A synthesis of sulfoxides and disulfides under classical and ultrasonic conditions in presence of recoverable inorganic–organic hybrid magnetism nanocatalysts Fe3O4@Tryptophan-M (M: Cu, Co and Fe)

Article abstract of DOI:10.1016/j.poly.2019.04.037

Three inorganic–organic hybrid magnetic nanocatalysts have been reported as new catalysts for the synthesis of sulfoxides and disulfides. Also, the effect of ultrasound irradiation on the synthesis of sulfoxides and disulfides was checked. The time of rea

Full text of DOI:10.1016/j.poly.2019.04.037

Accepted Manuscript
A synthesis of sulfoxides and disulfides under classical and ultrasonic conditions
in presence of recoverable inorganic-organic hybrid magnetism nanocatalysts
Fe3O4@Tryptophan-M (M: Cu, Co and Fe)
Nazanin Moeini, Mohammad Ghadermazi, Arash Ghorbani-Choghamarani
PII:
S0277-5387(19)30284-0
https://doi.org/10.1016/j.poly.2019.04.037
POLY 13897
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
12 December 2018
11 April 2019
22 April 2019
Please cite this article as: N. Moeini, M. Ghadermazi, A. Ghorbani-Choghamarani, A synthesis of sulfoxides and
disulfides under classical and ultrasonic conditions in presence of recoverable inorganic-organic hybrid magnetism
nanocatalysts Fe3O4@Tryptophan-M (M: Cu, Co and Fe), Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.
2019.04.037
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A synthesis of sulfoxides and disulfides under classical and
ultrasonic conditions in presence of recoverable inorganic-organic
hybrid magnetism nanocatalysts Fe3O4@Tryptophan-M (M: Cu,
Co and Fe)
Nazanin Moeini,a Mohammad Ghadermazi*,a Arash Ghorbani-Choghamaranib
a University of Kurdistan, Department of Chemistry, Faculty of Science, Sanandaj, Iran
e-mail: mghadermazi@yahoo.com
b Department of Chemistry, Faculty of Science, Ilam University, P.O. Box, 69315516, Ilam, Iran
e-mail: arashghch@gmail.com or a.ghorbani@mail.ilam.ac.ir
KEYWORDS: Ultrasound irradiation, Magnetism nanocatalyst, Sulfoxides, Disulfides.
ABSTRACT
Three inorganic-organic hybrid magnetic nanocatalysts have been reported as new catalysts for
the synthesis of sulfoxides and disulfides. Also, the effect of ultrasound irradiation on the synthesis
of sulfoxides and disulfides was checked. The time of reaction was reduced and the yield was
improved in the ultrasonic method compared with the conventional method. Magnetic
nanoparticles were used for several times without any significant change in activity. Easy
synthesis, high catalytic activity, mild reaction conditions, eco-friendliness, ease of separation of
the catalysts and good reusability are their advantages.

Address correspondence to University of Kurdistan, Department of Chemistry, Faculty of Science,
Sanandaj, Iran.; e-mail: mghadermazi@yahoo.com (M. Ghadermazi)
1

1 | INTRODUCTION
In recent decades, scientists have paid much attention to magnetic nanoparticles. Their
unique properties include shape control, monodisperse magnetic nanoparticle, eco-friendly
composition, thermal and chemical stability and easy separation. Magnetic nanoparticles
can be divided into several categories: spinel-type ferromagnets (CoFe2O4), iron oxides
(Fe3O4 and γ-Fe2O3), alloys (CoPt3 and FePt) and pure metals (Fe and Co).[1-5] Widder et
al reported the magnetic drug delivery properties of magnetic nanoparticles in the 1970s.[6]
Other uses of these compounds are protein separation and gene separation in
biotechnology.[ 7,8] Also, the magnetic nanoparticles (MNPs) used as a catalyst support in
organic synthesis. They have been used in a wide range of catalytic reactions, such as
hydrogenations, oxidations, C-C, C-S, C-N and etc.[9,10] For example copper(II)-
coordinated magnetic nanoparticles have been used as catalysts for the oxidation of sulfides
and thiols.[11]The synthesis of effective and easily separable catalysts in organic and
industrial reactions is also one of the most important applications of these nanoscale
compounds. [12]
Organosulfur compounds such as mercaptans, sulfides, and thiophenes are present in the
liquid fuels. These compounds reduce fuel quality and cause air pollution. Therefore,
sulfides and mercaptans must be removed from the fuel. For this purpose, oxidation of these
compounds is one of the best methods.[13,14] Moreover, sulfoxides and disulfides as the
products of sulfides and thiols oxidation are significant compounds in medicine and
industry. Sulfoxides are important synthetic intermediates in anti-atherosclerotic,
psychotropic, vasodilator drugs, as well as omeprazole and pesticide fipronil.[15-18]
Disulfides also enhance tensile strength in rubber.[19] Oxidation of sulfides and thiols is
done in the presence of some oxidants including sodium metaperiodate, m
[20]
chloroperoxybenzoic acid
and HNO3.[21] Because of some disadvantages of these
oxidation agents (toxicity, producing by-products like sulfone), H2O2 was suggested as a
green and inexpensive oxidant which its side product is water.[22]
In recent decades ultrasound irradiation has increasingly been considered as an
environmentally sound, simple and convenient method and time and cost-effective
approach in chemical synthesis. [23] Ultrasound irradiation is an important technique in the
synthesis of organic compounds. Some advantages of ultrasound method are accelerating
2

