(PhCH2PPh3)+Br3-: A versatile reagent for the chemoselective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1080/10426500903176539

Benzyltriphenylphosphonium tribromide (BTPTB), as a stable solid reagent, is easily prepared by the reaction of benzyltriphenylphosphonium bromide with Br2. This reagent can be used as an efficient reagent for the chemoselective oxidation of dialkyl and a

Full text of DOI:10.1080/10426500903176539

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Phosphorus, Sulfur, and Silicon and the
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(PhCH2PPh3)+Br3 : A Versatile Reagent
for the Chemoselective Oxidation of
Sulfides to Sulfoxides
F. Shirini a , G. H. Imanzadeh b , A. R. Mousazadeh a b & A. R.
Aliakbar a
a Department of Chemistry, College of Science , University of
Guilan , Rasht, Iran
b Department of Chemistry, College of Science , University of
Mohaghegh-Ardabili , Ardabil, Iran
Published online: 02 Aug 2010.
To cite this article: F. Shirini , G. H. Imanzadeh , A. R. Mousazadeh & A. R. Aliakbar (2010)
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(PhCH2PPh3)+Br3 : A Versatile Reagent for the Chemoselective Oxidation of Sulfides to
Sulfoxides, Phosphorus, Sulfur, and Silicon and the Related Elements, 185:8, 1640-1644, DOI:
10.1080/10426500903176539
To link to this article: http://dx.doi.org/10.1080/10426500903176539
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Phosphorus, Sulfur, and Silicon, 185:1640–1644, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500903176539
(PhCH2PPh3)+Br3−: A VERSATILE REAGENT
FOR THE CHEMOSELECTIVE OXIDATION OF SULFIDES
TO SULFOXIDES
F. Shirini,1 G. H. Imanzadeh,2 A. R. Mousazadeh,1,2
and A. R. Aliakbar1
1Department of Chemistry, College of Science, University of Guilan, Rasht, Iran
2Department of Chemistry, College of Science, University of Mohaghegh-Ardabili,
Ardabil, Iran
Benzyltriphenylphosphonium tribromide (BTPTB), as a stable solid reagent, is easily pre-
pared by the reaction of benzyltriphenylphosphonium bromide with Br2. This reagent can be
used as an efficient reagent for the chemoselective oxidation of dialkyl and aryl-alkyl sulfides
to their corresponding sulfoxides in the presence of diaryl sulfides and primary alcohols.
All reactions were performed in a refluxing mixture of methanol and water in very short
reaction times.
Keywords Benzyltriphenylphosphonium bromide; benzyltriphenylphosphonium tribromide;
chemoselective oxidation; sulfides; sulfoxides
Sulfoxides, as useful synthetic intermediates which are used for the construction of various
chemically and biologically significant molecules,1 are usefully prepared by the oxidation
of sulfides. Various methods are available for the oxidation of sulfides to sulfoxides.2–19
However, some of these methods suffer from drawbacks such as low yields of the products,
long reaction times, low selectivity, overoxidation of sulfoxides to sulfones, the use of
expensive reagents, and the use of corrosive acids and hazardous peracids. Consequently,
introduction of new methods and procedures to circumvent these problems is still in demand.
Very recently, benzyltriphenylphosphonium tribromide (BTPTB) was used in the
tetrahydropyranylation of alcohols20 as well as the thioacetalization of carbonyl com-
pounds.21 In continuation of our ongoing research program on the development of new
methods for the oxidation of organic compounds,22–26 in this article, in addition to the
introduction of a new and high yielding method for the preparation of BTPTB, we report
the applicability of this reagent in the chemoselective oxidation of dialkyl and aryl-alkyl
sulfides to their corresponding sulfoxides (Scheme 1).
We carried out our initial studies with dibenzyl sulfide to find the optimal reaction
conditions. These studies showed that the best result was obtained in a refluxing mixture
Received 1 May 2009; accepted 10 July 2009.
We are thankful to the University of Guilan and Mohaghegh-Ardabili University Research Councils for the
partial support of this work.
Address correspondence to F. Shirini, Department of Chemistry, College of Science, University of Guilan,
Rasht 41335, Iran. E-mail: shirini@guilan.ac.ir
1640

CHEMOSELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES
1641
(PhCH2PPh3) Br3
Y-S-Z
Y-S(O)-Z
CH3CN : H2O (2:1, v/v), reflux
Y, Z= Alkyl
Y= Alkyl, Z= Aryl
Scheme 1
of CH3CN:H2O (2:1, v/v) when the relative ratio of BTPTB to substrate was 2:1. After
optimization of the reaction conditions, different types of dialkyl and aryl-alkyl sulfides
were subjected to oxidation under the determined conditions. As shown in Table I, the
corresponding sulfoxides were obtained in very short reaction times in high yields. Our
investigations clarified that primary alcohols are resistant to the oxidation under the selected
conditions, and diaryl sulfides are oxidized slower than the other sulfides using this method,
and the reaction is incomplete in most cases (Table I, entries 7, 9, 10). Therefore this
methodology can be used for the selective oxidation of dialkyl and aryl-alkyl sulfides in the
presence of primary alcohols and diaryl sulfides (Table I, entries 7 and 11). It is important
to note that results show no sulfone formation using this method, even after prolonged
heating.
A plausible mechanism of this oxidation is shown in Scheme 2 based on reported
pathway in literature,11 our observations, and obtained results.
(Ph3PCH2Ph) Br3
(Ph3PCH2Ph) Br
Br
Br
R1 S R2
Br
Br2
R1 S R2
R1 S R2 Br
H2O
O
R1 S R2 + 2 HBr
Scheme 2
Table II, which is used for the comparison of the efficiency of our method with some
of those reported in the literature, shows that the current method is simpler, more efficient,
and less time consuming.
In conclusion, we have reported the application of benzyltriphenylphosphonium tri-
bromide in efficient oxidation of dialkyl and aryl-alkyl sulfides. Very short reaction times,
high yields of the products, simple procedure, and easy and clean work-up are important ad-
vantages of this method. The chemoselective oxidation of the above-mentioned substrates
in the presence of diaryl sulfides and primary alcohols, as one of the most important ad-
vantages, make this method attractive for the chemists. We believe that the present method
could be an important addition to the existing methodologies.

1642

CHEMOSELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES
1643
Table II Comparison of efficiency of various reagents in the oxidation of dibenzyl sulfide
Entry
Reagent
Time (min)
Yield (%)
Reference
1
2
3
4
5
6
7
8
9
HIO3/wet SiO2
H2O2/Mn(III)-Schiff base complex
170
100
24 (h)
20
14
120
92
95
10
87
96
92
90
98
92
5
7
14
12
13
11
17
3-Carboxypyridinium chlorochromate/AlCl3
H2O2/MoO3
Cetyltrimethylammonium tribromide
H2O2/NBS
PVP-HNO3/KBr
BTPTB
11 (h)
15
Immediately
15
This work
EXPERIMENTAL
Chemicals were purchased from Merck, Fluka, and Aldrich Chemical Companies.
Products were separated and identified by the comparison of their mp, IR, NMR, and bp
with those reported for the authentic samples. 1H NMR spectra were recorded on JNM-EX
90A or Bruker-DRX MH2 NMR spectrometers.
Preparation of BTPTB
A 0.5 M solution (1 mL) of Br2 in MeOH was added dropwise to a stirring solution
of benzyltriphenylphosphonium bromide (0.3 mmol, 0.13 g) in EtOH (5 mL) in an ice
bath, and the mixture ws stirred for 1 h, then filtered. The yellow solid residue was washed
with distilled water and dried at room temperature and recrystallized from CH3CN to give
BTPTB in 85% yield. (According to the previously reported method, BTPTB was prepared
in 70% yield.21)
Oxidation of Sulfides Using BTPTB
BTPTB (2 mmol, 1.2 g) was added to a solution of the substrate (1 mmol) in MeCN-
H2O (2:1, v/v) (3 mL). The mixture was stirred at reflux temperature. After completion
of the reaction, most of the solvent was evaporated, and Et2O (5 mL) was added. The
mixture was filtered, and the solid residue was washed with Et2O (5 mL). The filtrate was
washed with a saturated solution of NaHCO3 and H2O and dried over MgSO4. Evaporation
of the solvent followed by column chromatography on silica gel gave the corresponding
sulfoxides in high yields.
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