An efficient clean methodology for the C-S coupling to aryl thioethers and S-S homocoupling to aromatic disulfides catalyzed over a Ce(IV)-leucine complex immobilized on mesoporous MCM-41

Article abstract of DOI:10.1039/c9nj02270h

A novel Ce(iv)-anchored l-leucine covalently bonded to mesoporous MCM-41 has been synthesized by a non-hydrothermal post-functionalization approach. It has been thoroughly characterized by sophisticated physicochemical techniques. The material was applied in the efficient green synthesis of aromatic sulfides by C-S coupling using molecular sulfur and haloarenes. Another catalytic application was in the synthesis of symmetric disulfides by the homocoupling of aromatic thiols in the presence of H₂O₂ as an oxidant. The ligand-free protocol is simple, clean and free from hazardous chemicals. Moreover, the catalyst is reusable for several times, thus making the methodology sustainably viable.

Full text of DOI:10.1039/c9nj02270h

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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An efficient clean methodology for the C-S coupling to aryl thioethers and S-S
homocoupling to aromatic disulfides catalyzed over Ce(IV)-Leucine complex immobilized
mesoporous MCM-41
a
a
b
Hojat Veisi, Taibeh Tamoradi,* and Bikash Karmakar*
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Abstract
A novel Ce(IV) anchored L-Leucine covalently bonded over mesoporous MCM-41 has been
synthesized by non-hydrothermal post-functionalization approach. It has been characterized
thoroughly by sophisticated physicochemical techniques. The material was applied in the
efficient green synthesis of aromatic sulfides by C-S coupling using molecular sulfur and
haloarenes. Another catalytic application was performed in the synthesis of symmetric disulfides
by homocoupling of aromatic thiols in presence of H O as oxidant. The ligand-free protocol is
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simple and clean, free from hazardous chemicals and the catalyst is reusable for several times
thus making the methodology sustainably viable.
aDepartment of Chemistry, Payame Noor University, 19395-4697 Tehran, Iran. Email:
t.tabss@yahoo.com
bDepartment of Chemistry, Gobardanga Hindu College, 24-Parganas (North), India. Email:
bikashkarm@gmail.com

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1
Introduction
Organosulfur motifs containing one or more C-S bonds are significant class of compounds that
have broad spectrum of applications in pharmacological terrain. Many of the sulfur compounds
construct the backbone of different natural products. Organo thioethers finds importance as
intermediates in organic synthesis,1-3 in material science4-6 and as potential drug candidates in the
treatment of cancer, Parkinson’s and Alzheimer’s disease.7-9 One of the most practicable
pathways in constructing aryl-sulfur bonds is the cross-coupling reactions between the organic
halides with thiols catalyzed over transition metals.10-11 However, the methodology encompasses
drawbacks like slow aerial oxidative homocoupling of thiols resulting disulfides as well as
catalyst deactivation by S poisoning.12-13 Still, considering its importance, researchers are
incessantly searching for alternative protocols in order to avoid such difficulties.14-20 In
accordance with the trend we pursued an approach using molecular sulfur in place of thiols
reacting with haloarenes thus averting the chance of disulfide formation.
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The aryl disulfides are another important group of organosulfur derivatives. They have drawn
significant attentions due to their large number of implications. They are the building blocks of
several S containing synthetic compounds as well as natural products.21-25 The disulfides are used
in the metabolism of several marine alkaloids.26-28 They possess a wide range of crucial functions
in Biochemistry, food chemistry and pharmaceuticals.29-36 These disulfanes have also been used
for the preparation of self-assembled monolayers (SAMs) and monolayer-protected clusters
(
MPCs) with a number of versatile properties.37-38 Several groups have worked on the synthesis
of disulphides following oxidative homocoupling method. 39-40 Instead of using H O , a widely
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used oxidant in this protocol, we brought novelty by carrying out the same using molecular

