Transition-metal-free highly efficient aerobic oxidation of sulfides to sulfoxides under mild conditions

Article abstract of DOI:10.3390/molecules15010083

A highly efficient transition-metal-free catalytic system Br ₂/NaNO₂/H₂O has been developed for a robust and economic acid-free aerobic oxidation of sulfides. It is noteworthy that the sulfide function reacts under mild co

Full text of DOI:10.3390/molecules15010083

Molecules 2010, 15, 83-92; doi:10.3390/molecules15010083
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Transition-Metal-Free Highly Efficient Aerobic Oxidation of
Sulfides to Sulfoxides under Mild Conditions
Hua Zhang 1,*, Chunyu Chen 1, Renhua Liu 2, Qiang Xu 1 and Weiqie Zhao 1
1
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of
Technology, No.158-139, Zhongshan Road, Dalian 116012, China;
E-Mails: chenchunyu321@yahoo.cn (C.Y.C.); xuqianguser@163.com (Q.X.).;
zyzhao@chem.dlut.edu.cn (W.Q.Z.)
2
School of Pharmacy, East China University of Science and Technology, Shanghai 200237, China;
E-Mail: liurh@ecust.edu.cn (R.L.)
* Author to whom correspondence should be addressed; E-Mail: huazhang@chem.dlut.edu.cn.
Received: 30 November 2009; in revised form: 17 December 2009 / Accepted: 23 December 2009 /
Published: 28 December 2009
Abstract: A highly efficient transition-metal-free catalytic system Br2/NaNO2/H2O has
been developed for a robust and economic acid-free aerobic oxidation of sulfides. It is
noteworthy that the sulfide function reacts under mild conditions without over-oxidation to
sulfone. The role of NaNO2 as an efficient NO equivalent for the activation of molecular
oxygen was identified. Under the optimal conditions, a broad range of sulfide substrates
were converted into their corresponding sulfoxides in high yields by molecular oxygen.
The present catalytic system utilizes cheap and readily available agents as the catalysts,
exhibits high selectivity for sulfoxide products and releases only innocuous water as the
by-products.
Keywords: aerobic oxidation; sulfids; Br2; NaNO2; molecular oxygen
1. Introduction
The oxidation of sulfides into the corresponding sulfoxides is one of the most important functional
group transformations in organic synthesis [1]. Sulfoxides as a class of important compounds are
generally used as synthetic intermediates for the construction of various chemically and biologically

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significant molecules [2–3], as well as for the synthesis of drugs and natural products [3–5], Therefore,
many efficient methods have been developed for the oxidation of sulfides to sulfoxides. However, it is
difficult to control the results while the sulfoxides could be further oxidized to sulfones. A large
number of methods have been developed to overcome this drawback; however, most of these reactions
require a stoichiometric amount of oxidant, resulting in undesirable waste [6–8]. Hydrogen peroxide
and molecular oxygen has recently been utilized as an attractive and environmentally benign oxidant
for the oxidation of sulfides, because they are inexpensive, easy to handle, safely stored, and produce
only water as a side-product [9–12]. Hydrogen peroxide is usually applied in catalytic sulfide
oxidations system since it is an environmentally benign oxidant [13–18]. However, selective oxidation
by molecular oxygen is even more attractive because of its obvious economical and environmental
advantages [19–22].
Recently, the use of molecular oxygen as terminal oxidant has received great attention for both
economic and environmental benefits, and many highly efficient systems have been developed for
catalytic aerobic sulfide oxidation using rhenium [23], palladium [24], ruthenium [25], copper [26,27]
manganese [26], iron [27–29], cobalt [30] and gold-based catalysts [31]. However, the metal catalysts
can be expensive and may hinder the purification of products, and cause some environment problems.
Only a few catalyst systems, for example, small amounts of cheap metal salts, provide an efficient
catalyst for aerobic oxidation of sulfides under mild conditions. Therefore, the development of an
efficient nonmetallic catalyst for the aerobic oxidation of sulfides under mild conditions has long been
desired. Our research was inspired by the mechanism of selective catalysis of thioether oxidations
with dioxygen by Bosch et al. [32] who reported the facile and selective oxidation of various aliphatic
and aromatic sulfides by the combination of NO2 and dioxygen at room temperature and below. The
catalysis of thioether autoxidation was also effected by small amounts of other nitrogen oxides, such as
nitric oxide and nitrosonium salts. NaNO2 as an alternative NO source for their property were
identified as an efficient NO equivalent for the activation of molecular oxygen [33,34]. In this context,
a process for the highly efficient aerobic oxidation of a wide range of sulfides utilizing a
DBDMH/NaNO2 and HBr/t-BuONO catalyst system that was free from any transition-metal co-
catalysts has been disclosed by our research group [35,36]. On the other hand, the halogens and their
derivatives are considered environmentally unfriendly, despite their advantages such as low price, easy
handling, commercial availability, and relatively high stability. The biggest disadvantage of the
utilization of molecular bromine in the oxidation of sulfides is the formation of byproducts such as
sulfonic or sulfinic acids and bromosubstituted sulfides or sulfones [37]. If Br2 was only used in
catalytic amounts but not in stoichiometric amount, the transformation of sulfides into the
corresponding sulfoxides could be more attractive and effective oxidation procedure using molecular
oxygen. A novel metal-free catalytic system Br2/NaNO2/H2O for the highly selective oxidation of a
variety of sulfides using molecular oxygen as the terminal oxidant in a small amount of water at
ambient temperature (Scheme 1) is reported in this paper.

