Selective synthesis of sulfoxides and sulfones by tantalum(V) catalyzed oxidation of sulfides with 30% hydrogen peroxide

Article abstract of DOI:10.1016/j.tetlet.2008.12.064

The reaction of sulfides with 30% hydrogen peroxide catalyzed by tantalum(V) chloride in acetonitrile, i-propanol, or t-butanol selectively provided the corresponding sulfoxides in high yields. On the other hand, the reaction of sulfides with 30% hydrogen peroxide-catalyzed by tantalum(V) chloride or tantalum(V) ethoxide in methanol effectively gave the sulfones.

Full text of DOI:10.1016/j.tetlet.2008.12.064

Tetrahedron Letters 50 (2009) 1180–1183
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Selective synthesis of sulfoxides and sulfones by tantalum(V) catalyzed
oxidation of sulfides with 30% hydrogen peroxide
Masayuki Kirihara , Junya Yamamoto , Takuya Noguchi a,b, Yoshiro Hirai b
a,*
a
a
Department of Materials and Life Science, Shizuoka Institute of Science and Technology, 2200-2 Toyosawa, Fukuroi, Shizuoka 437-8555, Japan
Faculty of Science, Department of Chemistry, Toyama University, 3190 Gofuku, Toyama 930-8555, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of sulfides with 30% hydrogen peroxide catalyzed by tantalum(V) chloride in acetonitrile, i-
propanol, or t-butanol selectively provided the corresponding sulfoxides in high yields. On the other
hand, the reaction of sulfides with 30% hydrogen peroxide-catalyzed by tantalum(V) chloride or tanta-
lum(V) ethoxide in methanol effectively gave the sulfones.
Received 8 October 2008
Revised 4 December 2008
Accepted 8 December 2008
Available online 24 December 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Oxidation
Hydrogen peroxide
Tantalum(V) chloride
Sulfide
Sulfoxide
Sulfone
Sulfoxides and sulfones are useful synthetic intermediates for
the construction of several important organic molecules.1 More-
over, there are many biologically important compounds containing
a sulfoxide- or sulfone moiety. Sulfoxides and sulfones are gener-
ally prepared via oxidation of the corresponding sulfide; however,
it is often very difficult to stop the oxidation at the sulfoxide stage.
For the synthesis of sulfones from sulfides, other oxidizable func-
tional groups, such as alcohols or olefins, sometimes react as well
producing undesirable compounds.1
Mn), low chemoselectivity (Ti, Fe, V, W, and Re), and rare elements
in the earth (Re and Au).
Recently, we reported several new synthetic methods for
environmentally benign reactions using aqueous 30% hydrogen
5
peroxide. During the course of these studies, it was found that
tantalum(V) chloride and tantalum(V) ethoxide are excellent
catalysts for the oxidation of sulfides using aqueous 30% hydrogen
peroxide (Scheme 1).
Higher valent vanadium compounds [V(V) or V(IV)] have been
used as catalysts for hydrogen peroxide oxidation, utilizing various
A large number of methods have been developed to overcome
this drawback; however, most of these reactions require a stoichi-
6
synthetic methods. In contrast, although tantalum is a group 5
2
ometric amount of oxidant, resulting in undesirable waste. Aque-
element like vanadium, tantalum compounds have seldom been
used as oxidation catalysts despite being ideal for ‘green chemis-
ous 30% hydrogen peroxide has recently been utilized as an
attractive and environmentally benign oxidant3 for the oxidation
of sulfides, because it is inexpensive, easy to handle, safely stored,
and produces only water as a side-product. Since the oxidation of
sulfides with aqueous hydrogen peroxide in the absence of any
catalysts requires a long reaction time, several transition metal
7a,b
try’, because they are far less toxic than vanadium compounds.
Recently, we found that the treatment of 1,3-dithianes with 30%
hydrogen peroxide in the presence of a catalytic amount of
tantalum(V) chloride and sodium iodide provided the carbonyl
4
b
4c
4d
4e,f
4g,h
4i
4j
4k,l
4m
(
Ti, Mo, Fe, V, , W,
Re, Cu, Mn,
and Au ) com-
4a
O
pounds have been used as catalysts.
cat. TaCl , 30%H O
2 2
5
Although transition metal-catalyzed reactions produce the
products in high yields, there are still some problems such as the
use of acidic or base conditions (Mn), long reaction times (Fe and
Mo), using hazardous solvents and/or reagents (Mo, V, Cu, and
S
CH CN or iPrOH or tBuOH, RT
R1
O
R2
3
S
R1
R2
O
cat. TaCl or cat. Ta(OEt) , 30%H O
5
5
2
2
S
R1
R2
CH OH, 45ºC
3
*
Corresponding author. Tel.: +81 538 45 0166; fax: +81 538 45 0110.
