Mild and efficient deoxygenation of sulfoxides to sulfides with Cp 2TiCl2/gallium system

Article abstract of DOI:10.1080/00397911.2010.516382

The Cp₂TiCl₂/Ga system was found to be a new reagent for reducing a variety of sulfoxides to the corresponding sulfides in good to excellent yields under mild conditions. Copyright

Full text of DOI:10.1080/00397911.2010.516382

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Mild and Efficient Deoxygenation of
Sulfoxides to Sulfides with Cp2TiCl2/
Gallium System
Byung Woo Yoo a & Sang Ki Min a
a Department of Advanced Materials Chemistry , Korea University ,
Jochiwon , Chungnam , Korea
Published online: 11 Jul 2011.
To cite this article: Byung Woo Yoo & Sang Ki Min (2011) Mild and Efficient Deoxygenation of
Sulfoxides to Sulfides with Cp2TiCl2/Gallium System, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 41:20, 2993-2996, DOI:
10.1080/00397911.2010.516382
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Synthetic Communications1, 41: 2993–2996, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.516382
MILD AND EFFICIENT DEOXYGENATION OF
SULFOXIDES TO SULFIDES WITH CP2TiCl2/GALLIUM
SYSTEM
Byung Woo Yoo and Sang Ki Min
Department of Advanced Materials Chemistry, Korea University,
Jochiwon, Chungnam, Korea
GRAPHICAL ABSTRACT
Abstract The Cp2TiCl2/Ga system was found to be a new reagent for reducing a variety
of sulfoxides to the corresponding sulfides in good to excellent yields under mild
conditions.
Keywords Deoxygenation; gallium; sulfide; sulfoxide; titanium
INTRODUCTION
The deoxygenation of sulfoxides to sulfides is a valuable transformation in the
application of organosulfur compounds in organic synthesis and biochemical reac-
tions. In particular, sulfoxides deserve much attention as important chiral auxiliary
in asymmetric synthesis.[1] Accordingly, a good number of methods have been
developed for the reduction of sulfoxides to the corresponding sulfides.[2] However,
they often suffer from serious disadvantages, such as functional group incompati-
bility, difficult workup procedures, harsh reaction conditions, or lack of chemoselec-
tivity. Some of these methods are associated with limitations of poor yields and
prolonged reaction times. Therefore, there still exists a need for improved methods
based on mild and operationally simple procedures for the reduction of sulfoxides.
Recently, gallium metal has drawn increasing attention for its unique properties such
as low toxicity, high stability in water, and low vapor pressure.[3] We envisaged that
the Cp2TiCl2=Ga system could be an efficient reducing agent for the reduction of
sulfoxides to sulfides. We have investigated the reactions of the Cp2TiCl2=Ga system
Received December 8, 2009.
Address correspondence to Byung Woo Yoo, Department of Advanced Materials Chemistry,
Korea University, Jochiwon, Chungnam 339 700, Korea. E-mail: bwyoo@korea.ac.kr
2993

