Selective Oxidation of Sulfides to Sulfoxides Using Modified Keggin Heteropolyacids as Catalyst

Article abstract of DOI:10.1080/10426507.2013.865128

The use of modified Keggin phosphomolybdic heteropolyacids (HPA) (V and Al) as catalysts in the selective oxidation of sulfides to the corresponding sulfoxides is reported. Excellent yields were obtained (13 examples: 79%-95%), using 35% (w/v) aqueous hydrogen peroxide as oxidant, and acetonitrile as solvent at 25°C. The relationship between the electron densities of the sulfur atoms, which was estimated by molecular theoretical calculations, and the oxidative relativities of the sulfur-containing compounds was also investigated. The results indicate that the time for 100% conversion decreases with an increase in electron density.

Full text of DOI:10.1080/10426507.2013.865128

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Phosphorus, Sulfur, and Silicon and the
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Selective Oxidation of Sulfides to
Sulfoxides Using Modified Keggin
Heteropolyacids as Catalyst
a
a
a
Valeria Palermo , Ángel G. Sathicq , Patricia G. Vázquez & Gustavo
a
P. Romanelli
a
Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr.
Jorge J. Ronco” (CINDECA-CCT-CONICET), Universidad Nacional de
La Plata. Calle 47 N° 257, B1900AJK La Plata, Argentina
Accepted author version posted online: 13 Mar 2014.Published
online: 21 Aug 2014.
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To cite this article: Valeria Palermo, Ángel G. Sathicq, Patricia G. Vázquez & Gustavo P. Romanelli
2014) Selective Oxidation of Sulfides to Sulfoxides Using Modified Keggin Heteropolyacids as
Catalyst, Phosphorus, Sulfur, and Silicon and the Related Elements, 189:10, 1423-1432, DOI:
0.1080/10426507.2013.865128
(
1
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Phosphorus, Sulfur, and Silicon, 189:1423–1432, 2014
Copyright ꢀ
Taylor & Francis Group, LLC
C
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2013.865128
SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES
USING MODIFIED KEGGIN HETEROPOLYACIDS
AS CATALYST
´
Valeria Palermo, Angel G. Sathicq, Patricia G. V a´ zquez,
and Gustavo P. Romanelli
Centro de Investigaci o´ n y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco”
◦
(
CINDECA-CCT-CONICET), Universidad Nacional de La Plata. Calle 47 N 257,
B1900AJK La Plata, Argentina
GRAPHICAL ABSTRACT
Abstract The use of modified Keggin phosphomolybdic heteropolyacids (HPA) (V and Al)
as catalysts in the selective oxidation of sulfides to the corresponding sulfoxides is reported.
Excellent yields were obtained (13 examples: 79%–95%), using 35% (w/v) aqueous hydrogen
◦
peroxide as oxidant, and acetonitrile as solvent at 25 C. The relationship between the electron
densities of the sulfur atoms, which was estimated by molecular theoretical calculations, and
the oxidative relativities of the sulfur-containing compounds was also investigated. The results
indicate that the time for 100% conversion decreases with an increase in electron density.
Keywords Modified Keggin heteropolyacids; Catalyst; Sulfide oxidation; Sulfoxides
Received 27 September 2013; accepted 6 November 2013.
´
Address correspondence to Angel G. Sathicq and Gustavo P. Romanelli, Centro de Investigaci o´ n y Desarrollo
en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA-CCT-CONICET), Universidad Nacional de La Plata. Calle
◦
4
7 N 257 B1900AJK, La Plata, Argentina. E-mail: gpr@quimica.unlp.edu.ar; agsathicq@quimica.unlp.edu.ar
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
1
423

