Catalytic oxidation of sulfides to sulfoxides by poly(4-vinyl pyridinium nitrate), silica sulfuric acid and ammonium bromide as a catalyst

Article abstract of DOI:10.1016/S1872-2067(10)60124-6

A new catalytic procedure for the chemoselective oxidation of sulfides to sulfoxides was studied. A variety of aliphatic and aromatic sulfides were subjected to sulfoxidation by combining poly(4-vinyl pyridinium nitrate), silica sulfuric acid (SiO₂-OSO₃H), and catalytic amounts of ammonium bromide in CH₂Cl₂ in the presence of a few drops of water at room temperature. Excellent yields were obtained.

Full text of DOI:10.1016/S1872-2067(10)60124-6

CHINESE JOURNAL OF CATALYSIS
Volume 31, Issue 11, 2010
Online English edition of the Chinese language journal
RESEARCH PAPER
Cite this article as: Chin J Catal, 2010, 31: 1347–1350.
Catalytic Oxidation of Sulfides to Sulfoxides by Poly(4-vinyl
pyridinium nitrate), Silica Sulfuric Acid and Ammonium
Bromide as a Catalyst
Arash GHORBANI-CHOGHAMARANI1,*, Sara SARDARI2
1Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran
2Islamic Azad University, Branch-Omidiyeh, Ahvaz, Iran
Abstract: A new catalytic procedure for the chemoselective oxidation of sulfides to sulfoxides was studied. A variety of aliphatic and
aromatic sulfides were subjected to sulfoxidation by combining poly(4-vinyl pyridinium nitrate), silica sulfuric acid (SiO2-OSO3H), and
catalytic amounts of ammonium bromide in CH2Cl2 in the presence of a few drops of water at room temperature. Excellent yields were
obtained.
Key words: sulfoxide; sulfide; poly(4-vinyl pyridinium nitrate); ammonium bromide; silica sulfuric acid
Sulfoxides are useful for stereocontrol in asymmetric syn-
thesis and they are important targets of pharmaceutical interest
[1]. They have been used as reagents in transformations such as
the oxidation of thiols [2,3], the l,4-diacetoxylation of
1,3-diene [4], asymmetric alkylation [5], and the enantioselec-
tive synthesis of dihydroisoindol-4-ols [6]. Sulfoxides are
generally prepared by the oxidation of their corresponding
sulfides. However, it is often very difficult to stop the oxidation
at the sulfoxide stage [7]. Many reagents and oxidizing media
exist for the oxidation of sulfides [8–16]. However, most of
them are not satisfactory because of the formation of envi-
ronmentally unfriendly by-products, over-oxidation to sul-
fones, low product yields, tedious workup, toxicity, and reagent
expense.
In a 50-ml round-bottomed flask, 2 ml of HNO3 (65%) and
3.03 g of poly(4-vinyl pyridine) was stirred for 1 h and then
kept at 50 ºC for 48 h to quantitatively obtain dry poly(4-vi-
nylpyridinium nitrate) as a white crystalline solid.
1.2 Oxidation of 2-(phenylthio)ethanol (1g) to
2-(phenylsulfinyl)ethanol (2g) using poly(4-vinyl pyridin-
ium nitrate) and silica sulfuric acid in the presence of a
catalytic amount of ammonium bromide
In a typical procedure, silica sulfuric acid (0.65 g) and two
drops of water were added to a suspension of NH4Br (0.005 g,
0.05 mmol), poly(4-vinyl pyridinium nitrate) (0.672 g), and
2-(phenylthio)ethanol (0.154 g, 1 mmol) in dichloromethane.
The resulting mixture was stirred at room temperature for 8 h
(the reaction progress was monitored by TLC) and then fil-
tered. The residue was washed with CH2Cl2 (20 ml). Anhy-
drous Na2SO4 (1.5 g) was added to the filtrate and filtered off
after 20 min. Finally, CH2Cl2 was evaporated and
2-(phenylsulfinyl) ethanol 2g was obtained as a colorless oil
(0.168 g, 99%). 1H NMR (200 MHz, CD3SOCD3): į 7.51–7.94
(m, 5H), 5.89 (s, 1H), 3.63–3.88 (m, 2H), 2.83–3.01 (m, 2H);
13C NMR (50 MHz, CD3SOCD3): į 144.9, 132.1, 131.1, 123.6,
60.4, 55.8 (Ref. [23]).
1 Experimental
All chemicals were purchased from Fluka, Merck, and Al-
drich. The oxidation products were characterized by comparing
1
their spectral (IR, H NMR, and 13C NMR) and physical data
with authentic samples [8–22].
1.1 Preparation of poly(4-vinyl pyridinium nitrate)
Received date: 25 May 2010.
*Corresponding author. Tel: +98-841-2227022; Fax: +98-841-2227022; E-mail: arashghch58@yahoo.com, a.ghorbani@mail.ilam.ac.ir
Foundation item: Supported by the research facilities of Ilam University, Ilam, Iran.
Copyright © 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60124-6

