Reduction of sulfoxides catalyzed by oxo-complexes

Article abstract of DOI:10.1016/j.tetlet.2010.09.071

This work reports the catalytic activity of the oxo-complexes HReO ₄, MoO₂(acac)₂, WO₂Cl₂, and VO(acac)₂ in the reduction of sulfoxides with PhSiH₃ or HBcat. The results obtained showed that the catalyst systems PhSiH ₃/HReO₄ (5 mol %) and HBcat/HReO₄ (5 mol %) are highly efficient for the deoxygenation of sulfoxides. The complex MoO ₂(acac)₂ was also efficient, but the reactions required more time and heating. Finally, the complexes WO₂Cl₂ and VO(acac)₂ showed a moderate activity.

Full text of DOI:10.1016/j.tetlet.2010.09.071

Tetrahedron Letters 51 (2010) 6132–6135
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Tetrahedron Letters
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Reduction of sulfoxides catalyzed by oxo-complexes
⇑
Ivânia Cabrita, Sara C. A. Sousa, Ana C. Fernandes
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 July 2010
Revised 14 September 2010
Accepted 17 September 2010
Available online 24 September 2010
This work reports the catalytic activity of the oxo-complexes HReO4, MoO2(acac)2, WO2Cl2, and VO(acac)2
in the reduction of sulfoxides with PhSiH3 or HBcat. The results obtained showed that the catalyst sys-
tems PhSiH3/HReO4 (5 mol %) and HBcat/HReO4 (5 mol %) are highly efficient for the deoxygenation of
sulfoxides. The complex MoO2(acac)2 was also efficient, but the reactions required more time and heat-
ing. Finally, the complexes WO2Cl2 and VO(acac)2 showed a moderate activity.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Sulfoxides
Oxo-complexes
Silanes
Boranes
High valent oxo-molybdenum and -rhenium complexes were
known for their abilities to catalyze oxygen-transfer reactions to
sulfides, phosphines, and olefins.1,2 Recently, we have demon-
strated the reversal role of oxo-molybdenum and -rhenium com-
plexes by showing their abilities to catalyze the hydrosilylation
of aldehydes and ketones3,4 and organic reductions, such as the
reduction of imines,5 amides,6 aromatic nitro compounds,7 esters,8
sulfoxides,9–12 pyridine N-oxides,9 and alkenes.13
As the deoxygenation of sulfoxides to the corresponding sul-
fides is an important reaction that has found utility in organic syn-
thesis and in biochemical reactions, over the years, several
methods have been developed to reduce sulfoxides.14–36 However,
many of these are limited by side reactions, low yields, lack of che-
moselectivity or harsh conditions. For these reasons, the search for
alternative efficient and highly chemoselective methods for the
reduction of sulfoxides remains an important target in organic
synthesis.
Recently, we have demonstrated that high valent oxo-molybde-
num and -rhenium complexes are excellent catalysts for the deox-
ygenation of sulfoxides.9–12 In the literature there are some other
examples of the reduction of sulfoxides catalyzed by rhenium com-
plexes, but these methods are less efficient.37–42 In continuation of
our studies on the use of high valent oxo-complexes in the deoxy-
genation of sulfoxides, in this work we explored the catalytic activ-
ity of other oxo-rhenium, -molybdenum, -tungsten, and -vanadium
complexes in this reduction using silanes or boranes as reducing
agents.
The reaction of phenyl sulfoxide was chosen as the model reac-
tion for studying the influence of catalysts, silanes, boranes, and
critical parameters such as solvent and temperature (Tables 1–3).
The progress of the reactions was monitored by thin layer chroma-
tography and by 1H NMR.
In order to compare the catalytic activity of several oxo-com-
plexes, the reduction of phenyl sulfoxide was carried out with
PhSiH3 or catecholborane (HBcat) in the presence of the catalysts
43
HReO4, MoO2(acac)2, WO2Cl2, and VO(acac)2 as summarized in
Table 1.
Among the oxo-complexes tested, HReO4 (5 mol %) was the
most effective catalyst, reducing completely the phenyl sulfoxide
in 30 min at room temperature with PhSiH3 (120 mol %) (Table 1,
entry 1). Similar reduction using HBcat (200 mol %) instead of
PhSiH3 was much faster, and required only 5 min (Table 1, entry 2).
The dioxo-molybdenum complex MoO2(acac)2 (10 mol %) was
also very efficient in the reduction of phenyl sulfoxide with PhSiH3
(120 mol %) or HBcat (200 mol %) under nitrogen atmosphere.
However, these reactions required more time and heating (120 °C)
(Table 1, entries 3 and 4).
The catalytic activity of WO2Cl2 was also explored in the reduc-
tion of phenyl sulfoxide with PhSiH3 or HBcat. These reactions were
carried out with 10 mol % of catalyst in refluxing toluene under
nitrogen atmosphere. The reduction with PhSiH3 (120 mol %) affor-
ded the corresponding sulfide in 30% conversion after 22 h (Table 1,
entry 5). Similar reduction using HBcat gave the sulfide in 50%
conversion after 6 h (Table 1, entry 6).
Finally, the oxo-vanadium complex VO(acac)2 showed a moder-
ate activity in the reduction of phenyl sulfoxide using PhSiH3 or
HBcat in refluxing toluene (Table 1, entries 7 and 8).
In the absence of catalyst, no reduction was observed with
PhSiH3 (Table 1, entry 9) and only a small amount of sulfide was
⇑
Corresponding author. Tel.: +351 218419264; fax: +351 218464457.
E-mail address: anacristinafernandes@ist.utl.pt (A.C. Fernandes).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.071

