A highly efficient, green, rapid, and chemoselective oxidation of sulfides using hydrogen peroxide and boric acid as the catalyst under solvent-free conditions

Article abstract of DOI:10.1016/j.tetlet.2010.04.103

We report boric acid as a highly efficient and eco-friendly catalyst for the selective oxidation of sulfides to sulfoxides or sulfones, in excellent yields under solvent-free conditions, using 30% hydrogen peroxide as an oxidant. Various sulfides possessing functional groups such as alcohol, ester, and aldehyde are successfully and selectively oxidized without affecting sensitive functionalities.

Full text of DOI:10.1016/j.tetlet.2010.04.103

Tetrahedron Letters 51 (2010) 3501–3503
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A highly efficient, green, rapid, and chemoselective oxidation of sulfides
using hydrogen peroxide and boric acid as the catalyst under solvent-free
conditions
*
Amin Rostami , Jamal Akradi
Department of Chemistry, Faculty of Science, University of Kurdistan, Sanandaj, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 February 2010
Revised 7 April 2010
Accepted 23 April 2010
Available online 28 April 2010
We report boric acid as a highly efficient and eco-friendly catalyst for the selective oxidation of sulfides to
sulfoxides or sulfones, in excellent yields under solvent-free conditions, using 30% hydrogen peroxide as
an oxidant. Various sulfides possessing functional groups such as alcohol, ester, and aldehyde are suc-
cessfully and selectively oxidized without affecting sensitive functionalities.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Oxidation
Hydrogen peroxide
Boric acid
Catalyst
Sulfide
Sulfoxide
Sulfone
The chemoselective preparation of chiral and achiral sulfoxides
and sulfones is extremely important in organic chemistry. Sulfox-
ides and sulfones are valuable intermediates for the synthesis of
fine chemicals and biologically active compounds.1 Furthermore,
sulfoxidation catalysis has assumed relevance in the desulfuriza-
tion of fuels, due to environmental constraints.2
The main synthetic route for the preparation of these valuable
materials is via oxidation of the corresponding sulfides. There are
several reagents available for this key transformation which is typ-
ically achieved using stoichiometric or catalytic amounts of both
organic and inorganic reagents.3–5
The use of ‘green oxidants’ such as molecular oxygen and
hydrogen peroxide is attractive, since they are readily available,
inexpensive, and environmentally benign, with formation of water
as the only by-product.6 Oxidation of sulfides with H2O2 is slow;
hence extensive studies have been undertaken to develop new cat-
alysts for this reaction.7–14,3 Disadvantages of this sulfoxidation
reaction include difficulties in stopping the oxidation at the sulfox-
ide stage, utilization of environmentally unfavorable reagents, sol-
vents, and catalysts, the preparation of complex catalysts, removal
or recovery of the expensive catalyst, metal residues in the prod-
ucts, and the formation of large amounts of toxic waste. Hence
there is a need to develop new catalysts which can overcome these
drawbacks.
Boric acid is commercially available, environmentally compati-
ble, cheap, easy to handle, and stable. It was recently shown to be a
good substitute for conventional acidic catalytic materials.15–19 The
boric acid/H2O2 system offers an ideal combination for the chosen
oxidations. For these reasons, and in continuation of our studies on
environmentally benign chemical processes,20–22 we report the
chemoselective oxidation of sulfides to sulfoxides or sulfones with
H2O2 in the presence of a catalytic amount of boric acid by control-
ling the amount of H2O2 and catalyst (Scheme 1). First, in order to
optimize the reaction conditions, we evaluated the influence of
different amounts of hydrogen peroxide and catalyst on the
oxidation of diphenyl sulfide, a relatively unreactive substrate, as
a model compound in terms of time and product yield (Table 1).
As shown in Table 1, the reaction was incomplete in the absence
of a catalyst even after 24 h (entries 3 and 8). When boric acid was
O
S
O
O
boric acid (cat.), H2O2
solvent-free, r.t.
S
S
R1
R2
+
R1
R2
R1
R2
R1, R2 = Ar, benzylic, linear, cyclic
* Corresponding author.
E-mail address: a_rostami372@yahoo.com (A. Rostami).
Scheme 1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.103