Arash GHORBANI-CHOGHAMARANI et al. / Chinese Journal of Catalysis, 2010, 31: 1347–1350
Table 1 Oxidation of dibenzyl sulfide by poly(4-vinyl pyridinium ni-
H2
C
H2
C
trate), silica sulfuric acid, and catalytic amounts of NH4Br in different
solvents at room temperature
H
C
H
C
HNO3
Solvent
Time (min)
22
Yielda (%)
Acetonitrile
Acetone
98
n
n
+
b
1440
1440
15
—
N
N
ꢀ
Chloroform
Dichloromethane
n-Hexane
98
97
H NO3
Scheme 1. Preparation of poly(4-vinyl pyridinium nitrate).
b
1440
1440
1800
—
b
Ethanol
—
Ethyl acetate
88c
2 Results and discussion
Reaction conditions: substrate 1 mmol, poly(4-vinyl pyridinium nitrate)
0.336 g, NH4Br 0.05 mmol, silica sulfuric acid 0.3 g, two drops of water.
aIsolated yield. bNo reaction. cSulfoxide purified by TLC.
Our recent work has focused on the in situ generation of the
+
bromonium ion (Br+) and the nitronium ion (NO2 ) and their
application to different organic reactions [17–27]. In con-
tinuation of this work we decided to explore catalytic and
metal-free media for the in situ generation of Br+. We thus
prepared poly(4-vinyl pyridinium nitrate) by the reaction of
poly(4-vinylpyridine) with nitric acid (Scheme 1).
We found that poly(4-vinyl pyridinium nitrate) is an oxidant
in the presence of acids. Therefore we decided to introduce
Table 2 Oxidation of sulfides 1 to their corresponding sulfoxides 2
O
S
S
R1
R2
R1
R2
1
2
Amount
Entry
1
Substrate
Product
Time (min)
17
Yielda (%)
83
TON
16.6
I (g)
II (g)
III (mmol)
S
1a
2a
0.336
0.3
0.05
S
2
3
1b
1c
1d
1e
1e
1e
1f
2b
2c
2d
2e
2e
2e
2f
0.336
0.336
0.336
0.336
0.336
0.336
0.336
0.670
0.336
0.504
0.3
0.3
0.3
0.3
0.3
—
0.05
0.05
0.05
0.05
—
45
20
92
88
98
87
18.4
17.6
19.6
17.4
—
Cl
S
S
4
50
S
5
26
S
S
b,c
6
1440
1440
15
—
c,d
7
0.05
0.05
0.20
0.05
0.20
—
—
S
8
0.3
0.65
0.3
0.5
97
99
99
97
19.4
4.95
19.8
4.85
S
OH
9
1g
1h
1i
2g
2h
2i
480
20
S
10
11
S
4200
S
S
12
13
1j
2j
0.336
0.336
0.3
0.3
0.05
0.05
140
23
98
98
19.6
19.6
1k
2k
S
S
14
15
1l
2l
0.336
0.336
0.3
0.3
0.05
0.05
30
15
90
96
18.0
19.2
C11H23
S
S
C11H23
OMe
1m
2m
16
1n
2n
0.336
0.3
0.05
25
95
19.0
O
S
H
17
1o
2o
0.336
0.3
0.05
35
60
12.0
O
Other conditions: two drops of water, dichloromethane as solvent, room temperature. aIsolated yield. bIn the absence of NH4Br. cReaction not complete. dIn
the absence of silica sulfuric acid.

