Synthesis and characterization of tribenzyl ammonium-tribromide supported on magnetic Fe3O4 nanoparticles: a robust magnetically recoverable catalyst for the oxidative coupling of thiols and oxidation of sulfides

Article abstract of DOI:10.1007/s11164-016-2790-6

Abstract: Taking into account the principles of green chemistry, magnetic nanoparticles, especially Fe₃O₄ nanoparticles, open up a new chapter in modern organic synthesis to inset a fascinating, stupendous and efficient catalytic strategy for facilitating catalyst recovery in various chemical reactions. Inspired by this topic, tribenzyl ammonium-tribromide immobilized on magnetic nanoparticles (Fe₃O₄–TBA-Br₃) as a bromine source was successfully synthesized and its catalytic activity in the oxidative coupling of thiols and oxidation of sulfides was investigated. It is the first report on the use of the immobilized bromine source on Fe₃O₄ nanoparticles as a nanomagnetic recyclable catalyst for the oxidative coupling of thiols. The nanosolid catalyst could be easily recovered by a simple magnetic separation and reused for several cycles without significant degradation in catalytic activity. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-016-2790-6

Res Chem Intermed
DOI 10.1007/s11164-016-2790-6
Synthesis and characterization of tribenzyl ammonium-
tribromide supported on magnetic Fe O nanoparticles:
3
4
a robust magnetically recoverable catalyst
for the oxidative coupling of thiols and oxidation
of sulfides
1
1
•
Lotfi Shiri
Mosstafa Kazemi
•
Arash Ghorbani-Choghamarani
1
Received: 30 July 2016 / Accepted: 19 October 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Taking into account the principles of green chemistry, magnetic nanoparti-
cles, especially Fe O nanoparticles, open up a new chapter in modern organic synthesis
3
4
to inset a fascinating, stupendous and efficient catalytic strategy for facilitating catalyst
recovery in various chemical reactions. Inspired by this topic, tribenzyl ammonium-
tribromide immobilized on magnetic nanoparticles (Fe O –TBA-Br ) as a bromine
3
4
3
source was successfully synthesized and its catalytic activity in the oxidative coupling of
thiols and oxidation of sulfides was investigated. It is the first report on the use of the
immobilized bromine source on Fe O nanoparticles as a nanomagnetic recyclable
3
4
catalyst for the oxidative coupling of thiols. The nanosolid catalyst could be easily
recovered by a simple magnetic separation and reused for several cycles without sig-
nificant degradation in catalytic activity.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2790-
6) contains supplementary material, which is available to authorized users.
&
Lotfi Shiri
Lshiri47@gmail.com
Arash Ghorbani-Choghamarani
Arashghch58@yahoo.com
Mosstafa Kazemi
Mosstafakazemi@gmail.com
1
Department of Chemistry, Faculty of Basic Sciences, Ilam University,
P.O. Box 69315-516, Ilam, Iran
123

L. Shiri et al.
Graphical Abstract
Keywords Bromine source Á Oxidation Á Thiols Á Sulfides Á External magnet Á
Recyclable catalyst
Introduction
Oxidative coupling of thiols and oxidation of sulfides are two fundamental
transformations in organic chemistry because their products, namely disulfides and
sulfoxides, respectively, are two important classes of organic sulfur compounds for
which their application as precursors or intermediates in organic synthesis are
highly regarded among organic chemists [1, 2]. Besides synthetic applications,
interest in disulfides and sulfoxides has been amplified when it was revealed that
they not only play a crucial role in the synthesis of natural products and valuable
physiologically and pharmacologically active molecules but also exist as important
integral and supplementary parts in many pharmaceutical and biological active
molecules such as cystine, DTNB (Ellman’s reagent or 5,50-dithio-bis(2-nitroben-
zoic acid), omeprazole and fipronil [3–6]. These fascinating applications have
recently stimulated organic chemists to apply more and more focus on new, clean
and economic methodologies for the synthesis of disulfides and sulfoxides. During
recent years, a diverse range of reagents, catalysts and oxidants have been reported
for the performance of these transformations [7]. A series of tremendous properties
such as being inexpensive and environmentally benign as well as producing water as
1
23

