18
M. Bagherzadeh et al. / Catalysis Communications 23 (2012) 14–19
Table 4
Recently reported catalytic systems for deoxygenation of sulfoxides.
Ent.
Substrate
Catalyst
Reductant
PPh
Time
Solvent
Temp.
Conv.
TON
TOF (h−1)
Ref.
1
2
3
4
5
6
7
8
9
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2a
2a
MoO(O
AuHAP
2
)(phox)
2
3
b1 min
1 h
12 h
5 min
30 min
2 h
5 min
5 h
6 h
DCE
RT
>99
>99
>99
92
55
97
>99
>99
50
30
>99
87
20
62.5
20
92
55
20
20
10
5
>1200
62.5
1.7
552
110
10
240
2
0.9
Present work
[12]
[13]
[14]
[15]
[16]
[18]
[18]
[18]
Ph
Ph
2
SiH
SiH
2
dioxane
toluene
THF
THF
THF
30 °C
100 °C
RT
RT
reflux
RT
reflux
reflux
reflux
120 °C
120 °C
Cu(acac)
Re
Mo
MoO
HReO
MoO
WO Cl
VO(acac)
MoO Cl
ReIO (PPh
2
2
2
2
O
7
HBcat
PhSiH
PhSiH
HBcat
HBcat
HBcat
PhSiH
2
(OtBu)
6
3
3
2
Cl
2
4
THF
2
(acac)
2
toluene
toluene
toluene
toluene
toluene
2
2
10
11
12
2
3
30 h
20 h
20 h
3
10
8.7
0.1
0.5
0.4
[18]
[19]
[19]
2
2
H
H
2
2
3
)
2
2
is one of the fastest catalytic system for reduction of sulfoxides up to
now (Table 4, entry 1). Among the previously reported catalysts, oxo-
rhenium complexes are characterized by remarkable catalytic activity
in deoxygenation of sulfoxides (Table 3, entries 4,7). However the
this protocol, only the S=O moiety of sulfoxides was deoxygenated
(Table 5, entry 1), whereas epoxide, aldehyde, ester and amide,
which are sensitive towards reduction, remained intact under the
present conditions (Table 5, entries 2–5). Surprisingly, although
high turnover frequency and low cost of PPh
3
compared to silanes, cate-
/PPh catalytic system
MoO(O
but was completely inactive for the deoxygenation of sulfone S=O
bonds (Table 5, entries 6–7). These results revealed that MoO(O
(phox) /PPh catalytic system shows excellent chemoselectivity
2 2
)(phox) was effective catalyst for the reduction of sulfoxide,
cholborane (HBcat) makes the MoO (O )(phox)
2
2
2
3
more attractive as a protocol for reduction of sulfoxides.
2
)
Moreover, several attempts have been accomplished to extend the
2
3
mentioned MoO(O
ation of other compounds (Table 5). The results showed that using
2
)(phox)
2
/PPh
3
catalytic system for the deoxygen-
for the catalytic reduction of sulfoxides. Such a considerable chemos-
electivity may be useful as a means for selective deoxygenation of sulf-
oxides in presence of various functional groups in complex biological
systems.
Table 5
a
Reduction of various compounds by MoO(O
2
)(phox)
2
/PPh
3
. .
4
. Conclusion
Ent.
1
Substrate
Time
immediately
Conversionb
>99
In summary, we have developed a novel, molybdenum based cat-
alytic systems for oxidation of sulfides and reduction of sulfoxides.
With MoO(O )(phox) /TBHP various sulfides were transformed to
2
2
corresponding sulfoxide in excellent yields and selectivity, under
mild conditions. Moreover, we have demonstrated that the present
MoO(O
2
)(phox)
2
complex is also highly efficient catalyst for the che-
at room temperature.
2
24 h
trace
moselective reduction of sulfoxides with PPh
3
Catalytic systems described in this paper have the following remark-
able advantages: 1) fast reaction times; 2) high conversion, selectivity
and chemoselectivity; 3) mild reactions conditions and 4) use of
cheap and readily available materials. We believe that these unique
properties make molybdenum(VI) oxo–peroxo complexes an ideal
platform to study various catalytic processes.
3
4
24 h
24 h
trace
trace
Acknowledgements
We acknowledge the Research Council of Sharif University of
Technology for the research funding of this project.
5
24 h
trace
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a
Reaction condition: substrate (0.4 mmol), PPh
3 2 2
(1.5 equiv.), MoO(O )(phox)
(
5 mol%), 1 mL DCE, room temperature.
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b
Determined by GC using decane as an internal standard.