E-mail address: kirihara@ms.sist.ac.jp (M. Kirihara).
Scheme 1.
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.12.064

M. Kirihara et al. / Tetrahedron Letters 50 (2009) 1180–1183
1181
cat. TaCl ,
cat. I ,
5
S
S
S
S
S
S
OH
-
I
1
2
I
I
R
R
R1
R2
R1
R2
H O
2
2
O
sulfoniun ion intermediate
S
S
H O
2
1
2
AcOEt-H O
R1
R2
R
R
2
H2O
+
I-
S
R
S
R
S
S
S
S
OH
S
S
O
1
2
HO
HO
R1
R2
R1
R2
sulfoxide intermediate
H O
2
Scheme 2.
compounds in high yield.5b This reaction is the result of oxidation
of the sulfur atom of the 1,3-dithianes via sulfonium ion interme-
diates and/or sulfoxide intermediates (Scheme 2), and therefore by
extension, it would be possible to oxidize sulfides under similar
reaction conditions.
Table 2
0
.02 eq. catalysts
30 % H (5.0 eq.)
CH CN, r.t.
O
S
O
O
S
O
2 2
S
Ph
Me
Ph
Me + Ph
Me
The oxidation of thioanisole (1a) with 30% hydrogen peroxide
using tantalum(V) chloride as a catalyst in acetonitrile in the pres-
ence or absence of sodium iodide was investigated first (Table 1).
Thioanisole was oxidized with hydrogen peroxide in the absence
of catalysts to produce the sulfoxide (2a); however, it took quite
some time to complete the oxidation (run 1). Tantalum(V) chloride
was an excellent oxidation catalyst, and was effective even in the
absence of sodium iodide (runs 2–4). The sulfoxide (2a) was effi-
ciently obtained in all cases. The sulfone (3a) was the minor prod-
uct (less than 10% yield) even for a long reaction time (24 h, run 4).
Urea hydrogen peroxide (UHP) was not a good oxidant for the
1a
3
2
a
3a
Run
Catalyst (equiv)
Time (h)
Yielda (%)
1
a
2a
3a
1
2
3
4
TaCl
Ta(OEt)
5
l.75
5.0
0
0
0
0
96
72
94
98
4
28
6
5
Ta
—
2
O
5
20.0
20.0
2
a
Determined by 1H NMR (methyl protons).
8
oxidation of sulfide catalyzed by tantalum(V) chloride (run 5).
ical less hindered) solvents (run 4: CH
0: EtOH) is greater than that in less polar (or sterical more
hindered) solvents (run 5: CH CH CN, run 11: iPrOH, and run 13:
tBuOH). The best conditions to prepare the sulfoxide (2a) selec-
tively are the reaction in CH CN at room temperature (run 4), in
3
CN, run 7: MeOH, and run
Interestingly, a stoichiometric amount of sodium iodide inhibited
the oxidation of thioanisole with hydrogen peroxide catalyzed by
tantalum(V) chloride (run 7).
The oxidation of 1a with 30% hydrogen peroxide was then
investigated using several tantalum(V) compounds as catalysts in
acetonitrile (Table 2). Tantalum(V) chloride and tantalum(V) eth-
oxide have catalytic activity for the oxidation of 1a with hydrogen
peroxide. However, tantalum(V) oxide was ineffective.
1
3
2
3
iPrOH at 45 °C (run 12), or in tBuOH at 45 °C (run 14). The ideal
condition to synthesize the sulfone (3a) is the reaction in MeOH
at 45 °C (run 9).
In order to further examine the synthesis of sulfoxides (2),
several sulfides (1) were treated with 30% hydrogen peroxide in
The oxidation of 1a with 30% hydrogen peroxide using tanta-
lum(V) chloride as a catalyst was examined in several solvents
(
Table 3). The less polar solvents constituting a heterogeneous sol-
vent system with aqueous hydrogen peroxide were not effective
runs 1–3). In the polar solvents, 1a was efficiently oxidized into
Table 3
(
the corresponding sulfoxide (2a) and/or sulfone (3b) (runs 4–14).
