Ferric nitrate in the presence of catalytic amounts of KBr or NaBr: An efficient and homoselective catalytic media for the selective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1007/s00706-008-0015-4

A mild, efficient, and highly selective oxidation method of sulfides to sulfoxides using Fe(NO₃)₃·9H₂O and catalytic amounts of KBr or NaBr in the presence of wet SiO₂ (50% w/w) has been developed. A variety of aliphatic and aromatic sulfides were selectively oxidized at room temperature in good to excellent yields.

Full text of DOI:10.1007/s00706-008-0015-4

Monatsh Chem (2009) 140:65–68
DOI 10.1007/s00706-008-0015-4
ORIGINAL PAPER
Ferric nitrate in the presence of catalytic amounts of KBr
or NaBr: an efficient and homoselective catalytic media
for the selective oxidation of sulfides to sulfoxides
Kamal Amani Æ Mohammad Ali Zolfigol Æ
Arash Ghorbani-Choghamarani Æ Maryam Hajjami
Received: 21 February 2008 / Accepted: 2 June 2008 / Published online: 22 August 2008
Ó Springer-Verlag 2008
Abstract A mild, efficient, and highly selective oxidation
method of sulfides to sulfoxides using Fe(NO3)3Á9H2O and
catalytic amounts of KBr or NaBr in the presence of wet
SiO2 (50% w/w) has been developed. A variety of aliphatic
and aromatic sulfides were selectively oxidized at room
temperature in good to excellent yields.
molecular rearrangements and functional group transfor-
mations [7–11].
Although a number of methods for sulfoxidation have
emerged in the recent years, such as H2O2/Mn(III) [12],
stabilized iodine-2-iodoxybenzoic acid (SIBX) [13],
immobilized vanadyl alkyl phosphonate/NaBrO3/TBHP/
H2O2 [14], PhIO/Mn(III) complexes [15], H2O2/bisflavini-
um perchlorate [16], amberlyst 15/H2O2 [17], 9- and
Keywords Homoselectivity Á Sulfide Á Ferric nitrate Á
KBr Á NaBr Á Oxidation
27-armed
tetrakis(diperoxotungsto)phosphate-cored
dendrimers/H2O2 [18], vanadium(IV) complexes of
dibromo- and diiodo-functionalized chiral Schiff bases [19],
magnesium monoperoxyphthalate (MMPP) [20], NBS/
b-cyclodextrine [21], silica-based tungstate/H2O2 [22],
pyridinium tribromide [23], (Bu4N)3[PMo12O40]/FAp/urea–
H2O2 [24], carboxypyridinium chlorochromate/AlCl3 [25],
(Bu4N)2S2O8 [26], and MoO2Cl2/H2O2 [27], low selectivity,
long reaction times, low yields of products, toxicity,
expensive reagents and catalysts, and overoxidation to
sulfones are still serious obstacles for this transformation.
Introduction
An important aspect of clean technology is the use of
environmentally friendly catalysts [1]. The development of
efficient and new catalytic systems for various organic
transformations is an active research area aiming at further
improvements towards milder reaction conditions [2]. The
selective oxidation of sulfides to sulfoxides is an important
reaction both in industrial processes and in basic research
[3]. Sulfoxides are useful synthetic intermediates in the
synthesis of drugs and natural products [4–6]. Sulfoxides
have also been used extensively in C–C bond-forming,
Results and discussion
To overcome the above-mentioned disadvantages and in
continuation of our ongoing program on the functionali-
zation of organic compounds [28–38], we became
interested in the use of new catalytic media, based on the in
situ generation of Br+, for the selective oxidation of sul-
fides to sulfoxides.
K. Amani (&) Á M. Hajjami
Department of Chemistry, Faculty of Science,
University of Kurdistan, P.O. Box 6617715175, Sanandaj, Iran
e-mail: amani_71454@yahoo.com
M. A. Zolfigol (&)
Initially, we screened different solvents for the oxidation
of dibenzyl sulfide as our model reaction. We found that
dichloromethane is the best solvent in terms of activity and
selectivity (Table 1).
Faculty of Chemistry, Bu-Ali Sina University,
P.O. Box 6517838683, Hamadan, Iran
e-mail: Zolfi@basu.ac.ir
A. Ghorbani-Choghamarani
Department of Chemistry, Faculty of Science, Ilam University,
P.O. Box 69315516, Ilam, Iran
We wish to report here the selective oxidation of dif-
ferent types of sulfides 1 to the corresponding sulfoxides 2
123

