl groups [21,
2]. Organic groups have been reported to desorb at temperatures above 260 °C. The organic
groups on the catalyst are found to have a mass percentage loss about of 15%, while the CPTES-
Fe O and DETA-Fe O have the mass loss, at 5% and 9%, respectively.
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4
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The magnetic properties of Fe O nanoparticles and Br -Fe O were characterized by VSM. The
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room temperature magnetization curves of Fe O MNPs and Br -Fe O were shown in Figure S 3
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4
(
Supplemental Matgerials). The magnetic measurement shown that the Br -Fe O have a
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-1
saturated magnetization value of 47.86 emu g . As expected, the Ms value of Br -Fe O is
3
3
4
decreased due to the silica coating and the organic layer of the grafted catalyst (from 74.09 to
-1
4
7.86 emu g ) [21].
The XRD patterns of the DETA-Fe O and Br -Fe O were shown in Figure S 4 (Supplemental
3
4
3
3
4
Materials). The XRD patterns of the DETA-Fe O and catalyst, showed several peaks at 2Ɵ
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4
values 35.1º, 41.5º, 50.6º, 63.1º, 67.5º and 74.6º related to crystal planes in the Fe O cubic
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4
lattice (ESI, Figure S4). This XRD patterns correspond with the standard Fe O nanoparticles
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4
XRD spectrum [8]. These results showed that the surface modification of the Fe O nanoparticles
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4
do not lead to their phase change.
We tested the catalytic activity of Br -Fe O in the oxidation of sulfides into sulfoxides (Scheme
3
3
4
2
). The reaction conditions were optimized whit the oxidation of methylphenyl sulfide using
H O , in the presence of different amounts of catalyst and in various solvents (Table 1). As
2
2
shown in Table 1, 0.005 g (0.01 mmol) of Br -Fe O using H O (0.5 mL) under solvent free
3
3
4
2
2
conditions was found to be ideal reaction conditions for the oxidation of methylphenyl sulfide (1
mmol).
After the optimization of the reaction condition, various sulfides including aromatic sulfides
(Table 2, entries 1-4), aliphatic sulfides (Table 2, entries 6-10), allyl sulfide (Table 2, entry 5)
and cyclic sulfide (Table 2, entry 11) have been examined in optimized conditions. The
experimental procedure is very simple and all products were obtained in high yields. Because
oxidation of sulfides to sulfoxides were carried out in mild conditions, overoxidation for
oxidation of sulfides to sulfone was not observed. Also, other functional groups (which are
sensitive in oxidation procedures) were remained intact during the conversion of sulfides to
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sulfoxides (Table 2, entries 2, 5 and 6). For example, this oxidizing system allowed the
chemoselective oxidation of allyl methyl sulfide, 2-(phenylthio)ethanol and 2-
(methylthio)ethanol to the corresponding sulfoxides. Interestingly, primary hydroxyl groups in
these substrates remained intact during the oxidation reactions and the corresponding sulfoxides
were obtained in good yields (Table 2, entries 2, 5 and 6).
A plausible mechanism for oxidation of sulfides to sulfoxides in the presence of Br -Fe O is
3
3
4
outlined in Scheme 3.
The catalyst can be rapidly and easily recovered from the reaction mixture using an external
magnet (ESI, Figure S 5 Supplemental Materials). After the completion of the reaction, the
catalyst was easily separated from the product by an external magnet. The remaining magnetic
nanocatalyst was washed with ethylacetate to remove residual products. Then, the reaction vessel
was charged with fresh substrate and H O and subjected to the next. As shown in Figure S 6
2
2
(Supplemental Materials), the catalyst can be recycled up to 17 runs without any significant loss
of activity.
The efficiency of this nanocatalyst is demonstrated by comparison on the oxidation of
methylphenyl sulfide with the other catalysts in the literature (Table 3). This catalyst affords
good reaction yields and short reactions times than other catalysts. More important, in
comparison with other catalyst, Br -Fe O can be prepared using cheap and available materials,
3
3
4
and can be reused for 17 times without any significant loss of catalytic activity. Also, in
comparison with other catalyst, this new catalyst is comparable or may be better in terms of
price, recyclability, stability and easy separation.
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CONCLUSION
In summary, we have designed the Br -Fe O as a new source of bromine for the oxidation of a
3
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4
wide range of sulfides to sulfoxides using H O . The advantages of this protocol are the use of a
2
2
commercially available, cheap and chemically stabile oxidant, the mild reaction conditions,
operational simplicity, practicability and good to high yields. The products separation and
catalyst recycling are easy and simple with the assistance of an external magnet. The catalyst can
be reused 17 times without any significant loss of activity.
