Preparation of DSA@MNPs and application as heterogeneous and recyclable nanocatalyst for oxidation of sulfides and oxidative coupling of thiols

Article abstract of DOI:10.1007/s11164-015-2399-1

Abstract: Dopamine sulfamic acid-functionalized magnetic Fe₃O₄ nanoparticles (DSA@MNPs) were prepared by a very simple and inexpensive procedure using commercial materials and characterized by X-ray diffraction (XRD) analysis, thermogravimetric analysis (TGA), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and vibrating-sample magnetometry (VSM) techniques. They were employed as an efficient and recoverable catalyst in oxidation of sulfides to sulfoxides and oxidative coupling of thiols to disulfides using hydrogen peroxide as green oxidant at room temperature. This is the first report of application of DSA@MNPs in organic reactions. The supported magnetic nanoparticles could be easily recovered using an appropriate external magnet, minimizing catalyst loss during separation, and reused for several times without any loss of catalytic activity. Also, the amount of sulfamic acid function in the DSA@MNPs was calculated from ion-exchange pH analysis and back titration. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-015-2399-1

Res Chem Intermed
DOI 10.1007/s11164-015-2399-1
Preparation of DSA@MNPs and application
as heterogeneous and recyclable nanocatalyst
for oxidation of sulfides and oxidative coupling of thiols
1
Arash Ghorbani-Choghamarani
1
•
1
•
Hossein Rabiei
Banoo Ghasemi
•
Bahman Tahmasbi
1
Farideh Mardi
1
•
Received: 2 July 2015 / Accepted: 16 December 2015
Ó Springer Science+Business Media Dordrecht 2016
Abstract Dopamine sulfamic acid-functionalized magnetic Fe O4 nanoparticles
3
(DSA@MNPs) were prepared by a very simple and inexpensive procedure using com-
mercial materials and characterized by X-ray diffraction (XRD) analysis, thermogravi-
metric analysis (TGA), Fourier-transform infrared (FT-IR) spectroscopy, scanning
electron microscopy (SEM), transmission electron microscopy (TEM), and vibrating-
sample magnetometry (VSM) techniques. They were employed as an efficient and
recoverable catalyst in oxidation of sulfides to sulfoxides and oxidative coupling of
thiols to disulfides using hydrogen peroxide as green oxidant at room temperature. This
is the first report of application of DSA@MNPs in organic reactions. The supported
magnetic nanoparticles could be easily recovered using an appropriate external magnet,
minimizing catalyst loss during separation, and reused for several times without any loss
of catalytic activity. Also, the amount of sulfamic acid function in the DSA@MNPs was
calculated from ion-exchange pH analysis and back titration.
&
Arash Ghorbani-Choghamarani
a.ghorbani@mail.ilam.ac.ir; arashghch58@yahoo.com
1
Department of Chemistry, Ilam University, P.O. Box 69315516, Ilam, Iran
123

A. Ghorbani-Choghamarani et al.
Graphical abstract
Dopamine
OH
4
MNPs
MNPs
NH
OH
HO
HO
HO
OH
OH
OH
MNPs
OH
Fe3+
Fe2+
+
SH
S
S S
R
R
R
ClSO H
3
MNPs
MNPs
O
S
R1 R2 R1 R2
Keywords Hydrogen peroxide Á Dopamine Á Magnetic nanoparticles Á Sulfoxide Á
Disulfide
Introduction
Nanomaterials have provoked wide interest because of their unique properties,
being used in biology [1], physics [2], and chemistry [3]. In recent decades,
nanotechnology has pushed forward synthesis of functional magnetic nanoparticles
(
MNPs) for use as heterogeneous catalysts [4]. In general, catalysts can be divided
into two groups: homogeneous and heterogeneous systems [5]. In spite of the
various benefits of homogeneous catalysts, heterogeneous catalysts are widely
applied because of their facile recyclability [6].
Surface-functionalized iron oxide magnetic nanoparticles (MNPs), which have
been widely used in biotechnology [7] and analysis [8], offer incomparable
advantages over traditional catalysts because of their high activity, magnetic
recyclability, and reusability [9].
Mineralliquidacids(suchasHFandH SO )andLewisacids(suchasAlCl andBF )
2
4
3
3
are still used as catalysts for organic reactions such as alkylation [10], acylation [11],
oxidation, and isomerization [12]. In recent years, solid acid catalysts have served as
important functional materials in chemical reactions because of their superacidity, high
activity at low temperatures, and nontoxicity [13]. However, preparation of various
heterogeneous supports used previously, such as MCM-41 [3], SBA-15 [14], or various
nanoparticlessuchasTiO NPs [15], requireshighcalcinationtemperaturesorlongtime
2
and tedious conditions. Other previously reported materials such as heteropolyacids
[16], carbon nanotubes [17], ionic liquids [18], or various polymers [19] are more
1
23

