Efficient oxidation of sulfides to the sulfoxides using zirconium (IV) chloride, sodium nitrite and catalytic amounts of bromide ion as a novel oxidizing media

Article abstract of DOI:10.2174/157017809788489882

Chemo and homoselective catalytic oxidation of sulfides has been developed. A variety of aliphatic and aromatic sulfides are subjected to the sulfoxidation reaction using ZrCl₄, NaNO₂ and catalytic amounts of KBr or NaBr in the prese

Full text of DOI:10.2174/157017809788489882

Letters in Organic Chemistry, 2009, 6, 335-339
335
Efficient Oxidation of Sulfides to the Sulfoxides Using Zirconium (IV)
Chloride, Sodium Nitrite and Catalytic Amounts of Bromide Ion as a
Novel Oxidizing Media
Arash Ghorbani-Choghamarani*, Hamid Goudarziafshar*, Mohsen Nikoorazm and Somaieh
Yousefi
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran
Received March 08, 2009: Revised March 17, 2009: Accepted March 18, 2009
Abstract: Chemo and homoselective catalytic oxidation of sulfides has been developed. A variety of aliphatic and aro-
matic sulfides are subjected to the sulfoxidation reaction using ZrCl4, NaNO2 and catalytic amounts of KBr or NaBr in the
presence of wet SiO2 (50 % w/w) in acetonitrile at room temperature.
Keywords: Sulfoxides, Homoselectivity, Zirconium chloride (ZrCl4), Sodium nitrite (NaNO2).
INTRODUCTION
In order to find an appropriate catalyst for the selective
oxidation of sulfides to sulfoxides; a combination of ZrCl4,
NaNO2 and catalytic amounts of a metal halide (MX) in the
presence of wet SiO2 (50% w/w) was examined in acetonitril
for the oxidation of dibenzyl sulfide as a model reaction. The
results are summarized in Table 1. As is evident from Table
1, the best catalysts are KBr and NaBr.
Oxidation is one of the most fundamental reactions in
organic synthesis [1]. The design and development of het-
erogeneous catalysts is one of the most important objectives
in a current area of research interest. Also important aspect
of clean technology is the use of environmentally friendly
catalysts [2]. Nowadays the research is focused on the more
versatile, more selective, and more reactive catalyst [3]. Dev-
elopment of efficient and new catalytic systems for various
organic transformations is an active research area aiming at
further improvements towards milder reaction conditions [4].
Table 1. Oxidation of Dibenzyl Sulfide by ZrCl4, NaNO2 and
Catalytic Amounts of a Metal Halide (MX) in the
Presence of Wet SiO2 (50% w/w)a in CH3CN at
Room Temperature
Sulfides oxidation is one of the most important organic
processes, because corresponding sulfoxides are valuable
synthetic intermediates for the synthesis of chemically and
biologically important significant molecules [5-7]. Sulfox-
ides are also valuable in C-C bond-forming [8,9] and mo-
lecular rearrangements [10-12].
Entry
Catalyst
Time (Min)
Yield (%)b
1
2
3
4
5
NaCl
NaBr
NaI
21h
100
21h
80
--c
98
--c
98
--c
Although a number of efficient catalytic systems have
been developed for this purpose [13-27], but unfortunately,
some of these reagents are not satisfactory for the selective
oxidation of sulfides to the sulfoxides because of several
reasons such as overoxidation to sulfones, low selectivity,
low yields of products, toxicity, and expensive reagents or
catalysts.
KBr
KI
21h
aSubstrate/ZrCl4/NaNO2/MX/wet SiO2: 1 : 0.562 : 2.25 : 0.025 (mmol) : 0.2 g. bIsolated
yield. cReaction didn’t complete.
Table 2. Oxidation of Dibenzyl Sulfide Using ZrCl4, NaNO2
and Catalytic Amounts of KBr in the Presence of
Wet SiO2 (50 % w/w) in Different Solventsa
RESULTS AND DISCUSSION
Entry
Solvent
Time (h)
Yield (%)b
In order to overcome the above mentioned problems and
in continuation of our recent studies on the oxidation of
sulfides [28-30] we became interested to introduce a novel
catalytic system, based on in situ generation of X+ via
combination of zirconium (IV) chloride, sodium nitrite and a
metal halide.
1
2
3
4
5
6
7
Acetonitrile
Acetone
80 min
98
c
9
9
9
9
9
9
-
d
Chloroform
Dichloromethane
n-Hexane
-
e
-
d
-
d
Ethyl acetate
-
f
Ethanol
-
*Address correspondence to these authors at the Department of Chemistry,
Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran;
Tel/Fax: +98 841 2227022; E-mail: arashghch58@yahoo.com,
h.goudarziafshar@mail.ilam.ac.ir
aSubstrate/ZrCl4/NaNO2/KBr/wet SiO2: 1 : 0.562 : 2.25 : 0.025 (mmol) : 0.2 g.
bIsolated yield. cTrace conversion. dReaction didn’t complete. eReaction didn’t
complete and impurity of solfone was observed. fNo reaction.
1570-1786/09 $55.00+.00
© 2009 Bentham Science Publishers Ltd.