EXPERIMENTAL
Preparation of Catalyst (Br -Fe O )
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4
Fe O MNPs were prepared and functionalized with (3-chloropropyl)-triethoxysilane (CPTES)
3
4
by a previously reported procedure [17, 18]. The CPTES-Fe O (1.5 g) was dispersed in toluene
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4
(50 mL) in an ultrasonic bath for 10 min. DETA (2 mL) was added and stirred at 100 ºC for 30 h
under a N atmosphere. Then, the prepared functionalized magnetic nanoparticles were separated
2
by magnetic decantation and washed three times with ethanol to remove the unattached
substrates. The resulting product (DETA-Fe O ) was dried at room temperature. The prepared
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4
DETA-Fe O has been characterized by FT-IR, XRD, TGA and TEM. The obtained DETA-
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Fe O (1 g), was added to mixture of K CO (3 mmol) and benzyl bromide (5 mL) in DMF. The
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4
2
3
mixture was then stirred at 80 ºC for 24 h under a N atmosphere. The resulting powder
2
(
ammonium bromide-Fe O ) was separated by magnetic decantation. The magnetic powder was
3 4
washed with ethanol and dried at room temperature. Ammonium bromide-Fe O (0.5 g) was
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4
dispersed in CCl (5 mL) in an ultrasonic bath for 10 min. Subsequently, Br (2.5 mL) was added
4
2
and the mixture was stirred for 8 h at room temperature. Then, the final product was separated by
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magnetic decantation and washed with CCl to remove the unattached substrates. The product
4
(
Br -Fe O ) was stored in a refrigerator to use.
3 3 4
General procedure for the oxidation of sulfides to sulfoxides
A mixture of sulfide (1 mmol), hydrogen peroxide (1.2 mmol) and Br -Fe O (0.005 g, 0.01
3
3
4
mmol) were stirred at room temperature under solvent-free conditions and the progress of the
reaction was monitored by TLC. 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 x 5 mL). The organic layer was dried over anhydrous Na SO (1.5 g). Finally, the
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4
organic solvents were evaporated, and products were obtained in good to high yield.
Selected spectral data
Methyl(p-tolyl)sulfane (Table 2, entry 4): 1H NMR (400 MHz, CDCl ): δ = 2.23 (s, 3H), 2.56
3
(s, 3H), 7.18 (d, J= 3.2 Hz, 2H), 7.40 (d, J= 3.2 Hz, 2H) ppm.
1
-(Propylsulfinyl)propane (Table 2, entry 7): 1H NMR (400 MHz, CDCl ): δ = 1.12 (t, J= 6
3
Hz, 6H), 1.85-1.94 (m, 4H), 2.64-2.96 (m, 4H) ppm.
Tetrahydrothiophene 1-oxide (Table 2, entry 11): 1H NMR (400 MHz, CDCl ): δ = 2.26 (t,
3
J= 7.6 Hz, 4H), 3.05 (t, J= 7.6 Hz, 4H) ppm.
ACKNOWLEDGEMENTS
The authors acknowledge Ilam University, Ilam, Iran for financial support of this work.
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Table 1. Optimization of the reaction conditions for the oxidation of methylphenylsulfide (1
mmol) in the presence of Br -Fe O .
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4
Yield
entry
Solvent
Catalyst (mg)
Time (min)
a
(
%)
1
2
3
4
5
6
7
Solvent-Free
Solvent-Free
Solvent-Free
Ethanol
Acetonitrile
Ethylacetate
Water
5
50
98
99
98
97
98
20
81
10
15
5
40
35
100
140
300
120
5
5
5
a
Isolated yield.
1
3
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Table 2. Oxidation of sulfides into sulfoxides using H O in the presence of Br -Fe O .
2
2
3
3
4
Time
Melting Point
(°C)
a
entry
sulfide
product
2a
Yield (%)
(min)
1
2
3
4
50
98
97
96
92
30-32
Oil
2b
15
50
40
2c
Oil
2d
Oil
5
6
7
2e
2f
10
10
5
91
97
91
Oil
Oil
Oil
2g
8
9
2h
5
95
Oil
2i
2j
5
91
90
Oil
1
1
0
1
30
61-64
2k
5
92
Oil
b
Isolated yield
1
4
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Table 3. Comparison of Br -Fe O for the oxidation of methylphenyl sulfide with previously
3
3
4
reported catalysts.
a
Yield (%)
Entry
Substrate
Catalyst
Time (min)
[
Reference]
93 [23]
95 [15]
98 [15]
88 [24]
95[25]
1
2
3
4
5
Ph-SCH3
Ph-SCH3
Ph-SCH3
Ph-SCH3
Ph-SCH3
NBS
270
156
150
240
300
Ni-salen-MCM-41
Cd-salen-MCM-41
TsOH
VO F(dmpz)2
2
Heterocyclic amine salts of Keggin
heteropolyacids
6
Ph-SCH3
120
95 [26]
7
8
9
Ph-SCH3
Ph-SCH3
Ph-SCH3
Ph-SCH3
polymer-anchored Cu(II)
180
150
360
50
90 [27]
91.9 [28]
SiO –W –Im
2
2
DSA@MNPs
Br -Fe O
98 [29]
1
0
98 [This work]
3
3
4
a
Isolated yields
1
5
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Figure 1. FT-IR spectra of (a) magnetic nanoparticle Fe O , (b) CPTES-Fe O , (c) DETA-Fe O ,
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4
3
4
3
4
(
d) Ammonium Bromide-Fe O and (e) Br -Fe O .
3 4 3 3 4
1
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Scheme 1. Synthesis of Br -Fe O .
3
3
4
1
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Scheme 2. Br -Fe O catalyzed the oxidation of sulfides into sulfoxides.
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3
4
1
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Scheme 3. A possible mechanism for the oxidation of sulfides to sulfoxides in the presence of
Br -Fe O using H O .
3
3
4
2
2
1
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