Highly efficient rhenium-catalyzed deoxygenation of sulfoxides without adding any reducing agent

Article abstract of DOI:10.1016/j.tet.2012.07.071

This work reports a novel method for the deoxygenation of aromatic and aliphatic sulfoxides catalyzed by oxo-rhenium complexes without adding any reducing agent. The oxo-rhenium complex ReOCl₃(PPh₃) ₂ proved to be very efficient for the deoxygenation of several sulfoxides with tolerance of different functional groups.

Full text of DOI:10.1016/j.tet.2012.07.071

Tetrahedron 68 (2012) 8194e8197
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Highly efficient rhenium-catalyzed deoxygenation of sulfoxides without adding
any reducing agent
a
a
b
a,
*
~
Sara C. A. Sousa , Joana R. Bernardo , Carlos C. Romao , Ana C. Fernandes
a
ꢀ
ꢀ
Centro de Química Estrutural, Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Instituto de Tecnologia Química e Biologica da Universidade Nova de Lisboa, Av. da Republica, EAN, 2780-157 Oeiras, Portugal
b
ꢀ
ꢀ
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 aromatic and aliphatic sulfoxides catalyzed
by oxo-rhenium complexes without adding any reducing agent. The oxo-rhenium complex
ReOCl3(PPh3)2 proved to be very efficient for the deoxygenation of several sulfoxides with tolerance of
different functional groups.
Received 28 March 2012
Received in revised form 17 July 2012
Accepted 19 July 2012
Available online 23 July 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Deoxygenation
Oxo-rhenium complexes
Sulfoxides
Sulfides
1. Introduction
the oxo-rhenium complexes ReOCl3(PPh3)2, ReIO2(PPh3)2,
ReOCl3(dppm), Re2O7, CH3ReO3 (MTO), and HReO4 in reflux of
The reduction (deoxygenation) of sulfoxides to sulfides is a fun-
damental reaction in both chemistry and biology. Over the years,
several methods have been developed to reduce sulfoxides.1e28
Among the variety of catalysts used in this reduction, high-valent
oxo-molybdenum7,11,19,20,23 and oxo-rhenium1,6e8,11,26e28 com-
plexes have attracted considerable interest. These catalytic methods
involve the addition of at least 1 equiv of a phosphorus com-
pound,1,23,26e28 silane,7,8,20 borane,6,7,19 or hydrogen11 as reductant.
Very recently, we have also demonstrated that high-valent oxo-
rhenium(V) and (VII) complexes, in particular ReIO2(PPh3)2,
ReOCl3(PPh3)2, and Re2O7 catalyze the deoxygenation of epoxides to
the corresponding alkenes without adding any reducing agent.29 This
novel methodology proved to be efficient, affording the corresponding
alkenes in moderate to good yields with high reusability of the catalysts
ReIO2(PPh3)2 and ReOCl3(PPh3)2. In comparison to the other pro-
cedures for the deoxygenation of epoxides catalyzed by oxo-rhenium
complexes,30e36 this method is more economic, easy and eco-
friendly, minimizing the use and generation of hazardous substances.
chloroform under air atmosphere (Table 1). The progress of the
reductions was monitored by thin layer chromatography and by 1H
NMR. The best results were obtained with 10 mol % of
ReOCl3(PPh3)2, affording the sulfide in 87% yield along with 4% yield
of the corresponding sulfone, after 17 h at reflux temperature
(Table 1, entry 1). At room temperature, this reduction gave
a moderate yield of sulfide (Table 1, entry 2). Using only 5 mol % of
ReOCl3(PPh3)2 at refluxing CHCl3, the sulfide was obtained in 56%
Table 1
Deoxygenation of 4-chlorophenyl sulfoxide by oxo-rhenium(V) and (VII) complexesa
Entry Catalyst
Catalyst (mol %) Temperature Time (h) Yieldb (%) (A:B:C)
1
2
3
4
5
6
7
8
ReOCl3(PPh3)2 10
ReOCl3(PPh3)2 10
ReOCl3(PPh3)2
ReOCl3(dppm) 10
ReIO2(PPh3)2 10
Reflux
rt
17
24
24
24
24
24
21
24
(0:87:4)
(38:58:4)
(38:56:5)
(46:49:5)
(72:28:0)
(77:23:0)
(92:4:0)
5
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
2. Results and discussion
Re2O7
HReO4
MTO
10
10
10
Initially, we investigated the reduction of the test substrate 4-
chlorophenyl sulfoxide in the presence of a catalytic amount of
No reaction
a
All reactions were carried out with 1.0 mmol of sulfoxide.
Isolated yield.
* Corresponding author. Tel.: þ351 218419264; fax: þ351 218464457; e-mail
b
address: anacristinafernandes@ist.utl.pt (A.C. Fernandes).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.07.071