Synthesis and characterization of tribenzyl ammonium…
the only by-product has caused hydrogen peroxide to be more extensively employed
in oxidation processes compared to other oxidants [8].
The easy recovery of catalysts and their reusability are two prominent
characteristics in most of modern catalytic processes because either the employed
catalysts are often very expensive or the targeted products are often very valuable
from the economical and medicinal points of view [9, 10]. In this respect, as an
important class of separable materials, magnetic nanoparticles (MNPs) have
recently become a hot research topic in organic synthetic chemistry and material
sciences, because of their high potential in catalysis and numerous applications in
biomedicine [11]. Numerous recent studies reported in literature clearly demon-
strated that among the various MNPs employed as the core magnetic support, Fe O4
3
nanoparticles have received more special attention in modern catalysis research due
to their tremendous properties such as simple synthesis and functionalization,
abundance of unique activities, high stability, high surface area, low toxicity and
price and simple separation by magnetic forces [12–14]. These attractive and
valuable properties have caused Fe O nanoparticles to emerge as a promising
3
4
alternative to other catalyst supports [15].
Up to now, a diverse range of synthetic protocols have been developed for the
construction of organic compounds and synthetic intermediates using molecular
bromine [16]. To decrease the toxicity of molecular bromine (since molecular
bromine is toxic), during the recent decades, a series of tribromide-containing
compounds have been prepared and successfully applied as catalysts for organic
reactions to obtain the corresponding targeted synthetic products [17, 18]. The
immobilization of N-tribromide reagents on MNPs can be regarded as an efficient
and fascinating catalytic strategy to overcome this drawback. Inspired by this
catalytic strategy, we synthesized immobilized tribenzyl ammonium-tribromide on
Fe O nanoparticles (Fe O –TBA-Br ) and studied their catalytic activities in the
3
4
3
4
3
oxidative coupling of thiols and oxidation of sulfides. According to the best of our
knowledge, it is the first report on the use of the immobilized bromine source on
Fe O nanoparticles as a nanomagnetic recyclable catalyst for the oxidative
3
4
coupling of thiols.
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. Nanostructures were characterized using a Holland Philips
X-pert X-ray powder diffraction (XRD) diffractometer (Co Ka, radia-
-
1
tion = 0.154056 nm), at a scanning speed of 2° min from 10° to 100°. The
particle size and morphology were investigated using a JEOL JEM-2010 scanning
electron microscope at an accelerating voltage of 200 kV. The IR spectra of samples
were recorded in KBr disks using a NICOLET Impact 410 spectrometer. The
thermogravimetric analysis (TGA) curves are recorded using a PL-STA 1500 device
123

L. Shiri et al.
manufactured by Thermal Sciences. Supermagnetic properties of catalysts were
measured on a VSM (MDKFD, Iran.
Preparation of the magnetic Fe O nanoparticles
3
4
The mixture of FeCl Á6H O (5.838 g, 0.0216 mol) and FeCl Á4H O (2.147 g,
3
2
2
2
0
.0108 mol) were dissolved in 100 mL of deionized water in a three-necked bottom
250 mL) under N atmosphere. After that, under rapid mechanical stirring, 10 mL of
(
NH was added into the solution within 30 min with vigorous mechanical stirring.
2
3
After being rapidly stirred for 30 min, the resultant black dispersion was heated to
8
0 °C for 30 min. The black precipitate formed was isolated by magnetic decantation,
washed with double-distilled water until neutrality, and further washed twice with
ethanol and dried at room temperature.
Preparation of Fe O –APTMS
3
4
The obtained Fe O nanoparticles (1.5 g) were dispersed in 250 mL of ethanol/
3
4
water (volume ratio, 1:1) by sonication for 30 min, and then APTMS (2.5 mL) was
added to the mixture reaction. The reaction mixture was stirred using mechanical
stirring under N2 atmosphere at 40 °C for 8 h. Then, the nanoparticles were
redispersed in ethanol by sonication for five times and separated through magnetic
decantation. The nanoparticle product (Fe O –APTMS) was dried at room
3
4
temperature.
Preparation of Fe O –TBA-Br
3
4
The obtained Fe O –APTMS (1 g) was added to a mixture of K CO (2 mmol) and
3
4
2
3
benzyl bromide (2.5 mL) in DMF. The mixture was then stirred at 80 °C for 30 h
under N atmosphere. The resulting powder was separated by magnetic decantation.
The magnetic powder was washed with ethanol and dried at room temperature.
2
Preparation of Fe O –TBA-Br catalyst
4
3
3
The Fe O –TBA-Br (1 g) was dispersed in CCl (10 mL) by ultrasonic bath for
3
4
4
2
0 min. Subsequently, Br (4 mL) was added and the mixture was stirred for 8 h at
2
room temperature. Then, the final product was separated by magnetic decantation
and washed by CCl to remove the unattached substrates. The product (Fe O –
TBA-Br ) was stored in a refrigerator for use as a catalyst.
4
3 4
3
General procedure for the oxidative coupling of thiols to the disulfides
A mixture of thiol (1 mmol), hydrogen peroxide (0.5 mL) and the catalyst (0.005 g)
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 an external magnet and washed with ethyl acetate; next,
the product was extracted with ethyl acetate (5 mL 9 4). The organic layer was
1
23