336 Letters in Organic Chemistry, 2009, Vol. 6, No. 4
Ghorbani-Choghamarani et al.
O
S
ZrCl4, NaNO2
S
R1
R2
R1
R2
KBr or NaBr
Wet SiO2
CH3CN, rt
1
2
a
b
c
S
Ph
S
S
d
e
S
f
S
S
OH
g
j
h
i
S
S
OMe
O
S
S
k
S
S
OH
CHO
Scheme 1.
Table 3. Oxidation of Sulfides 1 to the Sulfoxides 2 Using ZrCl4 I, NaNO2 II and Catalytic Amounts of KBr III or NaBr IV in the
Presence of Wet SiO2 (50% w/w) in Acetonitril at Room Temperature
Substrate/Reagent/Catalystsa
Mp or bp (ºC)
found
Mp or bp (ºC)
reported
Entry Substrate Product
Time (Min)
Yield (%)b
I
II
III
IV
1
2
1a
1a
1b
1b
1c
1c
1c
1c
1c
1d
1d
1e
1e
1f
2a
2a
2b
2b
2c
2c
2c
2c
2c
2d
2d
2e
2e
2f
0.562
0.562
1.375
1.375
0.562
0.562
0.562
0.562
--
2.25
2.25
5.5
0.025
--
--
0.025
--
230
240
1380
1470
80
98
96
92c
94c
98
98
--d,e
--e,f
--g.h
87
85
96
95
93
95
98
98
98
98
99c
99c
87
86
78
80
116-120
116-120
67-68
67-68
130-134
130-134
---
117-121 [31]
117-121 [31]
73-74 [32]
73-74 [32]
132-134 [33]
132-134 [33]
---
3
0.15
--
4
5.5
0.15
--
5
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
5.5
0.025
--
6
0.025
--
100
1200
1260
1200
160
165
52
7
0.025
--
8
--
---
---
9
0.025
0.025
--
--
---
---
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
0.562
0.562
0.562
0.562
0.562
0.562
0.562
0.562
0.562
0.562
1.375
1.375
0.562
0.562
0.562
0.562
--
31
30-32 [33]
30-32 [33]
103-105 [32]
103-105 [32]
147-150 [33]
147-150 [33]
---
0.025
--
31
0.025
--
106-107
106-107
144
0.025
--
60
0.05
--
450
510
145
155
40
1f
2f
0.05
--
144
1g
1g
1h
1h
1i
2g
2g
2h
2h
2i
0.025
--
oil
0.025
--
oil
---
0.025
--
107-108
107-108
142-145
142-145
102-103
102-103
oil
102-105 [33]
102-105 [33]
143 [34]
0.025
--
20
0.12
--
420
450
260
250
175
1i
2i
5.5
0.15
--
143 [34]
---
1j
2j
2.25
2.25
2.25
2.25
0.025
--
---
1j
2j
0.025
--
1k
1k
2k
2k
0.025
oil [33]
oil [33]
--
0.025
190
oil
b
c
d
e
f
aSubstrate : wet SiO2= 1 mmol : 0.2 g. Isolated yield. Substrate : wet SiO2= 1 mmol : 0.4 g. Without wet SiO2. Reaction did not complete. In the absence of catalyst. gIn the
absence of ZrCl4. hNo reaction.