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oxygen. The use of molecular oxygen in different reactions has already made a benchmark in
sustainable chemistry.
In catalytic arena, heterogeneous catalysis has become an automatic choice in lieu of
homogeneous catalysis during the last few decades on the ground of their easy recovery and
reusability.41-42 The irksome work up to isolate the desired product and non recyclability of
catalyst has been the prime drawback. Different approaches have been proposed for the
immobilization of homogeneous transition metal complexes on heterogeneous supports like
silica, alumina, clay, grapheme oxide, zeolites and other metal oxides to circumvent the
drawbacks.43-46 Following that pathway nanocatalysts have come into prominence as an excellent
heterogeneous support having appreciable high surface to volume ratio and outstanding
recyclability. The nanomaterials, being a quasi-phasic system, construct an efficient bridge
between homogeneous and heterogeneous catalysis.47-49
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Since the discovery of Mobil scientists (M41S), ordered mesoporous silica have been quite
distinctive as heterogeneous support and catalyst due to specific advantages like ease of
preparation, uniform pore structure, excellent chemical and thermal stability, large internal
surface area, good accessibility, low cost and high concentration of surface hydroxyl groups in
order of post functionalization.50-53 As part of our endeavor for the catalytic sustainable green
synthesis, we opted for MCM‐41 mesoporous matrix as heterogeneous support. We used L-
Leucine amino acid as the chelating ligand for covalent bonding on the silica surface and
efficiently anchored Ce (IV) to the system as the active site. Thereafter, the composite catalytic
system was used for the synthesis of aryl thioethers by C-S coupling involving molecular S and
aryl halides and the synthesis of organic disulfanes by oxidative homocoupling of thiols using
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molecular O (Scheme 1).
Both the reactions were highly facile in our protocol affording
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high yields in short reaction time. The high surface area material, easy recovery and reusability
of catalyst with nominal change in activity in several runs, simple procedure, cheap and stable
reagents, easy work up has made the methodology sustainably and economically viable.
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S
X
Ce(IV)-Leucine@MCM-41
DMSO, 130ºC
+
S8
R
R
R
X=I, Br, Cl
Ce(IV)-Leucine@MCM-41
R'-SH
R'
S
S
R'
O , EtOH, rt
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Scheme 1 Ce(IV)-Leucine@MCM-41catalyzed synthesis of thiothers and disulfanes

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Spherical micelles
Rod-Like micelles
Hexagonal face
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Condensation of silica
Calcination
OH OH
Ce(IV)
Leucine
EtOH,
Reflux, 16h
H O
Reflux
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NH2
NH2
O
O
O
O
MCM-41@Leucine-Ce(IV)
MCM-41@Leucine
MCM-41
Scheme 2 Synthetic protocol for the synthesis of Ce(IV)-Leucine@MCM-41 catalyst
2 Experimental
2
.1 Materials and physical measurements
All solvents and chemicals were purchased from Merck and Sigma-Aldrich and used
without further purification. The elemental analysis and Ce content was determined by
Energy Dispersive X-ray spectroscopy (EDAX) and inductively coupled plasma-optical
emission spectrometry (ICP-OES) respectively. The surface morphology of the catalyst
was determined by SEM using FESEM (TESCAN MIRA3) and Powder X-ray diffraction
(
XRD) measurements using Co radiation with a wave length of 1.78897 Å, operating at
4
0 kV, respectively. Thermal stability of the material was determined by TGA

