Solvent effects in the oxidation of sulfides with NaBrO 3/Mg(HSO4)2

Article abstract of DOI:10.1081/SCC-120022465

The oxidation of alkyl and aryl sulfides into the corresponding sulfoxides by NaBrO₃/Mg(HSO₄)₂ is studied. The selectivity of the reactions are highly governed by the choice of solvents and reaction conditions. Both alkyl and aryl sulfides are efficiently oxidized in CH₃CN and under solvent-free conditions, but in competition reactions with alkyl and aryl sulfides in CH₃CN only alkyl sulfides are oxidized. In n-hexane alkyl sulfides are oxidized to sulfoxides and oxidation of aryl sulfides completely inhibited.

Full text of DOI:10.1081/SCC-120022465

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Solvent Effects in the Oxidation of Sulfides with
NaBrO /Mg(HSO )
3
4 2
a b
a
a
b
Ahmad Shaabani
, Ayoob Bazgir , Kamal Soleimani & Peyman Salehi
a
Department of Chemistry, Medicinal and Aromatic Plants and Drug Research Institute,
Shahid Beheshti University, Tehran, Iran
b
Department of Phytochemistry, Medicinal and Aromatic Plants and Drug Research Institute,
Shahid Beheshti University, Tehran, Iran
Published online: 21 Aug 2006.
To cite this article: Ahmad Shaabani , Ayoob Bazgir , Kamal Soleimani & Peyman Salehi (2003): Solvent Effects in the
Oxidation of Sulfides with NaBrO /Mg(HSO ) , Synthetic Communications: An International Journal for Rapid Communication
3
4 2
of Synthetic Organic Chemistry, 33:17, 2935-2944
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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 17, pp. 2935–2944, 2003
Solvent Effects in the Oxidation of Sulfides with
NaBrO /Mg(HSO )
3
4 2
1
,2,
1
1
Ahmad Shaabani, * Ayoob Bazgir, Kamal Soleimani,
2
and Peyman Salehi
1 2
Department of Chemistry, and Department of
Phytochemistry, Medicinal and Aromatic Plants and
Drug Research Institute, Shahid Beheshti University,
Tehran, Iran
ABSTRACT
The oxidation of alkyl and aryl sulfides into the corresponding sulf-
oxides by NaBrO /Mg(HSO is studied. The selectivity of the reac-
3
4 2
)
tions are highly governed by the choice of solvents and reaction
conditions. Both alkyl and aryl sulfides are efficiently oxidized in
CH
tions with alkyl and aryl sulfides in CH
3
CN and under solvent-free conditions, but in competition reac-
CN only alkyl sulfides are
3
oxidized. In n-hexane alkyl sulfides are oxidized to sulfoxides and
oxidation of aryl sulfides completely inhibited.
*
Correspondence: Ahmad Shaabani, Department of Phytochemistry, Medicinal
and Aromatic Plants and Drug Research Institute, Shahid Beheshti University,
P.O. Box 19396-4716, Tehran, Iran; Fax: þ98-21-2403041; E-mail: a-shaabani@
cc.sbu.ac.ir.
2
935
DOI: 10.1081/SCC-120022465
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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2
936
Shaabani et al.
Key Words: Sulfide; Sulfoxide; Oxidation; Solvent effect;
Magnesium hydrogen sulfate.
Sulfoxide synthesis constitutes an important research area as they are
[
1–9]
versatile building blocks in organic synthesis.
oxidizing agents can effect the conversion of sulfides into sulfoxides.
A great number of
10–15]
[
However, the susceptibility of sulfoxides to further oxidation narrows
the choice of reagents for the oxidation of sulfides to sulfoxides. As a
consequence, the introduction of new methods and/or further work on
technical improvements to overcome the limitations are still important
experimental challenges.
In connection with ourstudies on the oxidation of o gr anic com-
[
16–18]
pounds by oxohalogen acid derivatives
halogen in aqueous media,
and organic derivation of
we now study the possibility of the syn-
thesis of sulfoxides with NaBrO /Mg(HSO ) in aprotic organic solvents
[
10,19]
3
4 2
such as CH CN or n-hexane and also undersolvent-f re e conditions.
3
Sodium bromate is a cheap and a commercially available compound.
It is a very stable solid material that can be handled much more easily
than liquid bromine or hypobromous acid solution. Oxidation with
bromate results in bromide ion formation, which can be safely treated
or re cycled.
As can be seen from the Table 1, aryl and alkyl sulfides are converted
into the corresponding sulfoxides in good yields in acetonitrile at room
temperature. This is a highly useful reaction for the preparation of
sulfoxides (Sch. 1), which are important intermediates in the synthesis
[
20]
of many organic compounds.
The observation that benzyl phenyl
sulfide and dibenzyl sulfide (Entries 8 and 9) are oxidized to the
corresponding sulfoxides indicate the reactions proceed by way of an
oxygen transfer mechanism. If the reaction involved electron transfer
instead of oxygen transfer substantial amounts of benzaldehyde
[
21,22]
would have been formed.
bromate in the absence of Mg(HSO ) in CH CN, oxidized alkyl sulfides
In addition we obtained that sodium
4
2
3
(
see Entries 12–14 in Table 1), but was not able to oxidize aryl sulfides
see Entries 15–17 in Table 1) similar to our previous results in protic
(
solvents.
[23]
In competitive oxidation reactions of alkyl and aryl sulfides with
NaBrO /Mg(HSO ) in CH CN only alkyl sulfides were selectively
3
4 2
3
oxidized to corresponding sulfoxides and aryl sulfides were recovered
practically unchanged (Entries 1 and 2 in Table 2).
It is interesting that NaBrO /Mg(HSO ) in n-hexane is able to
3
4 2
transform dialkyl sulfides to corresponding sulfoxides (Table 3), but
under the same conditions, this reagent is not capable of oxidizing aryl

