Mild and efficient deoxygenation of sulfoxides with MoCl5/indium system

Article abstract of DOI:10.1080/00397910701544729

It has been demonstrated that dialkyl, diaryl, and aryl alkyl sulfoxides can be efficiently converted into the corresponding sulfides by the reaction with a MoCl5/In system in good to excellent yields under mild conditions. Copyright Taylor & Francis Group, LLC.

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Mild and Efficient Deoxygenation of
Sulfoxides with MoCl5/Indium System
Byung Woo Yoo a , Min Suk Song a & Min Chol Park a
a Department of Advanced Materials Chemistry , Korea University ,
Jochiwon, Chungnam, South Korea
Published online: 10 Sep 2007.
To cite this article: Byung Woo Yoo , Min Suk Song & Min Chol Park (2007) Mild and Efficient Deoxygenation
of Sulfoxides with MoCl5/Indium System, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 37:18, 3089-3093, DOI: 10.1080/00397910701544729
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Synthetic Communicationsw, 37: 3089–3093, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701544729
Mild and Efficient Deoxygenation of
Sulfoxides with MoCl5/Indium System
Byung Woo Yoo, Min Suk Song, and Min Chol Park
Department of Advanced Materials Chemistry, Korea University,
Jochiwon, Chungnam, South Korea
Abstract: It has been demonstrated that dialkyl, diaryl, and aryl alkyl sulfoxides can be
efficiently converted into the corresponding sulfides by the reaction with a MoCl5/In
system in good to excellent yields under mild conditions.
Keywords: deoxygenation, indium, molybdenum, sulfoxides
The deoxygenation of sulfoxides to sulfides is a valuable transformation in the
application of organosulfur compounds in organic synthesis. In particular,
sulfoxides deserve much attention as important chiral auxiliary in asymmetric
synthesis.[1] Accordingly, a good number of methodologies have been
developed for the reduction of sulfoxides to the corresponding sulfides.[2]
However, they often suffer from serious disadvantages, such as functional
group incompatibility, difficult workup procedures, harsh reaction conditions,
or not readily available reagents. Further some of these methods are associated
with limitations regarding low yields and prolonged reaction times. Therefore,
there still exists a need for new improved methods based on easily accessible
reagents and operationally simple procedures for the reduction of sulfoxides.
The use of low-valent oxophilic d-block metals have become important in
deoxygenation of various types of organic substrates.[3] In this regard, deo-
xygenations of
Received in Japan November 22, 2006
Address correspondence to Byung Woo Yoo, Department of Advanced Materials
Chemistry, Korea University, Jochiwon, Chungnam 339-700, South Korea. E-mail:
bwyoo@korea.ac.kr
3089

3090
B. W. Yoo, M. S. Song, and M. C. Park
sulfoxides and oximes are readily performed with low-valent tungsten or mol-
ybdenum generated by reacting WCl6 or MoCl5 with either NaI, or Zn.[4]
Recently, indium metal has drawn increasing attention for its unique proper-
ties such as low toxicity and high stability in water and air compared with
other metals.[5] We envisaged that the MoCl5/In system can be an efficient
reducing agent for the reduction of sulfoxides to sulfides. To the best of my
knowledge, there are no literature examples for the deoxygenation of sulfox-
ides with the MoCl5/In system to produce sulfides. We have investigated the
reactions of the MoCl5/In system with various sulfoxides and found that they
can be rapidly reduced to the corresponding sulfides in high yields [Eq. (1)].
Metal–metal salt binary systems have long been used as reducing agents for
many functional groups.[6] In this communication, the use of the MoCl5/In
system is reported for the selective reduction of sulfoxides to sulfides
in high yields under mild conditions. Recently we have reported that TiCl4/In
or Cp2TiCl2/In could be used for the deoxygenation of various sulfoxides.[7]
The new reducing system was generated by the addition of indium powder to a
stirred solution of molybdenum pentachloride in tetrahydrofuran (THF) under
nitrogen. In comparison with other procedures, the MoCl5/In system reduced
sulfoxides more rapidly (5 min) in high yields (87–96%) and showed a good
chemoselectivity. Some control experiments revealed that sulfoxides could
not be deoxygenated by MoCl5 or indium alone under the present condition,
and starting materials were recovered unchanged. A 2:1 ratio of indium and
MoCl5 was the best ratio in terms of yield and reaction time. The sulfoxides
are prepared by oxidation of the corresponding sulfides with sodium metaper-
iodate in aqueous methanol.[8] All the compounds obtained showed IR, NMR,
and mass spectral data compatible with the structure. To assess the scope and
limitations of this reagent system, the reaction was studied with various sulf-
oxides bearing other potentially labile functional groups. As shown in Table 1,
the methodology is equally applicable to dialkyl, diaryl, and aryl alkyl sulfox-
ides. The functional group tolerance of this method is evident from entries 3–
6, which show that bromo, methoxy, aldehyde, and vinyl are unaffected under
the reaction conditions. Thus we have been able to demonstrate the utility of
the easily accessible MoCl5/In system as a useful reagent for effecting
chemoselective deoxygenation of sulfoxides. Although the reaction
mechanism is still unclear, it can be rationalized as the result of a two-stage
process. In the first step, molybdenum(V) chloride is probably reduced by
indium to form low-valent molybdenum species, which, in the subsequent
step, would reductively deoxygenate sulfoxides 1 to give the corresponding
sulfides 2. The notable advantages of this methodology are mild reaction con-
ditions, fast reaction time, simple operation, and tolerance of some functional
groups. The utility of the MoCl5/In system as a new reducing agent is also
demonstrated by the high yields of dibenzyl sulfide (entry 11) and phenyl
benzyl sulfide (entry 12) obtained after the reduction of the corresponding
sulfoxides. Usually the sulfoxides that contain a benzyl group are difficult
to reduce by other reagents.[9]

