Rhenium-catalyzed deoxygenation of epoxides without adding any reducing agent

Article abstract of DOI:10.1016/j.tetlet.2011.10.085

This work reports a novel method for the deoxygenation of epoxides catalyzed by oxo-rhenium (V) and (VII) complexes without adding any reducing agent. This eco-friendly methodology was successfully applied to the deoxygenation of several epoxides with tolerance of different functional groups and high reusability of the catalysts ReIO₂(PPh₃) ₂ and ReOCl₃(PPh₃)₂.

Full text of DOI:10.1016/j.tetlet.2011.10.085

Tetrahedron Letters 52 (2011) 6960–6962
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Tetrahedron Letters
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Rhenium-catalyzed deoxygenation of epoxides without adding any
reducing agent
⇑
Sara C.A. Sousa, Ana C. Fernandes
Centro de Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, 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:
This work reports a novel method for the deoxygenation of epoxides catalyzed by oxo-rhenium (V) and
(VII) complexes without adding any reducing agent. This eco-friendly methodology was successfully
applied to the deoxygenation of several epoxides with tolerance of different functional groups and high
reusability of the catalysts ReIO2(PPh3)2 and ReOCl3(PPh3)2.
Received 30 June 2011
Revised 14 October 2011
Accepted 17 October 2011
Available online 20 October 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Epoxides
Deoxygenation
Oxo-rhenium complexes
Alkenes
Eco-friendly methodology
The deoxygenation of epoxides to alkenes is an important syn-
thetic transformation in both organic synthesis and biological
chemistry. The combination of epoxidation/deoxygenation would
provide a useful protection/deprotection sequence for the multiple
bond. However, the successful implementation of this strategy
would largely depend on the effective deoxygenation of epoxides
back to alkenes. Several methods have been developed to accom-
plish the deoxygenation of epoxides using several reagent systems
such as Cp2TiCl2/In,1 In/InCl,2 NbCl5/Zn,3 ZrCl4/NaI,4 CO/Au nano-
particules,5,6 alcohols/Au, or Ag nanoparticules.7
In continuation of our studies on the use of high-valent oxo-
complexes as excellent catalysts for organic reductions, in this
work we report an easy and eco-friendly methodology for the
deoxygenation of epoxides by oxo-rhenium (V) and (VII) com-
plexes without adding any reducing agent or usual oxygen sink.
The deoxygenation of 4-chlorostyrene oxide was chosen as the
model reaction for studying the influence of catalysts, and critical
reaction parameters such as solvent and temperature (Tables 1
and 2).
In order to compare the catalytic activity of oxo-rhenium (V)
and (VII) complexes, the deoxygenation of 4-chlorostyrene oxide
was carried out in the presence of the catalysts ReIO2(PPh3)2,
ReOCl3(PPh3)2, ReOCl3(dppm), Re2O7, and CH3ReO3 (MTO) in
refluxing toluene under air atmosphere (Table 1).
The high importance of the deoxygenation of epoxides justifies
the search for novel, efficient, cheaper, chemoselective, and eco-
friendly methodologies to carry out this reaction.
High-valent oxo-complexes, in particular, oxo-molybdenum8
and oxo-rhenium9 complexes are known for their abilities to cata-
lyze oxygen transfer reactions. More recently, these oxo-complexes
have also been successfully applied in the hydrosilylation of alde-
hydes and ketones,10,11 in C–C,12 C–S,12 C–N,13 and C–P14,15 bond
formation and also in several organic reactions such as the reduc-
tion of imines,16 amides,17 aromatic nitro compounds,18 esters,19
sulfoxides,20–25 pyridine N-oxides,20 and alkenes.26
Table 1
Deoxygenation of 4-chlorostyrene oxide by oxo-rhenium (V) and (VII) complexesa
Entry
Catalyst
Catalyst (%)
Time
Yieldb (%)
The methods reported in the literature for deoxygenation of
epoxides using oxo-rhenium complexes as catalysts involve the
addition of an equivalent amount of phosphorous compounds,27–31
dihydrogen (H2, 80–300 psi) under high temperature (150 °C),32 or
sulfite33 as the reductant.
1
2
3
4
5
6
ReIO2(PPh3)2
ReIO2(PPh3)2
ReOCl3(PPh3)2
Re2O7
ReOCl3(dppm)
MTO
10
5
10
10
10
10
35 min
19 h
30 min
30 min
1 h 30 min
24 h
78
50
67
62
59
49
a
⇑
Corresponding author.
E-mail address: anacristinafernandes@ist.utl.pt (A.C. Fernandes).
All reactions were carried out with 1.0 mmol of epoxide.
Isolated yield.
b
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.085

