Selective oxidation of sulfides to sulfoxides in water using 30% hydrogen peroxide catalyzed with a recoverable VO(acac)2 exchanged sulfonic acid resin catalyst

Article abstract of DOI:10.1016/j.molcata.2006.11.052

Various types of sulfides are selectively oxidized to sulfoxides in good to excellent yields in aqueous media using 30% aqueous hydrogen peroxide as an oxidant in the presence of catalytic amounts of a VO(acac)₂-exchanged strongly acidic polymeric resin catalyst in water at room temperature. The catalyst can be recovered and reused at least five times without loss of activity and selectivity.

Full text of DOI:10.1016/j.molcata.2006.11.052

Journal of Molecular Catalysis A: Chemical 268 (2007) 45–49
Selective oxidation of sulfides to sulfoxides in water using 30%
hydrogen peroxide catalyzed with a recoverable VO(acac)2
exchanged sulfonic acid resin catalyst
K. Leon Prasanth, H. Maheswaran∗
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 23 March 2006; received in revised form 24 November 2006; accepted 29 November 2006
Available online 5 December 2006
Abstract
Various types of sulfides are selectively oxidized to sulfoxides in good to excellent yields in aqueous media using 30% aqueous hydrogen
peroxide as an oxidant in the presence of catalytic amounts of a VO(acac)2-exchanged strongly acidic polymeric resin catalyst in water at room
temperature. The catalyst can be recovered and reused at least five times without loss of activity and selectivity.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Oxidation; VO(acac)2; Sulfides; Sulfoxides; Heterogeneous catalysis
1. Introduction
and cleanliness as it produces only water as by-product [16].
This feature has already stimulated the development of useful
procedures for H2O2 oxidation [17,18], and recently, Karimi et
al. have reported highly efficient synthesis of sulfoxides with
a recoverable silica-based tungstate interphase catalyst using
hydrogen peroxide as oxidizing agent [19]. With ever-increasing
environmental awareness, there is much incentive to develop
new processes that are robust, recyclable, and environmentally
very benign. In this paper, we disclose a new heterogeneous
protocol in aqueous media for the selective oxidation of sul-
fides to sulfoxides using 30% aqueous hydrogen peroxide as
oxidizing agent in the presence of catalytic amounts of a recov-
erable VO(acac)2-exchanged polymeric resin catalyst at room
temperature.
The selective oxidation of sulfides to sulfoxides is an impor-
tant method in organic synthesis because sulfoxides are useful
synthetic intermediates for the construction of various chem-
ically and biologically significant molecules [1]. To date, the
most important method of preparation involves the oxidation
of sulfides. Various oxidizing agents are used for this purpose
[2–15], and unfortunately, most of these reagents are not satis-
factory for medium to large-scale operations for several reasons.
The first reason is that the low content of the effective oxygen
that is available for the oxidation, generation of environmen-
tally unfavorable by-products, and high cost. Another reason is
that over oxidation of the sulfoxides to their sulfones during the
oxidation of sulfides. Indeed selective oxidation of sulfides to
sulfoxides remains a challenging task, and constitutes an active
research on its own particularly with the emphasis on ‘green
chemistry’. Thus, there is still a need for sulfoxidation proce-
dures that are efficient, more selective, and in particular conform
to a class of greener technologies. In this direction, use of aque-
ous hydrogen peroxide as oxidizing agent is highly preferred
because of its high effective oxygen content (as high as 47%)
2. Experimental
All organic reagents and solvents were reagent grade and pur-
chased from commercial vendors. Strongly acidic ion-exchange
resin INDION-770 was procured from Ion Exchange India
Ltd., and was used as received. All reactions were carried out
under nitrogen atmosphere, and monitored by thin layer chro-
matography (TLC). Column chromatography purifications were
performed using silica gel. All solvents were dried and degassed
before use. NMR spectra were measured in CDCl3 on a Bruker
300 MHz and Varian-200 MHz instruments with TMS as an
∗
Corresponding author. Tel.: +91 40 27193510; fax: +91 40 27160921.
E-mail address: maheswaran dr@yahoo.com (H. Maheswaran).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.11.052