Efficient Oxidation of Sulfides to the Sulfoxides Using Zirconium
Letters in Organic Chemistry, 2009, Vol. 6, No. 4
337
O
S
O
S
S
ZrCl4, NaNO2
+
KBr or NaBr
Wet SiO2
S
S
S
CH3CN, rt
O
100%
0%
Scheme 2.
Obviously, solvent plays an important role in the organic
reactions, and hence it was decided to investigate the solvent
effect and also to choose an appropriate solvent. However,
we screened different solvents for the oxidation of dibenzyl
sulfide as a typical example. The oxidation of dibenzyl sul-
fide was carried out using a mixture containing 1 mmol of
dibenzyl sulfide, ZrCl4 (0.562 mmol), NaNO2 (2.25 mmol),
KBr (0.025 mmol) and 0.2 g of wet SiO2 (50% w/w) in 5 mL
of solvent at room temperature. As it can be seen from
Table 2 acetonitril is the best solvent due to its selectivity
and reactivity for this transformation.
Additionally, this oxidizing system allowed the homo-
selective oxidation of thiantrene to thianthrene monosul-
foxide (Table 3, entries 20 and 21, Scheme 2). This result is
in close agreement with our previously works [28-30].
This selectivity can be proved exactly by 13CNMR of
thianthrene monosulfoxide (Fig. 1).
Also two sulfides containing primary hydroxyl group
were oxidized to their corresponding sulfoxides, and interest-
ingly primary hydroxyl group in these substrates was
remained intact in the during of the reaction (Table 3, entries
14, 15, 22 and 23 Scheme 3).
Consequently, we decided to apply a novel heterogene-
ous catalytic protocol for the selective oxidation of wide
range of sulfides 1 to the corresponding sulfoxides 2 with
combination of ZrCl4 I, NaNO2 II and catalytic amounts of
KBr III or NaBr IV in the presence of wet SiO2 (50% w/w)
in acetonitril at room temperature with good to excellent
yields (Scheme 1 and Table 3).
A possible mechanism of this oxidizing system is shown
in Scheme 4 based on our previously reported works [28-30].
In the first place, ZrCl4 might be hydrolyzed and
produced in situ HCl. Then nitrous acid (HNO2) can be
generated by reaction of HCl with NaNO2. Subsequently,
HNO2 react whit Br- to produce Br+, which is able to oxidize
sulfides to their corresponding sulfoxides.
To show the role of bromide ion as a catalyst in the
described system dibenzyl sulfide, as a typical sample, was
subjected to the oxidation without KBr or NaBr, however the
reaction didn’t complete after 21 hours (Table 3, entry 8).
Also sulfoxidation reaction did not complete in the absence
of wet SiO2, as source of H2O, until 20 hours (Table 3, entry
7).
The critical step of this mechanism is the in situ produc-
ing of HCl. To prove this claim dibenzyl sulfide, as a typical
example, was subjected to the oxidation reaction via
combination of HCl (37%), NaNO2 and KBr (HCl was used
instead of ZrCl4). Surprisingly it was observed that dibenzyl
sulfoxide was produced by this oxidizing system. Another
Fig. (1). 13C NMR of thianthrene monosulfoxide.