dissolution, increasing the reaction rates, enhancing product yields. [24-26] An investigation
of literature shows that condensation, substitution, oxidation, reduction, addition,
protection/deprotection, coupling, photochemical, and polymerization reactions have been
improved under this methodology.[27,28]
In this study, to develop greener and safer processes for oxidative coupling of thiols into disulfides
and oxidation of sulfides to sulfoxides, we report Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co
and Fe3O4@Tryptophan-Fe as new, highly efficient and stable, reusable nanocatalysts and simple
and inexpensive procedure. Also, reactions were compared under magnetic stirrer conditions and
in the presence of the ultrasonic irradiation.
2 | EXPERIMENTAL
2.1 | Materials
All chemicals were purchased from the Sigma-Aldrich and Merck. Powder X-ray diffraction
(XRD) was carried out on a Philips diffractometer of X’pert Company. The particle size and
microscopic morphology of nanocatalysts were visualized by FESEM-TESCAN MIRA3.
Magnetic properties were obtained on Vibrating Sample Magnetometer (VSM) MDKFD. All
melting points were recorded in the capillary tube on an Electrothermal 9100 apparatus. FT-IR
spectrum was obtained by KBr disc using a VRTEX 70 model BRUKER FT-IR
spectrophotometer. The metal content of the nanocatalysts was measured by Inductively Coupled
Plasma Optical Emission Spectrometry (ICP-OES). The thermal stability of the nanocatalysts was
studied by thermo-gravimetric analysis (TGA) using a Shimadzu DTG-60 instrument in the
temperature range of 50-600 ◦C.
2.2 | Preparation of magnetic Fe3O4 nanoparticles
MNPs were synthesized by using chemical co-precipitation method.[8] Fe(II) chloride and Fe(III)
chloride were added to the distilled water, the mole ratio for Fe2+ and Fe3+ ions is 1:2. The ammonia
solution was added to the salt mixture until pH of the suspension reaches about 10 under a nitrogen
atmosphere at 80 °C. The mixture was stirred mechanically for 30 min. Finally, the prepared black
precipitate was washed with distilled water and ethanol three times then dried in the oven.
3

2.3 | Preparation of the Fe3O4@Tryptophan nanoparticle
1 g of Fe3O4 Np was dispersed in 250 mL deionized water in the presence of nitrogen gas, and
then 0.408 g tryptophan was added. Stirring at 90 °C was continued for 24 h. Then the product
was washed with deionized water and dried in the oven.
2.4 | Preparation of the Fe3O4@Tryptophan-M nanoparticles
1 g of previous iron nanoparticle and 0.5 g of M(NO3)2.3H2O (M: Cu, Co and Fe) were mixed in
25 mL of ethanol under reflux condition for 15 h at 80 °C. After washing twice with ethanol, the
precipitates were isolated by applying a magnetic field.
2.5 | General procedure for oxidation of sulfides and thiols
For the fabrication of sulfoxide, 1mmol sulfide, 0.6 ml H2O2 (33%) and 0.005 g nanocatalyst were
stirred at room temperature. Also, for producing disulfides, 1mmol thiols and 0.4 ml H2O2 (33%)
with 0.008 g nanocatalyst in ethanol as a green solvent were mixed at 25 °C. After completion of
the reaction, the catalyst was removed by a magnet and the product was extracted with ethyl acetate
and dried using a vacuum dryer.
3 | RESULTS AND DISCUSSION
3.1 | Catalyst preparation
The mechanism of synthesis is shown in scheme 1. In the step 1, Fe3O4 synthesized with co-
precipitation method. In step 2, tryptophan was added to the previous substrate as ligand and
capping agent. In step 3, the metal was linked to this precursor with a coordination bond.
SCHEME 1
4