I. Cabrita et al. / Tetrahedron Letters 51 (2010) 6132–6135
6133
Table 1
Reduction of phenyl sulfoxide with PhSiH3 or HBcat catalyzed by high valent oxo-complexesa
Entry
Catalyst
Reducing agent
Solvent /Temp (°C)
Time
Conversionb (%)
1
2
3
4
5
6
7
8
9
HReO4 (5 mol %)
HReO4 (5 mol %)
PhSiH3
HBcat
PhSiH3
HBcat
PhSiH3
HBcat
PhSiH3
HBcat
PhSiH3
HBcat
THF/rt
THF/rt
30 min
5 min
7 h
5 h
22 h
6 h
30 h
2 h 30 min
24 h
24 h
100
100
100
100
30
50
40
55
MoO2(acac)2 (10 mol %)
MoO2(acac)2 (10 mol %)
WO2Cl2 (10 mol %)
WO2Cl2 (10 mol %)
VO(acac)2 (10 mol %)
VO(acac)2 (10 mol %)
Without catalyst
Toluene/reflux
Toluene/reflux
Toluene/reflux
Toluene/reflux
Toluene/reflux
Toluene/reflux
THF/rt
No reaction
20
10
Without catalyst
THF/rt
a
The reactions were carried out with 1.0 mmol of sulfoxide, PhSiH3 (120 mol %) or HBcat (200 mol %).
Conversion was determined by 1H NMR.
b
Table 2
a
Reduction of phenyl sulfoxide with different silanes or boranes catalyzed by HReO4
Entry
Reducing agent
Silane (mol %)
Temperature
Time
Conversionb (%)
1
2
3
4
5
6
7
8
9
PhSiH3
PhSiH3
PMHS
PhMe2SiH
PhMe2SiH
Et3SiH
Et3SiH
Ph3SiH
120
100
0.30
100
200
100
200
200
—
rt
rt
rt
rt
Reflux
rt
Reflux
Reflux
rt
rt
rt
rt
30 min
40 min
7 h 30 min
48 h
3 h 30 min
48 h
2 h 30 min
24 h
24 h
100
100
100
50
100
60
100
49
No reaction
100
55
Without silane
HBcat
HBcat
10
11
12
200
100
200
5 min
6 h 30 min
5 min
HBpin
100
a
All reactions were carried out with 1.0 mmol of sulfoxide and 5 mol % of HReO4.
Conversion was determined by 1H NMR.
b
lane, the reaction required more time (40 min) (Table 2, entry 2).
PMHS also afforded the complete reduction of phenyl sulfoxide,
but the reaction needed 7 h 30 min (Table 2, entry 3). In contrast,
moderate conversions were obtained, after 48 h, using 1 equiv of
PhMe2SiH or Et3SiH at room temperature (Table 2, entries 4 and
6). Adding 2 equiv of these silanes at refluxing THF, the sulfide
was obtained in 100% conversion in few hours (Table 2, entries 5
and 7). Triphenylsilane was less efficient, affording the sulfide in
49% conversion, after 24 h in refluxing THF, using 2 equiv of this si-
lane (Table 2, entry 8). Finally, no reaction was observed in the ab-
sence of silane (Table 2, entry 9).
We have also studied the reduction of phenyl sulfoxide with
HBcat and pinacholborane (HBpin). The reaction with HBcat
(200 mol %) was very fast (5 min), giving the sulfide in 100% con-
version at room temperature (Table 2, entry 10). Adding only
100 mol % of HBcat, this reduction was not complete, afforded
the sulfide in 55% conversion after 6 h 30 m (Table 2, entry 11).
The reduction of phenyl sulfoxide with HBpin was also successfully
accomplished, giving the sulfide in 100% conversion after 5 min
(Table 2, entry 12).
Table 3
Reduction of phenyl sulfoxide in different solventsa
Entry
Solvent
Time
Conversionb (%)
1
2
3
4
5
6
THF
CHCl3
Benzene
Toluene
CH2Cl2
CH3CN
30 min
6 h 30 min
19 h 30 min
24 h
48 h
24 h
100
100
100
100
70
55
a
All reactions were carried out with 1.0 mmol of sulfoxide, 120 mol % of PhSiH3,
5 mol % of HReO4 at room temperature.
b
Conversion was determined by 1H NMR.
formed with HBcat, after 24 h at room temperature (Table 1, entry
10), demonstrating the catalytic role of the oxo-complexes.
The reduction of phenyl sulfoxide was also studied with several
silanes, namely, phenylsilane, dimethylphenylsilane, triethylsilane,
triphenylsilane, and polymethylhydrosiloxane (PMHS). Of these si-
lanes, phenylsilane (120 mol %) was the most efficient reducing
agent, giving the sulfide in 100% conversion after 30 min at room
temperature (Table 2, entry 1). Using only 100 mol % of phenylsi-
The influence of the solvent on the reduction of sulfoxides was
also explored with the system PhSiH3/HReO4 (5 mol %) (Table 3).
Among the solvents tested, THF proved to be the best at room
temperature, affording the sulfide in 100% conversion after
30 min (Table 3, entry 1). Chloroform, benzene, and toluene also