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measurement in a Shimadzu DTG-60 instrument. Fourier transformed Infrared (FT-IR)
spectra of the catalysts were recorded in a Bruker VERTEX 80v instrument. Melting
points of thioethers and disulfides were determined with an electrothermal 9100
apparatus.
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.2 Preparation of Ce(IV)-Leucine@MCM-41 nanocatalyst
The mesoporous MCM-41 was synthesized following a previous reported method by
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Hajjami et al. 1.0 gm CTAB, the structure-directing template, was dissolved in a solution
o
containing 3.5 mL 2 (M) NaOH solution and 480 mL deionized water at 80 C under stirring. 5
mL TEOS, the silica source, was then added slowly to the mixture and stirred for 2 h under same
condition. The solid products were filtered, washed several times with deionized water and dried
o
o
o
at 60 C overnight. Finally it was calcined at 550 C for 5 h at ramping rate of 2 C / min to
afford MCM-41 as a white powder. Subsequently, Leucine@MCM-41 was synthesized by
adding 1.5 g of L-Leucine to 1.0 g suspension of MCM-41 in 30 mL distilled water and refluxing
o
at 100 C for 48 h. The resulting mixture was filtered, washed with ethanol/water and dried in
vacuum at 80 °C. Subsequently, 1.08 g Ce(NO ) .6H O was added into 1.0 g Leucine@MCM-41
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suspension in 30 mL ethanol under reflux conditions for 16 h. The resulting mixture was filtered,
washed thoroughly with ethanol and dried under vacuum to afford the final composite material
Ce(IV)-Leucine@MCM-41.
2
.3 General procedure for the thioetherification reactions
A mixture of molecular sulfur (1.0 mmol), aryl halide (1.0 mmol) and KOH (0.7 g) was stirred in
DMSO (2 ml) at 130 °C in presence of 0.05 g MCM-41@Leucine-Ce (IV) catalyst for requisite
time. After completion (by TLC), the catalyst was separated by filtration and washed with ethyl
acetate. The reaction mixture was added to water and extracted with ethyl acetate. The whole

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organic layer was dried over anhydrous Na SO , concentrated and purified on preparatory TLC
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plate to afford the corresponding aryl thioether.
2.4 General procedure for the thiol oxidation reactions
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To a stirred suspension of 0.05 g MCM-41@Leucine-Ce (IV) in 5 mL ethanol, 1.0 mmol
thiol and H O (0.4 mL) was added at room temperature fitted. Also, the reaction was
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tested in the presence of O as oxidant. After the completion of reaction (monitored by
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TLC), the catalyst was separated simply by filtration and the reaction mixture was
concentrated to get pure disulfide derivatives.
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. Results and Discussion
3.1 Catalytic Characterizations and Data analysis
Upon post-functionalization approached synthesis of the catalyst (Scheme 2), it was thoroughly
characterized by advanced analytical techniques like Scanning Electron Microscopy (SEM),
Energy Dispersive X-ray Spectroscopy (EDX), Powder X-ray diffraction, Thermal Gravimetric
Analysis (TGA), N adsorption-desorption isotherm, surface area analysis by BET method,
2
Fourier Transformed Infrared spectroscopy (FT-IR) and Inductively Coupled Plasma Atomic
Emission Spectroscopy (ICP-AES).
The structural morphology of MCM‐41 and Ce(IV)-Leucine@MCM-41 was ascertained using
scanning electron microscopy. Fig. 1 depicts the SEM images of the bare MCM‐41 (1a) and
Ce(IV)-Leucine@MCM-41 at different magnifications (1b-1d). Both the bare and functionalized
mesoporous materials are made up of quite homogeneous and uniform spherical particles. A
close look of the material represents its floppy nature and that is obviously due to its high
porosity. No significant changes were observed in surface engineering over MCM-41.

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Fig. 1 SEM images of calcined MCM-41at 2 μm (a) and Ce(IV)-Leucine@MCM-41 at 1 μm
b), 500 nm (c) and 200 nm (d).
(
The elemental identity was analyzed from EDX spectra of the Ce(IV)-Leucine@MCM-41
catalyst (Fig. 2). The evolved peaks represented the Si, C, N, O and Ce atoms. Presence of C, N
and O confirmed the amino acid attachment to Si surface. The exact Ce content in the catalyst
was found to be 0.46 mmol/g as determined by ICP-AES analysis.