3.2 |Catalyst characterization
Three catalysts (Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and Fe3O4@Tryptophan-Fe)
successfully synthesized in nanometer-scale form and characterization was performed using XRD,
FT-IR, SEM, EDX, ICP-OES, TGA and VSM methods.
To prove the chemical purity of the heterogeneous catalysts, energy dispersive x-ray analysis was
tested. The EDX spectrum shows peaks of elements Cu, Co, Fe, N, C and O which is given in
Figure 1 as components of the synthesized Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and
Fe3O4@Tryptophan-Fe nanoparticles.
FIGURE 1
Morphology of the hybrid nanomaterials was investigated by SEM (Figure 2). The shapes of
Fe3O4@Tryptophan-Cu
nanoparticles,
Fe3O4@Tryptophan-Co
nanoparticles
and
Fe3O4@Tryptophan-Fe nanoparticles are spherical.
FIGURE 2
The magnetic properties of the Fe3O4, Fe3O4@Tryptophan-Co, Fe3O4@Tryptophan-Cu
nanoparticles, and Fe3O4@Tryptophan-Fe nanoparticles were measured by vibrating sample
magnetometer (VSM) at room temperature (Figure 3). VSM measurements for uncoated Fe3O4
NPs (62.82emu g-1) is higher than Fe3O4@Tryptophan-Co, Fe3O4@Tryptophan-Cu nanoparticles,
and Fe3O4@Tryptophan-Fe nanoparticles in the same field. These results indicated that the
magnetization of Fe3O4 reduced with the immobilized nonmagnetic materials (organic layers and
metal complex) on the iron oxide layer.[7] As it can be seen, the magnetization curve of the particles
passes through the origin of the graph, and there is no coercivity force and residual magnetism in
the VSM curve. The results showed complete reversibility of the magnetization process that
confirms the superparamagnetic behavior of the core (magnetite nanoparticles). [8]
5

FIGURE 3
The XRD measurements were obviously conducted to identify the synthesized nanocatalysts. The
XRD pattern of Fe3O4 NPs (Figure 4a) showed diffraction peaks at 2Ө (35.27, 41.52, 50.56, 63.24,
67.49, 74.47), corresponding to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) reflections.
The XRD results confirmed the fabrication of magnetite phase of magnetic with inverse spinel
[11,12]
crystal structure.
In the XRD patterns of Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co
and Fe3O4@Tryptophan-Fe (Figure 4 b, c and d) the characteristic peaks of the iron oxide layer
have not been changed, therefore inverse spinel structure was retained after grafting of tryptophan
and copper, cobalt and iron salt on iron oxide layers.
FIGURE 4
ICP-OES analysis was carried out to ascertain the exact values of the copper, cobalt and iron
loaded on the substrate. The amounts of Cu, Co and Fe immobilized on magnetic nanoparticles
were 0.247, 0.164 and 0.159 mmolg-1, respectively.
The characteristic functional groups of the tryptophan as a ligand (Figure 5. a), Fe3O4@Tryptophan
(Figure 5 b), Fe3O4@Tryptophan-Co (Figure 5. c), Fe3O4@Tryptophan-Fe (Figure 5. d) and Fe3O4-
Tryptophan-Cu (Figure 5. e) were investigated using FT-IR spectroscopy in the range of 500-4000
cm-1. The presence of vibration bands at 550-600 cm- 1 were assigned to the presence of the
vibration metal-oxygen bond (Fe-O) in the Fe3O4@Tryptophan (Figure 5.b), Fe3O4-Tryptophan-
Cu (Figure 5. c), Fe3O4@Tryptophan-Co (Figure 5. d) and Fe3O4@Tryptophan-Fe (Figure 5. e).[
1,2]
The peaks between 2800 and 2900 cm-1 are due to the methylene groups (aliphatic C-H
stretching vibration)].[3] NH2 stretching band appeared at 3391 cm-1 in Fe3O4@Tryptophan while
in the Fe3O4-Tryptophan-Cu (Figure 5. c), Fe3O4@Tryptophan-Co (Figure 5. d) and
Fe3O4@Tryptophan-Fe (Figure 6. e) are shifted to 3389 cm-1, 3380 cm-1, 3358 cm-1. The FT-IR
measurement of Fe3O4@Tryptophan (Figure 5.b) and Fe3O4-Tryptophan-Cu (Figure 5. c),
Fe3O4@Tryptophan-Co (Figure 5. d) and Fe3O4@Tryptophan-Fe (Figure 5. e) show the bending
vibration of NH2 at 1402 cm-1, 1381 cm-1, 1383 cm-1 an1d 1383 cm-1.[29] These observations
6

