A Simple Metal Free Oxidation of Sulfide Compounds

Article abstract of DOI:10.1007/s10562-016-1932-1

Abstract: This work reports simple, efficient, selective protocol for the oxidation of sulfide compounds. Various sulfides were selectively and completely converted into their corresponding sulfoxides and sulfones using H₂O₂as an oxidant in presence of catalytic amount of caprylic acid. The reaction proceeds at room temperature to give sulfoxide and by increasing the reaction temperature to 50 °C, this system provides selective formation of sulfone with high conversion and excellent yields. Green, convenient, easy work-up, chemoselectivity, broad substrate scope and regeneration of catalyst are the important highlights of this protocol. Graphical Abstract: [Figure not available: see fulltext.] Using H₂O₂as an oxidant in presence of catalytic amount of caprylic acid various sulfides were selectively and completely converted into their corresponding sulfoxides and sulfones.

Full text of DOI:10.1007/s10562-016-1932-1

Catal Lett
DOI 10.1007/s10562-016-1932-1
A Simple Metal Free Oxidation of Sulꢀde Compounds
1
Ravindra B. Wagh · Jayashree M. Nagarkar
1
Received: 16 September 2016 / Accepted: 20 November 2016
Springer Science+Business Media New York 2016
©
Abstract This work reports simple, eꢁcient, selective
protocol for the oxidation of sulꢀde compounds. Various
sulꢀdes were selectively and completely converted into
their corresponding sulfoxides and sulfones using H O as
1 Introduction
Oxidation of sulꢀde is the most important synthetic aspects
for various industries such as pharmaceuticals, chemi-
cals and petrochemicals [1–4]. Now-a-days the chemical
research is directed towards developing green processes.
Innovation in a synthetic route becomes ineꢁcient without
a green approach. Oxidation of sulꢀde gives formation of
sulfoxide or sulfone compounds with the help of oxidant.
The main problem encountered during this type of oxida-
tion is chemoselectivity and due to this the reaction results
in the mixture of sulfoxide and sulfone. Therefore, selec-
tive oxidation of sulꢀde still remains one of the most stimu-
lating tasks for researchers. High atom economy is also
an essential contribution to the overall process eꢁciency.
Numerous reagents and oxidative procedures are available
for this transformation [5–11]. Use of oxidants such as
hydrogen peroxide with various transition metal catalysts
under homogenous and heterogeneous conditions for this
transformation is reported [12–17]. Oxidation also worked
well in supercritical carbon dioxide as an environmentally
benign reaction medium [18, 19]. Previous reports also sug-
gested that this type of oxidation of sulꢀdes is carried out
with the use of acid–oxidant system [20–24], deep eutectic
solvent–oxidant system [25], tetrazole amides-H O sys-
2
2
an oxidant in presence of catalytic amount of caprylic acid.
The reaction proceeds at room temperature to give sulfox-
ide and by increasing the reaction temperature to 50°C,
this system provides selective formation of sulfone with
high conversion and excellent yields. Green, convenient,
easy work-up, chemoselectivity, broad substrate scope and
regeneration of catalyst are the important highlights of this
protocol.
Graphical Abstract
O
Metal Free,
Easy Workup,
O
R
O
S
S
S
High Selectivity,
Excellent Yields
R
R'
R
R'
R'
Using H O as an oxidant in presence of catalytic amount
2
2
of caprylic acid various sulꢀdes were selectively and com-
pletely converted into their corresponding sulfoxides and
sulfones.
Keywords Oxidation · Sulꢀde · Sulfoxide · Sulfone ·
Caprylic acid · Medium chain fatty acid (MCFA)
2
2
tem [26], zirconium tetrachloride [27] and silica supported
sodium periodate [28], etc. However, by acknowledging the
all reported work in this ꢀeld, still hazardous metal contam-
ination or preparation and use of metal catalysts, solvents,
oxidants, long reaction time, high temperature, toxic waste
generation, tedious workup, formation of side products etc.
signiꢀcantly lower their appeal. Therefore, development
of new oxidation systems to overcome these drawbacks is
necessary.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1932-1) contains supplementary
material, which is available to authorized users.
*
Jayashree M. Nagarkar
jm.nagarkar@ictmumbai.edu.in
1
Department of Chemistry, Institute of Chemical Technology,
Matunga, Mumbai 400019, India
1
3