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Fig. 2 EDX pattern of Ce(IV)-Leucine@MCM-41
The crystalline morphology of MCM-41 and Ce(IV)-Leucine@MCM-41 was determined by
powder X-ray diffraction (XRD) study. Fig. 3 depicts the well-resolved peaks of bare MCM-41
with high diffraction intensity corresponding to the (100), (110), and (200) reflections attributed
to the P6mm hexagonal space group symmetry. The sharp low angle peaks clarifies high
mesoporosity of the material. However, surface immobilization of the Ce (IV) complexes on
MCM-41 results in the decrease in the diffraction intensity of the peak triad owing to the
difference in scattering contrast of the pores and the walls of the nanochannels of MCM-41. Still,
the presence of (100) peak in Ce(IV)-Leucine@MCM-41, although weak, confirms the structural
indifference upon post-functionalization.

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Fig. 3 Small angle XRD patterns of MCM-41 and Ce(IV)-Leucine@MCM-41
Fig. 4 assumes the FT-IR analysis of the catalysts. The outcome of bare MCM-41 (blue),
Leucine complexed silica (red) and Ce loaded final version of the catalyst (green) has been
-1
superposed in the figure. The absorption bands appeared at around 1100-1200 cm and 3000-
-1
3600 cm are due to Si–O–Si asymmetric stretching and the Si-OH stretching. The similarity in
FT-IR pattern of the post-functionalized materials with MCM-41 conveyed that the basic
structural morphology remained intact. However Ce connectivity with the basic matrix could not
be proved from FT-IR analysis.

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Fig. 4 FT-IR spectrum for bare of MCM‐41 (a), Leucine@MCM-41 (b) and Ce(IV)-
Leucine@MCM-41 (c)
Thermal stability of the catalyst and quantitative evaluation of the organic groups supported on
MCM-41 was determined by TGA analysis and the pattern has been shown in Fig. 5. The TGA
curve showed a minute weight loss below 200 °C due to removal of the adsorbed water as well
as of the surface OH groups. A steep fall in the curve was observed in the temperature range
2
00-250 °C indicating the major weight loss occurred in that region. This was mainly due to the
decomposition of organic layers on the surface of functionalized MCM-41. The further decrease
in mass was due to chemisorbed water.

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1
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 5 The TGA diagram of Ce(IV)-Leucine@MCM-41
The textural property of the catalyst was examined by N adsorption and desorption isotherm
2
experiment. Prior to that, the Ce(IV)-Leucine@MCM-41was degassed at 100 °C under an Ar
flow. Fig. 6 represents the isotherm and it clearly indicates to be a Type-IV nature along with a
hysteresis loop. These characterize a typical mesoporous material. The isotherm nature at lower
pressure region indicates the presence of micropores as well. Surface area of the bare MCM-41
and the functionalized one were determined by BET method and the results are documented in
Table 1. As expected, the functionalized MCM‐41 has smaller surface area, narrower pore
diameter and lower absorption volume than bare MCM-41.

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1
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 6 Nitrogen adsorption/desorption isotherms of Ce(IV)-Leucine@MCM-41.
Table 1
Texture parameters obtained from N adsorption/desorption studies.
2
Sample
S
Pore diameter by
Pore volume
BET
2
-1
(m g ) BJH method (nm)
3
-1
(cm g )
MCM-41
1113.7 2.39
1.39
Ce(IV)-Leucine@MCM-41 379.68 1.02
0.803

