Mild and efficient deoxygenation of sulfoxides to sulfides with triflic anhydride/potassium iodide reagent system

Article abstract of DOI:10.1055/s-2008-1067190

It was found that the combination of triflic anhydride/potassium iodide was an effective promoter for the deoxygenation of sulfoxides and gave the corresponding sulfides in excellent yield in acetonitrile at room temperature. It is worth mentioning that this reagent system is chemoselective and tolerates various functional groups, such as alkene, ketone, ester, aldehyde, acid, and oxime. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1067190

PAPER
2543
Mild and Efficient Deoxygenation of Sulfoxides to Sulfides with Triflic
Anhydride/Potassium Iodide Reagent System
Deoxygenation of
i
S
ulfox
u
S
ulfid
m
es ars Bahrami,*a,b Mohammad Mehdi Khodaei,*a,b Ahmad Karimia
a
Department of Chemistry, Razi University, Kermanshah 67149, Iran
Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah 67149, Iran
Fax +98(831)4274559; E-mail: kbahrami2@hotmail.com; E-mail: mmkhoda@razi.ac.ir
b
Received 15 April 2008; revised 10 May 2008
Table 1 Effects of Solvent on the Deoxygenation of Diphenyl Sulf-
oxide with Triflic Anhydride/Potassium Iodide
Abstract: It was found that the combination of triflic anhydride/po-
tassium iodide was an effective promoter for the deoxygenation of
sulfoxides and gave the corresponding sulfides in excellent yield in
acetonitrile at room temperature. It is worth mentioning that this re-
agent system is chemoselective and tolerates various functional
groups, such as alkene, ketone, ester, aldehyde, acid, and oxime.
O
Tf2O, KI
Ph
S
Ph
Ph
S
Ph
solvent, r.t.
Entry
Solvent
Time (min)
Isolated yielda (%)
Key words: deoxygenation, sulfoxide, sulfide, chemoselective,
triflic anhydride
1
2
3
4
MeCN
n-hexane
CHCl3
5
5
5
5
98
10
20
80
The deoxygenation of sulfoxides to the corresponding sul-
fides is an important reaction that has found considerable
utility in biochemical reactions.1 Although there are a
good number of methodologies available for the deoxy-
genation of sulfoxides to the corresponding sulfides,2–5
there remain the important problems of reaction condi-
tions, for example, the use of reagents that are difficult to
handle, functional group incompatibility, difficult workup
procedures, or harsh reaction conditions, such as high
temperatures or long reaction times. Therefore, a search
for readily available reagents and operationally simple
procedures for the deoxygenation of sulfoxides is still a
worthwhile goal.
MeOH
a Ratio sulfoxide/Tf2O/KI, 1:1:2.5.
Table 2 Effect of Increasing Amounts of Triflic Anhydride and Po-
tassium Iodide on the Deoxygenation of Diphenyl Sulfoxidea
Entry
Tf2O (mmol)
KI (mmol)
Isolated yield (%)
1
2
3
4
5
0.5
0.7
0.8
0.9
1
1
60
70
80
85
98
1.5
2
In continuation of our recent works on the deoxygenation
of sulfoxides6 and also the use of triflic anhydride in or-
ganic transformations,7 we describe the successful use of
the triflic anhydride/potassium iodide system as a method
to deoxygenate sulfoxides 1 to the corresponding sulfides
2 (Scheme 1).
2.5
2.5
a 1a (1 mmol), MeCN, r.t., 5 min.
The optimum solvent was studied by using diphenyl sulf-
oxide (1a) as the substrate (Table 1). Among the solvents
that were examined, acetonitrile was proved to be satis-
factory in terms of rate and yield of the reaction (Table 1,
entry 1).
The optimum molar ratio of sulfoxide to triflic anhydride
to potassium iodide (1:1:2.5) is found to be ideal for the
complete conversion of sulfoxide 1a into sulfide 2a in
acetonitrile at room temperature, while with lesser
amounts the reaction remains incomplete (Table 2).
O
Tf2O, KI
R1
S
R2
R1
S
R2
MeCN, r.t.
1
2
To extend the scope of the reaction and to generalize the
procedure, we investigated the deoxygenation of diaryl,
dibenzyl, aryl benzyl, dialkyl, and cyclic sulfoxides 1a–x
under the optimized reaction conditions. The results are
presented in Table 3.
R1, R2 = alkyl, aryl
Scheme 1
As shown, all the reactions were complete within a short
time and the sulfides 2a–x were obtained in almost quan-
titative yields as the sole deoxygenation products.
SYNTHESIS 2008, No. 16, pp 2543–2546
Advanced online publication: 24.07.2008
DOI: 10.1055/s-2008-1067190; Art ID: Z08808SS
x
x.
x
x
.
2
0
0
8
© Georg Thieme Verlag Stuttgart · New York