The oxidative ability of this reaction system in more polar (or ster-
TaCl (0.02 eq.)
O
S
5
O
O
S
30 % H O (5.0 eq.)
2 2
S
Ph
Me
Ph
Me + Ph
Me
solvent
Table 1
1a
2
a
3a
a
Catalysts
30 % H (5.0 eq.)
O
S
Run
Solvent
Time (h)
Temp.
Yield (%)
O
O
S
O
2 2
S
1a
2a
3a
Ph
Me
Ph
Me + Ph
Me
CH CN, r.t.
3
1
2
3
4
5
6
7
8
CH Cl2
5.5
e.5
5.5
3.0
3.0
7
0.5
8.5
1.5
0.5
0.5
0.3
0.5
1.5
rt
rt
rt
rt
rt
rt
rt
rt
45 °C
rt
rt
45 °C
rt
45 °C
28
50
17
0
11
0
0
0
0
7
23
2
49
48
25
5
1a
2
2
a
3a
Toluene
AcOEt
58
95
82
79
86
3
Run
TaCl
5
(equiv)
NaI (equiv)
Time (h)
Yielda (%)
CH
3
CH
3
CH
3
CN
CH
CH
2
CN
CN
7
1
a
2a
3a
2
21
14
97
99
6
1
2
3
4
—
—
—
—
—
—
0.10
1.00
20.0
0.5
1.75
24.0
30.0
1.0
0
0
0
0
43.2
0
98.0
93.8
95.2
90.8
56.8
88.2
5.1
2.0
6.2
4.8
9.2
0
MeOH
MeOH
MeOH
EtOH
iPrOH
iPrOH
tBuOH
tBuOH
0.10
0.02
0.02
0.02
0.10
0.10
9
1
10
11
12
13
87
66
95
64
96
b
5
34
0
36
1
0
6
7
11.8
0
5
0
24.0
94.9
14
3
a
Determined by 1H NMR (methyl protons).
b
a
Determined by 1H NMR (methyl protons).
UHP was used instead of 30% H
2
O
2
.

1
182
M. Kirihara et al. / Tetrahedron Letters 50 (2009) 1180–1183
Table 4
O
cat. TaCl , 30%H O
2 2
S
1
5
R1
R2
R1
S
R2
CH CN or iPrOH or tBuOH
3
2
R1
R
2
Solvent
TaCl
(equiv)
H
Time (h)
Temp.
Yield (%)
a
Entry
5
2 2
O (equiv)
1
2
3
4
5
6
7
8
9
Ph
Ph
CH
PhCH
2
@CH
2
CH
CH
CH
iPrOH
3
CN
CN
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.1
5
5
5
5
5
5
5
5
20
20
20
20
5.5
2
3.25
3
rt
rt
95
95
94
99
99
94
92
96
90
83
81
76
2
3
45 °C
45 °C
rt
45 °C
45 °C
rt
tBuOH
PhCH
2
2
PhCH
2
CH
3
CN
1
iPrOH
tBuOH
0.3
5.7
0.5
2
1.25
2
PhCH
Ph
CH
Ph
3
CH
CH
3
CN
CN
3
rt
1
1
1
0
1
2
iPrOH
tBuOH
0.1
0.1
0.1
45 °C
45 °C
50 °C
CH
3
CN
8.5
S
a
Isolated yields.
Table 5
O
O
cat. TaCl , 30%H O
2 2
S
5
R1
R2
S
3
R1
R2
º
MeOH, 45 C
1
Entry
R1
R2
TaCl
5
(equiv)
H
2
O
2
(equiv)
Time (h)
Yielda (%)
1
2
3
4
5
6
Ph
Ph
PhCH
PhCH
Ph
CH
PhCH
PhCH
2
@CH
2
CH
0.02
0.02
0.02
0.02
0.1
5
5
5
5
20
20
3
99
96
99
97
90
98
2
2.5
2.5
0.25
3.5
20
2
2
2
CH
Ph
3
0.1
S
a
Isolated yields.
the presence of catalytic amounts of tantalum(V) chloride in aceto-
nitrile, i-propanol, and t-butanol (Table 4). The sulfoxides were
selectively obtained in all cases. It is notable that the olefin moiety
was not oxidized at all (entry 2). Although 0.1 equiv of tantalum
chloride and 20 equiv of hydrogen peroxide were required for
the diaromatic sulfides, the corresponding sulfoxides were still
obtained in relatively high yield (entries 9–12).