2994
B. W. YOO AND S. K. MIN
with various sulfoxides and found that they can be rapidly reduced to the corre-
sponding sulfides in good yields [Eq. (1)]. Metal–
metal salt binary systems have long been used as reducing agents for many func-
tional groups.[4] In this communication, the use of the Cp2TiCl2=Ga system is
reported for the selective reduction of sulfoxides to sulfides in good yields under mild
conditions. Recently, we have reported that MoCl5=Ga could be used for the deox-
ygenation of various sulfoxides.[5] The new reducing system was generated by the
addition of gallium powder to a stirred solution of bis-(cyclopentadienyl) titaniu-
m(IV) dichloride in tetrahydrofuran (THF). In comparison with other procedures,
the Cp2TiCl2=Ga system reduces sulfoxides more rapidly (30 min) in greater yields
(85–95%) and showed good chemoselectivity. Some control experiments revealed
that sulfoxides could not be deoxygenated by Cp2TiCl2 or gallium alone under the
present condition, and starting materials were recovered unchanged. The optimum
molar ratio of sulfoxide to bis-(cyclopentadienyl)–titanium(IV) dichloride to gallium
of 1:2:4 is ideal for the complete conversion of sulfoxide into sulfide in THF at room
temperature in terms of yield and reaction time. The sulfoxides are prepared by oxi-
dation of the corresponding sulfides with sodium metaperiodate in aqueous meth-
anol.[6] 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 under the optimized reaction conditions. The sulfoxides
possessing bromo, chloro, methoxy, aldehyde, and alkene functionalities were
Table 1. Deoxygenation of sulfoxides with Cp2TiCl2=Ga system
Entry
R1
R2
Products
Time (min)
Yield (%)a
1
2
Ph
Ph
Ph
PhSPh
PhSCH3
20
30
30
30
25
30
30
20
25
20
25
30
30
93b
90b
88b
92b
89b
86b
85b
92b
94b
95b
87b
91b
93b
CH3
CH3
3
4-BrC6H4
4-ClC6H4
4-CH3OC6H4
4-CHOC6H4
Ph
4-BrC6H4SCH3
(4-ClC6H4)2S
(4-CH3OC6H4)2S
4-CHOC6H4SCH3
4
4-ClC6H4
4-CH3OC6H4
CH3
=
CH CH2
CH3
5
6
=
PhSCH CH2
7
8
4-CH3C6H4
4-CH3C6H4
Ph
4-CH3C6H4SCH3
(4-CH3C6H4)2S
PhSCH2CH3
(PhCH2)2S
9
4-CH3C5H4
CH2CH3
PhCH2
Ph
10
11
12
13
PhCH2
PhCH2
nC4H9
PhCH2SPh
(nC4H9)2S
11C4H9
aIsolated yields.
bThe products are commercially available and were characterized by comparison of their spectral data
with authentic samples.

DEOXYGENATION OF SULFOXIDES TO SULFIDES
2995
chemoselectively reduced to the corresponding sulfides in excellent yields without
affecting these groups (Table 1, entries 3–7). Thus, we have been able to demonstrate
the utility of the easily accessible Cp2TiCl2=Ga system as a useful reagent for effect-
ing 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, bis(cyclopentadienyl)–titanium(IV) dichloride is probably reduced by gallium
to form low-valent titanium species, which, in the subsequent step, would reductively
deoxygenate sulfoxides 1 to give the corresponding sulfides 2. The notable advan-
tages of this methodology are mild reaction conditions, fast reaction time, simple
operation, and tolerance of some functional groups. The utility of Cp2TiCl2=Ga
system as a new reducing agent is also demonstrated by the good 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.[7]
In conclusion, we believe that this procedure using the Cp2TiCl2=Ga system
will present a useful and efficient alternative to the existing methods for reduction
of sulfoxides to sulfides. Further investigation of the Cp2TiCl2=Ga system as reduc-
ing agent in organic synthesis is currently is progress.
GENERAL PROCEDURE FOR THE REACTION
A typical procedure for the deoxygenation of sulfoxides is as follows: Gallium
(230 mg, 2.0 mmol) and bis(cyclopentadienyl)–titanium(IV) dichloride (238 mg,
1.0 mmol). were mixed in anhydnous THF (4 mL). The resulting mixture was stirred
at room temperature for 1 h under sonication to get a solution of the low–valent
titanium-gallium complex. Sonication was carried out in a Bransonic ultrasonic clea-
ner bath, which delivered a 47-kHz wave with a fixed electrical power of 125 W.
Diphenylsulfoxide (101 mg, 0.5 mmol) was then added to this solution. The mixture
was stirred for 20 min at room temperature, and the progress of the reaction was fol-
lowed by thin-layer chromatography (TLC). After completion of the reaction, the
solvent was removed under reduced pressure. The residue was extracted with ether
and washed successively with brine. The organic layer was separated and dried over
anhydrous Na2SO4. The crude product was purified by silica-gel column chromato-
graphy (hexane–ethyl acetate ¼ 5:1) to afford diphenylsulfide (87 mg, 93%).
ACKNOWLEDGMENT
This work was financially supported by Korea University.
REFERENCES
1. (a) Carreno, M. C. Applications of sulfoxides to asymmetric synthesis of biologically active
compounds. Chem. Rev. 1995, 95, 1717; (b) Solladie, G. Asymmetric synthesis using
nucleophilic reagents containing a chiral sulfoxide group. Synthesis 1981, 185.
2. (a) See, for a review, Madesclaire, M. Reduction of sulfoxides to thioethers. Tetrahedron
1988, 44, 6537; (b) Schmizu, M.; Shibuya, K.; Hayakawa, R. Chemoselective deoxygena-
tion of sulfoxides with titanium tetraiodide. Synlett 2000, 1437; (c) Khurana, J. M.