46
K.L. Prasanth, H. Maheswaran / Journal of Molecular Catalysis A: Chemical 268 (2007) 45–49
Scheme 1. Preparation of catalyst.
internal reference. Atomic emission spectra were recorded on
an IRIS Interprid 11 XDL instrument. IR spectra were recorded
on a FT-IR (Perkin-Elmer Spectrum 1000) instrument.
was extracted by using dichloromethane (3 mL × 10 mL), dried
over anhydrous sodium sulfate and evaporated in vacuo to afford
the crude product. The crude product was purified by column
chromatography on silica gel (10% EtOAc in hexane) to afford
analytically pure methyl phenyl sulfoxide (130 mg; 93%). The
products were characterized by comparing the 1H-NMR spectra
with those reported in the literature [20]. The products were also
characterized by EI-mass spectroscopy.
2.1. Synthesis of VO(acac)2
To an aqueous suspension of vanadium pentoxide (5 g,
27.49 mmol) in 20 mL of water taken in a 500 mL beaker, 30%
hydrogen peroxide (37.37 mL, 329.88 mmol) was added drop-
wise in an ice-cold condition and the mixture was stirred till a
clear dark solution was formed. To this solution, distilled acetyl
acetone(19.84 mL, 192.5 mmol)wasaddeddrop-wiseverycare-
fullywithcontinuousstirring. Vigorouseffervescencetookplace
after 15 min, and further stirring for a period of 30 min led to
a precipitation of a brown colored microcrystalline compound
fromthereactionmixture. Thismixturewasthenallowedtoreact
at 70 ◦C for 15 min under vigorous stirring. During this time,
the brown precipitate turned olive green with shiny crystalline
appearance with the solution also turning green. The solution
was concentrated by heating on a steam bath for 30 min and
then placed in an ice-water bath for 15 min. The precipitate was
filtered through Whatman no. 42 filter paper, washed with ace-
tone, and dried in vacuo over fused calcium chloride. Yield:
11.7 g (80%).
3. Results and discussion
As shown in Scheme 1, the VO(acac)2-exchanged polymeric
resin catalyst was synthesized by treating strongly acidic cation
exchange resin (Sulfonic acid H+ form; INDION-770) with
VO(acac)2 in chloroform at room temperature for 12 h. Then,
the beads of polymer resin acquired dark blue color indicating
formation of the metal complex on the surface of the polymeric
material. FT-IR was used to study the structure of the cata-
lyst. The result from this study has strongly indicated that the
acetyl acetone functionality is no longer present in the immobi-
lized catalyst. Thus, most probable structure of the catalyst on
the polymer surface involves bonding of sulfonyl anions to the
V(IV) center so as to form the corresponding sulfonyl complex
as shown in Scheme 1. The quantity of vanadium present in the
Table 1
2.2. Synthesis of VO(acac)2 supported on strongly acidic
INDION-770 resin
Optimization studies for oxidation of methyl phenyl sulfidea
To a solution of VO(acac)2 (0.5 g) in chloroform (20 mL),
strongly acidic INDION-770 resin (5 g) was added and the mix-
ture was stirred at ambient temperature for 12 h under nitrogen
atmosphere. The solid catalyst was filtered, washed several times
with chloroform, and finally dried in vacuo. Measurement of
increase in the mass the mass of the resultant VO(acac)2-resin
indicates that 420 mg of VO(acac)2 is supported. The quantity
of vanadium present was estimated to be 0.283 mmol/g of resin
by atomic absorption spectroscopy.
Entry
Time
(min)
Solvent
Conversion
(%)a,b
Sulfoxide Sulfone
(%)b
(%)b
1
2
3
4
5
6
7
8
9
360
60
720
10
10
10
10
10
120
120
CH2C12
CH2C12
CHC13
CH3CN
CH3OH
H2O
H2O
H2O
H2O
H2O
54
80c
39
92
67
75
36
33
25
64
5
95
84
92
9
98
100
100
100
70
0
0
0
30
–
97d
2.3. Typical procedure for the oxidation of methyl phenyl
sulfide
94e
50c
10
No reactionf
–
A round bottom flask (10 mL capacity) was charged with
methyl phenyl sulfide (124 mg, 1 mmol), water (2 mL), and
catalyst (200 mg; 6 mol.%). Aqueous 30% hydrogen peroxide
solution (1.1 mmol) was added and the reaction mixture was
stirred for 10 min at ambient temperature. After completion of
the reaction (followed by TLC), the catalyst was filtered off
and washed with water, and product methyl phenyl sulfoxide
a
Reaction conditions: substrate (1 mmol), catalyst (6 mol.%), and 30% H2O2
(1.1 mmol) were stirred at room temperature in respective solvent (2 mL).
b
Determined from 1H-NMR spectra of the crude product.
Reaction was carried out using VO(acac)2.
Recovered catalyst was used (recycling experiments): I recycle.
Fifth recycle.
c
d
e
f
No catalyst was used.

