Selective and mild oxidation of sulfides to sulfoxides by H2O2 using DBUH-Br3 as catalyst

Article abstract of DOI:10.1016/j.cclet.2010.01.004

DBUH-Br₃ catalyzed selective conversion of sulfides to sulfoxides in the presence of H₂O₂ as oxidizing agent is described. The reaction was performed selectively at room temperature and relatively short reaction times.

Full text of DOI:10.1016/j.cclet.2010.01.004

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 651–655
www.elsevier.com/locate/cclet
Selective and mild oxidation of sulfides to sulfoxides
by H2O2 using DBUH-Br3 as catalyst
b
c
a
Mehdi Bakavoli a, , Alireza Motavalizadeh Kakhky , Ali Shiri , Mahdieh Ghabdian ,
*
Abolghasem Davoodnia a, Hossein Eshghi c, Mola Khatami c
a Department of Chemistry, School of Sciences, Islamic Azad University, Mashhad Branch, Mashhad, Iran
b Department of Chemistry, School of Sciences, Islamic Azad University, Neyshabur Branch, Neyshabur, Iran
c Department of Chemistry, School of Sciences, Ferdowsi University of Mashhad, 91775-1436 Mashhad, Iran
Received 4 November 2009
Abstract
DBUH-Br3 catalyzed selective conversion of sulfides to sulfoxides in the presence of H2O2 as oxidizing agent is described. The
reaction was performed selectively at room temperature and relatively short reaction times.
# 2010 Mehdi Bakavoli. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Sulfide; Sulfoxide; Selective oxidation; H2O2; DBUH-Br3
Sulfoxides are important intermediates in the synthesis of various natural products [1,2]. Omeprazole and Figpronil
pesticides are two typical examples of extensive applications of these important intermediates in pharmaceutical and fine
chemical industries [3]. Syntheses of sulfoxides are usually achieved by oxidation of their corresponding sulfides. During
the last years, various procedures for H2O2 oxidation of sulfides including the use of various heterogeneous catalyst
systems have been reported [4–6]. The application of this oxidant has been developed due to its effective oxygen content,
eco-friendly nature, low cost, and also safety in storage and operation [7,8]. Recently various heterogenous catalyst-
oxidizing agent systems including gold(III)–hydrogen peroxide [9], silica-immobilized vanadyl alkyl phosphonate–
sodium bromate[10], silica sulfuric acid–hydrogen peroxide [11], and titanium based catalyst-oxidizing agents have been
reportedfor oxidationofsulfides. Although, manyreagentshavebeenemployedfor theoxidationofsulfidestosulfoxides,
the most common problem encountered during the reaction is the over-oxidation of sulfoxides to their corresponding
sulfones[12]. Promptedbythesereportsandduetothepotentialoxidationofsulfur-containingcompoundsbyH2O2, inthe
presence of heterogenous catalysts, in this study wewish to report on a new and efficient method for selective oxidation of
sulfides to sulfoxides by H2O2 as oxidant and in the presence of DBUH-Br3 as catalyst (Scheme 1).
1. Experimental
All chemicals were purchased from Merck and Fluka chemical companies. The oxidation products were
characterized by comparison of their spectral and physical data with authentic samples.
* Corresponding author.
E-mail address: mbakavoli@yahoo.com (M. Bakavoli).
1001-8417/$ – see front matter # 2010 Mehdi Bakavoli. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.01.004

