Efficient Oxidation of Sulfides to Sulfoxides and Deoxygenation of Sulfoxides over Carbonaceous Solid Acid

Article abstract of DOI:10.1080/10426507.2014.978325

Carbonaceous Solid Acid (CSA) was found to be a highly efficient, environmentally friendly, recyclable heterogeneous solid acid for the oxidation of sulfides and deoxygenation of sulfoxides, in good to excellent yields under mild reaction conditions.

Full text of DOI:10.1080/10426507.2014.978325

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Phosphorus, Sulfur, and Silicon and the Related
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Efficient Oxidation of Sulfides To Sulfoxides and
Deoxygenation of Sulfoxides Over Carbonaceous Solid
Acid
a
a
a
Fariborz Atabaki , Ebrahim Abedini & Arash Shokrolahi
a
Chemistry Department, Malek-ashtar University of Technology, Shahin-shahr, P.O.Box
3145–115, I. R. Iran
8
Accepted author version posted online: 14 Nov 2014.
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and Deoxygenation of Sulfoxides Over Carbonaceous Solid Acid, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
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ACCEPTED MANUSCRIPT
EFFICIENT OXIDATION OF SULFIDES TO SULFOXIDES AND DEOXYGENATION
OF SULFOXIDES OVER CARBONACEOUS SOLID ACID
1
Fariborz Atabaki, Ebrahim Abedini and Arash Shokrolahi
Chemistry Department, Malek-ashtar University of Technology, Shahin-shahr, P.O.Box 83145-
115, I. R. Iran.
Abstract Carbonaceous Solid Acid (CSA) was found to be a highly efficient, environmentally
friendly, recyclable heterogeneous solid acid for the oxidation of sulfides and deoxygenation of
sulfoxides, in good to excellent yields under mild reaction conditions.
Keywords Carbonaceous Solid Acid; Hydrogen peroxide; Sulfide; Sulfoxide
1
E-mail: arshokrolahi@gmail.com; ar_shokrolahi@mut-es.ac.ir
1
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INTRODUCTION
Development of solid acids for fine chemicals synthesis has become a major area of research in
both industry and laboratory. Solid acids have many advantages such as easy catalyst separation
1
-9
from the product, simplicity in handling and fewer reactor and plant corrosion problems .
Organosulfur compounds, such as sulfoxides and sulfides are valuable compounds for the
production of a range of chemically and biologically molecules. For this reason in the recent
years the sulfides oxidation to sulfoxides and deoxygenation of sulfoxides has been the subject of
1
0-15
many studies
. However, some of these methods still suffer from serious disadvantages like
tedious work-up, harsh reaction conditions, intricate procedures and not readily available
reagents. Therefore, environmentally benign and convenient procedures for the oxidation of
sulfides and deoxygenation of sulfoxides are still required.
Aqueous 30% hydrogen peroxide is a very attractive oxidant, because it is readily available, high
effective-oxygen content, formation of water as the only by-product, inexpensive and safely
1
6-19
stored
. Nevertheless, oxidation of sulfides with H
2
2
O is slow and therefore various kinds of
2
0-29
acids and transition metal based systems have been used to activate hydrogen peroxide
.
Xuezheng Liang and co-workers reported the synthesis of a heterogeneous strong acid through
the copolymerization of p-toluenesulfonic acid (PTSA) and paraformaldehyde using sulfuric acid
as catalyst. This carbonaceous solid acid (CSA) has high density of sulfonic acid groups (SO H),
3
3
0, 31
high thermal and chemical stability
.
In the present study we report oxidation of sulfides to sulfoxides with H
2
2
O and deoxygenation
3
2-34
of sulfoxides with NaI
in the presence of CSA as a heterogeneous solid acid (Scheme 1).
2
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RESULTS AND DISCUSSION
Oxidation of sulfides
To start with diphenyl sulfide was selected as a model substrate for the optimization and studied
its oxidation with 30% H
2
2
O using CSA as catalyst. The results are summarized in Tables 1 and
2
.
The effect of various solvents on oxidation of diphenyl sulfide to diphenyl sulfoxide is shown in
Table 1. Dichloroethane was found suitable, solvent for reaction at room temperature. However,
ethanol as a non-toxic, environmentally benign media and inexpensive solvent was selected for
this reaction.