1
424
V. PALERMO ET AL.
INTRODUCTION
The oxidation of organic substrates with aqueous hydrogen peroxide is very attractive
from the viewpoint of industrial technology and synthetic organic chemistry since aqueous
1
–8
hydrogen peroxide is very cheap, environmentally clean and easy to handle.
The development of efficient catalytic systems for the oxidation of sulfides to sul-
foxides and sulfones has attracted attention because they are essential intermediates for the
synthesis of many drugs and natural products, such as ligands in asymmetric catalysis and
9
reagents for oxygen transfer. The increased interest in the sulfoxides has been due to their
importance as synthetic intermediates for the production of a wide range of chemically and
7
biologically active molecules. They often perform a major function as therapeutic agents
1
0
11
12
13
14
such as antiulcer, antihypertensive, cardiotonic agents, psychotronics, vasodilators,
antibacterial, and antifungal.15
There are many reagents available for the oxidation of sulfides such as halogen
compounds, nitrates, transition metal oxides, hydrogen peroxide, and oxygen. Most of
these reagents are not satisfactory for sulfoxide synthesis, for different reasons such as
low content of effective oxygen, overoxidation, formation of environmentally unfavorable
16
byproducts, and cost effectiveness. Recently methods include the use of different catalyst
in heterogeneous or solvent free-condition for example: DABCO tribromide immobilized
on magnetic nanoparticle under metal-free conditions, or graphene-γ -Fe2O3 magnetic
nano-composites.1
7,18
Heteropolyacids (HPA) with Keggin structures show activity as both acid and
redox catalysts. Due to their complex structure and reactive properties, they give ample
opportunity for novel scientific study.1
9–21
Vanadium heteropolycompounds have recently
1
7,22,23
been studied due to their importance as catalysts in different oxidation reactions,
as
well as benzyl alcohol,24 phenols, and amine oxidation and the selective oxidation of
25
26
1
7,23,27
sulfides to sulfoxides and sulfones using hydrogen peroxide.
We studied the use of modified Keggin HPA (V and Al) as catalyst in the selective
oxidation of sulfides to the corresponding sulfoxides, using 35% (w/v) aqueous hydro-
gen peroxide as oxidant, and acetonitrile as solvent (Scheme 1). The three catalysts used
were H3PMo12O40 (PMo), H6PMo11AlO40 (PMoAl), and H4PMo11VO40 (PMoV). Excel-
lent yields were obtained (13 examples: 79%–95%) using 35% (w/v) aqueous hydrogen
◦
peroxide as oxidant, 1 mmol% of PMoV and acetonitrile as solvent at 25 C. The relation-
ship between the electron densities of the sulfur atoms, which was estimated by theoretical
calculations, and the oxidative relativities of the sulfur-containing compounds was also
investigated.
Scheme 1 Selective oxidation of sulfides using HPA as catalyst.

SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES
1425
Table 1 Oxidation of diphenylsulfide using HPA-H2O2 system in acetonitrilea
H2O2 amount
(mmol)
Temperature
Time
(min)
Conversion
(%)
Selectivity
sulfoxide (%)
Selectivity
sulfone (%)
◦
Entry
Catalyst
( C)
1
2
3
4
None
PMo
PMoAl
PMoV
3
3
3
3
3
25
25
25
25
25
25
25
25
40
70
70
300
300
300
300
300
300
300
300
300
300
420
—
65
52
98
98
—
98
100
98
99
98
97
95
41
10
—
—
2
—
2
1
2
3
5
59
90
100
a
5
6
7
8
9
0
1
1
92
10
30
30
30
30
100
100
100
100
100
1
1
aUsing a mixture acetonitrile: t-butanol 4:1.
RESULTS AND DISCUSSION
The reaction conditions were first optimized using diphenylsulfide as substrate at
5 C (Scheme 1). The results of the study of the oxidation reaction of diphenylsulfide with
◦
2
the use of the three different HPAs (PMo, PMoAl, and PMoV) are summarized in Table 1.
◦
No conversion was obtained in the absence of the catalyst at 25 C, in a time period of
300 min using 3 mmol of 35% (w/v) hydrogen peroxide (Table 1, entry 1), but when HPA
is added, the reaction takes place efficiently.
When PMo is used (Table 1, entry 2), a conversion of 65% is observed at 300 min,
with a 98% of selectivity of the corresponding sulfoxide. Although PMoAl is less active
than PMo (52%, 300 min) it is more selective (100%), (Table 1, entry 3).
However, when vanadium is incorporated into the PMo structure, a marked increase
in the activity is observed, which produces 98% of conversion of diphenylsulfide, and a
sulfoxide selectivity of 98%, in the same time period (Table 1, entry 4).
The PMoV catalyst shows unique catalytic features for oxidation due to its bifunc-
tional character, which arises because of the redox nature of vanadium and the oxida-
28
29
tion/acidic character of the molybdophosphoric acid. As reported by Song et al., the
metal substitution may be modifying the energy and composition of the lowest unoccupied
molecular orbital (LUMO) and consequently, its redox properties. The substitution of vana-
dium ions into the molybdenum framework stabilizes the LUMOs because these orbitals
derive in part from vanadium d-orbitals, which have been assumed to be more stable than
those of molybdenum.
Several researchers have investigated vanadium incorporated molybdophosphoric
acid catalysts by replacing 1, 2, and 3 molybdenum atoms with the corresponding number
of vanadium atoms. The acidic, redox, and thermal stability properties of the vanadium
24
catalysts are influenced by the degree of vanadium substitution. In solutions, the reduction
potentials of heteropolyanions containing Mo and V are high as these ions are easily
reduced. Oxidative ability decreases generally in the order: V- > Mo- > W- containing
heteropolyanions, which means that the vanadium-containing heteropoly compounds are
the strongest oxidants.26

1
426
V. PALERMO ET AL.
Table 2 Oxidation of sulfides to sulfoxides using PMoV-H2O2 catalyst
Entry Substrate/atomic charge on S atom (Qs)
1
Product
Time (min)
300
Yields (%)
95
0
.165
1
.050
2
350
82
0
0
.220
.085
3
4
60
60
92
93
0
0
.085
.097
5
80
83
6
7
50
55
94
88
0
0
.073
.077
8
9
55
35
91
83
0
.076
−
0.031
(
Continued on next page)

SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES
1427
Table 2 Oxidation of sulfides to sulfoxides using PMoV-H2O2 catalyst (Continued)
Entry Substrate/atomic charge on S atom (Qs)
Product
Time (min)
30
Yields (%)
1
1
0
1
88
83
−
−
0.078
0.047
40
300
75
1
1
2
3
85
79
0
.168
0
.094
As PMoV was much more active, it was used in the next reaction optimization
essays. The effect of H2O2 amount on conversion and selectivity was studied, and four
different H2O2/substrate molar ratios were used: 1, 3, 10, and 30 (Table 1, entries 4, 6–8).
No significant effect on conversion was obtained and diphenylsufoxide selectivity was
excellent (between 95%–98%).
In order to continue with the study of the reaction optimization, the effect of tem-
perature on the conversion and selectivity of diphenylsulfide was analyzed using H2O2 in
◦
excess (H2O2/substrate ratio: 30:1) (Table 1, entries 8–11). As can be observed, at 25 C, the
reaction reaches 100% of diphenylsulfide conversion and is selective to sulfoxide (95%).
On increasing the reaction temperature, a higher selectivity to sulfone results. For exam-
◦
ple, at 70 C and for 420 min, 100% selectivity to diphenylsulfone is achieved (Table 1,
entry 11).
In order to explore a possible reaction mechanism, an additional experiment was
performed using a radical inhibitor such as t-butanol. The experimental conditions were:
1
4
mmol% of the PMoV catalyst, 1 mmol of diphenylsulfide, 3 mmol of 35% (w/v) H2O2,
mL of acetonitrile, and 1 mL of t-butanol; reaction for 300 min at 25 C. In these conditions,
◦
a similar conversion of diphenylsulfide (98%) and selectivity of the corresponding sulfoxide
(
(
99%) were obtained, compared to the initial experiment without the radical inhibitor
Table 1, entries 5 and 4, respectively). This result demonstrates that the reaction mechanism
is not via a free radical oxidation.
Once the reaction condition for the selective oxidation of sulfides to sulfoxides and
sulfones had been optimized, the reaction was extended to other starting substrates. Diaryl