Arash GHORBANI-CHOGHAMARANI et al. / Chinese Journal of Catalysis, 2010, 31: 1347–1350
poly(4-vinyl pyridinium nitrate) as a novel oxidizing polymer
H2
C
H
for the conversion of sulfides to sulfoxides in the presence of
silica sulfuric acid (SiO2-OSO3H) and catalytic amounts of
ammonium bromide.
C
n
+
N
ꢀ
H NO3
H2
C
H
C
Initially, to find an appropriate solvent for this transforma-
tion we screened different solvents for the oxidation of
dibenzyl sulfide as a typical example. The results are summa-
rized in Table 1. As is evident from Table 1, the oxidation
reaction proceeds more rapidly and selectively in dichloro-
methane.
H+
O
S
+ H2O
n
R1
R2
N
ꢀ
+
Br
NO2
H2O
Br
S
With optimal conditions in hand, we report on the
chemoselective oxidation of a variety of aliphatic and aromatic
sulfides 1 to their corresponding sulfoxides 2 by poly(4-vinyl
pyridinium nitrate) I, silica sulfuric acid (SiO2-OSO3H) II, and
a catalytic amount of ammonium bromide III in the presence of
two drops of water in dichloromethane at room temperature
with good to excellent yields (Table 2).
R2
R1
Br+
NO
S
R1
R2
Scheme 2. Mechanism of the sulfoxidation reaction of sulfides.
Sulfoxides were heterogeneously prepared under mild con-
ditions. Pure products were obtained easily by mixing a sulfide,
poly(4-vinyl pyridinium nitrate), silica sulfuric acid, and
catalytic amounts of NH4Br in the presence of water (2 drops)
in CH2Cl2 as solvent. This mixture was stirred at room tem-
perature. Eventually, the pure product was easily obtained by
simple filtration and evaporation of the reaction solvent.
As is evident from Table 2, a good turn over number (TON)
range for the catalyst was obtained. To determine the role of
ammonium bromide as catalyst, ethyl phenyl sulfide (as a
typical example) was selected for the oxidation reaction in the
absence of NH4Br. Interestingly, the reaction was not complete
after 24 h (Table 2, entry 6). We also observed that the sul-
foxidation reaction of ethyl phenyl sulfide was not complete in
the absence of silica sulfuric acid after 24 h (Table 2, entry 7),
which means that the presence of acid is necessary for this
reaction.
Entry 12 from Table 2 shows that this catalytic media ho-
moselectively oxidized thiantrene to thianthrene monosulfox-
ide. This result is in close agreement with our previous report
on the oxidation of sulfides to sulfoxides [17–22]. A 13C NMR
of the oxidation product (thianthrene monosulfoxide) shows
six peaks at į = 141.2, 130.9, 129.8, 129.4, 128.2, and 124.7.
It is of interest to note that the described system allows for
the chemoselective oxidation of 2-(phenylthio)ethanol (1g) to
2-(phenylsulfinyl)ethanol (2g) and the hydroxyl group remains
intact during the course of the reaction (Table 2, entry 9).
To investigate and develop the scope and limitations of this
oxidizing media we decided to examine different solid acids in
place of silica sulfuric acid. These reactions were carried out by
mixing benzyl phenyl sulfide (1 mmol), poly(4-vinyl pyridin-
ium nitrate) (0.336 g), the acid (2 mmol), and NH4Br (5 mol%)
in the presence of water (2 drops) and in CH2Cl2 (Table 3). As
shown in Table 3, a variety of solid acids are capable of this
transformation in addition to silica sulfuric acid.
Table 3. Oxidation of benzyl phenyl sulfide with poly(4-vinyl pyridin-
ium nitrate), the solid acid, two drops of water, and catalytic amounts of
A plausible mechanism for the described transformation is
outlined in Scheme 2.
NH4Br in dichloromethane at room temperature
O
S
S
3 Conclusions
Solid acid
Zn(HSO4)2
Zr(HSO4)4
Ca(HSO4)2
KHSO4
Time (min)
170
Yielda (%)
A novel catalytic procedure for the selective oxidation of
sulfides to sulfoxides under metal-free, mild, and heterogene-
ous conditions was investigated. The advantages of this
method are no environmental pollution, shorter reaction times,
high selectivity, non-toxic conditions, cost effective reagents
and catalyst, and easy workup.
90
99
98
35
265
b
1380
4320
30
—
c
Fe(HSO4)2
Al(HSO4)3
L-Alanine
NH2SO3H
Citric acid
—
97
d
1500
1500
1500
—
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d
—
d
—
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