[
[
Insert Table 1]
Insert Table 2]
Several oxidation reactions were performed with different amounts of hydrogen peroxide,
catalyst and without any catalyst in order to find the most adequate condition and optimizing of
the oxidation reaction. The yield of oxidation diphenyl sulfide to the corresponding sulfoxide as
a function of time is presented in the Table 2. The optimum ratio of sulfide to H
2
2
O (1:1.5 eq.) in
+
the presence of CSA (0.07 g equal to 0.34mmol H ) was found to be ideal for complete
conversion of sulfides to sulfoxide (Table 2 Entry 5).
The generality of the method was examined using different types of sulfides (Table 3). All the
substrates studied were selectively oxidized to their corresponding sulfoxides in good to
excellent yields at room temperature. To check the reusability of CSA for the formation
sulfoxide, the catalyst was removed by filtration and used for the next experiment under similar
3
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reaction conditions. This catalyst was reused without significant loss of activities after four runs
(
[
Table 3 Entry 1).
Insert Table 3]
In Table 4, we compared our results on the oxidation of diphenyl sulfide with data from some
other workers. As shown in table, the previously reported procedures suffer from one or more
disadvantages such as elevated reaction temperatures (entry 7, 8, 9), longer reaction times (entry
5
7
7
, 6), special efforts for the preparation of catalyst (entry 8), using a transition metal (entry 3, 4,
), no recovery catalyst (entry 10) and the need for volatile and toxic organic solvents (entry 2, 4,
, 8, 11).
[
Insert Table 4]
This procedure offers several major advantages: The use of an available, cheap, and chemically
stabile catalyst and oxidant; highly efficient for the oxidation of structurally diverse sulfides to
sulfoxides in good to high yields; and the method conforms to several of the guiding principles
of green chemistry.
Deoxygenation of sulfoxides
In continuation of this work, we investigated the deoxygenation of sulfoxides. In order to get an
insight into the optimum conditions for reduction of sulfoxides to the corresponding sulfide, the
reaction of diphenyl sulfoxide by sodium iodide and CSA in ethanol as a model experiment was
studied at different solid acid and sodium iodide ratios. Results of these optimization
experiments are presented in Table 5.
[
Insert Table 5]
4
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The effect of solvent on the deoxygenation efficiency of the reagent was evaluated by carrying
out the deoxygenation of diphenyl sulfoxide in ethanol and acetonitrile at room temperature. It
was found that reaction in polar aprotic solvent, such as acetonitrile faster than in ethanol (protic
solvent), because acetonitrile dissolve salts by ion–dipole interactions and enable formation of
naked anions. However ethanol as a non-toxic, environmentally benign media and inexpensive
solvent was also investigated for this reaction.
Different types of sulfoxides were subjected to deoxygenation reaction in the presence of sodium
iodide and CSA in ethanol. The deoxygenation reactions were performed under mild and
heterogeneous conditions at room temperature with good to excellent yields (Table 6).
After the first run, the reusability of the CSA was tested. The recycled solid acid was used for
three times. The obtained results demonstrating that CSA can be reused in these deoxygenation
reactions (Table 6 Entry 1).
[
Insert Table 6]
Table 7 compares the efficiency of this method with the some other method in the deoxygenation
of diphenyl sulfoxide to diphenyl sulfide. The data in Table 7 show that some protocols took
much longer time (entry 5, 9, 14), using a transition metal (entry 8, 10, 11, 12, 13), reduced
isolated yields (entry 7, 9), elevated reaction temperatures and poorly available reagents or harsh
conditions (3, 5, 8-14).
[
Insert Table 7]
In conclusion, the advantage of this methodology is the availability of the starting materials,
simplicity of oxidations and deoxygenation procedure under heterogeneous system, clean, easy
5
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work-up, high yields, cheapness, safe, eco-friendly, mild condition and recyclability of the solid
acid.
EXPERIMENTAL
All chemicals were commercial products. Melting points were measured on an Electrothermal
9
100 apparatus. Thin layer chromatography (TLC) was used to monitor all reactions and all
1
yields refer to isolated products. H-NMR spectra were recorded on a Bruker Avance AQS 300
MHz using TMS as an internal standard. The products were purified by column chromatography
and were identified by comparison of their m.p and NMR spectra with those reported for
authentic samples. GC measurements were carried out on an Agilent 6890 N gas chromatograph.