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I. Cabrita et al. / Tetrahedron Letters 51 (2010) 6132–6135
gave the sulfide in 100% conversion, but the reactions required
more time (Table 3, entries 2–4). In contrast, dichloromethane
and acetonitrile only afforded the sulfide in moderate conversions
(Table 3, entries 5 and 6).
(5–10 min), affording the sulfides in good to excellent yields. The
functional-group tolerance of this catalytic system was evident
from entries 4, 12, 14, 16, 18 of Table 4, which showed that -Cl, -
CO2R, -NO2, and double or triple bonds were unaffected under
these experimental conditions.
In conclusion, we have demonstrated that the catalyst HReO4 is
highly efficient for the reduction of a large variety of aryl, aryl alkyl,
and alkyl sulfoxides with PhSiH3 or HBcat at room temperature un-
der air atmosphere. Both the systems PhSiH3/HReO4 and HBcat/
HReO4 were also highly chemoselectives, tolerating several func-
tional groups such as -Cl, -NO2, -CO2R, and double or triple bonds.
Comparing the catalytic systems PhSiH3/HReO4 and HBcat/HReO4,
we concluded that the reactions with the second system were
much faster.
This novel methodology has other advantages such as high
yields, mild reaction conditions, fast reaction times, simple exper-
imental operation, use of the cheap commercially available and air
stable catalyst HReO4. We believe that this novel procedure repre-
sents a practical, useful, highly efficient and chemoselective alter-
native to the traditional methods for the deoxygenation of
sulfoxides.
To evaluate the general applicability of the catalytic system
PhSiH3/HReO4 (5 mol %), we carried out the reduction of several
sulfoxides bearing other potentially labile functional groups (Table
4, method A). These reactions were performed with 1.0 mmol of
substrate in THF at room temperature under air atmosphere.44
Generally, good to excellent yields of sulfides were obtained,
including sulfides bearing electron-withdrawing or electron-
donating groups. As shown in Table 4, this methodology is equally
applicable to diaryl, aryl alkyl, and dialkyl sulfoxides. The reduction
of sulfoxides containing several functional groups such as -Cl,
-CO2R, -NO2, and double or triple bond was successfully accom-
plished with the catalytic system PhSiH3/HReO4 (5 mol %) (Table
4, entries 3, 11, 13, 15, 17), showing the high chemoselectivity of
this novel methodology.
We have also explored the reduction of sulfoxides with the cat-
alytic system HBcat/HReO4 (5 mol %) at room temperature under
air atmosphere (Table 4, method B).44 The reactions were very fast
Table 4
a
Reduction of sulfoxides with PhSiH3 or HBcat catalyzed by HReO4
Entry
Substrate
Product
Method
Time
Yieldb (%)
A
B
30 min
5 min
95
85
1
2
A
B
1 h 30 min
5 min
86
89
3
4
A
B
1 h 30 min
5 min
94
90
5
6
A
B
1 h 30 min
5 min
92
90
7
8
A
B
2 h
91
89
9
10
10 min
A
B
2 h 45 min
5 min
98
98
11
12
A
B
3 h 10 min
10 min
85
98
13
14
A
B
2 h
92
89
15
16
5 min
A
B
50
55
66
17
18
5 min
A
B
55 min
5 min
100c
100c
19
20
a
b
c
All reactions were carried out with 1.0 mmol of sulfoxide, 5 mol % of HReO4, 120 mol % of PhSiH3 or 200 mol % of HBcat.
Isolated yields.
Conversion was determined by 1H NMR.

I. Cabrita et al. / Tetrahedron Letters 51 (2010) 6132–6135
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catalyzed by HReO4, to a solution of catalyst (5 mol %) and sulfoxide
(1.0 mmol) in THF (3 ml) was added PhSiH3 (1.2 mmol) or HBcat (2.0 mmol).
The reaction mixture was stirred at room temperature under air atmosphere
and the progress of the reaction was monitored by TLC and 1H NMR. Upon
completion, the reaction mixture was evaporated and purified by silica gel
column chromatography with the appropriate mixture of n-hexane and ethyl
acetate to afford the sulfides, which are all known compounds.
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