strongly suggested the immobilization of metals on the Fe3O4@Tryptophan layers and bonding of
metals to nitrogen in tryptophan.[30]
FIGURE 5
The thermos-gravimetric analysis (TGA) was investigated for determination on the
organic groups supported on the surface of Fe3O4 magnetic nanoparticles (Figure 6). The
small amount of weight loss in the range of 30-200 ◦C is because of the removal of hydroxyl
groups and adsorbed solvent on the magnetic nanoparticles [31]. The organic layers on the
◦
surface of Fe3O4 were decomposed in the range of 200-600 C. The weight loss of the
Fe3O4@Tryptophan-Cu,
Fe3O4@Tryptophan-Co
and
Fe3O4@Tryptophan-Fe
nanoparticles were observed in the Figure 6.
FIGURE 6
3.3 | Catalytic study
Three heterogeneous catalysts (Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and
Fe3O4@Tryptophan-Fe) were employed to produce sulfoxides and disulfides .The catalytic
activity and efficiency of them were investigated to produce sulfoxides (Scheme 2), disulfides
(Scheme 3). Also, we reported facile sonochemical synthesis of sulfoxides and disulfides upon
ultrasound irradiation.
Several synthesis parameters must be effectively controlled to reach a satisfactory outcome. The
progress of reaction was monitored by TLC (Thin Layer Chromatography), and different
parameters including amount of catalyst, various solvents were checked to obtain optimal reaction
conditions. The obtained data were collected in Table 1. As could see as in Table 1 (entry3), 0.6
mL of H2O2 (33%), and 0.005 g of Fe3O4@Tryptophan-Cu in solvent-free condition gave the best
results, so they were chosen as optimization reaction condition for oxidation of sulfide to sulfoxide.
The oxidation reaction was performed without catalyst and no desired product was obtained even
after 2 h (Table7, entry 9). Also the catalytic activity of synthesized Fe3O4 and Fe3O4@Tryptophan
were checked for oxidation reaction (Table7, entry 7 and 8).
7

SCHEME 2
TABLE 1
Under the optimum reaction conditions (0.005 g nanocatalyst, 0.6 ml of H2O2 (33%), free solvent),
catalytic activity of Fe3O4@Tryptophan-Cu was investigated in the oxidation reaction of various
sulfides derivatives (Table 2). As it presented in Tables 2, also the oxidation of various sulfides
were checked in the presence of Fe3O4@Tryptophan-Co and Fe3O4@Tryptophan-Fe as
nanocatalysts. They have short reaction times in high to excellent yields which showed efficiency
and good catalytic activity of these nanocatalysts in the producing sulfoxide. It is well known that
oxidation reaction time and yield were sensitive to type of catalyst effect. As shown in table 2, rate
of reaction in the presence of Fe3O4@Tryptophan-Cu was increased significantly. Mechanism for
the oxidation of sulfides in the presence of catalyst is reported in the literatures.[31]
TABLE 2
Also, we were interested in oxidation reaction of thiols to disulfides as important bio compounds.
The results which are described in Tables 3 showed the optimization conditions of oxidation
coupling reaction of thiols. Various amounts of Fe3O4@Tryptophan-Cu were tested and 0.008 g
gave the best result (Table 3 entry 4). In continue, both polar and nonpolar solvent (Ethyl acetate,
Acetonitrile, Ethanol, Solvent-Free) were checked. In order to find appropriate amount of H2O2,
we followed the effect of various amount of H2O2 on the model reaction. Next, as shown in Table
4, we turn our attention to the catalytic oxidation of different thiols in the presence of
Fe3O4@Tryptophan-M (Cu, Co and Fe). It was observed that the synthesizing disulfide in the
presence of these catalysts afforded good to excellent yields with short reaction times.
SCHEME 3
8

