Selective and efficient deoxygenation of sulfoxides with NbCl 5/indium system

Article abstract of DOI:10.1080/00397911.2012.684756

It has been found that dialkyl, diaryl, and aryl alkyl sulfoxides can be selectively converted to the corresponding sulfides with a NbC_l5/In system in good to excellent yields under mild conditions. Several functional groups (bromo, chloro, methoxy, aldehyde, and vinyl) were tolerated under the reaction conditions. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2012.684756

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Selective and Efficient Deoxygenation of
Sulfoxides with NbCl /Indium System
5
a
a
a
Byung Woo Yoo , Hyung Min Kim & Duckil Kim
a
Department of Advanced Materials Chemistry, Korea University,
Jochiwon, Chungnam, South Korea
Accepted author version posted online: 10 Jan 2013.Published
online: 28 Apr 2013.
To cite this article: Byung Woo Yoo , Hyung Min Kim & Duckil Kim (2013): Selective and Efficient
Deoxygenation of Sulfoxides with NbCl /Indium System, Synthetic Communications: An International
5
Journal for Rapid Communication of Synthetic Organic Chemistry, 43:15, 2057-2061
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1
Synthetic Communications , 43: 2057–2061, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.684756
SELECTIVE AND EFFICIENT DEOXYGENATION OF
SULFOXIDES WITH NbCl /INDIUM SYSTEM
5
Byung Woo Yoo, Hyung Min Kim, and Duckil Kim
Department of Advanced Materials Chemistry, Korea University,
Jochiwon, Chungnam, South Korea
GRAPHICAL ABSTRACT
Abstract It has been found that dialkyl, diaryl, and aryl alkyl sulfoxides can be selectively
5
converted to the corresponding sulfides with a NbCl /In system in good to excellent yields
under mild conditions. Several functional groups (bromo, chloro, methoxy, aldehyde, and
vinyl) were tolerated under the reaction conditions.
Supplemental materials are available for this article. Go to the publisher’s online edition
1
of Synthetic Communications to view the free supplemental file.
Keywords Deoxygenation; indium; niobium; sulfide; sulfoxide
INTRODUCTION
The deoxygenation of sulfoxides to the corresponding sulfides is a valuable
transformation that has found utility in organic synthesis and biochemical
[
reactions. In Particular, sulfoxides deserve much attention as important chiral
1]
[2]
auxiliaries in asymmetric synthesis. Therefore, a number of methods have been
developed for the deoxgenation of sulfoxides. However, many of these methods
are limited by undesirable side reactions, functional group incompatibility, difficult
workup procedures, and harsh reaction conditions. Further, some of these methods
are associated with limitations of poor yields and prolonged reaction times. As
a consequence, the search for a new and efficient method for the deoxygenation of
sulfoxides is still a worthwhile goal. Much effort has been devoted to the develop-
ment of milder conditions where the presence of various sensitive and reducible func-
tional groups can be tolerated. The use of low-valent oxophilic d-block metals has
[
3]
become important in deoxygenation of various types of organic substrates. In this
regard, deoxygenations of sulfoxides are readily performed with low-valent tungsten,
Received February 22, 2012.
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
2
057

2058
B. W. YOO, H. M. KIM, AND D. KIM
molybdenum, and titanium generated from WCl , MoCl , and TiCl in the presence
6
5
4
[4]
of metal (Zn, In, Sm). In continuation of our efforts toward low-valent metal
reagents for organic transformations, we considered that a combination of
NbCl with indium could bring about the deoxygenation of sulfoxides under mild
[5]
5
conditions. Recently, indium metal has drawn increasing attention for its unique
properties such as low toxicity and high stability in water and air compared with
[6]
other metals. Herein, we report an efficient and chemoselective method for the
deoxygenation of sulfoxides 1 to the corresponding sulfides 2 with a NbCl =In
system at room temperature.
5
RESULTS AND DISCUSSION
We have investigated the reactions of NbCl =In system with various sulfoxides
5
and found that they can be selectively reduced to the corresponding sulfides in good
yields (Scheme 1). Metal–metal salt binary systems have long been used as reducing
[7]
agents for many functional groups. Recently we have reported that TiCl =In
4
[
or Cp TiCl =In could be used for the deoxygenation of various sulfoxides. In
8]
2
2
comparison with other procedures, the NbCl =In system reduces sulfoxides in better
5
yields and shows good chemoselectivity. Some control experiments revealed that
sulfoxides could not be deoxygenated by NbCl or indium alone under the present
5
condition and starting materials were recovered unchanged. The new reducing
system was readily generated by the addition of indium powder to a stirred solution
of niobium pentachloride in tetrahydrofuran (THF) under sonication. To assess the
scope and limitations of this reagent system, the reaction was studied with various
sulfoxides bearing other potentially labile functional groups. As shown in Table 1,
the methodology is equally applicable to dialkyl, diaryl, and aryl alkyl sulfoxides.
We have tried reduction of diphenylsulfone and methylphenylsulfone under the
present reaction conditions. However, the starting materials were recovered intact.
Electron-donating or electron-withdrawing groups on the aromatic ring did
not seem to affect the reaction significantly either in the yield of the product or
the rate of the reaction. The functional group tolerance of this method is evident
from entries 3–7, which show that bromo, chloro, methoxy, aldehyde, and vinyl
functionalities are unaffected under the reaction conditions. Thus we have been able
to demonstrate the utility of the easily accessible NbCl =In system as a useful reagent
5
for effecting chemoselective deoxygenation of sulfoxides. Although the reaction
mechanism is still unclear, it can be rationalized as the result of a two-step process.
In the first step, it is assumed that reduction of niobium(V) chloride with indium
provides low-valent niobium species, which are involved in complexation with the
substrates. The complex weakens the S-O bond and renders the sulfur atom more
Scheme 1. Conversion of sulfoxides to their corresponding sulfides.