R. B. Wagh, J. M. Nagarkar
The use of green oxidant, i.e. hydrogen peroxide which
is cheap, attractive, readily available and eco-friendly with
water as a byproduct. The oxidative power of H O is high-
2 Experimental
2.1 Materials and Methods
2
2
lighted in several publications [29–31]. As per the prin-
ciples of green chemistry, the process should be ‘green’
which implies that green techniques/chemicals should be
used in order to make reaction conditions eco-friendly. The
inorganic acids such as HCl, H SO are not eco-friendly and
All chemicals were purchased from Sigma Aldrich, S.D
Fine Chemical, commercial suppliers and used without
further puriꢀcations. The MCFAS were procured from
Loba Chemie Pvt Ltd. The reaction was monitored by TLC
and GC analysis and performed on PerkinElmer Clarus
480. GC equipped with ꢂame ionized detector with capil-
lary column (Elite-1701, 30 m×0.32×0.25). The product
mass conformed by GC–MS–QP 2010 instrument (Rtx-
17, 30 m_25 mm ID, ꢀlm thickness 0.25 µm, column ꢂow:
2
4
thus use of such acids should be avoided which causes pol-
lution in environment. Medium chain fatty acids (MCFAS)
have more advantages as compaired to other acids such as
non-toxic in nature, easy to handle and reusable. Caprylic
acid is one of the medium chain fatty acid (MCFA) which
is cheap and easily available. It is obtained from butter and
other animal fats. It is issued as an algaecide, bactericide,
and fungicide in nurseries, greenhouses, garden centers,
and interiorscapes on ornamentals, dairy equipment, food
processing equipment, breweries, wineries, and beverage
processing plants in the manufacture of perfumes and ꢂa-
vors. In this context, with the aim to improve the eꢁciency
and eco-friendly conditions of catalytic processes, we have
developed new oxidation system, i.e. H O with caprylic
-1
2 mL min , 80–240 °C at 10 °C/min rise). The products
were puriꢀed by column chromatography using (60–120
mesh) silica gel with n-hexane and ethyl acetate as solvent.
1
The pure product H NMR spectroscopic data was recorded
on a Varian Mercury plus-300 spectrometer using CDCl as
3
a solvent and TMS as internal standard. Melting or boiling
points of the products were determined on a Galaxy scien-
tiꢀc melting/boiling point apparatus.
2
2
acid as a part of our research work on oxidation of organic
compounds [32]. Herein, we report selective oxidation of
sulꢀde to sulfoxide and sulfone with hydrogen peroxide in
presence of catalytic amount of caprylic acid (Scheme 1).
The present protocol is found to be eꢃective with ace-
tonitrile (MeCN) as a solvent. Higher selectivity was
achieved in the oxidation of sulꢀdes to sulfoxides and
sulfones. By increasing the reaction temperature to 50°C
for oxidation of sulꢀdes to sulfones provides excellent
conversion and maximum yield. Use of dichloromethane
2.2 General Procedure for Oxidation of Sulꢀde
to Sulfoxide and Sulfone Compounds (Table 3)
An oven-dried Schlenk ꢂask was allowed to cool
to room temperature and charged sequentially with
sulꢀde (1.0 mmol), MeCN (3.0 mL) and caprylic acid
(10/20 mol%). The reaction was then activated by slow
addition of 30% H O (1.2/2.4 equiv.) and stirred at room
2
2
temperature for the required time as given in Table 3. The
progress of reaction was monitored by GC. After comple-
tion of the reaction, the reaction was quenched by adding
aqueous solution of 10% Na SO to the reaction mixture.
(
CH Cl ) gives product separation easily. Moreover, the
2 2
sodium caprylate obtained after the workup is also used in
foods as abinder, emulsiꢀer in cosmetics etc. To best of our
knowledge, this is the ꢀrst oil/MCFA based oxidation sys-
tem which oꢃers high selectivity, purity, good to excellent
yields, metal free approach, regeneration of catalyst, easy
reaction condition with workup process which avoid expen-
sive catalyst and harsh reaction condition.
2
3
Then the product was extracted with CH Cl (30 mL) and
2
2
then washed with distilled water (10 mL). The organic
extract was dried over Na SO and the solvent was removed
2
4
under reduced pressure. The resultant product was puriꢀed
by column chromatography using silica gel (60–120 mesh)
with n-hexane and ethyl acetate as solvent to get the pure
product. The structure of the product was conꢀrmed by
1
GC–MS, M.P./B.P. and H NMR spectroscopic techniques.
2
.3 General Procedure for Oxidation of Sulꢀde
to Sulfone Compounds (Table 4)
An oven-dried Schlenk ꢂask was allowed to cool to
room temperature and charged sequentially with sulꢀde
(1.0 mmol), MeCN (3.0 mL) and caprylic acid (20 mol%).
The reaction was then activated by the addition of 30%
H O (2.4 equiv.) and stirred at 50°C for the required time
O
H O - Caprylic acid
2
O
R
O
2
S
S
S
MeCN, r.t.
R'
R
R'
R
R'
where, R,R' = alkyl or aryl
2
2
as given in Table 4. The progress of reaction was moni-
tored by GC. After completion of the reaction, the reaction
Scheme 1 Oxidation of sulꢀde compounds
1
3

