Simple and Friendly Sulfones Synthesis Using Aqueous Hydrogen Peroxide with a Reusable Keggin Molybdenum Heteropolyacid, Immobilized on Aminopropyl-Functionalized Silica

Article abstract of DOI:10.1080/10426500903299885

Keggin molybdenum heteropolyacid (H₃PMo₁₂O ₄₀), immobilized on aminopropyl-functionalized silica catalysts, were made using two immobilization methods: incipient wetness and equilibrium adsorption. The material prepared for the equilibrium adsorption technique was found to be a highly efficient, ecofriendly, and recyclable heterogeneous catalyst for the selective oxidation of sulfides to sulfones in excellent yields, under mild reaction condition using 35% w/v aqueous hydrogen peroxide as the oxidant.

Full text of DOI:10.1080/10426500903299885

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Simple and Friendly Sulfones
Synthesis Using Aqueous
Hydrogen Peroxide with a
Reusable Keggin Molybdenum
Heteropolyacid, Immobilized
on Aminopropyl-Functionalized
Silica
Valeria Palermo a , Gustavo P. Romanelli a & Patricia
G. Vázquez a
a Centro de Investigación y Desarrollo en Ciencias
Aplicadas “Dr. Jorge J. Ronco” (CINDECA),
Departamento de Química, Facultad de Ciencias
Exactas , Universidad Nacional de La Plata-CCT-
CONICET , La Plata, Argentina
Published online: 18 Nov 2009.
To cite this article: Valeria Palermo , Gustavo P. Romanelli & Patricia G. Vázquez
(2009) Simple and Friendly Sulfones Synthesis Using Aqueous Hydrogen Peroxide
with a Reusable Keggin Molybdenum Heteropolyacid, Immobilized on Aminopropyl-
Functionalized Silica, Phosphorus, Sulfur, and Silicon and the Related Elements,
184:12, 3258-3268, DOI: 10.1080/10426500903299885
To link to this article: http://dx.doi.org/10.1080/10426500903299885
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Phosphorus, Sulfur, and Silicon, 184:3258–3268, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500903299885
Simple and Friendly Sulfones Synthesis Using Aqueous
Hydrogen Peroxide with a Reusable Keggin Molybdenum
Heteropolyacid, Immobilized on
Aminopropyl-Functionalized Silica
Valeria Palermo, Gustavo P. Romanelli, and Patricia G.
´
Vazquez
Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas “Dr. Jorge J.
Ronco” (CINDECA), Departamento de Qu´ımica, Facultad de Ciencias
Exactas, Universidad Nacional de La Plata-CCT-CONICET, La Plata,
Argentina
Keggin molybdenum heteropolyacid (H3PMo12O40), immobilized on aminopropyl-
functionalized silica catalysts, were made using two immobilization methods: in-
cipient wetness and equilibrium adsorption. The material prepared for the equi-
librium adsorption technique was found to be a highly efficient, ecofriendly, and
recyclable heterogeneous catalyst for the selective oxidation of sulfides to sulfones
in excellent yields, under mild reaction condition using 35% w/v aqueous hydrogen
peroxide as the oxidant.
Keywords Functionalized silica; hydrogen peroxide; Keggin heteropolyacid; sulfide
oxidation
INTRODUCTION
Sulfones and their derivatives are widely applied. They can be used
as intermediates for pharmaceuticals and pesticides and for the mod-
ification of polymers.1 1,1-Diiodomethyl sulfone, heterocyclic sulfones,
and α-sulfonyloximes2 have been described as plant fungicides. Cer-
tain sulfones are effective herbicides,3 and others act as insecti-
cides and acaricides.4 Dihydropyridine sulfone derivatives are calcium
Received 25 June 2009; accepted 31 August 2009.
The authors thank Consejo Nacional de Investigaciones Cient´ıficas y Te´cnicas (CON-
ICET), Agencia Nacional de Promocio´n Cient´ıfica y Tecnolo´gica, and Universidad Na-
cional de La Plata for financial support. GPR and PGV are members of CONICET.
Address correspondence to Gustavo P. Romanelli, Centro de Investigacio´n y Desar-
rollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA), Departamento de Qu´ımica,
Facultad de Ciencias Exactas, Universidad Nacional de La Plata-CCT-CONICET, 1900
La Plata, Argentina. E-mail: gpr@quimica.unlp.edu.ar
3258