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NaBrO /Mg(HSO )
3
2937
4 2
Table 1. Oxidation of alkyl and aryl sulfides to corresponding sulfoxides by
NaBrO
Entry
3
/Mg(HSO
4
)
2
in CH
3
CN at room temperature.
Reaction
ꢀ
M.p. ( C) or
ꢀ
a
time (h) Yield (%)
Substrate
Product
b.p. ( C)
b
1
2
3
4
5
6
7
8
9
(CH
(CH
3
)
CH
2
S
(CH
(CH
3
)
CH
2
SO
0.5
1.10
1.5
2
85
85
90
90
45
90
85
85
75
88
85
85
80
75
—
—
—
—
—
190–192 (189)
b
3
2
)
2
S
3
2
)
2
SO
103–105 (104)
b
[CH
[CH
3
(CH
(CH
2
)
)
3
]
]
2
2
S
S
[CH
[CH
3
(CH
(CH
2
)
)
3
]
]
2
2
SO
SO
28–30/6 (32/6)
c
3
2
7
3
2
7
75–76 (71–72)
d
73–74 (70.5)
(C
6
H
5
)
2
S
(C
6
H
5
)
2
SO
24
c
C
C
6
6
H
H
5
SCH
SCH
3
C
C
6
6
H
H
5
SOCH
SOCH
3
2
30–33 (33–34)
147–148/13 (146/13)
c
5
2
CH
S
3
5
2
CH
3
2.5
3.5
2
c
(C
6
H
5
CH
CH
p-BrC
p-NO
(CH
2
)
2
(C
6
H
5
CH
CH
p-BrC
p-NO
(CH
2
)
2
SO
SOC
135–137 (133–135)
c
43–45 (42–44)
C
6
H
5
2
SC
SCH
SCH
6
H
5
C
6
H
5
2
6
H
5
c
1
0
1
2
3
4
5
6
7
8
9
6
H
4
3
6
H
4
SOCH
SOCH
3
1.5
1
75–77 (74–76)
139–141 (140–141)
c
1
1
1
1
1
1
1
1
1
2
C
6
H
4
3
2
C
6
H
4
3
e
e
e
e
e
e
f
b
3
)
2
S
3
)
2
SO
4
190–191 (189)
b
[CH
[CH
3
(CH
(CH
2
)
)
3
]
]
2
2
S
S
[CH
[CH
3
(CH
(CH
2
)
)
3
]
]
2
2
SO
SO
4
28–30/6 (32/6)
c
3
2
7
3
2
7
4
75–76 (71–72)
(C
6
H
5
)
2
S
no reaction
no reaction
no reaction
6
—
—
—
—
—
C
C
C
6
6
6
H
H
H
5
5
5
SCH
SCH
3
4
2
CH
3
4
SOCH
3
C
SO [CH
6
H
5
SOCH
(CH
3
72
24
f
[CH
3
(CH
2
)
7
]
2
3
2
)
7
]
2
SO
a
Isolated yield.
[24]
b
From Ref.
[25]
From Ref.
c
d
[4]
From Ref.
e
4 2
In the absence of Mg(HSO ) .
f
Isolated sulfoxide without any transformation to sulfone.
Scheme 1.