Preparation of DSA@MNPs and application as heterogeneous…
expensive. However, Fe O MNPs can be prepared in water in short time using
3
4
commercial materials such as FeCl Á4H O and FeCl Á4H O. In addition, one of the
3
2
2
2
attractive features of magnetic nanoparticles is the rapid (within 10 s) and efficient
100 %) separation of the catalyst using an appropriate external magnet, minimizing
(
catalyst loss during separation; this effect is not observed in the case of other supported
catalysts,especiallynanocatalysts,becausetheycannotbeeasilyrecoveredandrecycled
by filtration [20, 21].
This is the first report of preparation of DSA@MNPs for application as a solid
acid catalyst for oxidation of organosulfur compounds with importance in synthetic
chemistry, biochemistry, and industrial chemistry. In numerous studies reported in
literature, hydrogen peroxide (with only H O as byproduct) was applied as an
2
inexpensive and environmentally friendly oxidant for this conversion [22–25]. The
whole process for preparation of DSA@MNPs took less than a day, without tedious
conditions. More importantly, dopamine and chlorosulfuric acid are commercially
available and not expensive.
Experimental
Materials
All reagents and solvents used in this work were purchased from Sigma-Aldrich,
Fluka or Merck chemical companies and used without further purification. The
particle size and morphology were investigated by scanning electron microscopy
(
SEM, JEOL JEM-2010) at accelerating voltage of 200 kV. Thermogravimetric
analysis (TGA) curves were recorded using a PL-STA 1500 device manufactured
by Thermal Sciences. Particle size and morphology were investigated using
transmission electron microscopy (TEM, Zeiss-EM10C) at accelerating voltage of
8
0 kV. VSM measurements were performed using a MDKFD vibrating-sample
magnetometer. Magnetization measurements were carried out in an external field of
up to 15 kOe at several temperatures. IR spectra were recorded from KBr pellets on
a Bruker VRTEX 70 model FT-IR spectrophotometer.
Preparation of magnetic Fe O nanoparticles (MNPs)
3
4
3
Fe O nanoparticles were prepared by chemical coprecipitation of Fe and Fe
?
2?
3
4
ions at molar ratio of 2:1. A mixture of FeCl Á6H O (2.35 g, 8.7 mmol) and
3
2
FeCl Á4H O (0.86 g, 4.3 mmol) was dissolved in 40 mL deionized water under
2
2
nitrogen atmosphere with rapid mechanical stirring. Then, 10 mL NH OH solution
4
was added, and black precipitate formed immediately. The black magnetite
nanoparticles were isolated by magnetic decantation, washed several times with
deionized water, and further washed with ethanol and dried in vacuum [26].
Preparation of MNPs coated by 4-(2-aminoethyl)benzene-1,2-diol (dopamine)
The resulting MNP powder (2 g) was dispersed in 25 mL water using an ultrasonic
bath for 30 min. Subsequently, 4-(2-aminoethyl)benzene-1,2-diol (dopamine) (2 g)
123