A Simple Metal Free Oxidation of Sulꢀde Compounds
Table 1 Optimization of reaction parameters for diphenyl sulꢀde
O
S
O
O
S
S
H O - Caprylic acid
2 2
MeCN, r.t.
diphenyl sulfide
diphenyl sulfoxide
diphenyl sulfone
1
a
2a
3a
a
Conv. (%)
a
Entry
Solvent
MeCN
Oxidant H O (equiv.)
2
Caprylic acid
mol%)
Temperature (°C)
Time (min)
Selectivity
(%)
2
(
2
a
3a
1
2
3
4
5
6
7
8
9
0.8
1.0
1.2
1.4
1.9
2.2
2.4
2.4
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
10
10
10
10
20
20
20
20
–
RT
RT
RT
RT
RT
RT
RT
50°C
RT
RT
RT
RT
RT
RT
RT
RT
RT
220
120
30
59
78
59
77
97
81
35
13
–
–
1
100
100
100
100
100
100
63
3
70
19
65
87
100
100
5
85
110
90
30
–
24h
30
58
38
49
68
19
57
59
48
55
65
70
14
10
11
12
13
14
15
16
17
Toluene
DCM
10
10
10
10
10
10
10
10
45
7
30
58
9
MeOH
Hexane
CHCl3
Acetone
EtOAc
MeCN
30
85
17
5
30
24
30
65
8
30
62
3
30
53
5
TBHP (70%)
.2
30
64
9
6
3
0
78
13
20
2
1
00
90
1
8
MeCN
Oxone
.2
10
RT
30
16
1
6
3
0
21
30
33
42
62
49
58
67
19
25
32
38
53
39
46
52
2
00
5
1
9
0
MeCN
MeCN
Peracetic acid
.2
10
10
RT
RT
30
1
6
3
0
4
1
00
9
2
m-CPBA
.2
30
10
12
15
6
3
0
1
00
Reaction conditions: 1a (1 mmol), solvent (3 mL), caprylic acid, 30% H O , temp. RT (30–35°C)
2
2
a
Conversion and selectivity determined by GC with the area normalization method
was quenched by adding aqueous solution of 10% Na SO3
2
structure of the product was conꢀrmed by GC–MS, M.P./
1
B.P. and H NMR spectroscopic techniques.
to the reaction mixture. Then the product was extracted
with CH Cl (30 mL) and then washed with distilled water
2
2
(10 mL). The organic extract dried over Na SO and the
2 4
solvent removed under reduced pressure. The resultant
product was puriꢀed (if necessary) by column chroma-
tography using silica gel (60–120 mesh) with n-hexane
and ethyl acetate as solvent to get the pure product. The
3 Results and Discussion
Initially, for optimization purpose diphenyl sulꢀde 1a
(DPS) was selected as a substrate and oxidation of this
1
3