Synthesis and characterization of tribenzyl ammonium…
dried over anhydrous Na SO (1.5 g). Finally, the organic solvents were evaporated,
2
4
and the corresponding disulfides were obtained in high to excellent yields
88–98%).
(
General procedure for the oxidation of sulfides to the sulfoxides
The catalyst (0.010 g) was added to solution of sulfide (1 mmol) and H O (0.5 mL)
2
2
in ethanol (2 mL). The reaction mixture was stirred at room temperature, and the
progress of the reaction was monitored by TLC (acetone:n-hexane, 2:8). After
completion of the reaction, catalyst was separated by an external magnet and
washed with ethyl acetate; next, the product was extracted with ethyl acetate
(
5 mL 9 4). The organic layer was dried over anhydrous Na SO (1.5 g). Finally,
2 4
the organic solvents were evaporated, and the corresponding sulfoxides were
obtained in high to excellent yields (88–99%).
Spectroscopic data
All the products reported here are known compounds and the spectroscopic data was
matched literature values. Data for the some of the compounds are given below.
1
,2-Bis(4-methylphenyl)disulfane (2f)
1
H NMR (400 MHz, CDCl ): d = 2.62 (s, 6H), 6.88 (d, J = 8.8 Hz, 4H), 7.32 (d,
3
1
3
J = 8.8 Hz, 4H) ppm. C NMR (100 MHz, CDCl ): d = 22.1, 126.6, 128.8, 131.9,
3
1
35.5.
1
,2-Bis(4-fluorophenyl)disulfane (2g)
1
H NMR (400 MHz, CDCl ): d = 7.47 (d, J = 8.8 Hz, 4H), 7.56 (d, J = 8.8 Hz,
3
1
3
4
H) ppm. C NMR (100 MHz, CDCl ): d = 119.46, 129.48, 132.27, 145.32.
3
1
,2-Bis(4-bromophenyl)disulfane (2h)
1
H NMR (400 MHz, CDCl ): d = 7.54 (d, J = 8.8 Hz, 4H), 8.23 (d, J = 8.8 Hz,
3
1
3
4
H) ppm. C NMR (100 MHz, CDCl ): d = 115, 121.3, 131.9, 139.1.
3
1
,2-Dibenzyldisulfane (2m)
1
13
H NMR (400 MHz, CDCl ): d = 3.75 (s, 4H), 7.03–7.15 (m, 10H) ppm.). C
3
NMR (100 MHz, CDCl ): d = 43.35, 128.34, 128.51, 129.13, 137.49.
3
Methyl phenyl sulfoxide (4a)
1
13
H NMR (400 MHz, CDCl ): d = 2.68 (s, 3 H, CH ), 7.45–7.62 (m, 5 H, ArH). C
3
3
NMR (100 MHz, CDCl ): d 43.92, 123.47, 129.35, 131.05, 145.60.
3
123

L. Shiri et al.
Benzyl phenyl sulfoxide (4b)
1
H NMR (400 MHz, CDCl ): d = 4.02 (d, 1 H, J = 12.8 Hz), 4.11 (d, 1 H,
3
1
J = 12.8 Hz), 6.99–7.01 (d, 2 H), 7.25–7.50 (m, 8 H). C NMR (100 MHz,
3
CDCl ): d 63.57, 124.45, 128.27, 128.47, 128.88, 129.13, 130.39, 131.20, 142.74.
3
Benzyl 4-methylphenyl sulfoxide (4e)
1
H NMR (400 MHz, CDCl ): d = 2.43 (s, 3 H), 4.00 (d, 2 H, J = 12.8 Hz), 4.12 (d,
3
1
3
2
H, J = 12.4 Hz), 7.02–7.31 (m, 9 H). C NMR (100 MHz, CDCl ): d 21.49,
3
6
3.70, 124.50, 128.21, 128.48, 129.33, 129.59, 130.40, 130.52, 141.67.
4
-Chlorobenzyl 4-methylphenyl sulfoxide (4f)
1
H NMR (400 MHz, CDCl ): d = 2.43 (s, 3 H), 4.00 (s, 2 H), 6.92–6.94 (d, 2 H,
3
1
3
J = 8 Hz), 7.23–7.30 (m, 6 H). C NMR (100 MHz, CDCl ): d 21.49, 62.52,
3
1
24.42, 127.66, 128.59, 129.78, 131.69, 134.34, 139.19, 141.84.
Benzyl 4-bromophenyl sulfoxide (4g)
1
H NMR (400 MHz, CDCl ): d = 4.02 (d, 2 H, J = 12 Hz), 4.13 (d, 2 H,
3
1
3
J = 12 Hz), 6.99–7.59 (m, 9 H). C NMR (100 MHz, CDCl ): d 63.42, 125.66,
3
1
26.07, 128.48, 128.60, 130.40, 132.05, 141.83.
Dibenzyl sulfoxide (4j)
1
H NMR (400 MHz, CDCl ): d = 3.898 (d, 2 H, J = 12.8 Hz), 3.957 (d, 2 H,
3
1
3
J = 12.8 Hz), 7.29–7.43 (m, 10 H). C NMR (100 MHz, CDCl ): d 57.28, 128.40,
3
1
28.99, 130.15, 130.18.
Results and discussion
Preparation and characterization of Fe O –TBA-Br
3
3
4
Immobilized tribenzyl ammonium-tribromide on Fe O nanoparticles (Fe O –TBA-
3
4
3 4
Br catalyst) was successfully synthesized by using the surface modification strategy
3
as depicted in Scheme 1. The Fe O nanoparticles were prepared by a chemical co-
3
4
precipitation method [19] and coated with 3-aminopropyltrimethoxysilane
APTMS) by covalent bonds [19]. The immobilized ammonium bromide on
Fe O nanoparticles (Fe O –TBA-Br) was prepared through the reaction of the
(
3
4
3 4
supported APTMS with tribenzyl bromide in the presence of K CO in DMF
2
3
(
Fe O –TBA-Br with Br in tetrachloride carbon at ambient temperature for 8 h led
dimethylformamide) under reflux conditions for 30 h. Finally, the reaction of
3
4
2
to generation of immobilized tribenzyl ammonium-tribromide on Fe O nanopar-
3
4
ticles (Fe O –TBA-Br ).
3
4
3
1
23

