An efficient catalytic procedure for the selective oxidation of sulfides to sulfoxides by citric acid/Al(NO3)3.9H2O/MBr under mild and heterogeneous conditions

Article abstract of DOI:10.1002/jccs.200900036

A wide range of aliphatic or aromatic sulfides are selectively oxidized to the corresponding sulfoxides using Al(NO₃)₃.9H ₂O, citric acid and a catalytic amount of KBr or NaBr in the presence of wet SiO₂ (50% w/

Full text of DOI:10.1002/jccs.200900036

Journal of the Chinese Chemical Society, 2009, 56, 251-254
251
Communication
An Efficient Catalytic Procedure for the Selective Oxidation of Sulfides to
Sulfoxides by Citric Acid/Al(NO3)3.9H2O/MBr under Mild and
Heterogeneous Conditions
Arash Ghorbani-Choghamarani* and Somaieh Rezaei
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran
Awide range of aliphatic or aromatic sulfides are selectively oxidized to the corresponding sulfoxides
using Al(NO3)3.9H2O, citric acid and a catalytic amount of KBr or NaBr in the presence of wet SiO2 (50%
w/w) in CH2Cl2 under mild and heterogeneous conditions in good to high yields.
Keywords: Sulfides; Sulfoxides; Aluminum nitrate (Al(NO3)3.9H2O); Citric acid; Catalyst.
Oxidation is one of the most fundamental reactions in
organic synthesis.1 Recently, development of efficient and
new catalytic systems for various organic transformations
is an active ongoing research area and the scope for further
improvement towards milder reaction conditions.2 The oxi-
dation of sulfides to sulfoxides is of significant importance
in organic chemistry, both for fundamental research and for
a wide range of applications.3 Selective oxidation of sul-
fides to the sulfoxides remains a challenge and is interest-
ing because corresponding sulfoxides are valuable syn-
thetic intermediates for the synthesis of chemically and bi-
ologically important significant molecules.4-6 Sulfoxides
are also valuable in C-C bond-forming7,8 and molecular re-
arrangements.9-11 Additionally some of biologically active
sulfoxides play an important role as therapeutic agents.12,13
Even though a number of methods for the oxidation
of sulfides to sulfoxides has been emerged in the recent
years such as: H2O2/Peroxotungstate,14 oxaziridinium salt,15
LaCoO3/TBHP,16 silica gel supported CAN/NaBrO3,17 o-
methyl-substituted IBX,18 O2/Ru(TPP)Cl,19 H2O2/MoO3,20
H2O2/Ti4[(OCH2)3CMe]2(i-PrO)10,21 H2O2/acetic acid,22
H2O2/HAuCl4·4H2O,23 UHP/cis-[MoO2(phox)2],24 UHP/
Mn(N-OPh-sal) (acac) (EtOH),25 H2O2/silica sulfuric acid,26
but this transformation have received considerable atten-
tion, especially in the search for efficient, selective, cost ef-
fective, simple manipulation, mild reaction conditions and
environmentally friendly catalysts and oxidants.
tion of new reagents on the oxidation of organic com-
pounds,27-35 we became interested to introduce a new cata-
lytic media, based on in situ generation of X+ using alumi-
num nitrate {Al(NO3)3.9H2O}, citric acid and a metal ha-
lide for the oxidation of sulfides.
To find the best catalytic conditions for the oxidation
of sulfides, a combination of Al(NO3)3.9H2O, citric acid
and catalytic amounts of a metal halide (MX) in the pres-
ence of wet SiO2 (50% w/w) in dichloromethane at room
temperature was examined for the oxidation of benzyl
phenyl sulfide as a model reaction (Table 1). As is evident
from Table 1, the best catalysts are KBr and NaBr.
Subsequently, to find the best solvent for our reac-
tions, we screened different solvents for the oxidation of
benzyl phenyl sulfide as a typical example. The oxidation
of benzyl phenyl sulfide was carried out using a mixture
containing 1 mmol of benzyl phenyl sulfide, Al(NO3)3.
9H2O (1.1 mmol), citric acid (1.5 mmol), KBr (0.05 mmol)
and 0.3 g of wet SiO2 (50% w/w) in 5 mL of solvent at room
temperature. As it can be seen from Table 2 dichlorometh-
ane is the best solvent as selectivity and reactivity for this
transformation.
Finally, we disclosed a new heterogeneous catalytic
protocol for the selective oxidation of different types of
sulfides 1 to the corresponding sulfoxides 2 using combi-
nation of Al(NO3)3.9H2O I, citric acid II and wet SiO2
(50% w/w) in the presence of catalytic amounts of KBr III
or NaBr IV in dichloromethane at room temperature with
In continuation of our recent studies on the applica-
* Corresponding author. Tel/Fax: +98-811-8257404; E-mail: arashghch58@yahoo.com