R. B. Wagh, J. M. Nagarkar
Table 2 Comparison study between MCFAS and LCFAS
substrate was studied as a model reaction (Table 1) with
acetonitrile (MeCN) and caprylic acid. Oxidant plays a key
role in this reaction. The eꢃect of the oxidant on DPS was
studied by varying the amount of 30% H O from 0.8 to
a
Conv. (%)
Entry
Acid (10 mol%)
Time (min)
Selec-
a
tivity
(%)
2
2
2
.4 equiv. (Table 1, entry1–7). H O turned out to be the
2 2
2a
3a
best oxidant as it gave 100% conversion of the products
Table 1, entry 3–7) at room temperature. It was observed
that 1.2 equiv. of H O gives 97% of diphenyl sulfoxide 2a
1
2
3
4
5
6
Caproic acid
Caprylic acid
Capric acid
Lauric acid
Palmitic acid
Oleic acid
30
30
30
30
30
30
93
100
90
91
97
87
56
61
35
2
3
3
2
3
3
(
2
2
in 30 min (Table 1, entry 3) and 2.4 equiv. of oxidant gives
00% conversion of 1a to diphenyl sulfone 3a in 90 min
Table 1, entry 7). However, during conversion of 1a–3a,
58
1
64
(
38
time required for complete conversion was higher as com-
paired to that of 1a–2a. So we decided to increase the tem-
perature to 50°C which provided excellent yield and also
helped to reduce the time during the sulꢀde to sulfone con-
version (Table 1, entry 8). When the oxidation of diphenyl
sulꢀde was carried out using diꢃerent amounts of H O
Reaction condition: 1a (1 mmol), MeCN (3 mL), Acid (10 mol%),
30% H O (1.2 equiv.), temp. (30–35°C)
2
2
a
Conversion and selectivity determined by GC with the area normali-
zation method
2
2
such as 0.8 and 1.0 equiv. of oxidant, the reaction did not
go to completion (Table 1, entry 1–2). However, absence of
caprylic acid in model reaction showed incomplete conver-
sion of product even after 24 h (Table 1, entry 9). It is also
clear that the selectivity with respect to 3a also increases
as the caprylic acid amount increased from 10 to 20 mol%
increased the amount of oxidant and caprylic acid along
with increasing temperature to 50°C. This helped to give
selective and complete conversion of sulꢀde to sulfone
without formation of sulfoxide.
In order to generalize the scope of the reaction, a series
of structurally diverse sulꢀdes were subjected to oxidation
under optimized reaction conditions, and the results are
presented in Table 3. Oxidation of alkyl, aryl–alkyl, diaryl
suꢀdes etc. produced excellent yields of corresponding sul-
foxides or sulfones depending upon the reaction condition
(Table 3, entry 1–8). The sulfoxide and sulfone formation
occurred in speciꢀed time in Table 3. DPS 1a was easily
and selectively oxidized to 2a and 3a with excellent yields
(Table 3, entry 1). Similarly, methyl phenyl sulꢀde 1b also
oxidized to 2b and 3b selectively (Table 3, entry 2). The
inꢂuence of electron donating or withdrawing group on
substrate was not signiꢀcant in this protocol. The substrates
bearing electron donating and withdrawing groups were
smoothly oxidized to the corresponding sulfoxide and sul-
fone selectively (Table 3, entry 3, 4). Dibenzyl sulꢀde 1e
and tetrahydrothiophene 1f also oxidized selectively with
good yields (Table 3, entry 5, 6). Aliphatic sulꢀdes such as
dimethyl sulꢀde 1g and dibutyl sulꢀde 1h were easily oxi-
dized (Table 3, entry 7, 8).
(
Table 1, entry 4–8).
In further investigation, we carried out the model reac-
tion in diꢃerent solvents such as toluene, dichloromethane
DCM), methanol, hexane, chloroform (CHCl ), acetone,
(
3
ethyl acetate (EtOAc) but found less conversion and mix-
ture of 2a and 3a (Table 1, entry 10–16). The oxidation was
investigated with various oxidants such as tert-butyl hydro-
gen peroxide (TBHP), oxone, peracetic acid and m-CPBA
but found less conversion of 1a–2a (Table 1, entry 17–20).
In order to establish the catalytic activity of caprylic
acid, we performed the model reaction with other MCFAS
and LCFAS to investigate eꢃect of medium chain fatty acid
(
MCFA) and long chain fatty acid (LCFA) on the oxidation
of 1a–2a (Table 2, entry 1–6). We found that higher con-
version was obtained only with caprylic acid while others
failed to give high conversion of 1a–2a. This proves that
caprylic acid is the most suitable catalyst for the oxidation
of sulꢀdes.
It was observed that higher conversion and selectivity
for oxidation of sulꢀde to sulfoxide or sulfone is solubil-
ity dependent and excellent under homogenous conditions.
Acetonitrile helps the reaction to proceed in homogene-
ous phase which provides proper mixing of substrate with
caprylic acid and oxidant. Acetonitrile also plays an impor-
tant role in this oxidation as the nitrogen atom of –CN sta-
bilizes the partial positive charge on the sulfur atom in the
transition state [23, 33]. Sulfoxide formation is an interme-
diate step during the oxidation of sulꢀde to sulfone. Since
sulfoxide is less nucleophilic in nature, formation of sul-
fone takes longer time as compared to that of sulfoxide. We
The oxidation of sulꢀde to sulfone at 50°C is achieved
with high yields and selectivity and the results are pre-
sented in Table 4. A clean synthetic method was devel-
oped, in which sulfones were redily precipitated as the pure
product with high yields (Table 4, entry 1–10). The sulfone
formation occurred in speciꢀed time as given in Table 4.
The product analysis study showed that sulfone was the
only product formed under present reaction conditions. It
is also clear that aromatic or aliphatic sulꢀdes are chem-
oselectively oxidized to sulfones (Table 4, entry 1–10).
Also no eꢃect on sulfone formation was observed due to
1
3