K.L. Prasanth, H. Maheswaran / Journal of Molecular Catalysis A: Chemical 268 (2007) 45–49
47
heterogeneous complex was estimated to be 0.283 mmol/g of
resin by atomic absorption spectroscopy.
The oxidation of methyl phenyl sulfide, as a standard
substrate, was first studied in the presence of the VO(acac)2-
exchanged polymeric resin catalyst using 30% aqueous
hydrogen peroxide at ambient temperature in a variety of sol-
vents (Table 1). The course of the oxidation reaction is strongly
influenced by the nature of the solvent used for the reaction.
For example, in chlorinated solvents such as dichloromethane
and chloroform the oxidation reaction occurred in a sluggish
manner with poor conversion (Table 1; entries 1 and 3). In con-
trast, in polar organic solvents such as acetonitrile and methanol,
Table 2
VO(acac)2-exchanged polymer resin catalyzed selective oxidation of sulfides to sulfoxides with 30% H2O2 in watera
Entry
1
Substrateb,c
Time (min)
Sulfoxide
Yieldd
95
20
2
3
05
10
89
95
4
5
10
40
97
95
92
87
98
92
89
78
82
6
40
7
130
160
160
160
160
160
8
9
10
11
12
13
14
140
25
79
97
15
720
74
a
Reaction conditions: substrate (1 mmol), resin catalyst (6 mol.%), and 30% H2O2 (1.1 mmol) were stirred at room temperature in water as solvent (2 mL).
Entries 5–8: reactions were carried out using 9:1 water:methanol as solvent (2 mL).
Entries 14 and 15 was carried out in methanol (2 mL) as solvent.
Determined from 1H-NMR spectra of the crude product.
b
c
d