S. C. A. Sousa, A. C. Fernandes / Tetrahedron Letters 52 (2011) 6960–6962
6961
Table 2
chlorostyrene oxide in moderate yields (Table 1, entries 3–5). Fi-
nally, the least reactive catalyst is MTO, producing the 4-chlorosty-
rene in only 49% yield after 24 h (Table 1, entry 6).
Deoxygenation of 4-chlorostyrene oxide by ReIO2(PPh3)2 in different solventsa
The influence of the solvent on the deoxygenation of epoxides
was also explored (Table 2). Toluene was the best solvent at reflux
temperature, affording the 4-chlorostyrene in 78% yield after only
35 min (Table 2, entry 1). At room temperature, the alkene was
obtained in only 12% yield after 24 h (Table 2, entry 2). THF also
gave good yield of 4-chlorostyrene, but the reaction required 2 h
(Table 2, entry 3). In chloroform, benzene, and dichloromethane,
the product was obtained in moderate to low yields (Table 2,
entries 4–6). Finally, the deoxygenation of 4-chlorostyrene oxide
in acetonitrile gave a mixture of 4-chlorostyrene (20%) and 4-chlo-
robenzaldehyde (22%) (Table 2, entry 7).
To evaluate the general applicability of this novel methodology,
the deoxygenation of several epoxides using 10 mol % of
ReIO2(PPh3)2, ReOCl3(PPh3)2, and Re2O7 in refluxing toluene was
explored (Table 3).34 Generally, moderate to good yields of alkenes
were obtained. Both terminal and internal epoxides were deoxy-
genated by this novel method.
Entry
Solvent
Temperature
Time
Yield (%)
1
2
3
4
5
6
7
Toluene
Toluene
THF
Reflux
rt
35 min
24 h
2 h
24 h
7 h
78b
12c
Reflux
Reflux
Reflux
Reflux
Reflux
76c
CHCl3
56c
Benzene
CH2Cl2
CH3CN
35c
24 h
24 h
22c
20c (22)c,d
a
b
c
All reactions were carried out with 0.5 mmol of epoxide.
Isolated yield.
Yields determined by 1H NMR using 4-(methylthio)benzaldehyde (0.5 mmol) as
an internal standard.
d
Yield of 4-chlorobenzaldehyde.
We observed that the deoxygenation of the cis-stilbene oxide
produced only cis-stilbene and the trans-stilbene oxide afforded
only the trans-stilbene (Table 3, entries 10–13). However, the
deoxygenation of the commercial mixture of cis and trans ethyl
3-phenyl-2-oxiranecarboxylate, only gave the isomer ethyl
(2E)-3-phenyl-2-propenoate (Table 3, entries 4–6).
The best result was obtained using as the catalyst ReIO2(PPh3)2
(10 mol %), affording the 4-chlorostyrene in 78% yield after 35 min
(Table 1, entry 1). Using only 5 mol % of this catalyst, the reaction
required more time (19 h) and the alkene was obtained in moder-
ate yield (Table 1, entry 2). The oxo-complexes ReOCl3(PPh3)2,
Re2O7, and ReOCl3(dppm) also catalyzed the deoxygenation of 4-
Table 3
Deoxygenation of epoxides by oxo-rhenium (V) and (VII) complexesa
Entry
Substrate
Product
Catalyst
Time
Yieldb
1
2
ReIO2(PPh3)2
ReOCl3(PPh3)2
35 min
30 min
78
67
3
Re2O7
30 min
62
4
5
ReIO2(PPh3)2
ReOCl3(PPh3)2
2 h 15 min
1 h
73
79
6
Re2O7
3 h
64
7
8
ReIO2(PPh3)2
ReOCl3(PPh3)2
35 min
35 min
70
65
9
Re2O7
2 h
45
10
11
ReIO2(PPh3)2
ReOCl3(PPh3)2
15 min
40 min
68
54
12
13
ReIO2(PPh3)2
ReOCl3(PPh3)2
25 min
20 min
63
57
14
15
ReIO2(PPh3)2
ReOCl3(PPh3)2
30 min
1 h
67
48
16
17
ReIO2(PPh3)2
ReOCl3(PPh3)2
35 min
1 h 30 min
58c
40c
18
19
ReIO2(PPh3)2
ReOCl3(PPh3)2
24 h
24 h
51c
30c
a
b
c
All reactions were carried out with 1.0 mmol of epoxide and 10.0 mol % of catalyst.
Isolated yield.
Yields determined by 1H NMR using 4-(methylthio)benzaldehyde (1.0 mmol) as an internal standard.