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M. Bakavoli et al. / Chinese Chemical Letters 21 (2010) 651–655
Scheme 1.
1.1. General procedure for the selective oxidation of sulfides to sulfoxides
To a mixture of various sulfides (1 mmol) and DBUH-Br3 (0.1 mmol, 0.039 g) in CH3CN (15 mL), H2O2 30% (v/v)
(1.4 mmol) was added gradually. After the completion of the reaction which was monitored by TLC using n-hexane:
EtOAc (7:3), the solvent was removed and CH2Cl2 (20 mL) was added and the mixture was washed with water
(15 mL), 5% HCl solution (15 mL) and water (15 mL), respectively. The organic phase was then dried over anhydrous
1
Na2SO4 and the solvent was removed under reduced pressure. The resulting products were determined by their H
NMR spectrums and compared with the authentic samples.
2. Results and discussion
The synthesis and X-ray structure of DBUH-Br3 as the catalyst have already been reported elsewhere (Fig. 1)
[13].
In order to get an insight into the optimum catalytic conditions for the selective oxidation of sulfides to sulfoxides,
the oxidation reaction of methylphenylsulfide by hydrogen peroxide as a model experiment was studied at different
temperatures and catalyst/H2O2 ratios. The results are summarized in Table 1. Different solvents were also screened
Fig. 1. Molecular structure and atomic labeling scheme (50% probability level).
Table 1
The data for optimization of oxidation of methylphenylsulfide using H2O2.a.
Entry
Catalyst (mol%)
H2O2 (mmol)
Temperature (8C)
Sulfide (%)
Sulfoxide (%)
Sulfone (%)
1
2
50
30
15
10
5
1.4
1.4
1.4
1.4
1.4
1.4
0
25
0
0
82
100
100
100
21
18
0
25
3
25
0
0
4
25
0
0
5
25
79
75
88
14
0
0
6
0
25
25
0
7
10
10
10
10
10
10
10
25
12
0
8
1.9
1.4
0.8
1.4
1.4
1.4
25
81
5
9
25
100
16
0
10
11
12
13
25
84
0
0
40
100
100
89
0
60
0
0
Reflux
0
11
a
Selectivities are based on 1H NMR spectroscopic integration.

M. Bakavoli et al. / Chinese Chemical Letters 21 (2010) 651–655
653
Table 2
Selective oxidation of various functionalized substrates to sulfoxides using H2O2 in the presence of DBUH-Br3.
Entry
Substrate
Product
Time (min)
Yield (%)a
95
[lit.]
[14]
1
60
2
3
60
90
80
[15]
[16]
120
4
5
6
7
8
150
45
85
90
95
66
75
[17]
[17]
[18]
[19]
[20]
100
150
90
9
120
100
80
75
[17]
[17]
10
11
12
90
95
85
[21]
[19]
100
13
150
70
[18]
a
Isolated yield.

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M. Bakavoli et al. / Chinese Chemical Letters 21 (2010) 651–655
Scheme 2.
for the oxidation of methylphenylsulfide, and as the result acetonitrile was found to be the best solvent in terms of
activity and selectivity.
In order to show the catalytic role of DBUH-Br3, the reaction of methylphenylsulfide was performed in the absence
of DBUH-Br3. The reaction was not completed within 24 h (Table 1, entry 6). Consequently, it was performed in
different molar ratios of the catalyst in the presence of oxidant in order to obtain the optimum reaction conditions
(Table 1, entries 1–5).
The chemoselectivity of the system was also investigated. Several sulfides containing an unsaturated C–C bond,
nitrile or an alcoholic group were subjected to the sulfoxidation reaction, and it was observed that the functional
groups remained intact during the reaction (Table 2, entries 4, 6, 7, 13).
The suggested mechanism of this selective oxidation is shown in Scheme 2. DBUH-Br3 might in situ generate Br2.
Subsequently, Br+ can be produced via oxidation with H2O2. The final step of oxidation involves nucleophilic attack of
sulfur of the sulfide compound on the Br+ followed by concerted oxygen transfer from water to give the corresponding
sulfoxide.
In summary, we have described an efficient method for the selective oxidation of sulfides to sulfoxides under very
mild reaction conditions. Also, chemoselectivity, easy and clean work-up, green oxidant, high yields and the easy
preparation of the catalyst were the remarks of this method which could be the priority of the other existing
methodologies.
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