Synthesis of the Carbonaceous Solid Acid (CSA)
In a typical synthesis, p-TsOH (0.8 g), paraformaldehyde (0.36 g) and sulfuric acid (0.8 mL) was
stirred at 80 ºC for 24 h and then maintained at 120 ºC to form a black solid. The product was
washed with boiling distilled water until impurities such as sulfate ions were no longer detected
3
1
in the wash water . The density of prepared acid (CSA) determined using 0.01 M NaOH as
-1
titrant in an acid-base potentiometric titration and the result obtained 4.9 mmol g .
General Experimental Procedure for the Preparation of Sulfoxides
+
In a typical reaction, a mixture of CSA (0.07 g equal to 0.34mmol H ), diphenyl sulfide (1
mmol), hydrogen peroxide (aq. 30%, 1.5mmol) and ethanol (5 mL) was stirred at r.t. for 240
min. The progress of reaction was monitored by TLC method. At the end of the reaction, catalyst
6
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was removed by filtration and the resulting solution was concentrated under reduced pressure.
Then, residue was purified by column chromatography on silica gel (n-hexane – ethyl acetate 1 :
5
) to afford 0.192 g (93%) of diphenyl sulfoxide as a white solid. The resulting products were
1
21,35
determined by their H-NMR spectra and compared with the authentic samples
.
General Experimental Procedure for the Deoxygenation of Sulfoxides
A mixture of diphenyl sulfide (1 mmol), NaI (2.5 mmol), and CSA (0.45 g equal to 2.2 mmol
+
H ) and ethanol (5 mL) was stirred at r.t. for 210min. The progress of the reaction was monitored
by TLC. When the starting sulfoxide had completely disappeared, the mixture was neutralized by
adding NaOH solution. The product was extracted with EtOAc (3× 5 mL). The combined
extracts were washed with 10% aq Na
2
S
2
O
3
.The organic layer dried over MgSO . The filtrate
4
was evaporated, and then residue was purified by column chromatography on silica gel.
7
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Table 1 The effect of various solvents on oxidation of diphenyl sulfide to diphenyl sulfoxide
Reaction time
a
Entry
Solvent
Yield (%)
(
min)
80
1
1,2-Dichloroethane
Acetonitrile
Xylene
95
90
60
93
2
150
360
240
3
5
Ethanol
a
Isolated yield.
Table 2 Optimization of the amounts of H
2
O
2
and CSA for the oxidation of diphenyl sulfide to
diphenyl sulfoxide in Ethanol.
CSA
a
Entry
H
2
O
2
(eq.)
Time (min)
Yield (%)
+
(
mmol H )
0
1
2
3
4
5
6
1.5
0.5
0.5
1.0
1.5
2.5
1440
350
300
300
240
210
60
60
0.24
0.34
0.34
0.34
0.34
65
90
93
b
90 (5)
a
Isolated yield.
b
diphenyl sulfone. Conversion was determined by GC.
1
2
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Table 3 Oxidation of sulfides to sulfoxides with hydrogen peroxide in the presence
Of CSA in Ethanol
a
Substrate
Diphenyl sulfide
Time (min)
240
80
Yield (%)
ref
21
b
1
93,90,95,91
2
Methyl phenyl sulfide
p-Nitrophenyl methyl sulfide
Benzyl phenyl sulfide
Benzyl phenyl sulfide
Dibenzyl sulfide
91
89
93
90
93
90
95
90
92
90
85
21
3
4
5
6
7
8
9
160
170
65
35a
35b
35b
35b
21
90
Di n-butyl sulfide
65
Tetrahydrothiophene
Ethyl phenyl sulfide
Allyl phenyl sulfide
p-Bromophenyl methyl sulfide
Dibenzothiophene
65
35c
35b
35d
35b
35e
90
1
0
1
2
90
1
1
120
240
a
Isolated yield.
b
The same reagent was used for each of the four runs.