S.C.A. Sousa et al. / Tetrahedron 68 (2012) 8194e8197
8195
yield (Table 1, entry 3). The oxo-rhenium complexes ReOCl3(dppm),
ReIO2(PPh3)2, and Re2O7 only afforded moderate to low yields of
sulfide (Table 1, entries 4e6), and MTO and HReO4 proved to be
inefficient catalysts for this reduction (Table 1, entries 7 and 8).
The reduction of the test substrate 4-chlorophenyl sulfoxide
catalyzed by ReOCl3(PPh3)2 (10 mol %) was also studied in different
solvents. Chloroform was the best solvent at reflux temperature,
giving the corresponding sulfide in 87% yield and the sulfone in 4%
yield (Table 2, entry 1). At room temperature, the sulfide was iso-
lated in 58% yield, the sulfone in 4% yield, and the sulfoxide was
recovered in 38% yield (Table 2, entry 2). In benzene and toluene
the sulfide was obtained in 62e60% yields and the sulfone in
32e29% yields (Table 2, entries 3 and 4). Moderate yields of sulfide
(63e36%) were also afforded in THF, CH2Cl2, MeOH and CH3CN
(Table 2, entries 5e8). In contrast, in EtOH the reduction gave the
sulfone as the major product (65%) and the sulfide as the minor
product (33%) (Table 2, entry 9).
functional groups using 10 mol % of ReOCl3(PPh3)2 in reflux of
chloroform under air atmosphere. As shown in Table 3, the re-
actions were complete, affording the sulfides in excellent yields.
This method is compatible with the functional groups eNO2, eCl,
eCOCH3, eCO2R, and furfuryl ring (Table 3, entries 1e3, 8, 9,
and 10).
The efficiency of this novel methodology is also demon-
strated by the good yields obtained in the reduction of benzyl
phenyl sulfoxide and dibenzyl sulfoxide (Table 3, entries 6 and
7), since several methods fail completely with these substrates
or only provide poor yields. In these reductions was also ob-
served the formation of benzaldehyde in 3% and 5% yields,
respectively.
In comparison to other reported procedures for the reduction of
sulfoxides catalyzed by oxo-molybdenum or oxo-rhenium com-
plexes using phosphines,1,23,26,27 silanes,7,8,20 boranes6,7,19 or hy-
drogen11 as reducing agents, this new method is more economic
and eco-friendly, minimizing the use of hazardous substances. Of
course, the reaction requires the existence of a still unidentified
oxygen sink or sinks, since it is not a disproportionation of the
sulfoxide into the corresponding sulfide and sulfone. The search for
such species, which may be solvent dependent, is presently under
way in our laboratory.
Table 2
Deoxygenation of 4-chlorophenyl sulfoxide by ReOCl3(PPh3)2 in different solventsa
Entry
Solvent
Temperature
Time (h)
Yield (%)b
(A:B:C)
3. Conclusion
1
2
3
4
5
6
7
8
9
CHCl3
CHCl3
Benzene
Toluene
THF
CH2Cl2
MeOH
CH3CN
EtOH
Reflux
rt
17
24
17
24
24
24
24
24
18
(0:87:4)
In conclusion, we have developed a novel, efficient, and practical
method for the deoxygenation of aromatic and aliphatic sulfoxides
to the corresponding sulfides catalyzed by oxo-rhenium complexes
with the following advantages: (1) excellent catalytic activity; (2)
absence of an added reducing agent; (3) good chemoselectivity; (4)
applicability to a variety of sulfoxides; (5) use of commercial and
easy-handle catalysts.
(38:58:4)
(0:62:32)
(0:60:29)
(29:63:3)
(33:61:3)
(53:45:0)
(59:36:4)
(0:33:65)
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
a
These results extend the scope of the use of ReOCl3(PPh3)2 as an
effective catalyst for deoxygenation reactions without the need to
add any reducing agents. All these features make this procedure
one of the more practical, easier, and eco-friendlier methods for the
deoxygenation of sulfoxides yet reported in the literature.
All reactions were carried out with 1.0 mmol of sulfoxide.
Isolated yield.
b
To assess the scope and limitations of this novel methodology,
we carried out the reduction of several sulfoxides bearing different
Table 3
a
Deoxygenation of sulfoxides catalyzed by ReOCl3(PPh3)2
Entry
1
Substrate
Product A
Product B
Time (h)
17
Yield (%)b (A:B)
98 (98:0)
d
2
d
17
96 (96:0)
3
17
96 (91:5)
(continued on next page)