A. Ghorbani-Choghamarani et al.
was added. The solution was stirred mechanically for 24 h at room temperature.
Then, dopamine-functionalized magnetic Fe O nanoparticles were separated using
an external magnet, washed with deionized water and n-hexane three times, and
3
4
dried at room temperature under vacuum.
Preparation of sulfamic acid-functionalized magnetic Fe O nanoparticles
4
3
The MNPs–dopamine (0.9 g) was dispersed in n-hexane (10 mL) using an ultrasonic
bath for 20 min. Then, chlorosulfuric acid (0.2 mL) was added dropwise over a period
of 20 min, and the mixture was stirred for 2 h at room temperature. Functionalized
MNPs were separated by magnetic decantation, washed four times with dry CH Cl
2
2
and two times with EtOH, and dried at room temperature (Scheme 1).
General procedure for oxidation of sulfides to sulfoxides
Dopamine sulfamic acid-functionalized magnetic Fe O nanoparticles
DSA@MNPs) (0.02 g) were added to solution of sulfide (1 mmol) and 33 %
3
4
(
H O (0.5 mL) in ethanol (10 mL), the mixture was stirred at room temperature for
2
2
the specified time, and the progress of the reaction was monitored by thin-layer
chromatography (TLC). After completion of the reaction, the catalyst was separated
using an external magnet. The product was extracted with CH Cl , washed with
2
2
water (5 mL), and dried under vacuum at room temperature.
General procedure for oxidative coupling of thiols
Dopamine sulfamic acid-functionalized magnetic Fe O
(
nanoparticles
DSA@MNPs) (0.004 g) were added to solution of thiol (1 mmol) and H O
3
4
2
2
Scheme 1 Synthesis of DSA@MNPs
1
23

Preparation of DSA@MNPs and application as heterogeneous…
3
3 % (0.5 mL) in ethanol (10 mL). The reaction mixture was stirred at room
temperature, and the progress of the reaction was monitored by TLC. After
completion of the reaction, the catalyst was separated using an external magnet, and
the product was extracted with CH Cl and dried over anhydrous Na SO . Finally,
2
2
2
4
the solvent was removed by simple evaporation to give the corresponding pure
disulfides.
Selected spectral data
1
-(Propylsulfinyl)propane (Table 5, entry 2) H NMR (400 MHz, CDCl3):
1
d = 1.13 (t, J = 6 Hz, 6H), 1.86–1.95 (m, 4H), 2.64–2.97 (m, 4H) ppm.
1
-((Methylsulfinyl)methyl)furan (Table 5, entry 3) H NMR (400 MHz, CDCl3):
2
d = 2.57 (s, 3H), 4.05 (d, J = 14 Hz, 1H), 4.16 (d, J = 14 Hz, 1H), 6.44 (d,
J = 3.2 Hz, 1H), 7.17–7.56 (m, 2H) ppm.
1
(Methylsulfinyl)methyl)benzene (2c, Table 5, entry 4) H NMR (250 MHz,
(
CDCl ): d = 2.46 (s, 3H), 3.94 (d, J = 17 Hz, 1H), 4.13 (d, J = 17 Hz, 1H),
3
7
.34 (s, 5H) ppm.
1
Tetrahydrothiophene 1-oxide (Table 5, entry 6) H NMR (400 MHz, CDCl3):
d = 2.27 (t, J = 7.6 Hz, 2H), 3.06 (t, J = 7.6 Hz, 2H) ppm.
1
Methyl 3-(methylsulfinyl)propanoate (Table 5, entry 7) H NMR (250 MHz,
CDCl ): d = 2.85 (t, J = 7.6 Hz, 2H), 2.95 (s, 3H), 3.36 (t, J = 7.5 Hz, 2H),
3
3
.75 (s, 3H) ppm.
1
1
,2-Di-p-tolyldisulfane (Table 6, entry 2) H NMR (400 MHz, CDCl ): d = 2.35
3
(
s, 6H), 7.14 (d, J = 7.9 Hz, 4H), 7.41 (d, J = 7.9 Hz, 4H) ppm.
1
,2-Bis(benzo[d]thiazol-2-yl)disulfane (Table 6, entry 4) H NMR (400 MHz,
1
CDCl ): d = 7.38 (t, J = 3.2 Hz, 2H), 7.46 (t, J = 3.2 Hz, 2H), 7.78 (d,
3
J = 3.2 Hz, 2H), 7.96 (d, J = 3.2 Hz, 2H) ppm.
1
1
,2-Bis(4,6-dimethylpyrimidin-2-yl)disulfane (Table 6, entry 5)
H
NMR
(
400 MHz, CDCl ): d = 2.39 (s, 12H), 6.76 (s, 2H) ppm.
3
0
1
,2 -Disulfanediyldiethanol (Table 6, entry 7) H NMR (400 MHz, CDCl3):
2
d = 2.58 (br, 2H), 2.89 (t, J = 5.6 Hz, 4H), 3.91 (t, J = 5.6 Hz, 4H) ppm.
1
,2-Di(naphthalen-2-yl)disulfane (Table 6, entry 9) H NMR (400 MHz, CDCl3):
1
d = 7.47 (m, 4H), 7.65 (m, 2H), 7.76 (m, 2H), 7.81 (m, 4H), 8.11 (s, 2H) ppm.
1
1
,2-Bis(4-bromophenyl)disulfane (Table 6, entry 10)
H
NMR (400 MHz,
CDCl ): d = 7.35 (d, J = 8.2 Hz, 4H), 7.43 (t, J = 8.2 Hz, 4H) ppm.
3
123