66
K. Amani et al.
Table 1 Oxidation of dibenzyl sulfide with Fe(NO3)3Á9H2O and
catalytic amounts of KBr in the presence of wet SiO2 (50% w/w) in
different solventsa
Table 2 Oxidation of sulfides 1 to the corresponding sulfoxides 2
using Fe(NO3)3Á9H2O (I) in the presence of catalytic amounts of KBr
(II) or NaBr (III) in dichloromethane at room temperature
Entry
Solvent
Time/min
Yieldb/%
Entry Substrate Product
Substrate/
reagents/
catalystsa
Time/min Yieldb/%
1
2
3
4
5
6
a
Acetonitrile
Acetone
60
300
45
91
c
–
Reference I
II
III
Chloroform
Dichloromethane
n-Hexane
95
95
1
1a
1a
1b
1b
1c
1c
1c
1c
1d
1d
1d
1d
1e
1e
1f
2a
2a
2b
2b
2c
2c
2c
2c
2d
2d
2d
2d
2e
2e
2f
1.2 0.05
–
90
89
15
d
2
1.2
1.5 0.3
1.5
1.2 0.05
–
0.05 95
360
0.3 360
15
0.05 15
84
100
300
–
c
3
–
63c
84c
95
Ethyl acetate
–
4
–
Substrate/Fe(NO3)3Á9H2O/KBr/wet SiO2: 1:1.2:0.05 (mmol):0.3 g
Isolated yield
5
–
b
c
d
6
1.2
1.2
–
–
98
No reaction
d, e
7
–
–
–
240
110
20
–
Reaction completed but an equal mixture of sulfone and sulfoxide
8
1.2 0.05
1.2 0.05
92f
98
96
98f
98f
98
98
83
95
87
84
was observed
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
a
1.2
1.2 0.05
1.2
1.2 0.05
1.2
1.2 0.05
1.2
1.2 0.05
1.2
1.2 0.05
1.2
1.2 0.05
1.2
1.4 0.2
1.4
1.2 0.05
1.2
–
0.05 15
65
0.05 70
20
0.05 20
40
0.05 95
10
0.05 15
240
0.05 240
140
0.05 130
using Fe(NO3)3Á9H2O (I) and catalytic amounts of KBr (II)
and/or NaBr (III) in the presence of wet SiO2 (50% w/w) in
dichloromethane at room temperature with good to excel-
lent yields (Scheme 1; Table 2).
–
–
–
–
As can be seen from Table 2, the reaction time increases
in the absence of wet SiO2 as a source of H2O (Table 2,
entries 8, 11, 12). Also, to show the catalytic role of KBr
and NaBr in the described system, dibenzyl sulfide (as a
typical example) was subjected to sulfoxidation without
potassium and sodium bromide; however, the reaction did
not complete after 240 min and a sulfone impurity was
observed (Table 2, entry 7).
–
1f
2f
–
1g
1g
1h
1h
1i
2g
2g
2h
2h
2i
–
–
g
–
–
g
–
–
–
90
88
95
90
73
78
1i
2i
–
It is of interest that an excellent homoselectivity was
observed in the oxidation of thianthrene. We wish to
introduce the term ‘‘homoselectivity’’ in this article. We
think that this keyword is suitable to be applied in
special reactions. When there are two or more identical
functional groups in a molecule, only one of them reacts
selectively in the course of the reaction; this selectivity
can be called homoselectivity. This oxidizing system
allowed the homoselective oxidation of thianthrene to
thianthrene monosulfoxide (Table 2, entries 21 and 22;
Scheme 2).
1j
2j
–
20
35
40
1j
2j
–
0.2
–
1k
1k
2k
2k
–
0.05 40
Substrate:wet SiO2 = 1 mmol:0.3 g
b
c
d
e
Isolated yield
GC yield
In the absence of KBr and NaBr
Reaction did not complete and a mixture of sulfone and sulfoxide
was observed
f
Without wet SiO2
To show the chemoselectivity of the described system,
two sulfides containing a hydroxy group were subjected to
g
No reaction
O
S
Fe(NO3)3.9H2O
S
R1
R2
R1
R2
MBr (Cat.) wet SiO2
CH2Cl2, rt
1a − 1k
2a − 2k
M = K or Na
g
c
a
b
d
e
f
h
i
j
k
S
Ph
S
S
S
S
OMe
S
S
S
S
S
S
OH
Me
Me
OH
O
S
Scheme 1
123