1
428
V. PALERMO ET AL.
4
3
3
2
2
1
1
00
50
00
50
00
50
00
9
0
1
2
8
0
0
1
2
5
1
3
7
60
3
4
6
8 7
50
0
,070
0,075
0,080
0,085
0,090
0,095
0,100
1
1
9
5
0
0
10
-0,10
-0,05
0,00
0,05
0,10
0,15
0,20
0,25
Qs
Figure 1 Reaction time for 100% conversion of sulfide vs. Qs (charge on sulfur atom in the corresponding
a
a
sulfide) . Number 1–13 corresponding to the different sulfides (see Table 1).
(Table 2, entries 1–2), aryl-alkyl (Table 2, entries 3–8), dialkyl (Table 2, entries 9–11), and
more sophisticated compounds (Table 2, entries 12–13) were used as starting materials.
Table 2 lists the results for the selective oxidation of these sulfides to the corre-
sponding sulfoxides. All the reactions were run within a reasonable reaction time and the
sulfoxides were obtained in excellent yields (79%–95%), and only traces of sulfones and
other secondary products were detected. Therefore, the reaction condition is selective to
the transformation of sulfides to sulfoxides. For example, in Table 2, entry 5, it can be seen
that the formyl group was not affected by the reaction condition. Similarly, the hydroxyl
group (Table 2, entry 7) was not affected under the same reaction condition and the other
group present in the sulfides remained unchanged.
The reactivity of the different sulfur-containing compounds was investigated for 13
examples using computational methods. The relationship between the electron densities
of the sulfur atoms, which were estimated by Hyperchem 5, and the oxidative relativities
of the sulfur-containing compounds was also investigated. The electron densities on the
sulfur atom of the 13 sulfides, which varied between −0.078 and 0.168, are shown in
Table 2.
Figure 1 shows a relationship between the substrate reactivity, evaluated as the reac-
tion time for a 100% conversion of sulfide and the electron densities on the sulfur atom.
From the results obtained, it could be observed that dialkyl sulfides (−0.031, −0.047,

SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES
1429
and −0.078) are more reactive than alkyl-aryl sulfides (0.076–0.097) and alkyl-aryl sul-
fides are more reactive than diaryl sulfides (0.165 and 0.220). This different reactivity is
associated with the electronic density on the sulfur, since a higher electronic density on
this atom facilitates the attack on the electrophilic form of the hydrogen peroxide-catalyst
system.
A plausible mechanism of this reaction involves the formation of peroxo-heteroatom
(Mo, V, Al) species and the subsequent nucleophilic attack of the sulfur atom in the
sulfide on the peroxo species. Indeed, it is known that sulfides are oxidized to sulfox-
ides by electrophilic oxidants. Mechanistically, it is believed that the electrophilicity of
the peroxide oxygen in H2O2 is increased by an oxometal group (M Od) in the HPA
30
(Scheme 2).
Scheme 2 Plausible hydrogen peroxide activation with the heteropolyacid and electrophilic attack to diphenyl-
sulfoxide.
The overoxidation of sulfoxide to sulfone requires the presence of an oxidant with
nucleophilic character. To verify this asseveration, we performed an additional experiment
using diphenylsulfoxide as substrate and the standard reaction conditions. The experimental
conditions were: 1 mmol% of the PMoV catalyst, 1 mmol of diphenylsulfoxide, 3 mmol of
◦
3
5% (w/v) H2O2, and 5 mL of acetonitrile; reaction for 300 min at 25 C. In these conditions,
the conversion of diphenylsulfoxide was only 4%, which means that the electrophilic
oxidant-catalyst system is not efficient to oxidize sulfur with a high positive electronic
density (1.050).
CONCLUSIONS
In conclusion, in this research, we found a convenient and selective procedure for
oxidizing sulfides to sulfoxides, using 35% (w/v) aqueous hydrogen peroxide and a catalytic
amount of HPA. The yields were excellent, and all reagents are cheap and the catalyst
preparation is easy. The reaction for the sulfoxide preparation was carried out at room
◦
temperature (25 C), in a reasonable reaction time. The incorporation of V into the structure
of PMo increases the catalytic activity. The PMoV catalyst shows unique catalytic features
for oxidation due to its bifunctional character, which arises because of the redox nature of
vanadium and the oxidation/acidic character of the molybdophosphoric acid. The different
reactivity of sulfides is associated with the electronic density on the sulfur atom, since a
higher electronic density on this atom facilitates the attack by the electrophilic form of the
hydrogen peroxide/catalyst system.