A. Ghorbani-Choghamarani et al.
Results and discussion
Characterization of DSA@MNPs as solid acid catalyst
The morphology and size of the DSA@MNPs were investigated by scanning
electron microscopy (SEM, Fig. 1) and transmission electron microscopy (TEM,
Fig. 2). It can be seen that most of the particles are quasispherical with
homogeneous average diameter of about 15–25 nm.
Formation of magnetite crystal phase in the DSA@MNP aggregate powder was
identified from the X-ray diffraction pattern. Figure 3 shows the XRD patterns for
Fe O MNPs and DSA@MNPs. As shown in Fig. 3, seven characteristic peaks
3
4
(
good agreement with the standard Fe O XRD spectrum. This analysis shows that
2h = 21.32°, 35.27°, 41.59°, 50.68°, 63.17°, 67.55°, and 74.51°) were observed, in
3
4
the surface modification of the Fe O nanoparticles did not lead to phase changes.
3
4
The average diameter of the DSA@MNPs was calculated to be 24 ± 1.6 nm
from the XRD results using Scherrer’s equation: D = kk/bcos h, as confirmed by
TEM (Fig. 2) and SEM (Fig. 1) techniques.
Figure 4 shows Fourier-transform infrared (FT-IR) spectra of (a) magnetic Fe O4
3
nanoparticles, (b) MNPs–dopamine, and (c) DSA@MNPs. The FT-IR spectrum for
-
Fe O alone shows stretching vibration at 3200–3400 cm which incorporates the
1
3
4
contributions from both symmetrical and asymmetrical modes of O–H bonds
attached to the surface of magnetic nanoparticles. The strong absorption near
-
1
73 cm is characteristic of Fe–O bond stretching vibration. In curve (b), which is
5
the spectrum of dopamine-functionalized MNPs, a peak near 1625 cm is assigned
-
1
-
1
to N–H bending vibrations and peaks near 1425 cm
stretching vibrations. The reaction of terminal NH groups with ClSO H was
are assigned to C–N
2
3
-
1
characterized by a 1000–1300 cm band which can indicate SO H groups.
3
Also, in the FT-IR spectrum of MNPs–dopamine, the presence of anchored
dopamine group is confirmed by N–H stretching vibration modes appearing at about
-
400 cm . Reaction of MNPs–dopamine with chlorosulfuric acid produced
1
3
Fig. 1 SEM image of
DSA@MNPs
1
23