252 J. Chin. Chem. Soc., Vol. 56, No. 2, 2009
Ghorbani-Choghamarani and Rezaei
Table 1. Oxidation of benzyl phenyl sulfide by Al(NO3)3.9H2O,
citric acid and catalytic amounts of a metal halide (MX)
in the presence of wet SiO2 (50% w/w)a in CH2Cl2 at
room temperature
Scheme I
Entry
Catalyst
Time (Min)
Yield (%)b
1
2
3
4
5
NaCl
NaBr
NaI
24 h
60
--c
97
--c
98
--c
24 h
45
KBr
KI
24 h
a Substrate/Al(NO3)3.9H2O/citric acid/MX/wet SiO2: 1 : 1.1 : 1.5
: 0.05 (mmol): 0.3 g.
b Isolated yield.
c Reaction didn’t complete and impurity of sulfone was observed.
Table 2. Oxidation of benzyl phenyl sulfide using
Al(NO3)3.9H2O, citric acid and catalytic amounts of
KBr in the presence of wet SiO2 (50% w/w) in different
solventsa
Entry
Solvent
Time (h)
Yield (%)b
c
1
2
3
4
5
6
Acetonitrile
Acetone
7
-
d
dia and presence of water are necessary for this transforma-
tion.
7
-
d
Chloroform
Dichloromethane
n-Hexane
7
-
45 min
98
c
Additionally, this oxidizing catalytic media allowed
the homoselective33-35 oxidation of thiantrene to thian-
threne monosulfoxide (Table 3, entries 16 and 17, Scheme
II).
7
7
-
c
-
Methanol
a Substrate/Al(NO3)3.9H2O/citric acid/KBr/wet SiO2: 1 : 1.1 : 1.5
: 0.05 (mmol): 0.3 g.
b Isolated yield.
To show the chemoselectivity of the described system
a sulfide containing hydroxyl group were subjected to the
sulfoxidation reaction, but alcohol oxidation did not occur
under mentioned conditions, and primary hydroxyl group
remained intact in the course of the reaction (Table 3, en-
tries 12 and 13, Scheme III).
c No reaction.
d Trace conversion.
good to excellent yields (Scheme I and Table 3).
To show the role of bromide ion as a catalyst in the de-
scribed system benzyl phenyl sulfide, as a standard sample,
was subjected to the oxidation without KBr or NaBr, how-
ever the reaction didn’t complete after 22 hours.
A plausible mechanism of this oxidation is shown in
Scheme IV based on our previously reported papers,33-35
our observations and obtained results.
In summary herein we report an efficient method for
the selective oxidation of sulfides to sulfoxides under mild
and heterogeneous conditions with good to high yields.
Also our investigations shown that the sulfoxidation
reaction did not complete either in the absence of citric acid
and/or in the absence of wet SiO2; which means acidic me-
Scheme II

Chemoselective Oxidation of Sulfides to the Sulfoxides
J. Chin. Chem. Soc., Vol. 56, No. 2, 2009 253
Table 3. Oxidation of sulfides 1 to the corresponding sulfoxides 2 using combination of
Al(NO3)3.9H2O I, citric acid II and wet SiO2 (50% w/w) in the presence of catalytic
amounts of KBr III or NaBr IV in dichloromethane at room temperature
Substrate/Reagent/Catalysta
Entry Substrate Product
Time (Min) Yield (%)b
I
II
III
IV
1
1a
1a
1b
1b
1c
1c
1d
1d
1e
1e
1f
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2f
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.3
1.3
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.5
1.5
1.1
1.1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.05
--
--
0.05
--
45
60
98
97
98
94
98
98
98
97
98
94
88
98
98
97
98
40
98
94
97
98
96
98
2
3
0.05
--
60
4
0.05
--
40
5
0.05
--
50
6
0.05
--
80
7
0.05
--
35
8
0.05
--
35
9
0.15
--
16 h
17 h
95
10
11
12
13
14
15
16
17
18
19
20
21
22
0.15
--
0.05
--
1f
2f
0.05
--
70
1g
1g
1h
1h
1i
2g
2g
2h
2h
2i
0.05
--
6 h
5.75 h
170
130
55
0.05
--
0.05
--
0.05
--
0.05
--
1i
2i
0.05
--
30
1j
2j
0.2
--
16 h
22 h
20
1j
2j
0.2
--
1k
1k
2k
2k
0.05
--
0.05
30
a substrate : wet SiO2 = 1 mmol: 0.3 g.
b Isolated yield.
Scheme III
Scheme IV
Oxidation of methyl phenyl sulfide 1c to methyl
phenyl sulfoxide 2c with Al(NO3)3.9H2O, citric acid
and NaBr
Also chemoselectivity, the cheapness and availability of
the reagents and catalyst, easy and clean work-up and high
yields make this method attractive for the chemists.
NaBr (0.005 g, 0.05 mmol), Al(NO3)3.9H2O (0.412 g,
1.1 mmol) citric acid (0.288 g, 1.5 mmol) and wet SiO2
(50% w/w), (0.2 g) was added to a solution of methyl
phenyl sulfide 1c (0.124 g, 1 mmol) in CH2Cl2 (5 mL). The
resulting reaction mixture was stirred at room temperature
EXPERIMENTAL
The chemicals and solvents were supplied by Fluka,
Merck and Aldrich chemical companies without further
purifications.

254 J. Chin. Chem. Soc., Vol. 56, No. 2, 2009
Ghorbani-Choghamarani and Rezaei
16. Patil, M. L.; Maujan, S. R.; Borate, H. B.; Uphade, B. S.
Arkivoc 2007, 70.
for 80 min (the reaction progress was monitored by TLC)
and then filtered. The residue was washed with CH2Cl2 (4 ´
5 mL). Anhydrous Na2SO4 (1.5 g) was added to the filtrate
and filtered off after 20 min. Finally CH2Cl2 was removed
and methyl phenyl sulfoxide 2c was obtained in 98% yield
(0.137 g).
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ACKNOWLEDGMENT
This work was supported by the research facilities of
Ilam University, Ilam, Iran.
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