A Simple Metal Free Oxidation of Sulꢀde Compounds
Table 3 Oxidation of sulꢀde
to sulfoxide and sulfone
compounds
O
S
O
R
O
H O - Caprylic acid
2
2
S
S
MeCN, r.t.
R
R'
R
R'
R'
1
a
2a
3a
a
b
Entry
Substarte
Sulfoxide
c
Sel.
(%)
Sulfone
c
Sel.
(%)
d
d
Time
(
Yield
(%)
Time
(min)
Yield
(%)
min)
S
1
2
30
97
96
94
93
90
85
95
100
97
96
95
a
1
S
30
98
98
100
99
1b
S
3
0
c
1
3
H CO
3
S
4
5
30
30
97
97
94
94
100
100
98
99
96
94
1
d
O N
2
S
e
1
S
6
7
25
25
98
97
88
82
95
90
98
98
86
92
a
Reaction conditions: sulꢀde
1 mmol), MeCN (3 mL),
1
f
(
caprylic acid (10 mol%),
0% H O (1.2 equiv.), temp.
3
S
2
2
(
b
30–35°C)
1
g
Sulꢀde (1 mmol), MeCN
3 mL), Caprylic acid
20 mol%), 30% H O2
(
(
(
c
2
S
25
98
84
95
99
88
2.4 equiv.), temp. (30–35°C)
8
1h
Selectivity by GC
d
Isolated yield
electron donating or withdrawing group present on sulꢀdes
Table 4, entry 3–7). This indicates that both the protocols,
by adding aqueous solution of 10% Na SO to the reac-
2 3
(
tion mixture. Then the product was extracted with CH Cl
2 2
one in which without heating sulfoxide is formed selec-
tively at room temperature and the other in which sulfone is
formed by heating the reaction mass at 50°C provide excel-
lent yields and purity. This shows that both the protocols
are very selective and easily controllable.
(30 mL) and then washed with distilled water (10 mL). The
aqueous layer obtained from this method was again acidi-
ꢀed with 0.1 N HCl till the pH became acidic. Then aqueos
layer was extracted with CH Cl (25 mL). This extract was
2
2
dried over Na SO and the solvent was removed under
2
4
The regeneration of a catalyst was carried out and then
the activity was tested. The oxidation of diphenyl sulꢀde 1a
to diphenyl sulfone 3a was carried out by using the catalyst.
After completion of the reaction, the reaction was quenched
reduced pressure. The resultant caprylic acid was reused in
next cycle. The results of the recycling experiments with
the yields of corresponding sulfone was still higher after
three cycles and then slightly decreases Fig. 1. For the
1
3

R. B. Wagh, J. M. Nagarkar
Table 4 Oxidation of sulꢀde to
sulfone compound
O
R
O
H O - Caprylic acid
2
2
S
S
°
MeCN, 50 C
R
R'
R'
4a
5a
b
Entry
Substarte
Time
(
Yield
(%)
min)
S
1
30
30
98
a
4
S
2
98
98
4b
S
3
0
c
4
3
4
H CO
3
S
30
97
4d
O N
2
S
5
30
30
98
98
e
4
S
OCH3
6
7
4
f
S
Cl
3
0
98
4
g
S
8
30
97
92
4
h
S
2
5
9
4i
Sulꢀde (1 mmol), MeCN
(
(
(
a
3 mL), caprylic acid
20 mol%), 30% H O2
2.4 equiv.), temp. (50°C)
S
2
10
25
95
4
j
Isolated yield
1
3

A Simple Metal Free Oxidation of Sulꢀde Compounds
1
1
02
00
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(
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In summary, we have developed a simple, highly eꢁcient
and environmentally benign methodology for the oxida-
tion of sulꢀde compounds by using new oxidation system
1
1
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(
H O -caprylic acid). This MCFA based oxidation is a
2 2
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metal and additive free protocol for the oxidation of sulꢀde
compounds. Here, these two protocols have a good sub-
strate scope with excellent yields as well as high selectivity.
The transformation worked well with the liquid and solid
sulꢀdes with homogeneous reaction mixture. Due to its
green, convenient and easy work up procedures with regen-
eration of catalyst, we believe that new oxidation system
surpasses previously reported systems. These developed
protocols promise broad applications in organic synthesis.
Importantly, signiꢀcant yield and quality is achieved which
is the noticeable achievement of this protocols. Other appli-
cations of this new oxidation system in organic reactions
are under way in our laboratory.
2
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