Preparation of DSA@MNPs and application as heterogeneous…
Fig. 2 TEM images of DSA@MNPs
DSA@MNPs, in which the presence of SO H moiety is confirmed by
3
-
1
9
vibration. All of those bands reveal that the surface of the Fe O nanoparticles was
98–1220 cm
bands in the FT-IR spectrum, overlapped by C–H stretching
3
4
successfully modified with dopamine sulfonic acid.
Functionalization of the Fe O nanoparticles with dopamine sulfamic acid can be
inferred from the TGA results. Thermogravimetric analysis (TGA) curves of Fe O
3
4
3
4
MNPs, MNPs–dopamine, and DSA@MNPs are outlined in Fig. 5. The TGA curves
present different mass loss ranges. MNPs–dopamine presents two weight losses.
The weight loss in the first step, between 35 and 150 °C, can be attributed to
removal of surface water and surface hydroxyl groups. The second step of weight
loss, between 450 and 680 °C, is due to loss of organic groups. The TGA curve for
123

A. Ghorbani-Choghamarani et al.
0
5
10
15
20
25
30
35
40
45
Fig. 3 XRD patterns of DSA@MNPs (green line) and Fe O (red line). (Color figure online)
3 4
Fig. 4 FT-IR spectra of
a magnetic Fe O nanoparticles,
3 4
b MNPs–dopamine, and
c DSA@MNPs
Fig. 5 TGA diagrams of Fe
MNPs (blue line), MNPs–
dopamine (green line), and
3
O
4
100
90
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
DSA@MNPs (red line). (Color
figure online)
3
6
236
436
636
836
◦
Temperature( C)
the DSA@MNPs exhibits three weight loss steps. The first region shows mass loss
of about 7.2 %, attributable to removal of physically adsorbed solvent and surface
hydroxyl groups. The second weight loss is concentrated at temperature around
2
50 °C, being due to decomposition of sulfamic acid and formation of sulfur
1
23

Preparation of DSA@MNPs and application as heterogeneous…
dioxide. The third weight loss occurs near 500 °C, corresponding to decomposition
of organic groups [27].
The thermal stability of the DSA@MNPs was also determined, since synthesis of
many organic compounds is usually carried out at high temperature. As shown in
Fig. 5, this catalyst was stable even at 220 °C.
To determine the acid amount on the surface, the prepared catalyst (10 mg) was
added to aqueous NaCl solution (1 mol/L, 10 mL) with initial pH of 7.9. The
mixture was stirred for 0.5 h, after which the pH of the solution had decreased to
4
.1, indicating ion exchange between sulfamic acid protons and sodium ions; this is
equal to a loading of 0.7 mmol/g of sulfuric acid group. This result was confirmed
by back titration of the catalyst.
Catalytic studies
After characterization of the prepared nanocatalyst, in the present work we were
also interested in using this compound as an active and stable, magnetically
separable nanocatalyst for oxidation of sulfides and oxidative coupling of thiols
(
Scheme 2).
Oxidation of sulfides to sulfoxides
To determine the best reaction medium, different parameters were investigated for
oxidation of dibenzyl sulfide as model reaction, including use of different molar
ratios of H O , amounts of catalyst, and various solvents (Table 1).
2
2
Initially, the amount of catalyst was optimized; it was observed that the best
results were obtained with 0.02 g catalyst (Table 1, entry 3). Also, these experi-
ments showed that oxidation reaction did not occur in the absence of the catalyst
(
Table 1, entry 7).
Subsequently, the influence of different solvents was examined (Table 1,
entries 1–3). Among the solvents examined, the highest yield was achieved when
using ethanol (Table 1, entry 3).
Also, the amount of H O in oxidation of tetrahydrothiophene was optimized
2
2
(
entry 1). Meanwhile, the best results were observed when using 0.5 mL H O
Table 2). We found that the reaction did not occur in the absence of H O (Table 2,
2 2
2
2
(
Table 2, entry 4).
To determine the role played by iron (Fe) or Fe O MNPs and MNPs–dopamine
during the reactions, oxidation of tetrahydrothiophene was examined in the presence
3
4
Scheme 2 DSA@MNP-
catalyzed oxidation of sulfides to
sulfoxides and oxidative
coupling of thiols into disulfides
2 2
using H O
123