Synthesis and characterization of tribenzyl ammonium…
3 4 3
Scheme 1 General route for the synthesis of Fe O –TBA-Br
The synthesized nanomagnetic catalyst was comprehensively characterized by
Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis
(
TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray
spectroscopy (EDX) and vibrating sample magnetometer (VSM) analysis
techniques.
The successful immobilization of tribenzyl ammonium-tribromide on Fe O4
3
nanoparticles (Fe O –TBA-Br catalyst) was confirmed by the FT-IR spectra. The
3
4
3
FT-IR spectra for the Fe O MNPs (a), Fe O –APTMS (b), Fe O –TBA-Br (c) and
3
4
3
4
3 4
Fe O –TBA-Br are shown in Fig. 1. As seen on Fig. 1, the FT-IR analysis of Fe O
3 4
3
4
3
-
1
MNPs (a) shows a strong peak at 579 cm , which is attributed to the presence of an
Fe–O bond and it guarantees the formation of Fe O MNPs. In the curve of Fe O
3 4
3
4
-
1
MNPs (a), a broad peak is observed at 3401 cm , which it is the characteristic of
both symmetrical and unsymmetrical modes of O–H bonds linked to the surface iron
-
1
atoms. Also, the existence of a stretch peak at 1625 cm is attributed to an
adsorbed water layer. The presence of the APTMS moiety on the surface of the
-
Fe O MNPs is confirmed by a characteristic peak at 1002 cm which is ascribed
1
3
4
to the Fe–O–Si stretching vibration. The FT-IR analysis of the Fe O –APTMS
3
4
-
1
(
b) exhibits a broad bond at 3434 cm that is assigned to an NH group, and two
2
-
1
weak peaks at 2922 and 2854 cm are ascribed to the C–H stretching vibration of
the alkyl chain of amine. In the curve of Fe O –TBA-Br (c), the characteristic weak
-
peaks at 2926 and 1642 cm are attributed to C–H and C=C stretching vibration.
3
4
1
-
1
Also, the broad peak at nearly 1370–1400 cm
in the Fe O –TBA-Br (c) is
3 4
?
attributed to C–N stretching vibration [20]. But, Br exhibits a signal between 180
3
-
and 200 cm [20], which is not recognizable with the FT-IR spectrum.
1
The morphology and size of the Fe O –TBA-Br catalyst was investigated by
3
4
3
SEM as shown in Fig. 2. From SEM images, it can be seen that the catalyst was
made up uniformly and the average size of Fe O –TBA-Br is about 10 nm.
The components of Fe O –TBA-Br were analyzed by using energy-dispersive
3
4
3
3
4
3
EDX (Fig. 3). As shown in Fig. 3, the characteristic peaks of Fe, O, C, N, Si and Br
showed well that the tribenzyl ammonium-tribromide was successfully immobilized
123