DEOXYGENATION OF SULFOXIDES
2059
Table 1. Deoxygenation of various sulfoxides with NbCl
5
=indium
1
R
2
R
a,b
Entry
Products
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
CH
PhSPh
3.0
1.0
3.0
2.5
3.0
3.0
1.5
0.5
0.5
4.0
3.5
2.0
92
96
94
90
93
89
88
91
90
95
93
92
3
PhSCH
4-BrC SMe
(4-ClC
(4-CH OC
4-CHOC
PhSCH=CH
3
4-BrC
4-ClC
6
H
4
Me
4-ClC
4-CH OC
6 4
H
6
H
4
6
H
4
6 4 2
H ) S
4-CH OC
3
6
H
4
3
6
H
4
3
6 4 2
H ) S
4-CHOC
Ph
6
H
4
CH
3
6 4 3
H SCH
CH
4-CH
CH
2
=CH
2
4-CH
4-CH
3
C
C
6
6
H
H
4
3
C
6
H
4
(4-CH
4-CH
3
C
6
H
4
)
2
S
3
4
3
3
C
6
H
4 3
SCH
1
1
1
0
1
2
PhCH
PhCH
2
PhCH
Ph
2
(PhCH
PhCH SPh
(nC
2 2
) S
2
2
4
nC H
9
nC
4
H
9
4 9 2
H ) S
a
b
Refers to isolated yields.
The products were characterized by comparison of their spectroscopic and physical data with reported
values.
susceptible to the reduction. In the subsequent step, the reduction proceeds by
a reductive cleavage of the polarized S-O bond through a single electron transfer
[
from indium metal to the niobium-substrate complex. The in situ generation of
9]
deoxygenating reagent, NbCl (or NbCl ), from the NbCl =indium system would
3
4
5
present the possibility of regeneration of the reagent from the oxidized form
NbOCl . The resulting NbOCl can be recycled upon reduction with indium to
NbOCl. The substoichiometric use of NbCl supports the possibility of such reagent
3
3
5
regeneration. The reducing property exhibited by metal–metal salt combinations
proceeds through transfer of one electron from the metal surface to the substrate.
In such combinations, the elementary metal part needs to be more electropositive
than the metal part of the salt. The method offers advantages such as mild reaction
conditions, simple experimental operation, and tolerance of some functional groups.
The utility of NbCl =In system as a new reducing agent is also demonstrated by the
5
good yields of dibenzyl sulfide (entry 10) and phenyl benzyl sulfide (entry 11)
obtained after the reduction of the corresponding sulfoxides. Usually the sulfoxides
[
10]
that contain a benzyl group are difficult to reduce by other reagents.
EXPERIMENTAL
Melting points were determined by the open glass capillary method and are
uncorrected. All reagents were used as received. THF was distilled from sodium
1
benzophenone immediately prior to use. The H NMR spectra were recorded on
1
13
a FT-Bruker AF-300 (300 MHz for H NMR; 75 MHz for C NMR) using tetra-
methyl silane (TMS) as an internal standard. Thin-layer chromatographic (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).
A general procedure for the deoxygenation of sulfoxides is as follows: Indium
powder (230 mg, 2.0 mmol) and niobium pentachloride (135 mg, 0.5 mmol) were
mixed in freshly distilled THF (5 mL). The resulting mixture was sonicated for

2060
B. W. YOO, H. M. KIM, AND D. KIM
0
.5 h to produce a solution of the low-valent niobium–indium complex. Diphenyl-
sulfoxide (202 mg, 1.0 mmol) was then added to this solution, and the reaction
mixture was stirred for 3 h at room temperature. The progress of the reaction was
followed by TLC. On completion, the solvent was removed under reduced pressure,
and the residue was extracted successively with ethyl acetate and washed with water
and brine. The organic layer was separated and dried over anhydrous Na SO . The
2
4
crude product was purified by column chromatography on a silica gel (hexane–ethyl
acetate ¼ 2:1) to give a pale yellow liquid, which was identified as diphenylsulfide
[11a] 1
171 mg, 92%).
13
H NMR (300 MHz, CDCl ): d 7.38–7.25 (m, 10H). C NMR
(
(
3
þ
75 MHz, CDCl ): d 135.7, 131.1, 129.2, 127.1. GC=MSD (m=z): 186 (M ). All of
3
the products were characterized by comparison of their spectroscopic and physical
data with reported values. Please see the spectroscopic data of products, available
online.
CONCLUSION
In conclusion, we have discovered that NbCl =In system mediates an efficient
5
and selective conversion of sulfoxides to the corresponding sulfides with good yields
under mild conditions. We believe that this procedure will present a useful and
practical alternative to the conventional methods for the deoxygenation of sulfoxides
to sulfides. Further investigations of more useful applications are currently in
progress.
ACKNOWLEDGMENT
This work was financially supported by Korea University.
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