The treatment of several sulfides with 30% hydrogen peroxide
in the presence of catalytic amounts of tantalum(V) chloride in
methanol at 45 °C effectively afforded the corresponding sulfones
(3) in all cases (Table 5). The olefin was inert under the reaction
condition (entry 1). For the diaromatic sulfides, 0.1 equiv of tanta-
lum chloride and 20 equvi of hydrogen peroxide were required to
produce 3 in high yields (entries 5 and 6).
Table 6
O
O
The oxidation of sulfides (1) with 30% hydrogen peroxide using
tantalum(V) ethoxide as a catalyst was further examined in meth-
anol (Table 6). The corresponding sulfones (3) were obtained in
high yield.
cat. Ta(OEt) , 30%H O
S
5
2
2
R1
R2
S
3
R1
R2
MeOH, 45oC
1
To investigate the reaction mechanism, 1a was treated with an
equimolar amount of tantalum(V) chloride in aqueous acetonitrile
Entry R1
R2
TaCl
equiv)
H
O
Time
(h)
Yielda
(%)
5
2
2
(
(equiv)
1
2
3
4
5
Ph
Ph
Ph
PhCH
Ph
CH
CH
PhCH
PhCH
Ph
3
0.02
CH 0.02
0.02
5
5
5
5
1
99
100
99
97
98
2
@CH
2
1.3
2.5
2.5
3
TaCl
(1.0 eq)
5
2
S
N. R.
0.02
0.1
Ph
Me
2
2
CH CN-H O or MeOH-H O, r.t., 24 h
3
2
2
20
1a
a
Isolated yields.
Scheme 3.

M. Kirihara et al. / Tetrahedron Letters 50 (2009) 1180–1183
1183
TaCl , Ta(OEt)
The reaction is monitored by thin layer chromatography (TLC).
After 1 disappears from the TLC, saturated aqueous sodium thio-
sulfate (15 mL) is added to the reaction mixture, followed by
extraction with ethyl acetate (20 mL Â 3). The combined organic
phase is washed with brine, dried over anhydrous magnesium sul-
fate, and evaporated. Chromatography on silica gel gives pure
products (2 or 3).
5
5
H O
2
Ta O
XnTa(OH)5-n
O O
2
5
S
R1
R2
H O and/or
2
2
solvent
R OH
ROOH and/or
O
Oxidant
S
NH
R1
R2
References and notes
R C
R C
N
OOH
1.
(a) Drabowicz, J.; Kielbsinski, P.; Mikolajczyk, M.; in Patai, S.; Rappoport, Z.;
Stirling, C. (Eds.), The Chemistry of Sulphone and Sulphoxide, John Wiley and
Sons, NY, 1988.; (b) Carreno, M. C. Chem. Rev. 1995, 95, 1717–1760; (c)
Frenanez, I.; Khiar, N. Chem. Rev. 2003, 103, 3651–3706.
S
R1
R2
Tantalum(V) Peroxides
complexed with solvent )
(
2. (a) Oae, S.; Kawai, T.; Furukawa, N. Tetrahedron Lett. 1984, 35, 69–72; (b)
Nicolaous, K. C.; Magolda, R. L.; Sipio, W. J.; Lysenko, W. E.; Joullie, M. M. J. Am.
Chem. Soc. 1980, 102, 3784–3793; (c) Leonard, N. J.; Johnson, C. R. J. Org. Chem.
Scheme 4.
1962, 27, 282–284; (d) Iranpoor, N.; Firouzabadi, H.; Pourali, A.-R. Synlett 2004,
347–349; (e) Hajipour, A. R.; Kooshki, B.; Ruoho, A. E. Tetrahedron Lett. 2005, 46,
5503–5506; (f) Ozanne-Beaudenon, A.; Quideau, S. Tetrahedron Lett. 2006, 47,
or aqueous methanol in the absence of hydrogen peroxide. In this
case, 1a was completely unreacted and was quantitatively recov-
ered. This result means that tantalum(V) chloride itself is not the
oxidizing agent in the reaction system (Scheme 3).8
5869–5873.
3
.
Jones, C. W. in Application of Hydrogen Peroxide and Derivatives, RCS Crean
Technology Monographs. Formerly of Solvay Interox R&D, Windnes, UK. 1999.;
Noyori, R.; Aoki, M.; Sato, K. Chem. Commun. 2003, 1977–1986.