L. Shiri et al.
3 4 3 4 3 4 3 4 3
Fig. 1 FT-IR spectra of Fe O MNPs (a) Fe O –APTMS (b), Fe O –TBA-Br (c) and Fe O –TBA-Br .
(Color figure online)
on Fe O nanoparticles. As a result of this analysis, it can be concluded that the
3
4
Fe O –TBA-Br nanomagnetic catalyst has been successfully synthesized.
3
3
4
The magnetic property of the catalyst was investigated by VSM at ambient
temperature. The curves of magnetization for the Fe O nanoparticles (red line) and
Fe O –TBA-Br (green line) are depicted in Fig. 4. As shown on Fig. 4, the
3
4
3
4
3
-
1
saturation magnetization of Fe O –TBA-Br was found to be at about 38 emu g ,
-
which is much lower than bare Fe O MNPs (about 55 emu g ). The decrease of
3
4
3
1
3
4
the saturation magnetization can be related to the presence of tribromide groups
onthe surface of the Fe O nanoparticles supports.
3
4
TGA is an effective and fascinating spectra technique to investigate the thermal
stability of the immobilized catalyst relative to that of MNPs. Furthermore, the bond
formation between the MNPs and the catalyst can be as well interpreted and verified
by TGA. The overlaid TGA curves of Fe O nanoparticles (green line), Fe O –
3
4
3 4
APTMS (red line) and Fe O –TBA-Br (blue line) are displayed in Fig. 5. The
3
4
3
initial mass loss at temperatures at below 200 °C is due to the removal of physically
adsorbed solvent and surface hydroxyl groups. Organic groups have been reported
to desorb at temperatures above 260 °C. The mass percentage loss of about 3.5%
1
23

Synthesis and characterization of tribenzyl ammonium…
Fig. 2 SEM image of the Fe
3
O
–TBA-Br
4 3
1
1
500
000
FeKα
5
00
FeLα
O Kα
BrLα
SiKα
FeKβ
BrKα
BrKβ
keV
0
0
5
10
15
3 4
Fig. 3 EDX spectrum of the Fe O –TBA-Br3
123

L. Shiri et al.
Fig. 4 Magnetization curves for Fe
temperature. (Color figure online)
3
O
4
MNPs (red line) and Fe
3
O
4
–TBA-Br
3
(green line) at room
3 4 3 4 3 4 3
Fig. 5 TGA curves of Fe O MNPs (green line), Fe O –APTMS (red line) and Fe O –TBA-Br (blue
line). (Color figure online)
between 250 and 475 °C for Fe O –APTMS is ascribed to the thermal decompo-
3
4
sition of the 3-aminopropyl silane groups. In the curve of Fe O –TBA-Br , a weight
3
4
3
percentage loss about 4% over the range of 200–620 °C is also observed, which is
attributed to the breakdown of the tribenzyl ammonium-tribromide moieties. As the
result of this analysis, it can be concluded that the effective grafting of tribenzyl
ammonium-tribromide groups (TBA-Br ) on the Fe O nanoparticles is verified.
3
3 4
XRD is an effective spectra analysis technique to achieve better reorganization of
the major properties of the magnetite structures. In fact, the formation of a
magnetite crystal phase in the nanomagnetic catalyst in aggregate powder form can
also be identified by the XRD. The XRD patterns of the prepared Fe O MNPs (blue
3
4
1
23

Synthesis and characterization of tribenzyl ammonium…
3 4 3
Fig. 6 The XRD patterns of Fe3O4 MNPs (a) and Fe O –TBA-Br (b)
line or a) and Fe O –TBA-Br (green line or b) are presented in Fig. 6. In the XRD
3
4
3
pattern of Fe O –TBA-Br , six characteristic peaks at 2h = 35.1°, 41.4°, 50.6°,
3
4
3
6
3.2°, 67.5° and 74.5° were observed, which are assigned to the (220), (311), (400),
422), (511) and (440) crystallographic faces of magnetite (in good agreement with
the standard Fe O XRD spectrum reported in literature) [14]. This analysis clearly
(
3
4
affirmed the theme that the surface modification and conjugation of the Fe O
3
4
nanoparticles did not lead to a phase change.
Catalytic studies
After characterization of the catalyst, the catalytic activity of Fe O –TBA-Br in the
3
4
3
oxidative coupling of thiols to the disulfides and also oxidation sulfides to the
sulfoxides at ambient temperature was studied. To find out the optimum reaction
conditions and understand the effects of various parameters (such as solvent type,
catalyst and hydrogen peroxide amounts) on these oxidation processes, the room
temperature oxidative coupling of benzo[d]thiazole-2-thiol was chosen as a model
substrate (Table 1). In the preliminary stage of this investigation, the room
temperature oxidative coupling of benzo[d]thiazole-2-thiol using H O (0.5 mL) in
2
2
EtOH was performed using different amounts of catalyst. As shown in Table 1, at a
lower catalyst amount (3 mg), a 92% yield of benzo[d]thiazole-2-thiol was observed
after 20 min (Table 1, entry 1). On increasing the catalyst weight to 5 mg, the yield
of benzo[d]thiazole-2-thiol increased up to 95% after 10 min (Table 1, entry 2). The
increase in the product yield with an increase in the catalyst weight can be attributed
to an increase in the availability of catalytically active sites required for this
oxidation reaction. On further increasing the amount of catalyst, namely, amounts
123