Sulfones Synthesis Using Aqueous Hydrogen Peroxide
3259
antagonists,5 fluorosulfones are very effective leukotriene antago-
nists with antiallergic and anti-inflammatory activity,6 and aryl- or
heteroaryl-substituted sulfones exhibit activity against retroviruses.7
Piperidinyl sulfones are used as light stabilizers in polymers, partic-
ularly polyolefins,8 and certain heterocyclic sulfones can be used as
corrosion protection agents for metals.9
Sulfones are generally synthesized by oxidation of sulfides via sul-
foxides as intermediates. The oxidizing agents used include potassium
permanganate,10 sodium perborate,11 dimethyldioxirane,12 and peri-
odic acid in the presence of ruthenium tetroxide as a catalyst.13 In
industrial processes, wet clays such as kaolin or montmorillonite are
used in the oxidation of various sulfides in dichloromethane to give the
corresponding sulfone in excellent yields.14 A common oxidizing agent
in the laboratory is hydrogen peroxide in acetic acid. It is also often
used in industry, but requires reaction temperatures of 70◦C and up
to reflux in many cases.15,16 In the presence of various metal catalysts,
oxidation with hydrogen peroxide proceeds at 20◦C and 1 atm of pres-
sure. The sulfides can be oxidized selectively with good yield to the
corresponding sulfones by using titanium, vanadium, molybdenum, or
tungsten salts17 or oxides,18 and heteropolyacids as catalysts.19
On the other hand, oxidation reactions are some of the most use-
ful chemical transformations, despite the fact that they are among the
most problematic processes, due to the production of large amounts
of pollutant materials. Significant efforts have been devoted to substi-
tuting the stoichiometric oxidant, such as Cr (VI) and Mn (VII), with
a cleaner catalytic system.20 In this context, hydrogen peroxide is an
ideal oxidant for liquid-phase reactions because it produces only wa-
ter in the reaction, is safe to store and use, and is cheap and readily
available.21 During the last several years, very useful procedures in-
volving polioxometalatos (POM) and heteropolyacids (HPA) catalysis
and hydrogen peroxide as oxidant have been developed to promote the
oxidation of organic substrates, but the application of POM catalyst
still suffers from some drawbacks related to the nature of polyoxomet-
alates, particularly their low area and the high solubility, which makes
recycling difficult and causes environmental problems.
A further step in the development of environmentally benign chemi-
cal processes is the replacement of current homogeneous oxidation pro-
cedures for heterogeneous processes. Immobilized catalysts have been
of great interest due to their advantages, such as simplified product
workup, separation, isolation, and catalyst reuse. Previous studies of
Keggin-type HPAs supported on silica by the incipient wetness method
have shown that the leaching of HPA is rather significant, but in recent

3260
V. Palermo et al.
papers it has been reported that the anchoring of heteropolyacids onto
the amine groups–modified silica surface minimizes the leaching of the
cluster during the different reaction cycles.22
In the present work, we report the results obtained with a
H3PMo12O40 Keggin heteropolyacids supported on silica-amine func-
tionalized as a recoverable heterogeneous catalyst for the oxidation of
sulfide to sulfone using 35% w/v aqueous hydrogen peroxide.
RESULTS AND DISCUSSION
To test the catalytic activity of Keggin molybdenum heteropolyacid im-
mobilized on an aminopropyl-functionalized silica catalyst, we selected
the oxidation of sulfides using 35% w/v H2O2 as oxidizing agent. We
first screened the oxidation of diphenylsulfide using different catalysts,
solvents, and reaction conditions (Table I). In a blank experiment, no
sulfone was detected in the absence of the catalyst (for 7 h) at 65◦C
using an excess of hydrogen peroxide (35% w/v, 0.75 mL) and acetoni-
trile or ethanol as solvent (Table I, entries 1 and 2). However, when a
commercial HPA sample such as H3PMo12O40 (PM12) was used (Table I,
entry 3), a yield of 89% is observed at 3 h of reaction using acetonitrile
as solvent at 65◦C. As can be observed (Table I, entry 4), upon increas-
ing the temperature (80◦C), the conversion rate increases and the yield
is 91% at 1 h. The replacement of acetonitrile by a more friendly solvent
such as ethanol increases the catalytic activity of PM12. A 92% yield of
sulfone is observed at 0.5 h and at a temperature of 78◦C (Table I, entry
5). This protocol needs homogeneous reaction conditions, and therefore
the catalyst is difficult to recover and reuse. Scheme 1
SCHEME 1
Since the replacement of current homogeneous oxidation procedures
by environmentally acceptable protocols based on recoverable catalysts
is one of the major tasks in green chemistry, solid oxidation catalysts
play an important role.22 Therefore, we prepared three catalyst types:
(i) the immobilization of PM12 on the inner surface of the silica [as
prepared in the Experimental section (PM12SiO2)] and (ii) the im-
mobilization of PM12 of the amino-functionalized silica. In this con-
text, two methods, incipient wetness (PM12NH2SiO2i) and equilibrium

