Deoxygenation of sulfoxides and reductive coupling of sulfonyl chlorides, sulfinates and thiosulfonates using Silphos [PCl3-n(SiO 2)n] as a heterogeneous phosphine reagent

Article abstract of DOI:10.1055/s-2005-868488

Deoxygenation of sulfoxides to thioethers and reductive coupling of sulfonyl chlorides, sodium sulfinates and thiosulfonates to their corresponding disulfides were carried out by a heterogeneous phosphine reagent, Silphos [PCl_3-n(SiO₂)_n] and molecular I₂ in dry refluxing MeCN in high yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-868488

LETTER
1447
Deoxygenation of Sulfoxides and Reductive Coupling of Sulfonyl Chlorides,
Sulfinates and Thiosulfonates Using Silphos [PCl (SiO ) ] as a Heterogeneous
3-n
2 n
Phosphine Reagent
D
N
eoxygenation of
a
S
ulfoxidessser Iranpoor,* Habib Firouzabadi,* Arezu Jamalian
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
Fax +98(711)2280926; E-mail: iranpoor@chem.susc.ac.ir; E-mail: firouzabadi@chem.susc.ac.ir
Received 3 January 2005
1
1
RR SO
+
I2 (cat.)
RR S
Abstract: Deoxygenation of sulfoxides to thioethers and reductive
coupling of sulfonyl chlorides, sodium sulfinates and thiosulfonates
to their corresponding disulfides were carried out by a heteroge-
neous phosphine reagent, Silphos [PCl3-n(SiO ) ] and molecular I
+
MeCN, reflux
Silphos
filterable
Silphos oxide
2 n
2
Scheme 1
in dry refluxing MeCN in high yields.
Key words: deoxygenation, thioethers, reductive coupling, sulfo-
nyl chlorides, disulfides, Silphos, heterogeneous phosphine
In order to optimize the reaction conditions, dibenzyl sulf-
oxide was chosen and converted to its corresponding thio-
ether by the Silphos/I system. A few preliminary tests
2
were carried out to optimize the reaction conditions. The
results are summarized in Table 1.
The reduction of sulfoxides to sulfides is an important
transformation in the application of organosulfur com-
pounds in organic synthesis, so it continues to attract con-
siderable attention and much effort has been devoted to
the development of milder procedures where various sen-
Table 1 Optimization of Different Conditions for the Transforma-
tion of Dibenzyl Sulfoxide (1.0 mmol) to Dibenzyl Sulfide in Reflux-
ing MeCN
1
sitive functional groups can be tolerated. Generally, the
oxo-transfer reaction from sulfoxides to phosphates and Entry Molar ratio of Mass of Silphos (g)/ Reaction
Conver-
sion (%)
substrate/I2
1.0 mmol of substrate time (min)
phosphites is expected to be a thermodynamically favor-
2
able process. Tervalent phosphorus compounds, either
1
2
3
4
5
6
7
1:0.0
1.0
0.6
0.7
0.8
0.9
1.0
1.0
120
60
60
60
60
50
5
15
40
with electron-withdrawing or electron-donating groups
3
1:0.2
are effective for the deoxygenation of sulfoxides. Phos-
phites are preferred to phosphines due to their greater re-
activity and readiness to be removed in the oxidized form4
although, the reactions are time-consuming and require
high temperatures. A number of phosphorus containing
1:0.2
45
1:0.2
50
1:0.2
70
5
reagents such as PPh /CCl , 1,3,2-benzodioxaphos-
1:0.2
100
100
3
4
6
7
8
9
pholes, P(OEt) /I /NaI, P(NMe ) /I /NaI, P(OSiMe ) ,
3
2
2 3
2
3 3
1:0.3
1
0
11
12
13
PCl3,
PI3,
P I ,
ClP(OEt)2,
P(OPh)3/
2 4
2
MoO Cl (dmf) , have been reported in order to avoid
2
2
2
these limitations. They are either expensive, produce an Solvents such as THF, petroleum ether, MeCN and
undesirable phosphine-oxide side product, which is diffi- CH Cl were used for this process and MeCN turned out
2
2
cult to remove from the reaction mixture, have limited to be the most suitable one. Quantitative conversion of
chemoselectivity or have low vapor pressure and unpleas- dibenzyl sulfoxide to dibenzyl sulfide was achieved in the
ant odours leading to difficulty in handling.
presence of 0.2 or 0.3 molar equivalents of iodine after 50
and 5 minutes, respectively. It is important to note that this
transformation does not go to completion with Silphos in
the absence of molecular iodine (Table 1, entry 1).
Very recently, we introduced Silphos [PCl (SiO ) ] as a
3
-n
2 n
new heterogeneous phosphine reagent for the conversion
of alcohols and thiols to alkyl halides. This reagent is eas-
ily prepared from the reaction of silica gel and PCl3.14 The results obtained from the reduction of structurally dif-
Now, we wish to report that Silphos in the presence of ferent sulfoxides to sulfides under our optimized condi-
iodine is an efficient heterogeneous system for the reduc- tions are shown in Table 2. The data summarized in
tion of sulfoxides to thioethers in dry refluxing MeCN Table 2 reveal that dialkyl, alkyl aryl, and diaryl sulfox-
(
Scheme 1).
ides are reduced to their corresponding sulfides in high to
excellent yields with short reaction time (15 min).
Along with this study, we also applied Silphos/I for the
2
SYNLETT 2005, No. 9, pp 1447–1449
Advanced online publication: 25.04.2005
DOI: 10.1055/s-2005-868488; Art ID: D00205ST
0
1.
0
6.
2
0
0
5
reductive coupling of sulfonyl chlorides to their corre-
sponding disulfides (Scheme 2). The results are shown in
Table 3.
©
Georg Thieme Verlag Stuttgart · New York