L. Shiri et al.
Table 1 Optimization of the reaction conditions
a
Entry
Catalyst (mg)
H
2
O
2
(mL)
Solvent (temp. °C)
Time/min
Yield (%)
1
2
3
4
5
6
7
8
9
3
5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.3
0.4
0.6
0.5
EtOH (r.t)
EtOH (r.t)
EtOH (r.t)
EtOH (r.t)
EtOH (r.t)
20
10
92
98*
97
98
5
10
15
–
10
10
120
20
5
CH
3
CN (r.t)
96
95
93
93
5
Acetone (r.t)
THF (r.t)
25
5
100
85
5
2 2
CH Cl (r.t)
b
10
11
12
13
14
5
H
2
O (r.t)
60
30
5
EtOH (r.t)
EtOH (r.t)
EtOH (r.t)
EtOH (40 °C)
55
91
95
98
97
5
30
5
10
5
10
*
and bold value indicate the optimal conditions
Reaction conditions: the oxidative coupling of benzo[d]thiazole-2-thiol (1 mmol) using 30% H
presence of catalyst and solvent (2 mL)
2 2
O in the
a
Isolated yield
b
Substrate was insoluble in water
greater than 5 mg, any appreciable change or improvement wasn’t observed in
product yield after 10 min (Table 1, entries 3–4). The collected results in Table 1
clearly demonstrate that the reaction rate and product yield not only depended
strongly on the amount of the catalyst (Table 1, entries 1–4), but also that the
presence of catalyst is a vital topic in this process because it was found that once the
reaction was carried out in the absence of a nanomagnetic catalyst, only 5% yield of
the desired product was observed after 120 min (using column chromatography;
Table 1, entry 5). Next, using an optimal catalytic amount of Fe O –TBA-Br
3
3
4
(
5 mg), the room temperature oxidative coupling of benzo[d]thiazole-2-thiol in
several prevalent solvents was carried out to assess the role of solvent on the process
Table 1, entries 6–10). Among the various solvents, such as EtOH, CH CN,
(
3
acetone, CH Cl , THF and water it, was found that EtOH was the most effective
2
2
solvent and gave the highest yield of the desired product within a short period of
time, namely, less than 10 min (Table 1, entry 2). After selecting the ideal solvent,
using an optimal catalytic amount of Fe O –TBA-Br (5 mg), the room temperature
3
4
3
oxidative coupling of benzo[d]thiazole-2-thiol in EtOH was also studied with varied
amounts of 30% H O , and the results are represented in Table 1 (entries 11–13).
2
2
1
23

Synthesis and characterization of tribenzyl ammonium…
Finally, using the optimal catalyst and H O amount, the model reaction in EtOH at
2
2
4
0 °C was tested, but, any appreciable change or improvement wasn’t observed in
product yield after 10 min. From Table 1 it is found that 0.5 mL of hydrogen
peroxide exhibits the best performance for the present process. Therefore, the
highest yield and shortest reaction time of product 2d was obtained in the presence
of 5 mg Fe O –TBA-Br and 0.5 mL of 30% hydrogen peroxide in EtOH (2 mL) at
3
4
3
ambient temperature (Table 1, entry 2).
With the optimized reaction conditions in hand, to further understand the scope
and generality of this catalytic system, the applicability of various thiols were next
subjected to the oxidation reaction (Scheme 2). The results of this study are listed in
Table 2. It is noteworthy that in all of the oxidative couplings, no product was
observed and the desired products were obtained in short times and in high to
excellent yields. As shown in Table 2, a diverse range of thiols including aromatic
thiols (Table 2, entries 1–2) and heteroaromatic thiols (Table 2, entries 3–5),
benzilic thiol (Table 2, entry 13) and aliphatic thiols (Table 2, entries 14–15) were
successfully transformed into the corresponding disulfides. The obtained results
demonstrated well that the reaction rate, selectivity and product yield of the
oxidative reactions depend on the type of thiol moieties because aromatic thiols
were oxidized to the corresponding disulfides in shorter times and higher yields than
aliphatic thiols. Also, aromatic and heteroaromatic thiols with electron-donating as
well as electron-withdrawing substituents at ortho and para positions were well-
tolerated and did not exert any tangible effect on the yield of reactions (Table 2,
entries 6–12).
The tremendous and fascinating results obtained in the oxidative coupling of
thiols encouraged us to apply more and more focus on Fe O –TBA-Br and its
3
4
3
catalytic activity in other organic transformations. In this respect, to find out the
general applicability and efficiency of the catalyst, the catalytic activity of Fe O –
TBA-Br in the oxidation of sulfides to sulfoxides was also studied. Therefore,
3
4
3
methyl phenyl sulfide (4a) was selected as a model substrate and it was observed
that under the optimized reaction conditions for the oxidative coupling of thiols, the
methyl phenyl sulfide oxidized smoothly to corresponding sulfoxide as the only
product [as per thin layer chromatography (TLC)], and it is noteworthy that sulfone
as an over-oxidation product isn’t formed under these conditions. After ensuring the
transformation, to understand the optimum reaction conditions, the impact of
various parameters (such as solvent type, catalyst and hydrogen peroxide amounts)
on the process (methyl phenyl sulfide as a model substrate) was studied. The best
results (with respect to product yield and reaction time) for product 4a were
3 4 3
Scheme 2 Fe O –TBA-Br catalyzed the oxidative coupling of thiols to the disulfides
123