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2
938
Shaabani et al.
Table 2. The competitive oxidation reaction between alkyl and aryl sulfides by
NaBrO
3
/Mg(HSO
4
)
2
in CH
3
CN at room temperature.
Products
Reaction
time
Entry
1
Substrates
Yield (%)
[CH
3
(CH
SCH
2
)
7
]
2
S and
[CH
3
(CH
SCH
2
)
7
]
2
SO and
1 h
87
C
(CH CH ) S and
6
H
5
3
C
6
H
5
3
no reaction
85
no reaction
2
(CH CH ) SO and
3
45 min
3
2 2
2 2
C
6
H
5
SCH
2
CH
3
C
6
H
5
SCH
2
CH
3
Table 3. Selective oxidation of alkyl sulfides to alkyl sulfoxides by
NaBrO
3
/Mg(HSO
4
)
2
in n-hexane at room temperature.
a
ꢀ
Reaction Yield
time (t/h) (%)
M.p. ( C) or
ꢀ
Entry
Substrate
Product
b.p. ( C)
b
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
(CH
(CH
3
)
2
S
CH
(CH
(CH
3
)
2
SO
3
90 190–192 (189)
85 103–105 (104)
90 28–30/6 (32/6)
b
3
2
)
2
S
3
CH
2
)
2
SO
2.5
3.5
8
b
[CH (CH ) ] S
3
[CH (CH ) ] SO
3
2 3 2
2 3 2
c
(C H CH ) S
6 5
(C H CH ) SO
6 5
75 135–137 (133–135)
c
90 75–76 (71–72)
2 2
2 2
[CH
3
(CH
)
2
)
7
]
2
S
[CH
3
(CH
2
)
7
]
2
SO
2.5
6
(C H
5
2
S
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
24
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
C H SCH
24
24
24
24
24
24
24
24
24
24
24
72
24
6
6
6
5
5
5
3
2
C
C
H
H
SCH
CH
CH
3
2
SC
6
H
5
1
1
1
1
1
1
1
1
1
1
p-BrC H SCH
6 4 3
p-NO
2 6 4 3
C H SCH
e
e
e
e
e
e
f
(CH ) S
3 2
[CH (CH ) ] S
3
2 3 2
[CH
(C
3
(CH
2
)
7
]
2
S
6
H
5
)
SCH
2
S
C
6
H
5
3
C H SCH CH
6 2
5
3
6
(C H
5
)SOCH
3
(C
6
H
5
)SOCH
]
3
f
[CH
3
(CH
2
)
7
]
2
SO [CH
3
(CH
2
)
7
2
SO
a
Isolated yield.
[24]
b
From Ref.
[25]
From Ref.
c
d
[4]
From Ref.
In the absence of Mg(HSO
e
4
)
2
.
Isolated sulfoxide without any transformation to sulfone.
f