1
448
N. Iranpoor et al.
LETTER
Table 2 Reduction of Sulfoxides to Thioethers by Silphos/I Systema
2
Entry
Substrate
Yield (%)
96
Mp (°C) [lit.]
Bp (°C) [lit.]/(torr)
1
2
3
4
5
6
7
8
9
PhCH SOCH Ph
45 [44–47]15
–
2
2
PhCH SOPh
94
43 [42–44]15
–
2
PhSOCH3
91
–
84 [84–85]/1517
96 [97–98]/1216
97 [96–100]/(0.1)15
115 [116–117]/1416
222 [223–225]15
136 [138]15
88 [89]/1416
100 [101–102]/1516
–
PhCH SOCH
95
–
2
3
Ph SO
95
–
2
PhSO(n-C H )
93
–
4
9
PhSOCH CHCH
94
–
2
2
(CH CH=CH ) SO
89
–
2
2 2
PhSO(i-C H )
90
–
3
7
10
11
12
13
14
PhSO(n-C H )
92
–
3
7
(p-Cl-C H ) SO
96
93 [94]10
6
4 2
(n-C H ) SO
88
–
183 [185–185.5]18
35 [37.3]/76018
–
4
9 2
DMSO
100b
97
–
Dibenzothiophene 5-oxide
98 [99]20
a
The molar ratio of substrate/I applied for the oxo-transfer process was 1:0.3 using 1.0 g of Silphos in refluxing MeCN for 15 min.
Conversion yield by GC analysis.
2
b
Table 3 Reductive Coupling of 1.0 mmol of Sulfonyl Chlorides, Sodium Sulfinate or Thiosulfonate to the Corresponding Disulfides in the
a
Presence of Silphos/I System in Refluxing MeCN
2
Entry
Substrate
Reaction time (h) Yield (%)b
Mp (°C) [lit.]
Bp (°C) [lit.]/(torr)
1
2
3
4
5
6
7
8
9
PhSO Cl
10
10
12
8
96
57 [58–60]15
–
2
p-CH -C H SO Cl
92
44 [43–45]19
–
3
6
4
2
p-BrC H SO Cl
88
91 [93.5]19
–
6
4
2
2-NaphthylSO Cl
98
138 [139–140]18
–
2
CH SO Cl
12
12
5
85
–
107 [108-110]15
3
2
PhSO Na
95
58 [58–60]
–
–
–
–
–
2
PhSO SPh
94
57 [58–60]
2
p-CH C H SO OH
24
24
24
No reaction
No reaction
No reaction
–
–
–
3
6
4
3
p-CH C H SO Na
3
6
4
3
1
0
p-CH C H SO C H
3 6 4 3 2 5
a
Molar ratio of substrate/I (1:2) in the presence of 1.2 g of Silphos.
Isolated yield.
2
b
In addition to sulfonyl chlorides, a sodium sulfinate and
thiosulfonate ester was also reduced to the corresponding
disulfides using this reagent system. Applying this meth-
od for deoxygenation of sulfonic acid, sulfonate esters,
and sulfonic acid salts was not successful (Table 3).
RSO2Cl
+
Silphos
RRSS
+
filterable
Silphos oxide
I2
MeCN, reflux
In conclusion, the reported method is readily applicable
to the reduction of structurally different sulfoxides to
their thioethers. In addition, sulfonyl chlorides, sodium
Scheme 2
Synlett 2005, No. 9, 1447–1449 © Thieme Stuttgart · New York

LETTER
Deoxygenation of Sulfoxides
1449
sulfinates and thiosulfonate esters are also reducible to Acknowledgment
their corresponding disulfides using this reagent system.
The authors are grateful to the Shiraz University Research Council
and the organization for programming and the budget of Iran for the
support of this work.
Moreover, in comparison with the literature methods us-
ing phosphine reagents, Silphos is easily prepared, and in
an easy work up, the produced Silphos oxide can be
readily removed by a simple filtration.
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(
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2
2
reagent was determined by collecting the produced HCl in H O fol-
2
lowed by titration with 0.1 M aq NaOH. The results obtained from
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mol of chloride atom in its structure. To determine the amount of ac-
tive phosphorus content of the reagent, Silphos was reacted with ex-
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(
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(
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2
2
3
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–
1
3
200, 1100, 1000, 800, 680, 500 cm .
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(
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5
(
2
2
3
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1
(
3
tion of the solvent afforded pure dibenzyl sulfide (96%, 0.20 g), mp
(
4–47 °C, lit.15 mp 45 °C. H NMR (CDCl ): d = 7.37 (10 H, m),
1
4
3
1
3
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2
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(
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(
(
enough powdered Na S O was added with vigorous stirring to re-
2
2
3
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1
th
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Synlett 2005, No. 9, 1447–1449 © Thieme Stuttgart · New York