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1
4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
.2 Reaction Data analysis
After the synthesis and rigorous characterization of the catalysts, their application appeared
important. The endeavor was started with the optimization of reaction conditions in both the
schemes. At the outset we chose the thioetherification reaction of haloarenes using molecular
sulfur and a base as additive using Ce(IV)-Leucine@MCM-41 as catalyst. We went through the
investigations regarding standardization of reaction conditions by variation of different factors
like solvent, bases, temperature and catalyst load upon a model reaction of iodobenzene and S8.
The consequences are represented in Table 2. A screening of solvents was carried out with a
series of solvents in presence of KOH as base and 70 mg catalyst. There was no reaction output
in PEG, CH CN, H O and toluene. The reaction responded most excellently in DMSO and KOH
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
2
o
combination. However, the best result was achieved at 130 C and 70 mg of catalyst. There is a
definite role of base in the reaction mechanism. We varied different bases like NaOH, Na CO
2
3
and Et N with the standardized conditions so far but there were no better results further than that
3
with KOH.
Table 2. Optimization of reaction conditions for C–S coupling using Ce (IV)-Leucine@MCM-41
X
S
Cat
+
+
S8
KOH
Cat =
DMSO, 130ºC
X=I, Br, Cl
1
a-1j
2
a-2j
NH2
O
O
Solvent
Base
Temp
(°C)
130
Cat
(mg)
70
Time
(min)
200
Yield
(%)
24
Entry
DMF
PEG
KOH
KOH
1
2
130
70
200
N.R

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1
5
1
2
3
4
5
6
7
8
9
CH
3
CN
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
NaOH
130
130
130
130
80
70
200
200
200
200
200
200
200
200
200
200
200
200
N.R
17
3
4
5
6
7
8
9
EtOH
70
70
70
70
70
70
50
90
70
70
70
H
2
O
N.R
N.R
15
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Toluene
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
100
130
130
130
130
130
130
22
80
48
1
0
1
2
3
4
85
1
1
1
1
N.R
25
Na
2
CO
3
Et
3
N
30
aIsolated yields.
Upon optimization of conditions, we concentrated on the linearity of those over different
substrates. A wide variety of substituted halobenzenes were reacted with molecular S under
8
standard conditions. They were well compatible with the protocol. The results have been
depicted in table 3 and moderate to high yields were obtained in each case (1a-1j).

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1
6
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 3 C–S coupling in the presence of Ce(IV)-Leucine@MCM-41
Entry
Aryl Halide
Product
1a
Time (min)
Yield (%)a
M.p (°C)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
I
1
140
85
47
CH3
I
2
3
4
1b
1c
1d
200
225
210
80
76
82
65
67
65
Cl
Br
Br
5
6
7
8
1e
1f
155
130
100
135
79
88
88
85
46
CH3
Br
158
155
Oil
OH
I
1g
1h
OH
I
OCH3

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1
7
1
2
3
4
5
6
7
8
9
Br
OCH3
9
1i
1j
160
150
73
80
Oil
Oil
Br
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
10
OCH3
a
Isolated yield.
Based on our reaction study and literature analysis a plausible mechanstic pathway has been
drawn in Fig. 7. Initially, the S molecule breaks down in presence of KOH, forming a
8
dipotassium disulfide salt, which acts as the S source. This binds with the catalyst forming a
bicyclic ring. Aryl halide further adds to it forming complex bridged intermediates.
Subsequently, a second molecule of aryl halide adds to this intermediate and finally liberates the
diaryl thioether product retaining the catalytic site free for reuse.

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1
8
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
SPh
Ce
SPh
PhX
Ce
2
NH2
X
NH2
Ph
S
O
O
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ph
Ph
O
O
MCM41
MCM41
Ce(IV)
NH2
Ph
Ce
Ph
S S
O
S
S
S
S
O
S S OH S
Ce
S
S
S S
+
KOH
S
H N
MCM41
NH2
2
X
X
O
S S OH
O
+
O
O
S S
MCM41
MCM41
K
K
Ph
Ph
S
S
S
Ce
Ce
Ce
H N
Ce
S
H N
NH2
2
NH2
2
O
X
X
O
O
O
O
O
O
O
MCM41
MCM41
MCM41
MCM41
PhX
Fig. 7 Mechanistic pathway for the thioetherization reaction over Ce(IV)-Leucine@MCM-41
catalyst.
The endeavor was started with the optimization of reaction conditions in both the schemes. At
the outset we chose the oxidative homocoupling of 4-methylthiophenol using our catalyst and
H O as the oxidant at room temperature. Different factors like solvent, amount of H O and
2
2
2
2
amount of catalyst were examined on the outcome of reaction. The results are summarized in
table 2. The reaction was not very fruitful in solvents like ethyl acetate or acetonitrile. It was