TABLE 3
TABLE 4
The chemoselectivity of substrate containing a hydroxyl group (2-(Mercaptoethanol) in the
oxidation of thiol group was checked. In these oxidation reactions, as shown in scheme 4, hydroxyl
group remained intact and functional group of thiol was oxidized to disulfide.
SCHEME 4
Mechanism for the oxidative coupling of thiols to disulfides in the presence of catalyst is reported
in literatures. [32, 33]
In the last part of our study, the effect of ultrasound on chemical reactions was checked. As shown
in the table 5 and table 6, the oxidation of sulfides and thiols were done by conventional method
and sonication method in the presence of Fe3O4@Tryptophan-Cu. The chemical effects of
ultrasound afford significant improvements in catalytic chemical reactions. The result showed that
the time of reaction was reduced and the yields were improved in the ultrasonic condition
compared with the classical condition.
TABLE
TABLE
3.4. Reusability of the catalysts
The reusability of the Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and Fe3O4@Tryptophan-Fe
for oxidation of sulfides and oxidation coupling reaction of thiols were checked under the
9

mentioned condition. The magnetic nanocatalysts structure preserved after several consecutive runs
for oxidation of sulfide and thiol (Figure 7). The magnetic nanocatalysts were separated from
reaction by an external magnet and reused in the next run without any notable loss of catalytic
activity and selectivity for these reactions. The Cu(II) and Co(II) and Fe(II) contents of recovered
Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and Fe3O4@Tryptophan-Fe were determined by
ICP analysis, which was found to be 0.21 mmol g-1, 0.143 mmol g-1and 0.12 mmol g-1 for
oxidation of sulfides and 0.192 mmol g-1, 0.130 mmol g-1and 0.109 mmol g-1 for oxidation of
thiols, respectively.
FIGURE 7
3.5. The hot filtration test
The hot filtration test is used for the determination of leaching metal from the catalyst. In the
presence of Fe3O4@Tryptophan-Cu, the oxidation of methyl phenyl sulfide was done in the
optimization condition in halftime of the reaction. The yield of product was 62%. Simultaneously,
the synthesis of sulfide was done in the same condition, with the difference that the nanocatalyst
was removed from the mixture in the halftime of reaction and the reaction was continued. Finally,
the yield of reaction was 65%. This result confirmed that the leaching of copper from the
heterogeneous catalyst has not been happened.
3.6. Comparison of the catalysts
In this study, Fe3O4, Fe3O4@Tryptophan, Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and
Fe3O4@Tryptophan-Fe were compared with catalysts those reported previously. As shown in
Table 7, the oxidation reaction of sulfide and thiol in the presence of the synthesized catalysts had
a better yield and shorter time in compared to other reported catalysts.
TABLE 7
10

4. Conclusions
In summary, three nanocatalysts (Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and
Fe3O4@Tryptophan-Fe) were successfully synthesized through a facile method. Both the chemical
nature and the structure of the catalysts were confirmed using XRD, FT-IR, SEM, EDX, ICP-OES,
TGA and VSM methods. These sustainable magnetic nanocatalysts lead to the efficient oxidation
of an extensive range of sulfides, thiols with good yields and selectivity under mild conditions.
Furthermore, applying ultrasound irradiation in the oxidation of sulfur-containing compounds lead
to increased yields, decrease reaction times and make reaction condition to milder ones. The
simplicity, efficiency, good yield of products, and green protocol are notable features of these
novel nanocatalysts.
Acknowledgements
We are grateful to Erfan Ghadermazi and the University of Kurdistan for support of this work.
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TABLE
TABLE 1 Optimization of the oxidation of sulfides by Fe3O4@Tryptophan-Cu under various conditions.
Entry
Solvent
(2mL)
H2O2
(mL)
Catalyst
Time
(min)
Yield
(%)a
(g)
1
Solvent-Free
0.6
0
120
Trace
2
3
4
5
6
7
9
8
Solvent-Free
Solvent-Free
Solvent-Free
Solvent-Free
Solvent-Free
EtOH
0.6
0.6
0.6
0.4
0.2
0.6
0.6
0.6
0.003
0.005
0.007
0.005
0.005
0.005
0.005
0.005
100
45
81
96
98
82
70
45
40
20
40
75
150
5h
Ethyl acetate
Acetonitrile
6h
13h
a Isolated yield
13