A. Ghorbani-Choghamarani et al.
Table 1 Optimization of oxidation of dibenzyl sulfide using dopamine sulfamic acid-functionalized
3 4
magnetic Fe O nanoparticles under various conditions
a
Entry
Solvent
Catalyst (mg)
Time (min)
Yield (%)
1
2
3
4
5
6
7
Acetone
n-Hexane
EtOH
20
20
20
15
10
5
4 h
24 h
60
88
NR
98
75
65
53
–
EtOH
60
EtOH
60
EtOH
60
EtOH
–
24 h
The reactions were carried out with 1 mmol dibenzyl sulfide, H
solvent at room temperature
2 2
O (0.5 mL), and 10 mL ethanol as
a
Isolated yield
Table 2 Oxidation of tetrahydrothiophene in presence of DSA@MNPs in different amounts of H
room temperature
2
O
2
at
a
Entry
H
–
2
O
2
(mL)
Time (min)
Yield (%)
b
–
1
2
3
4
5
a
20
20
20
20
15
c
45
0.3
0.4
0.5
0.6
c
65
92
94
Isolated yield
b
c
Reaction proceeds in absence of H
2 2
O
Isolated yield obtained by plate chromatography
3 4
Table 3 Effect of Fe O MNPs and MNPs–dopamine in comparison with DSA@MNPs in oxidation of
tetrahydrothiophene as model compound under optimized conditions
a
Entry
Substrate
Catalyst
–
Yield (%)
b
1
2
3
4
a
Tetrahydrothiophene
Tetrahydrothiophene
Tetrahydrothiophene
Tetrahydrothiophene
Trace
c
36
Fe
3
O
4
MNPs
c
18
MNPs–dopamine
DSA@MNPs
92
Isolated yield after 20 min
b
c
Reaction proceeds in absence of catalyst
Isolated yield obtained by plate chromatography
of DSA@MNPs compared with Fe O alone and MNPs–dopamine. The results of
3
4
this comparison are presented in Table 3, revealing that the products were obtained
in 36 and 18 % yield in the presence of Fe O MNPs and MNPs–dopamine,
respectively. These results show that Fe O acted as catalyst support and did not
3
4
3
4
1
23

Preparation of DSA@MNPs and application as heterogeneous…
play any role in the oxidation reaction. Also, these results show that Fe ion did not
leach during the reaction process.
Oxidative coupling of thiols to disulfides
The next part of the study focused on the utility of this catalyst for oxidative
coupling of thiols into their corresponding disulfides. In this reaction, 2-mercap-
tobenzothiazole was selected as model substrate.
The influence of reaction parameters such as solvents and catalyst amount was
tested (Table 4). Among the solvents investigated, ethanol was the best choice due
to the high yield and selectivity. To investigate the effect of catalyst amount on the
outcome of reaction, the reaction was carried out with different amounts of catalyst;
the results are presented in Table 4 (entries 6–10). The best result was obtained with
0
.004 g catalyst (Table 4, entry 6).
The generality of this approach was demonstrated by facile oxidation of various
sulfides, as presented in Table 5, the sulfoxides being obtained in high yields. As
shown in Table 5, a variety of sulfides were successfully employed to prepare the
corresponding sulfoxides. The experimental procedure is very simple. Also, various
thiols with various functional groups were coupled in the presence of DSA@MNPs
under optimized conditions, obtaining the corresponding disulfide compounds in
good to excellent yield (Table 5).
A possible mechanistic path for the oxidation of sulfides is shown in Scheme 3.
One explanation for this transformation is in situ formation of peroxyacid using the
reaction of DSA@MNPs with H O , followed by oxygen transfer to the organic
2
2
substrate (Scheme 3a). Another explanation is that DSA@MNPs act as protic acid,
which polarizes the O–O bond in hydrogen peroxide to produce the reactive oxygen
transfer agent (Scheme 3b).
Table 4 Optimization of oxidative coupling of 2-mercaptobenzothiazole using dopamine sulfamic acid-
nanoparticles under various conditions
functionalized magnetic Fe
3
O
4
a
Entry
Solvent
Catalyst (mg)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
EtOH
4
4
4
45
240
44
48
30
45
45
45
45
45
98
Acetone
n-Hexane
97
91
Dichloromethane
Ethyl acetate
EtOH
4
4
4
3
2
1
–
98
90
98
EtOH
80
EtOH
75
EtOH
42
10
EtOH
Trace
Reactions carried out with 1 mmol 2-mercaptobenzothiazole, H
solvent at room temperature
2 2
O (0.5 mL), and 10 mL ethanol as
a
Isolated yield
123