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1
9
1
2
3
4
5
6
7
8
9
quite convincing in EtOH resulting 86% yield. However, the best result was achieved at solvent-
free conditions. Similarly, after a tedious experimentation, 0.4 mL H O and 0.5 g of catalyst was
2
2
optimized to produce the best conditions (entry 3, table 2).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 4. Optimization of the reaction conditions for S-S homocoupling of 4-
methylthiophenol using MCM-41@Leucine-Ce (IV).
Me
Cat
Me
SH
S
EtOH, R.T
S
NH2
O
Cat =
Me
3a-3j
O
4
a-4j
Yield (%)a
Entry
Solvent
H O (mL) Cat(mg)
Time (min)
40
2
2
1
2
3
4
5
6
Solvent-Free
Solvent-Free
Solvent-Free
Solvent-Free
EtOH
0.4
0.4
0.4
0.5
0.4
0.4
0.4
7
3
5
5
7
7
7
97
66
94
95
85
46
21
95
50
45
60
95
80
Ethyl acetate
Acetonitrile
7
a
Isolated yield.
After having the optimized conditions in hand we were on to the generality of conditions. A wide
variety of aromatic and aliphatic thiophenols were reacted in presence of H O (Table 5) and O
2
2
2
(
Table 6) as oxidant under stabilized conditions. Interestingly, all the thiophenols afforded good
to excellent yields in short reaction time at room temperature. The results have been shown in

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2
0
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 5. The reactions were completed within 40-120 minutes. All the products were known and
confirmed by melting point measurement and further authentication from references (2a-2j).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 5. S–S homocoupling using H O in the presence of Ce(IV)-Leucine@MCM-41
2
2
Entry
Substrate
Product
4a
Time (min) Yield (%)a M.p (°C)
SH
1
80
95
88-91
Br
SH
2
4b
65
98
59
S
3
4
4c
40
97
95
176-178
163-167
SH
N
N
N
SH
4d
120
5
6
4e
4f
40
50
91
94
70
39
SH
SH
O
N
7
4g
35
96
91
SH
8
9
4h
4i
30
75
92
93
104
HS
COOH
SH
280-283
COOH
SH
1
0
4j
60
95
135

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2
1
1
2
3
4
5
6
7
8
9
a
Isolated yield.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 6. S–S homocoupling using molecular O2 in the presence of Ce(IV)-
Leucine@MCM-41
Entry
Substrate
Product
2a
Time (min) Yield (%)a M.p (°C)
SH
1
95
92
90
Br
SH
2
2b
70
91
60
S
3
4
2c
50
90
91
177-180
165-167
SH
N
N
N
SH
2d
120
5
6
2e
45
60
95
92
67
40
SH
SH
2f
O
N
7
2g
50
94
90-92
SH
8
9
2h
2i
35
80
91
90
102
HS
COOH
SH
280-282
COOH

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2
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
SH
1
0
2j
70
88
134
a
Isolated yield.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Subsequently, we proposed a reasonable mechanism for this homocoupling of aryl thiols to
disulfanes over our catalyst (Fig. 8). The reaction proceeds through radical pathway where O2
acts as the initiator. It binds with the complexed Ce and further attachment of two successive
thiol molecules follows an internal rearrangement to leave behind the disulfane product.
S
S
R
R
H
R
H
R
S
S
O
Ce(IV)
NH2
Ce
NH2
O
O
O
O
H
R
H
MCM41
MCM41
H O
O
2
2
S
O
Ce
NH2
O
Ce
O
NH2
O
O
O
MCM41
O
Ce
NH2
MCM41
S
O
R
H
O
+ H O
2
MCM41