338 Letters in Organic Chemistry, 2009, Vol. 6, No. 4
Ghorbani-Choghamarani et al.
O
S
S
S
ZrCl4, NaNO2
OH
OH
O
+
KBr or NaBr
Wet SiO2
CH3CN, rt
100%
0%
O
S
S
ZrCl4, NaNO2
S
O
OH
OH
+
KBr or NaBr
Wet SiO2
CH3CN, rt
100%
0%
Scheme 3.
CDCl3): ꢁ = 2.78-3.05 (d, 3H), 7.54 (s, 5H) ppm; 13C-NMR
(22.5 MHz, CDCl3): ꢁ = 145.4, 130.3, 128.7, 122.9, 43.2
ppm; IR (KBr): = 1048, 1090, 1151, 1305, 1444, 1476,
1655, 2914, 3056 cm-1.
observed evidence for this mechanism is evolving of NO gas
from reaction vessel that can be converted to the nitrogen
brown-color oxides, which can be observed on the top of the
reaction solution.
Zr(OH)4 + 4HCl
ZrCl4 + 4H2O
O
S
Spectral data for Selected Products
1
R1
R2
HNO2 + NaCl
NO+
HCl + NaNO2
HNO2
2a: H-NMR (90 MHz, CDCl3): ꢁ = 3.97 (s, 2H), 6.94-
Br-
7.32 (m, 10H) ppm; IR (KBr): = 692, 745, 1036, 1442,
1600, 2960, 3059 cm-1.
H2O
1
Br
S
2b: H-NMR (90 MHz, CDCl3): ꢁ = 7.39-7.54 (m) ppm;
IR (KBr): = 694, 737, 1037, 1087, 1441, 1476, 1580, 3048
cm-1.
2c: 1H-NMR (90 MHz, CDCl3): ꢁ = 3.97 (s, 4H), 7.41 (s,
10H) ppm; IR (KBr) : = 699, 775, 1032, 1454, 1602, 2958,
3031 cm-1.
2d:1H-NMR (90 MHz, CDCl3): ꢁ = 2.78-3.05 (d, 3H),
7.54 (s, 5H) ppm; IR (KBr): = 1048, 1090, 1151, 1305,
1444, 1476, 1655, 2914, 3056 cm-1.
Br+
NO
S
R1
R2
Scheme 4.
In summary herein we report a novel catalytic method for
the selective oxidation of sulfides under mild conditions.
Also chemoselectivity, the cheapness and availability of the
reagents and catalysts, easy and clean work-up and high
yields make this method attractive for chemists.
1
2f: H-NMR (90 MHz, CDCl3): ꢁ = 2.93 (t, 2H), 3.88 (t,
2H), 4.72 (br, 1H), 7.40 (m, 5H) ppm; 13C-NMR (22.5 MHz,
CDCl3): ꢁ = 96.8, 132.0, 133.7, 134.6, 140.6, ppm; IR (K-
Br): = 1031, 1150, 1460, 1583, 1658, 2973, 3339 cm-1.
1
2g: H-NMR (90 MHz, CDCl3): ꢁ = 2.84 (t, 2H), 2.91 (s,
EXPERIMENTAL
General
3H), 3.33 (t, 2H), 3.70 (s, 3H) ppm; IR (KBr): = 1017,
1245, 1376, 1461, 1632, 1734, 2855 cm-1.
1
2i: H-NMR (90 MHz, CDCl3): ꢁ = 7.22-7.98 (m) ppm;
The chemicals and solvents were purchased from Fluka,
Merck and Aldrich chemical companies without further puri-
fications.
13C-NMR (22.5 MHz, CDCl3): ꢁ = 123.6, 124.4, 128.4,
129.0, 129.8, 130.7 ppm; IR (KBr): = 1025, 1087, 1247,
1436, 1569, 1638, 3054 cm-1.
Oxidation of Methyl Phenyl Sulfide 1d to Methyl Phenyl
Sulfoxide 2d with ZrCl4, NaNO2, wet SiO2 (50% w/w) and
NaBr
ACKNOWLEDGEMENT
NaBr (0.002 g, 0.025 mmol), ZrCl4 (0.131 g, 0.562
mmol), NaNO2 (0.155 g, 2.25 mmol) and wet SiO2 (50%
w/w), (0.2 g) was added to a solution of methyl phenyl
sulfide 1d (0.124 g, 1 mmol) in CH3CN (5 mL). The resul-
ting reaction mixture was stirred at room temperature for 165
min (the reaction progress was monitored by TLC) and then
filtered. The residue was washed with CH2Cl2 (4ꢀ5 mL).
Anhydrous Na2SO4 (1.5 g) was added to the filtrate and
filtered off after 20 min. Finally solvents were removed and
methyl phenyl sulfoxide 2d was obtained in 85 % yield
This work was supported by the research affairs of Ilam
University, Ilam, Iran.
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