4. (a) Kaczorowska, K.; Kolarska, Z.; Mitka, K.; Kowalski, P. Tetrahedron 2005, 61,
315–8327; (b) Mba, M.; Prins, L. J.; Licini, G. Org. Lett. 2007, 9, 21–24; (c)
8
A plausible reaction mechanism is shown in Scheme 4. Tanta-
lum(V) chloride or tantalum(V) ethoxide immediately reacts with
water to form tantalum(V) hydroxides. The resulting hydroxides
react with hydrogen peroxide or peroxides derived from the reac-
tion of hydrogen peroxide with the solvents (alcohols or nitriles) to
produce tantalum(V) peroxides. Since tantalum(V) compounds act
Jeyakumar, K.; Chand, K. Tetrahedron Lett. 2006, 47, 4573–4576; (d) Egami, H.;
Katsuki, T. J. Am. Chem. Soc. 2007, 129, 8940–8941; (e) Blakemore, P. R.; Burge, M.
S. J. Am. Chem. Soc. 2007, 129, 3068–3069; (f)Mohammadpoor-Baltork, I.;Hill, M.;
Caggiano, L.; Jackson, R. F. W. Synlett. 2006, 3540–3544; (g) Sato, K.; Hyodo, M.;
Aoki, M.; Zheng, X.-Q.; Noyori, R. Tetrahedron 2001, 57, 2469–2476; (h) Shi, X.-Y.;
Wei, J.-F. J. Mol. Catal. A 2008, 280, 142–147; (i) Yamazaki, S. Bull. Chem. Soc. Jpn.
1996, 69,
2955–2959; (j) Kelly, P.; Lawrence, S. E.; Maguire, A. R. Synlett 2007,
1
1e–o
as Lewis acid,
alcohols and nitriles coordinate with tanta-
1501–1506; (k) Hosseinpoor, F.; Golchoubian, H. Tetrahedron Lett. 2006, 47,
5
3
195–5197; (l) Alonso, D. A.; Nájera, C.; Varea, M. Tetrahedron Lett. 2002, 43,
459–3461; (m) Yuan, Y.; Bian, Y. Tetrahedron Lett. 2007, 48, 8518–8520.
lum(V) peroxides to produce tantalum(V) peroxide complexes.
The tantalum(V) peroxide complexes oxidize sulfides or sulfoxides,
and revert to tantalum(V) hydroxides (or their complex of the sol-
vent). The resulting tantalum(V) hydroxides react with peroxides
to form the tantalum(V) peroxide complexes again. Unfortunately,
the chemistry of tantalum(V) peroxides is almost unknown even in
inorganic chemistry. Therefore, the exact structure of tantalum(V)
5.
(a) Kirihara, M.; Okubo, K.; Koshiyama, T.; Kato, Y.; Hatano, A. ITE Lett. 2004, 5,
279–281; (b) Kirihara, M.; Harano, A.; Tsukiji, H.; Takizawa, R.; Uchiyama, T.;
Hatano, A. Tetrahedron Lett. 2005, 46, 6377–6380; (c) Kirihara, M.; Ogawa, S.;
Noguchi, T.; Okubo, K.; Monma, Y.; Shimizu, I.; Simosaki, R.; Hatano, A.; Hirai,
Y. Synlett 2006, 2287–2289; (d) Kirihara, M.; Asai, Y.; Ogawa, S.; Noguchi, T.;
Hatano, A.; Hirai, Y. Synthesis 2007, 3286–3289; (e) Noguchi, T.; Hirai, Y.;
Kirihara, M. Chem. Commun. 2008, 3040–3042.
9
6. Hirao, T.; in ACS Symposium Series 974 (Vanadium: The Versatile Metal),
American, pp 2–27.
peroxide complexes is unclear. Although the solvent effect is not
perfectly clear, it is assumed that the coordinative properties of
the solvents affect the oxidative ability of the tantalum(V) perox-
ides. Further details of this reaction and the structure of tanta-
lum(V) peroxide complexes are currently under investigation.
In the presence of sodium iodide, iodo cation equivalents or
iodine derived from the oxidation of the iodo anion reacts with a
sulfide to produce an iodosulfonium ion intermediate. The inter-
mediate subsequently reacts with water to form the sulfoxide
7. (a) Zenz, C.; Bartlett, J. P.; Thiede, W. H. Arch. Environ. Health 1962, 5, 542–546;
(
b) Browning, E. Toxicity of industrial Metals, 2nd ed.; Appleton-Century-Crofts:
New York, 1969. 340–347.