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NaBrO /Mg(HSO )
3
2939
4 2
Scheme 2.
Scheme 3.
sulfides (Sch. 2). Both alkyl and aryl sulfides are not oxidized by NaBrO3
in the absence of Mg(HSO ) in n-hexane.
4
2
Thus, we examined the possibility of the oxidation of sulfides to
sulfoxides with NaBrO /Mg(HSO ) undersolvent-f er e conditions. As
3
4 2
indicated in Sch. 3, both alkyl and aryl sulfides are converted into the
corresponding sulfoxides in good yields (Table 4). Best results are
obtained when a 0.6 mmol of Mg(HSO ) and 2 mmol of NaBrO are
4
2
3
mixed and the reductant is then added dropwise while the solid oxidant is
tumbled using a magnetic stirrer. Most of the reactions can be carried out
at room temperature, however, improved yields for solid reductant are
ꢀ
obtained when the reaction temperature is about 60 C. Both alkyl and
aryl sulfides are not oxidized by NaBrO in the absence of Mg(HSO )
under solvent-free conditions (see Entries 12–17 in Table 4).
3
4 2
We repeated some experiments with sulfoxides as raw materials to
investigate the possibility of oxidation by NaBrO /Mg(HSO ) in
CH CN, n-hexane and undersolvent-f er e conditions. In all of these
3
4 2
3

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2
940
Shaabani et al.
Table 4. Oxidation of alkyl and aryl sulfides to corresponding sulfoxides by
NaBrO
Entry
3
/Mg(HSO
4
)
2
undersolvent-f re e conditions.
Reaction Yield
a
ꢀ
M.p. ( C) or
ꢀ
Substrate
Product
time (h)
(%)
b.p. ( C)
b
1
2
3
(CH
(CH
3
)
2
S
(CH
(CH
3
)
2
SO
0.5
0.45
1.5
0.5
10
0.5
1.5
3
87
84
80
85
45
80
88
70
55
90
85
—
—
—
—
—
—
—
—
190–192 (189)
b
3
CH
2
)
2
S
3
CH
2
)
2
SO
103–105 (104)
b
[CH
[CH
3
(CH
(CH
2
)
)
3
]
]
2
2
S
[CH
[CH
3
(CH
(CH
2
)
)
3
]
]
2
2
SO
SO
28–30/6 (32/6)
c
d
4
3
2
7
S
3
2
7
75–76 (71–72)
e
73–74 (70.5)
30–33 (33–34)
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
(C
6
H
5
)
2
S
(C
6
H
5
)
2
SO
d
C
C
6
H
H
5
SCH
SCH
3
C
C
6
H
H
5
SOCH
SOCH
3
d
6
5
2
CH
S
3
6
5
2
CH
3
147–148/13 (146/13)
d
c
c
c
c
f
(C
6
H
5
CH
CH
p-BrC
p-NO
(CH
2
)
2
(C
6
H
5
CH
CH
p-BrC
p-NO
2
)
2
SO
SOC
135–137 (133–135)
d
43–45 (42–44)
C
6
H
5
2
SC
6
H
5
C
6
H
5
2
6
H
5
24
0.5
0.45
4
d
1
1
1
1
1
1
1
1
1
1
6
H
4
SCH
SCH
3
6
H
4
SOCH
SOCH
3
75–77 (74–76)
139–141 (140–142)
d
2
C
6
H
4
3
2
C
6
H
4
3
3
)
2
S
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
—
—
—
—
—
—
—
—
f
[CH
[CH
3
(CH
(CH
2
)
)
3
]
]
2
2
S
S
4
f
3
2
7
4
f
(C
6
H
5
)
2
S
24
24
24
24
24
f
C
C
6
H
H
5
SCH
SCH
3
f
6
5
2
CH
3
g
g
(C
6
H
5
)SOCH
3
(C
6
H
5
)SOCH
3
[CH (CH ) SO [CH
3
2
7
]
2
3
(CH
2
)
7
]
2
SO
a
Isolated yield.
[24]
b
From Ref.
ꢀ
c
At 60 C.
d
[25]
From Ref.
[4]
From Ref.
e
f
4 2
In the absence of Mg(HSO ) .
g
Isolated sulfoxide without any transformation to sulfone.
experiments no reaction occurred, which indicated the selectivity of the
present system (see Entries 18 and 19 in Tables 1, 3 and 4).
SUMMARY
1
.
.
Both alkyl and aryl sulfides are efficiently oxidized by
NaBrO /Mg(HSO ) in CH CN, but in competitive reactions
with alkyl and aryl sulfides only alkyl sulfides are oxidized.
In n-hexane, only alkyl sulfides are converted into the corre-
sponding sulfoxides in good yields, the reagent does not,
however, react with aryl sulfides.
3
4 2
3
2