8196
S.C.A. Sousa et al. / Tetrahedron 68 (2012) 8194e8197
Table 3 (continued )
Entry
Substrate
Product A
Product B
Time (h)
17
Yield (%)b (A:B)
95 (92:3)
4
5
6
17
17
95 (83:12)
95 (92:0)c
d
7
8
17
17
95 (70:25)d
91 (87:4)
9
d
17
85 (85:0)
10
d
d
17
21
83 (83:0)
85 (85:0)e
11
a
All reactions were carried out with 1.0 mmol of sulfoxide.
Isolated yield.
3% of benzaldehyde was formed.
5% of benzaldehyde was formed.
Yield determined by 1H NMR.
b
c
d
e
Further studies to extend this methodology to the de-
oxygenation of other substrates as well mechanistic studies are
now under investigation in our group.
4.2. General procedure for the reduction of sulfoxides cata-
lyzed by ReOCl3(PPh3)2
To a solution of ReOCl3(PPh3)2 (10.0 mol %) in CHCl3 (3 mL) was
added the sulfoxide (1.0 mmol). The reaction mixture was heated at
reflux temperature under air atmosphere (the reaction times are
indicated in the Tables 1e3) and the progress of the reaction was
monitored by TLC or 1H NMR. Upon completion, the reaction
mixture was evaporated and purified by silica gel column chro-
matography with n-hexane to afford sulfides and sulfones, which
are all known compounds.
4. Experimental section
4.1. General information
All the reactions were carried out in air atmosphere and without
any dry solvent. Sulfoxides and catalysts were obtained from
commercial suppliers and were used without further purification.
ReIO2(PPh3)2 was prepared using the method reported by Toste.37
Flash chromatography was performed on MN Kieselgel 60M
230e400 mesh. 1H NMR and 13C NMR spectra were measure on
a Bruker Avance II 400 MHz and 300 MHz spectrometers. Chemical
shifts are reported in parts per million (ppm) downfield from an
internal standard. 1H NMR and 13C NMR data of the products are
consistent with those of the commercial products.
Acknowledgements
This research was supported by FCT through project PTDC/QUI-
QUI/110080/2009. S.C.A.S. (SFRH/BD/63471/2009) and J.R.B. thank
FCT for grants. Authors thank the project Pest-OE/QUI/UI0100/2011
and the Portuguese NMR Network (IST-UTL Center) for providing
access to the NMR facilities.

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