Sulfones Synthesis Using Aqueous Hydrogen Peroxide
3261
TABLE I Oxidation of Diphenylsulfide to Sulfone Employing Differ-
ent Reaction Conditionsa
Entry
Catalyst (mg)
Solvent Time (h) Temp (◦C) Isolated yieldsb(%)
1
2
3
4
5
6
7
8
None
None
PM12 (50 mg)
PM12 (50 mg)
PM12 (50 mg)
PM12 (50 mg)
SiO2
NH2SiO2
PM12SiO2 (50 mg)
PM12NH2SiO2i (50 mg)
PM12NH2SiO2i (50 mg)
PM12NH2SiO2e (50 mg)
PM12NH2SiO2e (100 mg) Ethanol
PM12NH2SiO2e (50 mg) Ethanol
CH3CN
Ethanol
CH3CN
CH3CN
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
7
7
3
1
0.5
1.5
6
6
0.75
1
65
65
65
80
78
65
78
78
78
78
65
65
25
65
—
—
89
91
92
90
—
—
9
91, 10c
90, 10c
93, 40c
92, 90c, 88d
87
10
11
12
13
14
2.5
3
20
6
10e
aH2O2: 0.75 mL, except Entry 13: 1 mL and Entry 14: 0.20 mL.
bYield% of pure product.
cFirst reuse.
dSecond reuse.
eConversion of sulfide, 100%.
adsorption techniques were employed (PM12NH2SiO2e). As previously
reported, strong interaction of HPA acidic proton with amine groups
results in the highest HPA retention on silica surface when used as a
catalyst in polar media.22b
In order to demonstrate that the supports do not contribute on the
catalytic activity, experiments with SiO2 and amino group–modified
support, NH2-SiO2, were carried out. No sulfone was detected under
the same reaction conditions (Table I, entries 7 and 8).
The diphenylsulfide oxidations using the three supported PM12 cat-
alysts are summarized in Table I, entries 9–14. As can be observed
(Table I, entry 9), the PM12SiO2 gives a high conversion and 91% yields
of the corresponding sulfide at 78◦C, using 0.75 mL of 35% w/v aqueous
hydrogen peroxide in 45 min. Similarly, PM12NH2SiO2i gives 90% in 1
h (Table I, entry 10).
In addition, one of the main advantages of using a supported solid
catalyst in the liquid phase reaction is the ease of separation and reuse
of the catalyst in the catalytic cycles. In this way, the catalysts were
separated by filtration, washed with ethanol, dried, and then reused
in the second cycle with a fresh reaction mixture. The procedures were
repeated using the same conditions (Table I, entries 9 and 10). In both