A. Ghorbani-Choghamarani et al.
Table 5 Oxidation of sulfides to sulfoxides in presence of DSA@MNPs
Entry
1
Substrate
Product
Catalyst (g)
0.02
Time (h) Yield (%)a M.p (°C) [Refs.]
1
97
85
138 [28]
Oil [29]
2
3
0.02
0.02
15min
45 min
55min
95
95
Brown oil [28]
Oil [30]
4
5
0.02
0.02
6
98
32 [29]
6
7
0.02
0.02
20 min
40 min
92
95
Oil [31]
Oil [29]
8
9
0.02
5
97
114–116 [32]
0.02
0.02
5min
95
70
85–89 [28]
1
0
20min
103–105 [28]
a
Isolated yield
The reusability of catalysts is one of their most important advantages, making
them useful for commercial applications. We found that the DSA@MNPs could be
rapidly recovered and demonstrated remarkably excellent recyclability. To inves-
tigate this issue, the recyclability of this catalyst was examined in oxidation of
dibenzyl sulfide and 2-mercaptobenzothiazole. After completion of the reaction, the
1
23

Preparation of DSA@MNPs and application as heterogeneous…
2 2
Table 6 Oxidative coupling of thiols into disulfides using H O in presence of DSA@MNPs
Entry
1
Substrate
Product
Time (min) Yield (%)a
M.p (°C) [Refs.]
69–70 [33]
60
15
90
90
97
70
2
3
38–40 [31]
94–96 [28]
4
5
47
96
70
174–175 [31]
165–167 [34]
200
90
6
7
80
81
Oil [29]
Oil [31]
2
2
8
9
120
95
90
274–281[29]
134–136 [31]
1
1
20
80
1
0
9
7
89–90 [29]
a
Isolated yield
catalyst was separated using an external magnet. The remaining catalyst was
washed with ethanol to remove residual product and dried in a hot air oven at
1
00 °C for 2 h. Then, fresh substrate was added to the remaining catalyst without
any activation and employed for the next reaction. As shown in Fig. 6, the catalyst
could be recycled in up to four runs without significant loss of catalytic activity.
123

A. Ghorbani-Choghamarani et al.
Scheme 3 Proposed mechanism for oxidation of sulfides in presence of DSA@MNPs as catalyst
Sulfide
Fig. 6 Reusability of
DSA@MNPs in oxidation of
sulfide (dibenzyl sulfide) and
Thiol
100
90
80
70
60
50
40
30
20
10
0
2-mercaptobenzothiazole
1
2
3
4
Run
Conclusions
We synthesized novel and recyclable DSA@MNPs and examined their catalytic
activity for synthesis of sulfoxide and disulfide derivatives. This active catalyst is
thermally stable, inexpensive, and easy to prepare. The advantages of this procedure
are use of commercially available, ecofriendly, cheap, and chemically stable oxi-
dant, mild reaction conditions, and operational simplicity. Product separation and
catalyst recycling are easier and simpler with the assistance of an external magnet.
Acknowledgments Financial support for this work by the research affairs of Ilam University, Ilam, Iran
is gratefully acknowledged.
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