Deoxygenation of Sulfoxides
3091
Table 1. Reduction of sulfoxides with the MoCl5/In system
Time
(min)
Yield
(%)a,b
Entry
R1
R2
Products
PhSPh
PhSCH3
1
2
3
4
5
6
Ph
Ph
Ph
5
5
5
5
5
5
92[10a]
96[10a]
91[10a]
91[10a]
93[10b]
95[10b]
CH3
CH3
4-BrC6H4
4-ClC6H4
4-BrC6H4SCH3
(4-ClC6H4)2S
(4-CH3OC6H4)2S
4-CHOC6H4SC6-
H4CH3-4
4-ClC6H4
4-CH3OC6H4
4-CH3C6H4
4-CH3OC6H4
4-CHOC6H4
7
8
Ph
CH55CH2
CH3
PhSCH55CH2
4-CH3C6H4SCH3
(4-CH3C6H4)2S
PhSCH2CH3
(PhCH2)2S
5
5
5
5
5
5
5
90[10c]
89[10e]
89[10c]
91[10d]
90[10b]
87[10a]
88[10a]
4-CH3C6H4
4-CH3C6H4
Ph
9
4-CH3C6H4
CH2CH3
PhCH2
Ph
10
11
12
13
PhCH2
PhCH2
nC4H9
PhCH2SPh
(nC4H9)2S
nC4H9
aIsolated yields.
bSpectral data of products are consistent with reported values.
In conclusion, we believe that this procedure using the MoCl5/In system
presents a useful and efficient alternative to the existing methods for reduction
of sulfoxides to sulfides. Further investigation of the MoCl5/In system as a
reducing agent in organic synthesis are currently in progress.
EXPERIMENTAL
1
The H NMR spectra were recorded on a FT-Bruker AF-300 instrument
1
(300 MHz for H NMR; 75 MHz for 13C NMR) using TMS as an internal
standard. Infrared spectra were obtained on a Perkin-Elmer 16F PC FT-IR
Shimadzu. Thin-layer chromatography (TLC) analysis was performed on
silica-gel plates (Merck, 60 F-254). All products were purified by flash-
column chromatography using silica-gel 60 (79–230 mesh, Merck).
General Procedure for the Reaction
A typical procedure for the deoxygenation of sulfoxides is as follows: to a
solution of indium (230 mg, 2.0 mmol), diphenylsulfoxide (101 mg,
0.5 mmol) in anhyd. THF (1 mL) and molybdenum pentachloride (238 mg,
1.0 mmol) were added. The mixture was stirred at room temperature, and
the progress of the reaction was followed by TLC. After completion of the

3092
B. W. Yoo, M. S. Song, and M. C. Park
reaction (5 min), the reaction was quenched with aq. NaOH (10%) and
extracted with ether. The combined ethereal extracts were washed succes-
sively with brine (20 mL) and H2O (20 mL). The organic layer was
separated and dried over anhydrous Na2SO4. The crude product was
purified by silica-gel column chromatography (hexane–ethyl acetate ¼ 1:1)
to afford diphenylsulfide (86 mg, 92%). The structures of known
compounds were determined by comparison of their spectral data with those
previously reported.[10]
ACKNOWLEDGMENT
This work was financially supported by the Brain Korea 21 project in 2006.
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