TABLE 2 Oxidation of sulfides to the sulfoxides in the presence of Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co
and Fe3O4@Tryptophan-Fe.
Entr
y
Substrate
Fe3O4@Tryptophan-Cu
Time (min ) Yield (%)a
Fe3O4@Tryptophan-Co
Fe3O4@Tryptophan-Fe
M.p.
(°C)
Time (min ) Yield (%)a Time (min ) Yield (%)a
1
2
3
4
5
Benzylphenylsulfide
Tetrahydrothiophene
60
15
10
45
10
89
92
75
25
20
55
20
86
90
85
30
25
60
30
80
90
118-125
[29]
Oil [29]
3,3′-Thiodipropionic
acid
90
88
86
200-205
Oil [30]
Methylphenylsulfide
96
94
95
89
92
87
Thiodiglycolic acid
160-170
Oil [29]
6
7
Diethylsulfide
55
40
95
92
75
65
93
89
90
80
88
85
Dibenzylsulfide
140-143
[29]
8
Ethylphenylsulfide
50
87
70
85
75
86
Oil [29]
aIsolated yield, 1mmol sulfide, 0.6 ml H2O2 (33%) and in free-solvent condition with 0.005 g nanocatalyst.
TABLE 3 Optimization of the oxidation of thiols by Fe3O4@Tryptophan-Cu under various conditions.
Entry
Solvent
(2mL)
EtOH
H2O2
(mL)
0.4
Catalyst
Time
(min)
120
Yield
(%)a
14
(g)
0
1
2
3
4
5
6
7
8
9
EtOH
EtOH
0.4
0.4
0.4
0.6
0.2
0.4
0.4
0.4
0.004
0.006
0.008
0.008
0.008
0.008
0.008
0.008
80
60
20
18
30
55
45
100
72
80
89
90
80
76
80
55
EtOH
EtOH
EtOH
Solvent-Free
Ethyl acetate
Acetonitrile
a Isolated yield
14

TABLE 4 Oxidative coupling of thiols by Fe3O4@Tryptophan-M under optimization conditions.
Entry
Substrate
Fe3O4@Tryptophan-Cu
Fe3O4@Tryptophan-
Co
Fe3O4@Tryptophan-Fe
M.p
[Ref]
Time (min ) Yield
(%)a
Time (min) Yield
(%)
Time (min ) Yield
(%)a
1
2
2-Mercaptoethanol
18
15
83
78
30
35
82
78
30
35
80
75
Oil [2]
Mercaptosuccinic
acid
105-108
[11]
3
4
5
2-Mercaptobenzoic
acid
20
40
45
95
82
86
25
75
15
91
80
90
20
92
54
80
280-282
[11]
Benzyl mercaptan
235
45
77-79
[30]
Benzo[d]thiazole-2-
thiol
169-171
[11]
6
25
91
89
35
88
50
80
98-100
[11]
Benzo[d]oxazole‐2‐
thiol
7
8
Thiophenol
20
15
30
30
87
85
80
80
72
70
55 [32]
4-
96
38-40
[31]
Methylbenzenethiol
a Isolated yield, 1mmol thiols and 0.4 ml H2O2 (33%) with 0.008 g nanocatalyst in ethanol.
TABLE
′
aIsolated yield, 1mmol sulfide, 0.6 ml H2O2 (33%) and in free-solvent condition with 0.005 g nanocatalyst
15