1
430
V. PALERMO ET AL.
EXPERIMENTAL SECTION
General Remarks
All reagents were purchased from Merck and Aldrich and used without further
purification. All yields refer to isolated products after purification. The products were
1
13
characterized by spectroscopy data ( H and C NMR). The NMR spectra were recorded
on a 200 MHz Bruker spectrometer. The NMR spectra were measured in CDCl3 relative
to TMS (0.00 ppm). The organic phase was dried on anhydrous Na2SO4 and filtered for
its analysis by gas chromatography using Varian Scan 3400 cx equipment. The product
distribution was quantified by a Shimadzu C-R34 instrument. Reactions were monitored
by thin layer chromatography (TLC) analyses.
Catalytic Preparation
H3PMo12O40 (PMo) was purchased from Aldrich and used without further purifica-
31
tion. The other HPA, doped with Al and V, were prepared as in previous work and were
labeled H4PMo11VO40.12H2O (PMoV) and H6PMo11AlO40.12H2O (PMoAl), respectively.
General Procedure for the Oxidation of Sulfide
A solution of sulfide (1 mmol) and catalyst (0.01 mmol), in acetonitrile, was added
◦
to 35% (w/v) H2O2 (2 mmol). The mixture was stirred at 25 C for a time period (see
Tables 1 and 2). The solvent was evaporated and then H2O (5 mL) was added. The substrate
was extracted with toluene (2 × mL) and dried with anhydrous Na2SO4; filtration and
evaporation afforded the corresponding crude sulfoxides. The solids were purified by
1
recrystallization to afford the pure sulfoxides. The products were confirmed by H NMR
13
and C NMR analyses.
Sample Analysis
Samples were withdrawn from the organic phase at 15-min intervals. Each volume
sample was approximately 20 μL and it was diluted with 1–2 mL of acetonitrile, the con-
centrations were calculated with an internal standard method. Conversions were obtained
with GC analysis performed with a Varian GC 3400 instrument. The capillary column was
a 30 m Chromopack CP Sil 8 CB, 0.32 mm in diameter. The identification of products
was performed with GC–MS (detector HP 5971) for comparison with GC–MS authentic
samples. Detailed characterization data for previously known samples are presented in the
Supplemental Materials.
Electronic Densities of Sulfur Atom Determination
The densities of the sulfur atoms in the sulfide were determined after optimization
of their conformation. The conformations of the compounds were drawn by means of
32
the “model build” modulus available in HyperChem 5.0. Each molecular structure was
firstly preoptimized with the Molecular Mechanics Force Field (MM+) procedure and
the resulting geometry was further refined by means of the semiempirical method PM3
−1
(
Parametric Method-3). A gradient norm limit of 0.01 kcal Å was chosen.

SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES
1431
ACKNOWLEDGMENTS
We gratefully acknowledge the support of this study by CONICET, ANPCyT and
´
Universidad de La Plata. Valeria Palermo, Angel G. Sathicq, Patricia G. V a´ zquez, and
Gustavo P. Romanelli are members of CONICET.
SUPPLEMENTAL MATERIAL
Supplementary data for this article can be accessed on the publisher’s website,
www.tandfonline.com/gpss
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