L. Shiri et al.
Table 2 Fe
temperature
3
O
4
–TBA-Br
3
catalyzed the oxidative coupling of thiols to the disulfides in ethanol at room
a
Entry
Thiol
Product
Time (min)
Yield (%)
Mp (°C)
References]
[
1
2
3
4
5
6
7
8
9
Benzenethiol
2a
2b
2c
2d
2e
2f
40
15
45
10
35
10
15
20
25
20
45
35
45
40
35
91
92
90
98
87
98
92
97
93
95
92
91
94
92
90
59–61 [21]
134–135 [4]
56–57 [22]
172–173 [4]
93–95 [4]
39–41 [4]
Oil [23]
Naphthalene-2-thiol
Pyridine-2-thiol
Benzo[d]thiazole-2-thiol
Benzo[d]oxazole-2-thiol
4-Methylbenzenethiol
4-Fluorobenzenethiol
4-Bromobenzenethiol
2-Aminobenzenethiol
2-Mercaptobenzoic acid
2,6-Dichlorobenzenethiol
4,6-Dimethylpyrimidine-2-thiol
Phenylmethanethiol
2g
2h
2i
89–90 [22]
81–83 [24]
284–287 [5]
88–90 [22]
160–162 [24]
69–70 [5]
Oil [5]
10
11
12
13
14
15
a
2j
2k
2l
2m
2n
2o
2-Mercaptoethanol
2-Mercaptoacetic acid
Oil [5]
Isolated yield
observed when the process was carried out in the presence of 10 mg of Fe O –TBA-
3
4
Br and 0.5 mL of 30% hydrogen peroxide in EtOH (2 mL) at ambient temperature.
3
Accordingly, a number of aliphatic and aromatic sulfides were subjected to
oxidation under the optimized reaction conditions (Scheme 3) and the results of this
study are summarized in Table 3. It is noteworthy that all corresponding sulfoxides
were obtained in excellent yields within a relatively short period of time. The
presented results in Table 3 show well that aromatic sulfides are found to be more
reactive and provided better product yields than aliphatic sulfides.
The high potential of this catalyst is shown by comparison f our results on the
oxidative coupling of 4-methylbenzenethiol and oxidation of methyl phenyl
sulfide with the previously reported protocols in the literature (Table 4). As
3 4 3
Scheme 3 Fe O –TBA-Br catalyzed the oxidative coupling of thiols to the disulfides
1
23