3502
A. Rostami, J. Akradi / Tetrahedron Letters 51 (2010) 3501–3503
Table 1
The expected products were obtained in short times and in high
yields. It is notable that the sulfides were oxidized chemoselective-
ly in the presence of oxidation-prone functional groups such as OH,
and CHO (Table 2, entries 9–11). An ester group also remained in-
tact during the reaction (Table 2, entry 12).
Optimization of the amounts of H2O2 and boric acid for the selective oxidation of
diphenyl sulfide to diphenyl sulfoxide or diphenyl sulfone
Entry
H2O2
(equiv)
Boric acid
(mmol)
Time
Yield (%)a
Sulfoxide
Sulfone
In order to establish the catalytic activity of boric acid, we com-
pared our results on the oxidation of diphenyl sulfide with data
from the literature (Table 3). As shown in Table 3, the previously
reported procedures suffer from one or more disadvantages such
as elevated reaction temperatures,11,13,14 longer reaction times,11,14
special efforts for the preparation of catalyst,11–13 using a transi-
tion metal,10,14 and the need for volatile and toxic organic sol-
vents.10–14
1
2
3
4
5
6
7
8
9
0.6
1.2
1.2
1.2
1.2
2.4
3.6
3.6
3.6
3.6
0.1
0.1
0
0.02
0.05
0.2
0.2
0
30 min
5 min
24 h
30 min
30 min
60 min
25 min
24 h
50
100
80
40
70
—
—
—
—
—
—
—
—
—
—
70
100
50
30
65
0.05
0.1
60 min
60 min
10
This procedure offers several major advantages: (1) the use of a
commercially available, cheap, and chemically stabile catalyst and
oxidant; (2) highly efficient for the selective oxidation of structur-
ally diverse sulfides in good to high yields; (3) control over the
degree of oxidation offers access to sulfoxides or sulfones; (4)
excellent chemoselectivity; and (5) the method conforms to
several of the guiding principles of green chemistry. We believe
the present method to be an improvement with respect to other
procedures.
General procedure for the oxidation of sulfides to sulfoxides. The
sulfide (1 mmol) was added to a solution of 30% H2O2 (1.2 equiv,
0.5 g) and boric acid (0.1 mmol, 0.006 g), and the mixture was
stirred at room temperature for the time specified in Table 2. The
a
Conversions determined by GC.
added, the times were reduced considerably and the yield in-
creased to 100%. H2O2 (1.2 equiv) in the presence of boric acid
(0.1 mmol) was found to be ideal for the complete conversion of
sulfides to sulfoxides. The use of excess H2O2 (3.6 equiv) in the
presence of boric acid (0.2 mmol) led to the corresponding sulfone
in a clean reaction.
In order to generalize the scope of the reaction, a series of struc-
turally diverse sulfides was subjected to oxidation under the opti-
mized reaction conditions, and the results are presented in Table 2.
Table 2
Selective oxidation of sulfides to sulfoxides or sulfones under solvent-free condition
Entry
Substrate
Sulfoxidea
Sulfone
Time (min)
Immediate
Isolated yield (%)
Time (min)
25
Isolated yield (%)
95b
S
S
1
2
3
95
15
92d
93
90
30
94b,d
S
S
Ph
Immediate
90b
S
4
5
Immediate
Immediate
94
91
30
20
94b
92b
S
89c
92c
S
S
6
7
12
15
87
90
150
135
8
10
89
180
87c
S
S
OH
9
10
11
20
89
86
88
90
210
180
90c
S
S
Immediate
Immediate
87c
OH
O
H
No reaction
S
OMe
12
Immediate
85
180
No reaction
O
a
b
c
Reaction conditions: sulfide (1 mmol), 30% H2O2 (1.2 equiv), boric acid (10 mol %), rt.
Reaction conditions: sulfide (1 mmol), 30% H2O2 (3.6 equiv), boric acid (20 mol %), rt.
Reaction conditions: sulfides (1 mmol), 30% H2O2 (4.8 equiv), boric acid (30 mol %), rt.
d
The disulfoxide/disulfone was formed, in the case of disulfoxide on the basis of melting point, compared with an authentic sample, its stereochemistry was trans.23