3262
V. Palermo et al.
cases the yields of sulfone were only of 10%. These results confirm the
important leaching of both catalysts (PM12SiO2 and PM12NH2SiO2i,
respectively). For this reason, the effect of leaching was studied by
varying the reaction temperature. For the use of PM12NH2SiO2i as cat-
alyst and a temperature of 65◦C, the results obtained for the use and
the first reuse are 93% and 40%, respectively (Table I, entry 11). This
result confirms a decrease in the leaching for a lower reaction tempera-
ture. Under similar conditions, the catalyst obtained by the equilibrium
adsorption technique (PM12NH2SiO2e) was tested. A yield of 92% was
observed at 3 h of reaction time using ethanol as solvent and a temper-
ature of 65◦C (Table I, entry 12). The first and the second reuse give
90% and 88% of diphenylsulfone, respectively. Under this condition, we
confirm the absence of considerable leaching using the Keggin molybde-
num heteropolyacid immobilized on aminopropyl-functionalized silica
(PM12NH2SiO2e).
The same reaction was carried out at 25◦C using 100 mg of catalyst
and 1 mL of 35% w/v H2O2, (Table I, entry 13). In this condition, 87% of
diphenylsulfone was obtained after 20 h of reaction. When the amount
of hydrogen peroxide was reduced to 0.2 mL, under the same reaction
conditions, conversion of diphenylsulfide was 100%, but the yield of
sulfone only of 10%. In this case, the reaction was selective to obtain
diphenylsulfoxide (Table I, entry 14).
Once the reaction conditions for the selective oxidation of diphenyl-
sulfide to diphenylsulfone had been optimized, the reaction was ex-
tended to other starting substrates. Table II shows the results for the
selective oxidation of different sulfides to sulfones. All the reactions
were run within a short time, and the sulfones were obtained in very
good yields as practically the only oxidation product. For example, in
Table II entry 13, it is shown that the formyl group was not affected by
the reaction conditions.
It was observed that the reaction rates of sulfide oxidation with
diaryl and aryl alkyl groups were lower than those of other aryl alkyl
or dialkylsulfides (see Table II, entries 1, 3, 8, and 9, respectively).
Finally, it was also observed that the reaction rates for the sulfides with
an electron-withdrawing group at the ortho position of diphenylsulfide
was lower than those of diphenylsulfide (see Table II, entries 1 and
2), which indicates that the electron-withdrawing group and the diaryl
group produced a deactivating effect in the oxidation process.
As reported by Gregori et al.17d a plausible mechanism of this re-
action involves the formation of peroxo species and the subsequent
nucleophilic attack of the sulfur atom in the thioether on the peroxo
species. Indeed, it is well known that thioethers are oxidized to sul-
fones by electrophilic oxidants, which explains the lower reactivity of

Sulfones Synthesis Using Aqueous Hydrogen Peroxide
3263
TABLE II General Procedure of Oxidation of Sulfides to Sulfonesa,b
Entry
Substrate
Time
3
Product
Isolated yields (%)c TOFd
1
92
88
4860
3492
2
4
3
4
5
2
2
2
93
94
90
7344
7452
7128
6
7
2
2
86
96
6804
7596
8
1.5
92
9684
9
1
1
83
81
13140
12813
10
(Continued on next page)

3264
V. Palermo et al.
TABLE II General Procedure of Oxidation of Sulfides to Sulfonesa,b
(Continued)
Entry
Substrate
Time
20e
Product
Isolated yields (%)c TOFd
11
77
609
12
13
1
70
89
11073
5616
2.5
14
3
62
3276
aSee Experimental section for reaction conditions.
bWe performed also the oxidation reaction at 25 ◦C with substrates sensitive to the
oxidation. Double and triple bonds, alcohols, amines, aldehydes and esters as styrene,
phenyl acetylene, undecene, phenyl allyl ether, 1-decanol, aniline and ethyl acetate are
not oxidized if the reaction is carried out for 20 h at 25 ◦C. In this condition sulfide was
oxidized selectively to sulfone using 1 mL of 35% w/v H2O2.
cPure product yields.
dTurnover frequency (product mols × H3PMo12O40 mols−1 × h −1).
eAt 25◦C.
aromatic thioether due to delocalization of the electron density on the
sulfur.
This protocol also represents a clear alternative to preparing more
sophisticated molecules. For example in Table II, entry 14, Biginelli
derivatives can be oxidized to the corresponding sulfones using these
catalysts.
The characterization of commercial PM12 and supports were previ-
ously carried out.23 The FT-IR spectra of PM12NH2SiO2e show that the

