Aluminium nitrate/silica sulfuric acid/bromide ion: As an effective and catalytic oxidizing media for the selective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1002/jccs.200800176

Various types of sulfides are chemoselectively oxidized to the corresponding sulfoxides by treatment of Al(NO₃)₃.9H ₂O, silica sulfuric acid and a catalytic amount of NaBr in the presence of wet SiO₂ (50% w/w) i

Full text of DOI:10.1002/jccs.200800176

Journal of the Chinese Chemical Society, 2008, 55, 1191-1194
1191
Communication
Aluminium Nitrate/Silica Sulfuric Acid/Bromide Ion: As an Effective and
Catalytic Oxidizing Media for the Selective Oxidation of Sulfides to
Sulfoxides
Arash Ghorbani-Choghamarani,* Hamid Goudarziafshar, Maryam Hajjami and Jinoos Soltani
Department of Chemistry, Faculty of Science, Ilam University, P. O. Box 69315516, Ilam, Iran
Various types of sulfides are chemoselectively oxidized to the corresponding sulfoxides by treatment
of Al(NO3)3.9H2O, silica sulfuric acid and a catalytic amount of NaBr in the presence of wet SiO2 (50%
w/w) in CH2Cl2 at room temperature. The catalytic oxidation procedure is very simple and the products are
easily isolated in good yields.
Keywords: Sulfides; Sulfoxides; Homoselectivity; Aluminium nitrate; Silica sulfuric acid.
The design and development of heterogeneous cata-
lysts are one of the most important objectives in the current
area of research interest. The Important aspect of clean
technology is the use of environmentally friendly cata-
lysts.1 Recently, the development of efficient and new cata-
lytic systems for various organic transformations is an ac-
tive ongoing research area and the scope for further im-
provement towards milder reaction conditions.2
acid {SiO2-OSO3H} and a metal halide.
Silica sulfuric acid can be easily prepared by the reac-
tion of chlorosulfonic acid with silica gel (Scheme I).40
Scheme I
The chemistry of sulfoxides has been attractive to or-
ganic chemists. Because sulfoxides are valuable synthetic
intermediates for the construction of chemically and bio-
logically important significant molecules.3-5 Sulfoxides
have also been used extensively in organic synthesis such
as C-C bond-forming, molecular rearrangements and func-
tional group transformations.6-10 Although various oxidiz-
ing agents are used for this purpose,11-32 unfortunately,
some of these reagents are not satisfactory for the selective
oxidation of sulfides to the sulfoxides because of several
reasons. The first reason is that sulfoxides can undergo
overoxidation to sulfones and therefore it is important that
the catalyst has low reactivity towards the sulfoxides. Other
serious obstacles of the previous procedures are long reac-
tion time, low selectivity, low yields of products, toxicity,
and expensive reagents.
To find the best reaction conditions for the oxidation
of sulfides, a combination of Al(NO3)3.9H2O, silica sulfu-
ric acid and catalytic amounts of a metal halide (MX) in the
presence of wet SiO2 (50% w/w) in dichloromethane at
room temperature was examined for the oxidation of ben-
zyl phenyl sulfide as a model reaction, and the results are
summarized in Table 1. As can be seen from Table 1, the
best result is referred to Al(NO3)3.9H2O, silica sulfuric acid
and wet SiO2 (50% w/w) in the presence of catalytic amount
of NaBr.
Consequently, we presented a new heterogeneous
catalytic protocol for the selective oxidation of different
types of sulfides 1 to the corresponding sulfoxides 2 using
combination of Al(NO3)3.9H2O I, silica sulfuric acid II and
wet SiO2 (50% w/w) in the presence of catalytic amounts of
NaBr IV in dichloromethane at room temperature in excel-
lent yields (Scheme II and Table 2).
In continuation of our recent studies on the oxida-
tion reactions,33-39 we have been enthusiastic to introduce a
new catalytic media, based on in situ generation of X+ us-
ing aluminium nitrate {Al(NO3)3.9H2O}, silica sulfuric
To show the role of bromide ion as a catalyst in the de-
scribed system benzyl phenyl sulfide was subjected to the
* Corresponding author. Tel/Fax: +98-811-8257404; E-mail: arashghch58@yahoo.com