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2
3
1
2
3
4
5
6
7
8
9
Fig. 8 Proposed mechanism for oxidation of thiols in presence of Ce(IV)-Leucine@MCM-41 as
catalyst.
3.3 Study of catalyst reusability
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In green and sustainable catalysis reusability study of the catalyst is an obvious measure. This
study was done with our catalyst in both the reactions with a larger batch size. Initially, 0.5 gm
iodobenzene was reacted with S in presence of 200 mg catalyst. After completion of reaction,
8
the catalyst was recovered by filtration, washed well with ethyl acetate for several times, dried at
o
1
00 C for 3h and reused in further cycles. Similar procedure was followed with 4-methyl
thiophenol in the disulphide formation with O . The results have been shown in Fig. 9. The
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catalyst was so efficient that we used it for a successive 5 times and 7 times respectively for the
two reactions with slight change in catalytic activity.
Fig. 9 Reusability of Ce(IV)-Leucine@MCM-41 in the synthesis of diphenyl sulfide and 1,2 di-
p-tolyldisufane
3.4 Hot filtration and leaching test

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To determine the heterogeneity of the catalyst, a hot filtration test was conducted for the
synthesis of diphenyl sulfide by the reaction of iodobenzene and S under optimized conditions.
8
After a time lag of half period, a 43% yield was obtained from the reaction. But in the next half
time of the reaction, the catalyst was separated from the reaction mixture by filtration and
allowed to continue under identical reaction conditions. It was noteworthy that the overall yield
of the reaction was only 45% after complete time thus verifying the heterogeneous property of
the catalyst. This also confirmed that the active site Ce (IV) was not been leached out the whole
matrix. An ICP-AES study of the filtered reaction mixture confirmed the leached content of Ce
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IV) was only 0.15% of the original load which is catalytically inactive.
Conclusion
In summary, we report herein an improved and competent methodology for the synthesis of aryl
thioethers and symmetric disulfanes over Ce anchored L-Leucine capped mesoporous MCM-41
as a high surface area, reusable, and highly efficient catalyst affording moderate to high yields in
short time. The novel catalyst was synthesized following a post functionalization approach upon
the templated synthesized MCM-41. It was physicochemically characterized by FESEM, EDX,
ICP‐AES, XRD, FT‐IR, TGA and N adsorption desorption measurements. The catalyst was
2
isolated from the reaction mixture simply by filtration and reused several times with constant
efficacy.
Acknowledgments
We are thankful to Payame Noor University (PNU) for financial supports and Gobardanga Hindu
College for providing research facilities.

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Graphical Abstract
An efficient clean methodology for the synthesis of aryl thioethers and aromatic disulfides
catalyzed over Ce(IV)-Leucine complex immobilized mesoporous MCM-41
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a
a,
b,*
Hojat Veisi, Taibeh Tamoradi, Bikash Karmakar
aDepartment of Chemistry, Payame Noor University, 19395-4697 Tehran, Iran.
E-mail: t.tabss@yahoo.com
bDepartment of Chemistry, Gobardanga Hindu College, 24-Parganas (North), India.
E-mail: bikashkarm@gmail.com
Anchored Ce(IV) on the surface of MCM-41 mesoprous silica was used for the synthesis of aryl
thioethers and aromatic disulfides.

a
Address correspondence to Department of Chemistry, Payame Noor University, 19395-4697 Tehran,
Iran.; E-mail: t.tabss@yahoo.com (T. Tamoradi) or Department of Chemistry, Gobardanga Hindu College,
4-Parganas (North), India. E-mail: bikashkarm@gmail.com (B. Karmakar).
2