1
3
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Table 4 Comparison of the activity of various catalysts in the oxidation of diphenyl sulfide using
H
2
O
2
Entry
catalyst
CSA
Conditions
EtOH, rt
Time
240 min
30 min
25 min
30 min
6.5 h
Yield
93
ref
This work
26
1
2
3
4
5
6
7
8
9
Silica sulfuric acid
CH
Acetone, rt
CH CN
3
CN, rt
95
MoO
2
Cl
2
89
24
TaCl
5
3
93.8
98
35d
36
Amberlite IR-400
Amberlyst 15
MeOH,rt
MeOH, rt
7.5 h
98
36
H
5
PMo11Al0.5
V
0.5
O
40
CH
3
CN, reflux
Cl , reflux
CN, reflux
120 min
10 min
90 min
40 min
48 h
100
95
22
CBSA
C
2
H
4
2
27
L-proline
CH
3
93
37
1
1
0
1
HNO
3
EtOH, rt
92
39
◦
NBS
CH
3
CN 40 C
60
40
1
4
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Table 5 Optimization of the amounts of NaI and CSA for the deoxygenation of diphenyl
sulfoxideto diphenyl sulfide in Ethanol.
CSA
a
Entry
NaI (eq.)
Time (min)
Yield (%)
+
(
mmol H )
1
2.0
2.2
2.5
2.5
1.96
1.96
2.2
300
300
210
200
70-73
79
2
3
4
91
2.45
91-93
a
Isolated yield.
1
5
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Table 6 deoxygenation of sulfoxides to sulfides with NaI and CSA in Ethanol and Acetonitrile
Ethanol
Acetonitrile
Entry
Substrate
Time
Time
(min)
25
Yield
a
Yield (%)
ref
a
(
min)
210
60
(%)
95
93
90
95
93
93
91
95
93
b
1
Diphenyl sulfoxide
Methyl phenyl sulfoxide
Benzyl phenyl sulfoxide
Benzyl methyl sulfoxide
Dibenzyl sulfoxide
91,90,93,87
41a
41b
41c
41d
41e
41a
41f
41b
41g
2
3
4
5
6
7
8
9
95
90
88
91
93
89
96
93
15
80
15
80
15
60
15
Di n-butyl sulfoxide
Tetrahydrothiophene oxide
Ethyl phenyl sulfoxide
Allyl phenyl sulfoxide
p-Bromophenyl methyl
sulfoxide
100
210
60
15
25
15
60
15
1
0
200
93
20
90
14
p-Nitrophenyl methyl
sulfoxide
1
1
1
2
240
300
92
85
25
45
93
87
41h
41f
Dibenzothiophene-5-oxide
a
Isolated yield.
b
The same reagent was used for each of the four runs.
1
6
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Table 7 Comparison of the activity of various reducing system in the deoxygenation of diphenyl
sulfoxide to diphenyl sulfide
Entry
Reaction Conditions
CSA, Ethanol, NaI
Time
210 min
240 min
55 min
40 min
24 h
Yield (%)
ref
This work
34
1
2
3
4
5
6
7
8
91
94
Silica sulfuric acid, PEG-200, rt
TABCO, 1,3-Dithiane, CHCl
NBS, 1,3-Dithiane, CHCl , rt
Zn(OTf) , PhSiH , toluene ,100 °C
KI-TCT, CH CN, 25 °C
Mg, MeOH, rt
3
, rt
93
41g
3
94
41g
2
3
>99
96
41f
3
25 min
90 min
120 min
42a
75
41c
Mo(CO)
6
, EtOH, reflux
92
42b
2
,6-dihydroxypyridine, CH
reflux
3
CN,
9
14 h
24
42c
1
1
0
1
ReIO
MoO Cl
MoCl
Ultrasonication, THF, r.t.
CN, 0 °C
Woollins’ reagent, toluene, reflux
2
(PPh
(DMF)
/Gallium, under
3
)
2
, PhSiH
3
,THF, rt
20 min
97
89
42d
42e
2
2
2
, pinacol, 90  C
240 min
5
1
2
10 min
92
42f
1
1
3
4
TiI4, CH
3
10 min
20 h
83
99
41e
41a
1
7
ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
H O - CSA
2
2
O
S
R
S
R'
R
R'
KI- CSA
Scheme 1
1
8
ACCEPTED MANUSCRIPT