Ferric nitrate in the presence of catalytic amounts of KBr or NaBr
67
Scheme 2
O
S
O
S
S
Fe(NO ) .9H O
3 3
2
+
MBr (Cat.)
S
S
S
CH Cl , rt
2
2
O
M = K or Na
90-98%
0%
Table 3 Efficiency comparison
Entry
Catalyst
Oxidant
Time/min
Yielda/%
Reference
of various oxidizing systems for
the oxidation of methyl phenyl
sulfide
1
2
3
NaBr
KBr
Fe(NO3)3Á9H2O
Fe(NO3)3Á9H2O
H2O2 (aq)
20
98
96
82
This work
This work
[22]
15
Silica-based ammonium
tungstate
1.5 h
4
5
6
7
8
Dodecyl hydrogen sulfate
H2O2 (aq)
H2O2 (aq)
NaOCl
5
92
99b, c
[39]
[40]
[41]
[42]
[43]
HAuCl4Á4H2O
n-Bu4NBr
60
30
30
6 h
93
Human hemoglobin
Ti(OiPr)4
H2O2 (aq)
96
79b
(S)-1-phenylethyl
hydroperoxide
a
Isolated yield
b
95c
99
[44]
[45]
Conversion
9
PhSO2N=CHPhNO2-p
Oxone
30
13
c
Trace impurity of sulfone was
observed
10
CAN
NaBrO3
To show the efficiency of the described system in
comparison with previously reported procedures in litera-
ture, we compared our obtained results for the oxidation of
methyl phenyl sulfide with the best of the well-known data
from literature, as shown in Table 3.
S
2
1
H O + NO(g)
2
R
R
+
Br
Br
S
A plausible mechanism for this oxidation is shown
in Scheme 3, based on a reported pathway in literature
[31, 46], our observations, and the obtained results.
Initially, hydrolysis of ferric nitrate generates HNO3 in
situ; subsequently, auto ionization of nitric acid generates
in situ nitronium ion (NO2?). Then, Br- can be oxidized
and generates Br? in situ. The final step of oxidation
involves a nucleophilic attack of sulfur on the Br?, fol-
lowed by concerted oxygen transfer from water to sulfur
atoms of sulfonium bromide to give sulfoxides (Scheme 4).
One of the observed pieces of evidence for this mech-
anism is the evolution of NO gas from the reaction vessel
that can be converted to the nitrogen brown-color oxide
compounds, which can be observed on the top of the
reaction solution.
-
H O
2
Br
HNO
Fe(NO ) .9H O
3
H O
2
3 3
2
O
S
1
2
+
R
R
+ 2H
Scheme 3
H O
2
Fe(NO ) .9H O
HNO + Fe(OH)
3 3
2
3
3
+
-
+
3HNO
+
NO + 2NO + H O
2 3 3
3
+
-
+
2H + NO
+ Br
H O + NO + Br
2
2
Br
O
Br
OH
H O
2
+
S
S
+
-HBr
Br
+
S
S
In conclusion, we have reported a catalytic, simple, and
mild method for the synthesis of sulfoxides by the oxida-
tion of sulfides with catalytic-generated Br?. This method
offers the advantage of shorter reaction times, high che-
moselectivity, and easy workup. We believe that the
present methodology could be an important addition to the
existing methodologies.
+
-H
Scheme 4
the sulfoxidation reaction, and the primary hydroxyl group
remained intact in the course of the reaction (Table 2,
entries 15–16 and 23–24).
123

68
K. Amani et al.
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Experimental
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Chemicals were purchased from Fluka, Merck, and Aldrich
chemical companies. The oxidation products were char-
acterized by the comparison of their spectral (IR, 1H NMR,
and 13C NMR) and physical data with authentic samples
[25, 47–50].
Oxidation of dibutyl sulfide (1e) to dibutyl sulfoxide
(2e) by Fe(NO3)3Á9H2O/KBr as an example
KBr (0.006 g, 0.05 mmol) was added to a solution of
0.146 g dibutyl sulfide 1e (1 mmol) in 5 cm3 CH2Cl2,
followed by the addition of 0.484 g Fe(NO3)3Á9H2O
(1.2 mmol) and 0.3 g wet SiO2. 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 20 cm3 CH2Cl2 . Anhydrous
Na2SO4 (1.5 g) was added to the filtrate and then filtered
off after 20 min. Finally, CH2Cl2 was removed and the
yield was 0.158 g (98%).
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Acknowledgments Financial support for this work by the Center of
Excellence of Development of Chemical Methods (CEDCM) of the
Bu-Ali Sina University, Hamadan, Iran, is gratefully acknowledged.
34. Zolfigol MA, Bagherzadeh M, Mallakpour S, Chehardoli G,
Ghorbani-Choghamarani A, Koukabi N, Dehghanian M, Do-
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35. Zolfigol MA, Shirini F, Chehardoli G, Kolvari E (2007) J Mol
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