Molecules 2010, 15
Scheme 1. Oxidation of sulfides to the corresponding sulfoxides.
85
O
S
Br2/NaNO2/H2O
S
R1
R2
R1
R2
O2, room temperature
2. Results and Discussion
Initial investigation of aerobic oxidation was carried out using methyl phenyl sulfide as a substrate
with 3 mol % of Br2 and oxygen under 25 °C for 1 h. The preliminary result (21% conversion) clearly
indicated the role of Br2 as an active catalyst. We sought to find a co-catalyst to bridge the gap
between O2 activation and HBr reoxidation.The ready availability and unique redox property of
NaNO2 as a source of NO under acidic conditions attracted our attention. Although NaNO2 alone
showed little activity (10% conversion), when 3 mol % of Br2 and 5 mol % of NaNO2 were both
employed in aerobic oxidation, a highly efficient catalyst system emerged. The keys to the newly
developed oxidation process are the discovery of molecular oxygen being able to oxidize HBr to Br2 in
situ with NaNO2 as the catalyst and its suitability in an aqueous environment (Table 1).
Table 1. The catalysts effect on the reaction. a
Entry
NaNO2 (mol %)b
Br2 (mol %) b
Conversion (%) c
1
2
3
0
5
5
3
0
3
21
10
100
a
Reaction conditions: methyl phenyl sulfide (MPS) (10 mmol), O2 (80 mL/min), 25 °C,
b
c
H2O(1.5 mL); Br2 (0.3 mmol, 0.015 mL), NaNO2 (0.5 mmol,0.0345 g); Conversions were
determined by GC and counted with area normalization.
2.1. Aerobic oxidation of sulfides to sulfoxides
Ten mmol substrates of analogues of methyl aryl sulfides were reacted with 5 mol % of NaNO 2 in
the presence of 3–5 mol % Br2, 1.5 mL of water and oxygen (80 mL/min) in CH3CN under 25 °C in
1 h. The reactions were took place efficiently (Table 2).
Table 2. Catalytic aerobic oxidation of sulfide in a small amount of water. a
Time
(h)
Br2
Conversion Yield
Entry
Substrate
Product
(mol %)
(%)b
(%)c
O
S
S
1
1
1
3
3
100
96
O
S
S
2
100
92
O
O

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86
Table 2. Cont.
O
S
1
1
3
3
100
100
95
94
S
3
4
O
O
O
S
S
S
O
S
5
1
1
3
3
100
100
95
94
F
F
O
S
S
6
7
Cl
Cl
O
S
S
S
1
3
100
93
O
O
O
S
8
2
5
3
5
100
100
92
96
Cl
Cl
O
S
S
9
HOOC
HO
HOOC
HO
O
S
S
10
11
5
1
5
3
25
21 d
92
S
100
S
O
a Reaction conditions: substrates (10 mmol), NaNO2 (0.5 mmol,0.0345 g), H2O (1.5 mL), CH3CN
b
(20 mL), O2 (80 mL/min), 25 °C; Conversions were determined by GC with area normalization;
c isolated yields; d yield were determined by GC with area normalization.
As shown in Table 2, most of methyl aryl sulfides including electron-rich and nucleophilic-rich
aromatic compounds were converted into their corresponding sulfoxides in high isolated yields within
1 h (entries 1–7). 4-Chlorophenyl methyl sulfide and 4-(methylthio)benzoic acid showed relatively low
activities in the oxidation. To address this, we prolonged the reaction time to 2 h, 4-Chlorophenyl
methyl sulfide was thus converted into 4-chlorophenyl methyl sulfoxide with 100% conversion in high
isolated yields (entry 8). Increasing the amount of Br2 (to 5 mol %) in the catalyst system will improve
the reaction rate－the oxidation of 4-(methylthio)benzoic acid could be completed within 5 h (entry 9).
Simple alkyl sulfides such as dibutyl sulfide were also efficiently oxidized (entry 11). It is noteworthy