TABLE 6 Oxidative coupling of thiols by Fe3O4@Tryptophan-Cu under
Entry
Substrate
Conventional method
Sonication method
Time (min ) Yield (%)a
Time (min) Yield (%)
1
2
2-Mercaptoethanol
18
15
83
78
7
5
85
80
Mercaptosuccinic acid
3
4
5
6
8
9
2-Mercaptobenzoic acid
Benzyl mercaptan
20
40
45
25
20
15
95
82
86
91
89
96
5
95
90
90
95
92
98
10
5
Benzo[d]thiazole-2-thiol
Benzo[d]oxazole‐2‐thiol
Thiophenol
10
10
10
4-Methylbenzenethiol
a Isolated yield, 1mmol thiols and 0.4 ml H2O2 (33%) with 0.008 g nanocatalyst in ethanol
TABLE 7 Comparison of Fe3O4@Tryptophan@Cu, Fe3O4@Tryptophan-Co, Fe3O4@Tryptophan-Fe, and
Fe3O4@Tryptophan and without catalyst for oxidation of methyl phenyl sulfide and 4-Methylbenzenethiol with
previously reported catalysts.
Entry
Substrate
Catalyst
Time
(min)
120
Yield
(%)a
57
Ref
1
2
3
Ph–SCH3
Ph–SCH3
Ph–SCH3
2NaBO3-4H2O(I)/KBr
Ni–salen–MCM-41
[34]
[35]
[36]
156
90
Polymer-anchored Cu(II)
180
90
4
Ph–SCH3
silica vanadic acid (III) in
acetonitrile
5
94
[39]
5
6
Ph–SCH3
Ph–SCH3
Cu-SBTU@MCM-41
70
97
55
[40]
[41]
2NaBO3 - 4H2O/ silica
sulfuric acid/ KBr
840
7
Ph–SCH3
2NaBO3 - 4H2O
120
57
[41]
/2Na2CO3 - 3H2O2
silica sulfuric acid / KBr
8
Ph–SCH3
Ph–SCH3
Fe3O4@Tryptophan-Cu
Fe3O4@Tryptophan-Co
Fe3O4@Tryptophan-Fe
Fe3O4@Tryptophan
Fe3O4
45
55
96
95
92
25
27
This work
This work
This work
This work
This work
9
10
11
12
13
14
Ph–SCH3
60
Ph–SCH3
120
120
120
90
Ph–SCH3
Ph–SCH3
without catalyst
Trace This work
99 [30]
4-Methylbenzenethiol
Fe3O4-Adenine-Zn
16

15
16
17
18
19
20
21
22
23
24
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
70
60
97
96
94
97
96
85
70
10
18
[37]
Fe3O4‐adenine‐Ni
Fe3O4@Tryptophan@Ni
Ni-SMTU@boehmite
Cu-SBTU@MCM-41
Fe3O4@Tryptophan-Cu
Fe3O4@Tryptophan-Co
Fe3O4@Tryptophan-Fe
Fe3O4@Tryptophan
Fe3O4
[32]
93
[38]
40
[40]
15
This work
This work
This work
This work
This work
30
80
120
120
120
without catalyst
Trace This work
SCHEME captions
SCHEME 1 Synthesis of Fe3O4@Tryptophan-M (Cu, Co and Fe) nanocatalysts.
SCHEME 2 General scheme for oxidation of sulfide to sulfoxides.
SCHEME 3 Fe3O4@Tryptophan-M catalysed the oxidative coupling of thiols into disulfides.
SCHEME 4 The chemoselectivity of 2-Mercaptoethanol.
Figure captions
FIGURE
1 EDX spectrum of Fe3O4@Tryptophan-Cu nanoparticles (a), Fe3O4@Tryptophan-Co nanoparticles
(b) and Fe3O4@Tryptophan-Fe nanoparticles (c).
FIGURE 2 FESEM images of Fe3O4@Tryptophan-Cu nanoparticles (a), Fe3O4@Tryptophan-Co nanoparticles
(b) and Fe3O4@Tryptophan-Fe nanoparticles (c) at 500 nm (top) and 1 μm (down).
FIGURE 3 VSM curve of Fe3O4 (a), Fe3O4@Tryptophan-Co (b) Fe3O4@Tryptophan-Cu nanoparticles (c), and
Fe3O4@Tryptophan-Fe nanoparticles (d).
FIGURE 4 The XRD patterns of Fe3O4 (a), Fe3O4@Tryptophan-Cu (b) Fe3O4@Tryptophan-Co (c), and
Fe3O4@Tryptophan-Fe (d).
17

FIGURE 5 FT-IR spectra for the Tryptophan (a), Fe3O4@Tryptophan (b), Fe3O4@Tryptophan-Cu (c), and
Fe3O4@Tryptophan-Co (d) and Fe3O4@Tryptophan-Fe (e).
FIGURE 6 The TGA diagrams of Fe3O4 nanoparticles (a), Fe3O4@Tryptophan-Fe (b), Fe3O4@Tryptophan-Co (c) and
Fe3O4@Tryptophan-Cu (d).
FIGURE 7 Reusability of the Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and Fe3O4@Tryptophan-Fe for
oxidation of sulfides (a, b, c) and reusability of the Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co and
Fe3O4@Tryptophan-Fe for oxidation coupling reaction of thiols (d, e, f).
18

19

20

21

22

23

24

25

26

27

28

29