6962
S. C. A. Sousa, A. C. Fernandes / Tetrahedron Letters 52 (2011) 6960–6962
A variety of functional groups such as –Cl, –Br, –F, –NO2, and es-
Acknowledgments
ter remained unaffected during deoxygenation of the correspond-
ing epoxides (Table 3, entries 1–9, 14, 15, 18, and 19). The high
chemoselectivity of this methodology is also demonstrated in the
deoxygenation of 4-chlorostyrene oxide performed in the presence
of other functional compounds such as benzophenone, N-(4-form-
ylphenyl)acetamide, dibenzyl sulfoxi-de, bis-(4-chlorophenyl)sul-
fone, 4-(trifluoromethyl)benzonitrile, without affecting the
functional groups ketone, aldehyde, amide, sulfoxide, sulfone, or
cyano. However, these reductions required more time (1.5–2 h).
The deoxygenation of the substrates 4-chlorostyrene oxide,
ethyl 3-phenyl-2-oxiranecarboxylate, and 4-nitrostyrene oxide
catalyzed by Re2O7 (10 mol %) afforded the corresponding alkenes
in 45–72% yield (Table 3, entries 3, 6, and 9). Nevertheless, some
exceptions were observed in the deoxygenation of styrene oxide,
and cis- or trans-stilbene oxides with this catalyst, producing a
mixture of the alkene and benzaldehyde in low yields.
The reusability of oxo-rhenium complexes ReIO2(PPh3)2
(10 mol %) and ReOCl3(PPh3)2 (10 mol %) was evaluated using
4-chlorostyrene oxide as the test substrate, by the sequential addi-
tion of fresh substrate to the reaction mixture. The reactions were
followed by 1H NMR, and the results obtained demonstrate that
the complex ReIO2(PPh3)2 can be reused in 15 cycles and the
catalyst ReOCl3(PPh3)2 in 5 cycles with the same catalytic activity.
In contrast to the simplicity of this novel methodology, the
mechanism of the reaction is still unknown. However, when the
catalytic deoxygenation of styrene oxide was monitored by 1H
NMR, we observed the formation of 1-phenyl-1,2-ethanediol,
which disappears at the end of the reaction. The formation of the
diol should result from the hydrolytic ring opening promoted by
the Lewis acidic character of oxo-rhenium complexes. Then, we
investigated the deoxygenation of 1-phenyl-1,2-ethanediol with
ReIO2(PPh3)2, ReOCl3(PPh3)2, and Re2O7 under the same catalytic
conditions and we observed the deoxygenation of the diol with
the formation of styrene, in moderate yields. This result also sug-
gests that this method can be explored for the deoxygenation of
diols to the corresponding alkenes.
In comparison to the other procedures catalyzed by oxo-
rhenium complexes reported in the literature,27–33 this new meth-
od has fast reaction times; is also more economic, easy, and
eco-friendly, minimizing the use and generation of hazardous
substances. Furthermore, the reactions can be carried out under
air atmosphere with readily available laboratory equipment, in
contrast to catalytic hydrogenation,32 which demands handling
of hydrogen gas and high-pressure equipment.
In conclusion, we have developed a novel, practical, and
eco-friendly method for the deoxygenation of epoxides to the
corresponding alkenes catalyzed by oxo-rhenium (V) and (VII)
complexes with the following advantages: (1) moderate to good
catalytic activity; (2) absence of a reducing agent; (3) high che-
moselectivity; (4) use of commercial and easy-to-handle catalysts;
(5) applicability to a variety of epoxides; (6) high reusable capacity.
To the best of our knowledge, this is the first example of the
deoxygenation of epoxides catalyzed by oxo-rhenium complexes
without adding a reducing agent. All these features make this
procedure one of the easiest, practical and eco-friendly procedure
for the deoxygenation of epoxides reported in the literature.
Further studies to improve the yields of the deoxygenation
through catalyst modifications and mechanistic studies are now
in progress in our group.
This research was supported by FCT through projects PTDC/QUI/
71741/2006 and PTDC/QUI-QUI/110080/2009. S.C.A.S thanks FCT
for grant (SFRH/BD/63471/2009). Authors thank the project PEst-
OE/QUI/UI0100/2011, the Portuguese NMR Network (IST-UTL
Center) for providing access to the NMR facilities and the
Portuguese MS Network (IST Node) for the ESI measurements.
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