1192 J. Chin. Chem. Soc., Vol. 55, No. 6, 2008
Ghorbani-Choghamarani et al.
media is necessary for this transformation by described cat-
alytic system.
Table 1. Oxidation of benzyl phenyl sulfide by Al(NO3)3.9H2O I,
silica sulfuric acid II and catalytic amounts of a metal
halide (MX) III in the presence of wet SiO2 (50% w/w)a
in CH2Cl2 at room temperature
Additionally, this oxidizing catalytic media allowed
the homoselective,39 oxidation of thiantrene to thianthrene
monosulfoxide (Table 2, entry 9, Scheme III).
Substrate/Reagents/Catalystb
Time Yield
Entry Catalyst
(Min) (%)c
I
II
III
To show the chemoselectivity of the described sys-
tem, a sulfide including hydroxyl group were subjected to
the sulfoxidation reaction, but alcohol oxidation did not oc-
cur under the above-mentioned conditions, and primary
hydroxyl group remained intact in the course of the reac-
tion (Table 2, entry 7, Scheme IV).
1
2
3
4
5
6
NaCl
NaBr
NaI
1.2
1.2
1.2
1.2
1.2
1.2
0.2
0.2
0.2
0.2
0.2
--
0.05
0.05
0.05
0.05
--
240
45
---d
98
240
70
---d
KBr
--
92
--e
--d
16 h
7 h
NaBr
0.05
A plausible mechanism of this oxidation is shown in
Scheme V based on reported pathway in literatures,35,39 our
observations, and the obtained results.
a substrate : wet SiO2 = 1 mmol : 0.2 g.
b I and III refer to mmol of Al(NO3)3.9H2O and MX; II refers to
the gram of the silica sulfuric acid.
c Isolated yield.
In summary, we report an efficient method for the se-
lective oxidation of sulfides to sulfoxides under mild and
heterogeneous conditions with good to high yields. In addi-
tion, chemoselectivity, the cheapness and availability of the
reagents and catalyst, easy and clean work-up and high
yields make this method attractive for chemists.
d Reaction didn’t complete.
e Reaction didn’t complete and impurity of sulfone was observed.
oxidation reaction without NaBr, however, the reaction did
not complete after 16 hours and impurity of sulfone was
observed on the TLC (Table 2, entry 3).
Also, in Table 2, entry 4, which is shown that the
sulfoxidation reaction did not complete in the absence of
silica sulfuric acid as source of proton; which means acidic
EXPERIMENTAL
The chemicals and solvents were supplied by Fluka,
Merck and Aldrich chemical companies without further
Scheme II
Scheme III

Catalytic Oxidation of Sulfides to Sulfoxides
J. Chin. Chem. Soc., Vol. 55, No. 6, 2008 1193
Table 2. Oxidation of sulfides 1 to the corresponding sulfoxides 2 using combination of
Al(NO3)3.9H2O I, silica sulfuric acid II and wet SiO2 (50% w/w)a in the presence of
catalytic amounts of NaBr IV in dichloromethane at room temperature
Substrate/Reagent/Catalystb
Time
Yield
(%)c
Entry Substrate Product
Ref.d
(Min)
I
II
IV
1
1a
1a
1a
1b
1c
1d
1e
1f
2a
2a
2a
2b
2c
2d
2e
2f
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
0.2
0.2
--
0.05
--
45
16 h
7 h
35
98
--e
--f
12, 14, 15, 39-42
2
--e
--f
3
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
4
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
97
96
98
99
98
99
19
92
80
81
12, 15, 39-42
14, 15, 39-42
12, 15, 39-42
15, 39
5
20
6
20
7
3.5 h
30
8
39
9
1g
1h
1i
2g
2h
2i
40
39, 41
10
11
12
13
15
15, 39
25
41
1j
2j
20
42
1k
2k
20
15, 41
a substrate : wet SiO2 = 1 mmol : 0.2 g.
b II refer to gram of silica sulfuric acid; I and IV refer to mmol of aluminium nitrate and catalyst.
c Isolated yield.
d All products are known and were identified using comparison of their physical and spectral data (IR,
NMR, or MS) with those reported in the literature.
e In the absence of catalyst; reaction didn’t complete and impurity of sulfone was observed.
f In the absence of silica sulfuric acid; reaction didn’t complete.
purifications.
Scheme IV
Oxidation of methyl phenyl sulfide 1c to methyl
phenyl sulfoxide 2c with Al(NO3)3.9H2O, SiO2-
OSO3H, wet SiO2 (50% w/w) and NaBr
NaBr (0.005 g, 0.05 mmol), Al(NO3)3.9H2O (0.45 g,
1.2 mmol), silica sulfuric acid (0.2 g) and wet SiO2 (50%
w/w), (0.2 g) was added to a solution of methyl phenyl sul-
fide 1c (0.124 g, 1 mmol) in CH2Cl2 (5 mL). The resulting
reaction mixture was stirred at room temperature for 20
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 96% yield
(0.134 g).
Scheme V
ACKNOWLEDGEMENTS
We thank the research facilities of Ilam University,
Ilam, Iran, for financial support.
Received August 14, 2008.

1194 J. Chin. Chem. Soc., Vol. 55, No. 6, 2008
Ghorbani-Choghamarani et al.
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