Synthesis and characterization of tribenzyl ammonium…
3 4 3
Table 3 Fe O –TBA-Br catalyzed the oxidation of sulfides into sulfoxides in ethanol at room
temperature
Entry Sulfide
Product Time
(
Yield
a
(%)
Mp (°C)
[References]
min)
1
2
3
4
5
6
Methyl phenyl sulfide
Benzyl phenyl sulfide
Ethyl phenyl sulfide
4a
4b
4c
4d
4e
4f
25
40
60
10
35
50
97
95
94
93
92
87
Oil [5]
115–117 [4]
Oil [24]
Methyl furfuryl sulfide
Benzyl 4-methylphenyl sulfide
Oil [5]
120–121 [25]
157–159 [25]
4-Chlorobenzyl 4-methylphenyl
sulfide
7
8
9
1
1
1
1
1
1
a
Benzyl 4-bromophenyl sulfide
2-(Methylthio)ethanol
2-(Phenylthio)ethanol
Dibenzyl sulfide
4g
4h
4i
45
10
10
55
90
88
87
91
89
92
94
90
93
141–142 [25]
Oil [24]
Oil [5]
0
1
2
3
4
5
4j
137–138 [4]
87–89 [4]
Oil [5]
Dodecyl sulfide
4k
4l
280
15
40
65
10
Dibutyl sulfide
Dipropyl sulfide
4m
4n
4o
Oil [5]
Diethyl sulfide
Oil [5]
Tetrahydrothiophene
Oil [5]
Isolated yield
3 4 3
Table 4 Comparison of Fe O –TBA-Br for the oxidative coupling of 4-methylbenzenethiol and oxi-
dation of methyl phenyl sulfide with previously reported catalysts
a
Entry Substrate
Catalyst
Time (min) Yield (%)
References
1
2
3
4
5
6
7
8
9
1
1
1
a
4-Methylbenzenethiol Ni-salen-MCM-41
4-Methylbenzenethiol Fe NPs@SBA-15
4-Methylbenzenethiol Zr-oxide@MCM-41
4-Methylbenzenethiol Cu(II) Schiff base MCM-41
4-Methylbenzenethiol Mn–ZSM-5
25
45
95
94
98
83
82
98
93
88
95
98
93
97
[6]
[26]
30
[5]
72
[22]
360
10
[27]
4-Methylbenzenethiol Fe
Methyl phenyl sulfide NBS
Methyl phenyl sulfide TsOH
3
O
4
–TBA-Br
3
This work
[28]
270
240
156
300
20
[29]
Methyl phenyl sulfide Ni-salen-MCM-41
Methyl phenyl sulfide Zr-oxide@MCM-41
Methyl phenyl sulfide Cu(II) Schiff base MCM-41
[6]
0
1
2
[5]
[22]
Methyl phenyl sulfide Fe
3
O
4
–TBA-Br
3
25
This work
Isolated yield
shown in Table 4, the previously reported methods suffer from one or more
drawbacks such as longer reaction time, using transition metal catalysts and low
yields of products.
123

L. Shiri et al.
Reusability of the Fe O –TBA-Br catalyst
3
4
3
The recovery and reusability of catalyst is a valuable advantage in modern catalysis
research, which makes catalysts very notable from commercial and economical
points of view. In this respect, the recyclability of Fe O –TBA-Br was investigated
3
4
3
in the oxidative coupling of 4-methylbenzenethiol and oxidation of methyl phenyl
sulfide. At the end of the process, the Fe O –TBA-Br was readily separated from
3
4
3
the reaction mixture by an external magnet, washed several times with ethyl acetate
and dried to remove residual product. Then, the recovered catalyst was reused for a
subsequent fresh batch of the reaction. The catalytic activity was studied for five
successive times and it showed similar activity (Fig. 7). Accordingly, one of the
fascinating advantages of this new catalytic system is the simple, rapid and efficient
separation of the catalyst by using a suitable external magnet, which minimizes the
loss of catalyst during separation.
Also, the SEM image of the recovered catalyst is shown in Fig. 8. From the
comparison of Figs. 8 with 2, it can be concluded that there aren’t any significance
differences in the morphologies of freshly prepared catalysts and the recovered
catalyst after operation for five consecutive reaction cycles.
Conclusion
In this research, the tribenzyl ammonium-tribromide supported onto magnetic
Fe O nanoparticles (Fe O –TBA-Br ) was introduced as a novel, stable, versatile
3
4
3
4
3
and recyclable nanomagnetic catalyst for the oxidative coupling of thiols and
oxidation of sulfides. The synthesized catalyst exhibits excellent activity in the
synthesis of disulfides and sulfoxides in an economical and eco-friendly strategy.
The Fe O –TBA-Br catalyst with high activity could be readily recovered from
3
4
3
reaction media by an external magnet and reused at least five successive times
1
9
9
8
8
00
5
0
5
0
5
4
3
2
1
Thiol
Sulfide
Run Number
3 4 3
Fig. 7 Reusability of Fe O –TBA-Br in the oxidative coupling of 4-methylbenzenethiol and oxidation
of methyl phenyl sulfide
1
23

Synthesis and characterization of tribenzyl ammonium…
3 4 3
Fig. 8 SEM image of the recovered catalyst (Fe O –TBA-Br ) after five cycles
without any noticeable loss of activity. According to the best of our knowledge, this
is the first report on the use of the immobilized bromine source on magnetic Fe O
3
4
nanoparticles as a nanomagnetic recyclable catalyst for the oxidative coupling of
thiols. The high to excellent yields of products, simplicity operation, use of ethanol
as a solvent, easy magnetic separation and reusability as well as high activity of the
catalyst are notable advantages of this strategy. As a result of this study, we believe
that this catalytic system will open up a new avenue to introduce new, versatile and
efficient bromine sources (as reusable nanomagnetic catalysts) for the synthesis of
biologically and pharmaceutically active molecules.
Acknowledgements This work was supported by the research facilities of Ilam University, Ilam, Iran.
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