48
K.L. Prasanth, H. Maheswaran / Journal of Molecular Catalysis A: Chemical 268 (2007) 45–49
the oxidation reaction proceeded rather fast, and gave mixture
of sulfoxides and sulfones with good selectivity toward sulfox-
ides (Table 1; entries 4 and 5). Formation of sulfones was fully
suppressed when highly polar water is used as solvent for the
reaction as this reaction gave exclusively sulfoxides (Table 1;
entry 6). Thus water is the best solvent for the reaction. This sol-
vent dependence could be attributed to swelling of the polymeric
catalyst in polar solvents, thus, enabling greater accessibility
of vanadium sites to the substrates for the oxidation reaction
[21]. The controlled reaction conducted under identical condi-
tions devoid of catalyst in water showed no oxidized product
formation within the stipulated reaction time (Table 1; entry
10).
To better understand the efficiency of our catalyst system,
we have studied the oxidation of methyl phenyl sulfide under
homogeneous conditions in the presence of VO(acac)2 and
aqueous 30% hydrogen peroxide. Both in aqueous media and
dichloromethane, the VO(acac)2 catalyst afforded the desired
products in poor yields (Table 1; entries 2 and 9). In contrast, the
heterogeneous VO(acac)2-exchanged resin catalyst selectively
affords sulfoxides in excellent yields. Thus, the heterogeneous
VO(acac)2-exchanged resin catalyst is the best catalyst in terms
of both catalytic activity and selectivity for sulfoxidation reac-
tion. To study the recyclability of the VO(acac)2-exchanged
resin catalyst, the oxidation of methyl phenyl sulfide was exam-
ined in water (Table 1; entries 7 and 8). After completion of
the reaction, the catalyst was filtered and washed well with
dichloromethane and water and reused in the sulfoxidation reac-
tion. The catalyst was found to be effective and gave quantitative
yields up to five times. No significant leaching of the metal ions
from the polymeric resin matrix was observed as indicated by
atomic absorption analysis of spent catalysts. Thus the cata-
lyst is reusable without significant loss of catalytic activity and
selectivity.
To study the scope of the procedure, a series of sulfides
having varied R groups containing aromatic and aliphatic
attached to the sulfur atom was subjected to the oxidation in
water using the VO(acac)2-exchanged polymeric resin cata-
lyst and 30% aqueous hydrogen peroxide as an oxidant and
the results are present in Table 2. A series of substrates,
diaryl, aryl–alkyl, alkyl–allyl, aryl–vinyl and dialkyl sulfides
was oxidized to the corresponding sulfoxides in high yields.
Of these substrates, aryl–alkyl and dialkyl sulfides were very
reactive as these substrates were oxidized to their correspond-
ing sulfoxides within 25 min in aqueous media (entries 1–4,
and 14). In contrast, both aryl–vinyl and aryl–allyl sulfides
were only moderately reactive as it took longer reaction times
for providing the corresponding sulfoxides in aqueous media
with good yields (entries 9–13). As anticipated, the oxida-
tion of diphenylsulfide took a much longer reaction time
(720 min) to give corresponding sulfoxide in moderate yields
(entry 15). The oxidation of diphenylsulfide, and 2-chloroethyl-
methylsulfide proceeded much slower in water; however, their
oxidation reaction was reasonably faster when methanol was
used as solvent (see foot note c in Table 2: entries 14 and
15). For substrates such as (2-chloro-ethylsulfanyl)benzene,
chloromethylsulfanylbenzene, methoxymethylsulfanylbenzene,
1-chloro-4-chloro-methylsulfanylbenzene, the oxidation reac-
tion proceeded very slowly in water quite possibly due to
poor dispersion of the sulfide in the aqueous media (entries
5–8). For this reason, their oxidation reactions were carried
out using a 9:1 mixture of water and methanol with 30%
aqueous hydrogen peroxide as oxidant (entries 5–8). The
additive methanol has substantially increased the speed of
the reaction, and gave excellent yields for the corresponding
sulfoxides.
A good functional group tolerance was observed under our
experimental conditions. For example, in the case of allylic and
vinylic sulfides, no overoxidation to the sulfones or epoxidation
of the double bond was observed, and only the corresponding
sulfoxides products were obtained in excellent yields (entries
9–13). A similar functional group tolerance was also observed
for substrates containing reactive chloro functional group in the
moiety for the oxidation reaction (entries 5–8). These substrates
selectively underwent oxidation at the sulfur atom without
undergoing further structural changes in their functional groups
1
as suggested by H-NMR and EI-mass spectroscopic analyses
of the products.
Being guided by the published reports on the peroxo-
chemistry of titanium [22] and vanadium [23], it is believed that
the catalyst interacts with peroxide to form a peroxometal inter-
mediate, thereby activating the bound peroxide for the oxidation
reaction. The ease of transfer of electrophilic oxygen from the
peroxometal species to the nucleophilic sulfide facilitates the
formation of sulfoxides with the eventual regeneration of the
catalyst.
4. Conclusion
The oxidation of sulfides to sulfoxides in aqueous media
is conducted using the VO(acac)2-exchanged polymeric resin
catalyst in the presence of 30% aqueous hydrogen peroxide at
ambient temperature. A variety of substrates with diverse struc-
tural features undergo oxidation to the corresponding sulfoxides
with excellent yields. It is important to note that the catalyst is
reused five times without significant loss of activity and selectiv-
ity. From an environmental standpoint, this procedure provides
a simple method for the formation of sulfoxides from sulfides,
which is a potentially competitive and an economically viable
process.
Acknowledgements
K. Leon Prasanth thanks Council of Scientific and Industrial
Research (CSIR), India for research fellowship. H. Maheswaran
thanks CSIR for providing funds through Task Force Project for
Green Technologies.
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