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that function selective oxidation can be achieved with this method, which is generally problematic in
classical oxidation processes. Under the modified conditions (Br2, 5 mol %), the conversion of 4-
(methylthio)phenol (entry 10) is only about 25% within 5 h. Recognizing the oxidation of 4-
(methylthio)phenol by molecular oxygen to the corresponding 4-(methylsulfoxyl)phenol, an
intermediate may be via conjugate transformation converted into a similar-benzoquinone compound,
then the ultimate compound is obtained by conjugate match once more. The key of the reaction is the
form of the intermediate, the more difficult the intermediate formed, the lower conversion the product
received (Scheme 2) [38].
Scheme 2. The catalytic oxidation of 4-(methylthio)phenol.
O
O
OH
OH
S
H2O
+ 2HBr
Br
+
2
Br
S
S
Br
S
O
O
2.2. Mechanism of aerobic sulfide oxidation
A possible overall mechanism of this new and transition-metal-free catalytic oxidation can be
described by the dual cycle in Scheme 3. Thus, transformation of sulfides into their corresponding
sulfoxides is fulfilled by the coupling of multiple redox reactions. Initially the electrophilic attack of
Br2 on sulfur in the organic phase leads to the sulfoxide [38]. The running of Cycle 1 continuously
produces the reoxidation of HBr to Br2 by NO2 [39], which is the key for a catalytic amount of Br2 to
benefit the oxidations of active sulfide substrates. NO2 is reduced to NO when it completes the
oxidation of HBr. NO is easily released from NaNO2 as an efficient NO equivalent .The oxidation of
NO into NO2 is a process easily accomplished with molecular oxygen (Cycle 2), and the coupling of
the two cycles furnishes a novel, coherent, and efficient aerobic sulfide oxidation system.
Scheme 3. Overall Catalytic Mechanism.
NaNO2
H2O
H2O
S
O2
Br2
1
NO
R1
R2
2
O
S
NO2
2HBr
R1
R2
3. Experimental
3.1. Materials
All methyl aryl sulfides (Wuling Science and Technology&Industry Co. Ltd.) and their analogues
(Zhejiang Shou & Fu Chemical Co. Ltd), dibutyl sulfide (Shandong Teng & Wu Spice Co.Ltd), was a

Molecules 2010, 15
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commercial product used as received without any further purification. Deionised water was used in all
experiments. All other solvents used were analytical grade.
3.2. Instrumentation
1
IR spectra were recorded on a Nicolet 20DXB FT-IR instrument. H-NMR spectra were recorded
on a Varian Inova 400MHz spectrometer. HPLC chromatograms were recorded with an Agilent 1100
system. GC chromatogram was recorded on a Varian 3700 apparatus. MS spectra was recorded on a
HP 6890/5973 instrument.
3.3. General procedure for catalytic aerobic sulfides oxidation
Oxidation of methyl phenyl sulfide is described as a representative example of aromatic
compounds: in a 100 mL Schlenk flask equipped with a mechanical stirrer were placed methyl phenyl
sulfide (10 mmol), acetonitrile (20 mL), water (1.5 mL), Br2 (3 mol %) and sodium nitrite (5 mol %).
The reaction mixture was stirred at 25 °C under a continuous stream of oxygen (80 mL/min). Upon
completion after 1 hour, as detected by GC, the solvent was removed and the residue extracted with
ethyl acetate. The organic extract was first washed with 5% sodium bicarbonate solution, then the
organic layer was finally dried over anhydrous sodium sulfate. The yield was calculated on the basis of
10 mmol of substrate. The 1H-NMR spectrum was recorded directly using the isolated product.
3.4. Spectroscopic data for products
Methyl phenyl sulfoxide (Table 2, entry 1): Yellow liquid; 1H-NMR (DMSO-d6): δ 2.73 (3H, s, CH3),
7.52-7.59 (3H, m, Ph), 7.67-7.71(2H, m, Ph); IR (KBr, cm-1): ν 1048, 749, 692; MS (EI), m/z: 140 [M]+.
4-Methoxyphenyl methyl sulfoxide (Table 2, entry 2): Yellow solid; 1H-NMR (DMSO-d6): δ 2.71(3H,
s, SOCH3), 3.83 (3H, s, OCH3), 7.13 (2H, d, J = 8.8 Hz, Ph), 7.63 (2H, d, J = 8.8 Hz, Ph); IR (KBr,
cm-1): ν 1027, 831; MS (EI), m/z: 170 [M]+.
1
2-Methoxyphenyl methyl sulfoxide (Table 2, entry 3): Yellow liquid; H-NMR (DMSO-d6): δ 2.70
(3H, s, SOCH3), 3.86 (3H, s, OCH3), 7.14 (1H, d, J = 7.6 Hz, Ph), 7.21 (1H, t, J = 7.6 Hz, J = 7.4 Hz,
Ph), 7.52 (1H, t, J = 7.4 Hz, J = 8.3 Hz, Ph), 7.63(1H, d, J = 8.3 Hz, Ph); IR (KBr, cm-1): ν 1038, 757;
MS (EI), m/z: 170 [M]+.
1
4-Methylphenyl methyl sulfoxide (Table 2, entry 4): Yellow solid; H-NMR (DMSO-d6): δ 2.35 ((3H,
s, CH3), 2.69 (3H, s, SOCH3), 7.4 (2H, d, J = 8.4 Hz, Ph), 7.56 (2H, d, J = 8.4 Hz, Ph); IR (KBr, cm-1):
ν 1046, 810; MS (EI), m/z: 154 [M]+.
4-Fluorophenyl methyl sulfoxide (Table 2, entry 5): Yellow solid; 1H-NMR (DMSO-d6): δ 2.73 (3H, s,
CH3), 7.40 (2H, t, J = 9.0 Hz, J = 8.8 Hz, Ph), 7.74 (2H, q, J = 8.8 Hz, J = 5.4 Hz, Ph); IR (KBr, cm-1):
ν 1048 , 834; MS (EI), m/z: 158 [M]+.
2-Chlorophenyl methyl sulfoxide (Table 2, entry 6): Yellow liquid; 1H-NMR (DMSO-d6): δ 2.81 (3H,