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NaBrO /Mg(HSO )
3
2941
4 2
3.
Both alkyl and aryl sulfides are efficiently oxidized by
NaBrO /Mg(HSO ) undersolvent- fe re conditions and sulf-
oxides are obtained as the sole products.
3
4 2
4
.
.
This oxidant system does not react with sulfoxides in CH CN,
3
n-hexane and undersolvent-f re e conditions.
Oxidation of alkyl and aryl sulfides in CH CN, n-hexane and
5
3
under solvent-free conditions reveals that in the absence of
Mg(HSO ) only alkyl sulfides are oxidized in CH CN.
4
2
3
EXPERIMENTAL
All products are known compounds and were identified by compar-
ison of theirphysical and spect ra l data with those of authentic samples.
Melting points were determined in open capillaries using an oil-bath and
are uncorrected. IR spectra were recorded as neat films or as KBr pellets
1
on a Shimadzu 470 spectrometer. H NMR spectra were recorded at
90 MHz on a JEOL EX-90 instrument with CDCl3 as solvent and
Me Si as an internal standard. All sulfides are commercial materials
4
and were purchased from Fluka, Aldrich and Merck Chemical
Companies.
Oxidation of Dibenzyl Sulfide in Acetonitrile;
Typical Procedure
Dibenzyl sulfide (0.43 g, 2 mmol) was added to a mixture of NaBrO3
(
(
0.32 g, 2 mmol) and Mg(HSO4)2 (0.12 g, 0.60 mmol) in acetonitrile
20 mL) and stirred at room temperature for 3.5 h. The resulting mixture
was filtered and washed with methylene chloride (20 mL). The filtrate was
washed with a saturated aqueous solution of NaHCO (5 mL) and dried
3
overanhyd ro us MgSO 4 and filtered. Evaporation of the filtrate afforded
dibenzyl sulfoxide as a colorless solid, which was purified by recrystalli-
zation from EtOH to afford the pure product, yield, 0.39 g (85%),
ꢀ
[24]
ꢀ
m.p. 135–137 C (lit. 133–135 C).
Oxidation of Dioctyl Sulfide in n-Hexane; Typical Procedure
Dioctyl sulfide (0.52 g, 2 mmol) was added to a mixture of NaBrO3
0.32 g, 2 mmol) and Mg(HSO ) (0.08 g, 0.40 mmol) in n-hexane (20 mL)
4 2
(

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942
Shaabani et al.
and stirred at room temperature for 2.5 h. The resulting mixture was
filtered and washed with methylene chloride (20 mL). The combined
organic layers were washed with a saturated aqueous solution of
NaHCO3 (5 mL) and dried over anhydrous MgSO4 and filtered.
Evaporation of the filtrate afforded dioctyl sulfoxide as a colorless
solid, which was purified by recrystallization from EtOH to afford the
ꢀ
[24]
ꢀ
pure product, yield, 0.49 g (90%), m.p. 75–76 C (lit. 71–72 C).
Oxidation of 4-Bromothioanisol in Solid State;
Typical Procedure
4-Bromothioanisol (0.41 g, 2 mmol) was added to a mixture of
NaBrO (0.32 g, 2 mmol) and Mg(HSO ) (0.12 g, 0.60 mmol) and stirred
3
4 2
ꢀ
magnetically at 60 C for 0.5 h. The product was separated by washing
with methylene chloride (15 mL). This filtrate was washed with a
saturated aqueous solution of NaHCO (5 mL) and dried over anhydrous
3
MgSO and filtered. Evaporation of the filtrate afforded the correspond-
4
ing sulfoxide as a pale yellow solid compound, which was purified by
recrystallization from EtOH to afford the pure product, yield, 0.39 g
[24]
ꢀ ꢀ
(
90%), m.p. 75–77 C (lit. 74–76 C).
ACKNOWLEDGMENT
Financial support by Research Council of Shahid Beheshti
University is gratefully acknowledged.
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Received in the UK November19, 2002