Sulfones Synthesis Using Aqueous Hydrogen Peroxide
3265
undergraded Keggin structure is presented. Nevertheless, the band
near to 1100 cm−1 is masked by the support. The band next to 900
cm−1 indicates a good interaction between silanols and HPA protons.
The diffraction lines of the bulk heteropolyacid, corresponding to crys-
talline structures, were not observed in the patterns of supported cata-
lyst. The potentiometric titration curve showed that the acid strength
of PM12NH2SiO2e is lower than the HPA, due the acid characteristic of
the amino-functionalized silica.
CONCLUSION
In summary, we have reported a new, versatile, and efficient methodol-
ogy for the rapid oxidation of sulfides to sulfones with aqueous 35%
w/v hydrogen peroxide as a clean oxidant, in the presence of cat-
alytic amounts of Keggin molybdenum heteropolyacid immobilized on
aminopropyl-functionalized silica. The catalyst prepared by the equi-
librium adsorption technique was recovered and reused without loss
of its catalytic activity. The main advantages of this procedure are the
operational simplicity; the use of a noncorrosive, reusable catalyst in
mild, clean oxidation; and the very good yields attained. The use of
an insoluble catalyst instead of soluble inorganic acids contributes to
waste reduction. Further investigations about the use of this catalyst
for the selective oxidation of sulfides in sulfoxides are in progress.
EXPERIMENTAL
Chemicals were purchased from Fluka, Merck, and Aldrich. The prod-
ucts were characterized by comparison of their spectral (1H and 13C-
RMN), TLC, and physical data with the authentic samples.
Preparation of Catalysts
Silica Preparation
Silica preparation by sol–gel technique was made using tetraethy-
lorthosilicate (34 mL), absolute ethanol (14 mL), and glacial acetic acid
(30 mL) with a molar ratio TEOS/EtOH/AcOH equal to 1:1.6:3.5. The
reactants were mixed in a glove box under nitrogen atmosphere at
20◦C. Then the mixture was removed from the controlled atmosphere,
and ethanol (30 mL) was added and gelation of the sols took place. The
wet gel was then aged in the same medium until dry silica particles
were obtained. This solid was dried at 40◦C. The solid was labeled SiO2.

3266
V. Palermo et al.
Functionalized Silica as Support
The catalyst was synthesized
by
addition
of
3-
aminopropyltrimethoxysilane (6.4 mL) to a suspension of silica
(4 g) previously prepared in refluxing toluene (128 mL). The mixture
was then stirred at this temperature for 24 h. The solid was filtered,
washed in a Soxhlet apparatus with diethyl ether and dichloromethane,
and then dried at 40◦C according to the literature.22b
Immobilization of H3PMo12O40 on Functionalized Silica
The immobilization of H3PMo12O40 (PM12) on functionalized silica
was achieved by two methods: incipient wetness and equilibrium ad-
sorption techniques.
Incipient wetness. The functionalized support was impregnated by
means of the incipient wetness method with a solution of 22% (w/v)
PM12 in acetonitrile, in order to obtain 23% (w/w) PM12 in the final
catalyst. Then it was dried at 70◦C for 24 h. The catalyst prepared was
named PM12NH2SiO2i.
Equilibrium adsorption technique. The impregnated solution was
prepared by dissolution of PM12 in absolute ethanol [4% (w/v)]. The
solution was contacted with functionalized silica, and then H2O was
added dropwise, obtaining 23% (w/w) in the final catalyst. The impreg-
nated solid was left without stirring for 24 h. After that, the obtained
solid was shaken for 3 h and then was kept undisturbed overnight.
Then it was shaken for 7 h, and the impregnated solid was allowed to
stand for 24 h. Finally, the impregnating solution was separated from
the solid and dried at 20◦C for 12 h. The catalyst prepared was named
PM12NH2SiO2e.
Characterization of Samples
The characterization of catalysts was performed by different tech-
niques: FT-IR, XRD, SBET, and potentiometric titration although not
all of them are presented in this work.
General Experimental Procedure for the Preparation
of Sulfones
A mixture of Keggin molybdenum heteropolyacid immobilized on
aminopropyl-functionalized silica catalyst (50 mg), sulfide substrate
(1 mmol), hydrogen peroxide (35% w/v, 7.7 mmol), and ethanol (4 mL)
was refluxed for the specified reaction times (Tables I and II). After

Sulfones Synthesis Using Aqueous Hydrogen Peroxide
3267
completion of the reaction, the catalyst was removed by filtration, and
the resulting solution was concentrated under reduced pressure. The
reaction mixture was directly charged on a small silica gel (Grace
Davison, Grade 62, 70 × 200 Mesh) column and eluted with ethyl
acetate–hexane, to obtain pure compounds in good yields (Tables I and
II).
Recycling of Catalyst
After reaction, the catalyst was filtered, washed thoroughly with
ethanol, dried under vacuum, and reused for the oxidation reaction,
following the procedure described above.
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