Molecules 2010, 15
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s, CH3), 7.56-7.59 (m, 2H), 7.62~7.68 (m, 1H), 7.81-7.86 (m, 1H); IR (KBr, cm-1): ν 1067, 759; MS
(EI), m/z: 174 [M]+.
1
1-(4-Methansulfinylphenyl)-ethanone (Table 2, entry 7): Yellow solid; H-NMR (DMSO-d6): δ 2.63
(3H, s, COCH3), 2.80 (3H, s, SOCH3), 7.83 (2H, d, J = 8.4 Hz, Ph), 8.13 (2H, d, J = 8.4 Hz, Ph); IR
(KBr, cm-1): ν 1674, 1047, 830; MS (EI), m/z: 182 [M]+.
4-Chlorophenyl methyl sulfoxide (Table 2, entry 8): Yellow solid; 1H-NMR (DMSO-d6): δ 2.78 (3H, s,
CH3), 7.67 (2H, d, J = 8.4 Hz, Ph), 7.73 (2H, d, J = 8.4 Hz, Ph); IR (KBr, cm-1): ν 1051, 822; MS (EI),
m/z: 174 [M]+.
4-Methanesulfinylbenzoic acid (Table 2, entry 9): White powder; 1H-NMR (DMSO-d6): δ 2.79 (3H, s,
CH3), 7.81 (2H, d, J = 8.0 Hz, Ph), 8.11 (2H, d, J = 8.0 Hz, Ph), 13.30 (1H, s, COOH); IR (KBr, cm-1):
ν 2773, 1690, 1083, 856; MS (EI), m/z: 184 [M]+.
1
Dibutyl sulfoxide (Table 2, entry 11): Yellow liquid; H-NMR (DMSO-d6): δ 0.91 (3H, t, CH3), 1.41
(2H, m, CH2), 1.608 (2H, m, CH2), 2.657 (2H, m, SOCH2); IR (KBr, cm-1): ν 737, 923, 1021, 1085;
MS (EI), m z: 162 [M]+.
4. Conclusions
In conclusion, we have successfully developed a highly efficient, transition-metal-free catalytic
process for the aerobic oxidation of sulfides with molecular oxygen. It should be noted that the
quantitative oxidation of methyl phenyl sulfide to the corresponding sulfoxides with excellent yields
can be achieved without acid under mild conditions. A catalyst system is based on molecular bromine
as an active catalyst in the presence of very inexpensive NaNO2 as co-catalysts. The results showed
that reaction with a low concentration of Br2 and NaNO2 in a small amount of water is highly selective
to the desired products without over-oxidation. This reliable non-metal catalytic system for aerobic
sulfide oxidation is applicable to a wide range of sulfides, including methyl aryl sulfides and their
analogues.
Acknowledgements
The research was supported by Program for Changjiang Scholars and Innovative Research Team in
University（IRT0711）.
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