A. Rostami, J. Akradi / Tetrahedron Letters 51 (2010) 3501–3503
3503
Table 3
Comparison of the activity of various catalysts in the oxidation of diphenyl sulfide using H2O2
Entry
Catalyst
Sulfoxide
Sulfone
Time (h)
Ref.
Conditions
Time (h)
Yield (%)
Conditions
Yield (%)
1
2
3
4
5
6
H3BO3
MoO2Cl2
H5PMo11Al0.5V0.5O40
Silica sulfuric acid
Carbon-based solid acid
TaCl5
Neat, rt
Immediate
0.35
2
6
0.17
2
95
89
100
95
98
90
Neat, rt
CH3CN, rt
CH3CN, reflux
CH3CN, rt
C2H4Cl2, reflux
CH3OH, reflux
0.6
0.55
4
0.75
0.17
3.5
95
91
95
95
95
90
This work
Acetone, rt
CH3CN, reflux
CH3CN, rt
C2H4Cl2, reflux
CH3CN or i-PrOH or t-BuOH, rt
10
11
12
13
14
2. Chica, A.; Gatti, G.; Moden, B.; Marchese, L.; Iglesia, E. Chem. Eur. J. 2006, 12,
1960–1967.
3. Romanelli, G. P.; Vázquez, P. G.; Tundo, P. Synlett 2005, 75–78.
4. Kaczorowska, K.; Kolarska, Z.; Mitka, K.; Kowalski, P. Tetrahedron 2005, 61,
8315–8327.
5. Bahrami, K. Tetrahedron Lett. 2006, 47, 2009–2012.
6. (a) Lane, B. S.; Burgess, K. Chem. Rev. 2003, 103, 2457–2473; (b) Sato, K.; Aoki,
M.; Noyori, R. Science 1998, 281, 1646–1647.
7. Sato, K.; Hyodo, M.; Aoki, M.; Zheng, X. Q.; Noyori, R. Tetrahedron 2001, 57,
2469–2476.
8. Choudary, B. M.; Bharathi, B.; Reddy, C. V.; Kantam, M. L. J. Chem. Soc., Perkin
Trans. 1 2002, 2069–2074.
9. Venkat-Reddy, C.; Verkade, J. G. J. Mol. Catal. A: Chem. 2007, 272, 233–240.
10. Jeyakumar, K.; Chand, D. K. Tetrahedron Lett. 2006, 47, 4573–4576.
11. Romanelli, G. P.; Bennardi, D. O.; Palermo, V.; Vazquez, P. G.; Tundo, P. Lett. Org.
Chem. 2007, 4, 544–549.
12. Shaabani, A.; Rezayan, A. H. Catal. Commun. 2007, 8, 1112–1116.
13. Zali, A.; Shokrolahi, A.; Keshavarz, M. H.; Zarei, M. A. Acta Chim. Slov. 2008, 55,
257–260.
14. Kirihara, M.; Yamamoto, J.; Noguchi, T.; Hirai, Y. Tetrahedron Lett. 2009, 50,
1180–1183.
progress was monitored by TLC or GC. After completion of the
reaction, the product was extracted with CH2Cl2 (3 Â 10 mL) and
the combined organics was washed with brine (15 mL) and dried
over anhydrous Na2SO4. The solvent was removed under reduced
pressure to give the corresponding pure sulfoxide in most cases.
Further purification was achieved by short-column chromatogra-
phy on silica gel with EtOAc/n-hexane (1/10) as eluent. All the
products are known and were characterized by IR, 1H NMR, and
by melting point comparisons with those of authentic sam-
ples.8,23–26
General procedure for the oxidation of sulfides to sulfones. The sul-
fide (1 mmol) was added to a solution of 30% H2O2 (3.6–4.8 equiv)
and boric acid (0.2–0.3 mmol), and the mixture was stirred at room
temperature for the time specified in Table 2. The progress was
monitored by TLC or GC. After completion of the reaction, the prod-
uct was extracted with CH2Cl2 (3 Â 10 mL) and the combined
organics was washed with brine (15 mL) and dried over anhydrous
Na2SO4. The solvent was removed under reduced pressure to give
the corresponding pure sulfone in most cases. Further purification
was achieved by recrystallization from EtOH. All the products are
known and were characterized by IR, 1H NMR, and by melting
point comparisons with those of authentic samples.7,8,25
15. Tu, S.; Fang, F.; Miao, C.; Jiang, H.; Feng, Y.; Shi, D.; Wang, X. Tetrahedron Lett.
2003, 44, 6153–6155.
16. Maki, T.; Ishihara, K.; Yamamoto, H. Org. Lett. 2005, 7, 5047–5050.
17. Chaudhuri, M. K.; Hussain, S. J. Mol. Catal. A: Chem. 2007, 269, 214–217.
18. Kondaiah, G. C. M.; Reddy, L. A.; Babu, K. S.; Gurav, V. M.; Huge, K. G.;
Bandichhor, R.; Reddy, P. P.; Bhattacharya, A.; Anand, R. V. Tetrahedron Lett.
2008, 49, 106–109.
19. Mukhopadhyay, C.; Tapaswi, P. K.; Butcher, R. J. Aust. J. Chem. 2009, 62, 140–
144.
20. Rostami, A.; Rahmati, S.; Khazaei, A. Monatsh. Chem. 2009, 140, 663–667.
21. Rostami, A.; Ahmad-Jangi, F.; Zarehbin, M. R.; Akradi, J. Synth. Commun. 2010,
40, 1500–1507.
22. Rostami, A.; Akradi, J.; Ahmad-Jangi, F. J. Braz. Chem. Soc., in press.
23. Savin, E. D.; Nedelkin, V. I.; Zverev, D. V. Chem. Heterocycl. Compd. 1997, 33,
333–337.
Acknowledgment
We are grateful to the University of Kurdistan Research
Councils for partial support of this work.
24. Habibi, D.; Zolfigol, M. A.; Safaiee, M.; Shamsian, A.; Ghorbani-Choghamarani,
A. Catal. Commun. 2009, 10, 1257–1260.
References and notes
25. Pouchert, C. J.; Behnke, J. The Aldrich Library of 13C and 1H FT NMR Spectra, 1st
ed.; Aldrich Chemical Company, Inc: Milwaukee, WI, 1993; Vol. 1.
26. Orito, K.; Hatakeyama, T.; Takeo, M.; Suginome, H. Synthesis 1995, 1357–1358.
1. Frenanez, I.; Khiar, N. Chem. Rev. 2003, 103, 3651–3706.