2996
B. W. YOO AND S. K. MIN
Deoxygenation of sulfoxides and selenoxides with nickel boride. Tetrahedron Lett. 1998, 39,
3829; (d) Yadav, J. S.; Reddy, B. V. S.; Srinivas, C.; Srihari, P. Ultrasound-promoted deox-
ygenation of sulfoxides by samarium-NH4Cl. Synlett 2001, 854; (e) Balicki, R. Mild and
efficient deoxygenation of sulfoxides with titanium(IV) chloride=sodium iodide reagent sys-
tem. Synthesis 1991, 155; (f) Miller, S. J.; Collier, T. R.; Wu, W. Efficient reduction of sulf-
oxides with 2,6-dihydroxypyridine. Tetrahedron Lett. 2000, 41, 3781.
3. Vijay, N.; Sindu, R.; Jayan, C. N.; Bindu, S. P. Indium- and gallium-mediated carbon–
carbon bond-forming reactions in organic synthesis. Tetrahedron 2004, 60, 1959.
4. (a) Yoo, B. W.; Choi, J. W. Mild and efficient deoxygenation of amine-N-oxides with
BiCl3=indium system. Synth. Communi. 2009, 39, 3550; (b) Bezbarua, M. S.; Bez, G.;
Barua, N. C. A facile procedure for the conversion of nitroolefins into carbonyl compounds
using Al-NiCl2 ꢀ 6H2O-THF system. Chem. Lett. 1999, 325; (c) Wang, W. B.; Shi, L. L.;
Huang, Y. Z. A novel reduction system, SbCl3-Al= or SbCl3-Zn=DMF-H2O, for conver-
sion of aldehydes to alcohols. Tetrahedron Lett. 1990, 31, 1185; (d) Barua, M.; Boruah,
A.; Prajapati, D.; Sandhu, J. S. A new method for the chemoselective reduction of aryl
and aroyl azides. Tetrahedron Lett. 1996, 37, 4559; (e) Rama, R. B.; Gitali, D.; Yoofisaca,
S. N.; Anil, K. S. A facile and efficient method for the reduction of sulfoxides into sulfides
with an Al-NiCl2 ꢀ 6H2O system. Synlett 2005, 358.
5. Yoo, B. W.; Park, M. C.; Shin, J. I. Rapid and mild deoxygenation of sulfoxides with
MoCl5=gallium system under ultrasonication. Bull. Korean Chem. Soc. 2009, 30, 1927.
6. Vogel’s Textbook of Practical Organic Chemistry, 5th ed. Longman Group Limited:
London, 1978, p. 792.
7. (a) Alper, H.; Kenney, E. C. H. Deoxygenation of sulfoxides by iron pentacarbonyl.
Tetrahedron Lett. 1970, 11, 53; (b) Chasar, D. W. Quantitative reduction of sulfoxides.
J. Org. Chem. 1971, 36, 613.