2544
K. Bahrami et al.
PAPER
Table 3 Deoxygenation of Sulfoxides to the Corresponding Sulfides Using Triflic Anhydride/Potassium Iodide
Entry
1
R1
R2
Time (min)
Producta
2a
2b
2c
Yieldb (%)
98
Ref. for data
Ph
Ph
5
3
4
3
3
3
4
3
2
2
4
3
3
3
3
5
3
3
2
5
6
5
5
5
8a
2
Bn
Bn
98
8b
8c
3
4-BrC6H4CH2
Ph
Bn
94
4
Bn
2d
2e
97
8b
8b
8d
8d
8b
8b
9a
5
Ph
4-MeC6H4CH2
4-FC6H4CH2
4-BrC6H4CH2
4-O2NC6H4CH2
Bn
97
6
Ph
2f
97
7
Ph
2g
2h
2i
96
8
Ph
93
9
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-ClC6H4
4-ClC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
naphth-1-yl
Ph
96
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
4-BrC6H4CH2
4-MeC6H4CH2
4-O2NC6H4CH2
Bn
2j
96
2k
2l
96
9b
9b
8d
9a
96
2m
2n
2o
2p
2q
2r
95
4-O2NC6H4CH2
Bn
95
96
8d
8d
9c
4-BrC6H4CH2
4-MeC6H4CH2
Bn
96
93
96
9d
10a
10b
10c
10d
10e
8b
Me
2s
92
Ph
CH2CO2Me
CH2CH=CH2
CH2CH=CH2
2t
91
Ph
2u
2v
2w
2x
93
CH2CH=CH2
2-C6H4C(O)-2-C6H4
Bu
95
90
Bu
92
a The products were characterized by comparison of their spectroscopic and physical data with those reported in the literature.8–10
b Yields refer to pure isolated products.
The sulfoxides possessing ester, alkene, and ketone func- A possible mechanism is shown in Scheme 3. Firstly, co-
tionalities were chemoselectively reduced to the corre- ordination of triflic anhydride to the oxygen proceeds to
sponding sulfides in excellent yields without affecting make the attack of iodide anion feasible. The resultant io-
these groups (Table 3 entries 20–23).
dinated species is in turn attacked by another iodide anion
to give the deoxygenated sulfide.
We also have monitored competitive deoxygenation of
sulfoxides in the presence of ester, aldehyde, acid, and In order to show the efficiency of this method, the results
oxime groups (Scheme 2).
of the deoxygenation of dibenzyl sulfoxide (1b) to diben-
zyl sulfide (2b) by our method are compared with those
reported by other methods. The results show that this
method is superior to some previously reported methods
in terms of yields, reaction times, and the amount of the
reagent used for successful deoxygenation (Table 4).
These observations clearly show that the method is appli-
cable for the selective deoxygenation of sulfoxides in the
presence of the previously mentioned functional groups
and can be considered as a useful practical achievement
for this transformation. Finally, sulfones failed to react
under the present reaction conditions.
In summary, we have found that the triflic anhydride/po-
tassium iodide system is an excellent and convenient
Synthesis 2008, No. 16, 2543–2546 © Thieme Stuttgart · New York

PAPER
Deoxygenation of Sulfoxides to Sulfides
2545
O
Table 4 Comparison of the Deoxygenation of Dibenzyl Sulfoxide
to Dibenzyl Sulfide by the Triflic Anhydride/Potassium Iodide Sys-
tem with the Literature
PhCO2Me
+
Ph
Ph
Ph
S
Ph
+
Ph
96%
S
Ph
PhCO2Me
100%
Reagent
Temp
r.t.
Time
3 min
2 h
Yield (%)
98
O
S
4-ClC6H4CHO
100%
Me
4-ClC6H4CHO
4-BrC6H4CO2H
Tf2O, KI, MeCN
+
Ph
S
Me
+
+
93%
NiCI2/NaBH4 (3:9), THF
PhSiH3, MoO2Cl2, toluene
2,6-dihydroxypyridine, MeCN
Ph3P, TiCl4 THF
0 °C
reflux
reflux
r.t.
8111
O
S
20 h
4 h
9512
+
Bn
Ph
S
Bn
4-BrC6H4CO2H
100%
9813
95%
O
9614
O
2 h
TiI4, MeCN
0 °C
r.t.
10 min 8515
20 min 9816
+
Ph
CH=NOH
+Ph CH=NOH
S
S
BF3·OEt2, NaI, MeCN
BBr3, CH2Cl2
O
–23 to 0 °C 40 min 9117
100%
95%
Scheme
2
Reagents and conditions: ratio substrate/Tf2O/KI
(1:1:1:2.5), MeCN, r.t.
Diphenyl Sulfide (2a)
Pale yellow liquid (Lit.8a pale yellow liquid).
1H NMR (200 MHz, CDCl3): d = 7.32–7.25 (m, 10 H).
13C NMR (50 MHz, CDCl3): d = 136.0, 131.1, 129.5, 127.0.
O
S
I
SO2CF3
R2
O:
O
S
O
KI
R1
R1
S
R2 + F3C
S
O
CF3
–
– CF3SO3
O
O
Benzyl 4-Chlorophenyl Sulfide (2m)
–
– CF3SO3
Pale pink solid; mp 52–53 °C (Lit.8d 51–52 °C).
1H NMR (200 MHz, CDCl3): d = 7.30–7.25 (m, 9 H), 4.1 (s, 2 H).
13C NMR (50 MHz, CDCl3): d = 137.3, 134.8, 132.4, 131.5, 129.0,
128.7, 128.6, 127.5, 39.3.
R1
R2
+
R1
S
R2
I2
S
+
I
I–
Dibutyl Sulfide (2x)
Scheme 3 Proposed mechanism for the deoxygenation of sulfox-
ides to the corresponding sulfides with triflic anhydride/potassium
iodide system
Colorless oil (Lit.8b colorless oil).
1H NMR (200 MHz, CDCl3): d = 3.60–0.41 (m, 18 H).
13C NMR (50 MHz, CDCl3): d = 46.1, 45.8, 29.7, 8.9.
deoxygenation reagent for sulfoxides. The present proce-
dure shows good chemoselectivity, and, under these con-
ditions, the reaction is rapid and equally applicable to
dialkyl, aryl alkyl, and diaryl sulfoxides. Thus, the use of
the triflic anhydride/potassium iodide system is a good ad-
dition to the existing methodologies for the deoxygen-
ation of sulfoxides.
Acknowledgment
We are thankful to the Razi University Research Council for partial
support to this work.
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Synthesis 2008, No. 16, 2543–2546 © Thieme Stuttgart · New York