8.
A referee asked what could be the effect of oxygen and other oxidizing agents
such as Oxone on the selectivity. We think that oxygen cannot be an oxidant in
this reaction system, because the sulfide (1a) is inert without hydrogen
peroxide as shown in Scheme 3. We examined the reaction depicted in Scheme
3
under air and oxygen atmosphere. Since it has been reported that Oxone
rapidly oxidizes sulfides to the corresponding sulfones without any catalysts,
we did not examine the reaction of sulfides with oxone in the presence of TaCl
5
.
Chemical Society, 2007 and references cited therein.
(
Scheme 5). It has been reported that the first step is fast and
9.
A referee pointed out that it would be better if comments on experimental results
and mechanistic aspects are discussed throughout the manuscript, instead of
having this listing of tables of results followed by some discussions in the last
part of the manuscript. We think that the active species of this oxidation must be
tantalum(V) peroxide complexes. Unfortunately, the chemistry of tantalum(V)
peroxide is almost unknown even in inorganic chemistry, it is impossible to
discuss the mechanistic aspects throughout the manuscript.
10
reversible, and the second step is very slow. The formation of
the iodosulfonium ions prevents the reaction of sulfides with tan-
talum(V) peroxide complexes, therefore sulfides did not produce
the oxidized products in the presence of the molar equivalent of
sodium iodide.
1
1
0. Higchi, T.; Gensch, K.-H. J. Am. Chem. Soc. 1966, 5, 5486–5487.
A general experimental procedure for the oxidation of sulfides
is thus the following: A mixture of sulfide (1.0 mmol), tantalum
chloride (7.1 mg, 0.02 mmol), and 30% hydrogen peroxide
1. Recent reports of organic syntheses using tantalum reagents: (a) Takai, K.;
Yamada, M.; Odaka, H.; Utimoto, K. J. Org. Chem. 1994, 59, 5852–5853; (b)
Aoyagi, Y.; Tanaka, W.; Ohta, A. J. Chem. Soc., Chem. Commun. 1994, 1225–1226;
(
c) Takai, K.; Yamada, M.; Odaka, H.; Utimoto, K.; Fujii, T.; Furukawa, I. Chem.
(
0.5 mL, 5 mmol) in a solvent (2 mL, acetonitrile or methanol) is
Lett. 1995, 315–316; (d) Takai, K.; Yamada, M.; Utimoto, K. Chem. Lett. 1995,
stirred at room temperature (acetonitrile) or 45 °C (methanol).
8
8
51–852; (e) Chandrasekhar, S.; Takhi, M.; Uma, G. Tetrahedron Lett. 1997, 38,
089–8092; (f) Chandrasekhar, S.; Takhi, M.; Reddy, Y. R.; Mohapatra, S.; Rao,
C. R.; Reddy, K. V. Tetrahedron 1997, 53, 14997–15004; (g) Chandrasekhar, S.;
Reddy, B. V. Synlett 1998, 851–852; (h) Chandrasekhar, S.; Ramachander, T.;
Takhi, M. Tetrahedron Lett. 1998, 39, 3263–3266; (i) Chandrasekhar, S.;
Mohanty, P. K.; Raza, A. Synth. Commun. 1999, 29, 257–262; (j)
Chandrasekhar, S.; Ramachandar, T.; Prakash, S. J. Synthesis 2000, 1817–
I
cat.Ta(V)
H O
2
2
1818; (k) Chandrasekhar, S.; Prakash, S. J.; Jagadehwar, V.; Narsihmulu, C.
Tetrahedron Lett. 2001, 42, 5561–5563; (l) Andes, C.; Harkins, S. B.; Murtuza, S.;
Oyler, K.; Sen, A. J. Am. Chem. Soc. 2001, 123, 7423–7424; (m) Shibata, I.; Nose,
K.; Sakamoto, K.; Yasuda, M.; Baba, A. J. Org. Chem. 2004, 69, 2185–2187; (n)
Chandrasekhar, S.; Ramachandar, T.; Shyamsunder, T. Indian J. Chem., Sect. B
O
S
"
"
" I "
S
I
H O
2
S
R1
R2
R1
R2
very slow
R1
R2
2004, 43B, 813–838; (o) Chandrasekhar, S.; Jaya Prakash, S.; Shyamsunder, T.;
Scheme 5.
Ramachandar, T. Synth. Commun. 2005, 35, 3127–3131.