The Pummerer-like reaction of 2,5-bis(trimethylsilyl) thiophene S-oxide with trifluoroacetic anhydride: Intermediary formation of sulfurane [10-S-4(C202)] (λ4-sulfane)

Article abstract of DOI:10.1002/hc.1066

On treatment of a 2,5-bissilylsubstituted thiophene S-oxide or dibenzothiophene S-oxide with trifluoroacetic anhydride, the equilibria between the corresponding substrates and hypervalent sulfur compounds (sulfurane or λ⁴-sulfane) have now been first detected by low temperature NMR experiments. By elevating the temperature to room temperature, in the case of the thiophene S-oxide, the sulfurane detected was converted to 3-thiolene-2-one quantitatively via a Pummerer-like reaction.

Full text of DOI:10.1002/hc.1066

Heteroatom Chemistry
Volume 12, Number 5, 2001
The Pummerer-Like Reaction of 2,5-Bis(tri-
methylsilyl)thiophene S-Oxide with Trifluoro-
acetic Anhydride: Intermediary Formation of
Sulfurane [10-S-4(C2O2)] (k4-Sulfane)
Soichi Sato,1 Shao-Zhong Zhang,1 and Naomichi Furukawa2
1Tsukuba Advanced Research Alliance Center and Department of Chemistry, University of Tsukuba,
Tsukuba, Ibaraki 305-8577, Japan
2Foundation for Advancement of International Science, Tsukuba, Ibaraki 305-0062, Japan
Received 18 January 2001; revised 26 February 2001
ophene S-oxides have been reported after develop-
ABSTRACT: On treatment of a 2,5-bissilylsubsti-
ment of convenient synthetic methods; for example,
the isolated thiophene S-oxides undergo [4 ם 2 ] cy-
tuted thiophene S-oxide or dibenzothiophene S-oxide
with trifluoroacetic anhydride, the equillibria between
cloaddition with olefins and acetylenes as dienophi-
the corresponding substrates and hypervalent sulfur
les, or cis-1,4-addition with halogenes [3]. However,
compounds (sulfurane or k4-sulfane) have now been
only Nakayama et al. [4] have reported a reaction on
first detected by low temperature NMR experiments. By
the sulfur atom in the thiophene S-oxides. In order
elevating the temperature to room temperature, in the
to estimate the reactivity of the sulfinyl site in the
case of the thiophene S-oxide, the sulfurane detected
thiophene S-oxide species, we tried to react the thi-
was converted to 3-thiolene-2-one quantitatively via a
ophene S-oxide derivatives with an acid anhydride.
Pummerer-like reaction. ᭧ 2001 John Wiley & Sons,
Generally, the reactions of chiral sulfoxides with acid
Inc. Heteroatom Chem 12:444–450, 2001
anhydrides undergo concurrent racemization and
oxygen exchange. In the case of sulfoxides having ␣-
protons as substrates, Pummerer-type reactions can
INTRODUCTION
conceivably proceed via the corresponding sulfo-
nium salts as intermediates. Detailed studies on pos-
sible reaction mechanisms were reported by Oae et
al. [5]. They proposed that the reactions proceed via
the corresponding sulfonium salts, and sulfuranes
formed as unstable intermediates. However, these
sulfurane intermediates were not detected directly.
In this article, we wish to report the first detection
of an equilibrium between thiophene S-oxide deriv-
atives and the corresponding hypervalent sulfur
compounds (sulfuranes or k4-sulfanes) as interme-
diates by low temperature NMR experiments in
which the thiophene S-oxide derivatives were caused
to react with acid anhydrides. Furthermore, a Pum-
merer-type reaction was observed involving the cor-
Although thiophene S-oxides are unstable species,
they can have interesting reactivities [1]. Recently,
new synthetic methods and structure determina-
tions of stable thiophene S-oxides having bulky sub-
stituents on the thiophene ring have been reported
[2]. Thiophene S-oxide species have mainly two re-
active sites. One is the diene site on the thiophene
ring. The other is the sulfinyl site (sulfoxide group).
A few reaction properties of the diene site in the thi-
Dedicated to Professor Naoki Inamoto on the occasion of his
72nd birthday.
Correspondence to: Soichi Sato and Naomichi Furukawa.
᭧ 2001 John Wiley & Sons, Inc.
444

The Pummerer-Like Reaction of 2,5-Bis(trimethylsilyl)thiophene S-Oxide with Trifluoroacetic Anhydride 445
SCHEME 1
FIGURE 1 VT 1H NMR spectra of the thiophene S-oxide with trifluoroacetic anhydride.
responding sulfurane intermediate by raising the
temperature to room temperature.
ure 1. This result suggests that an equilibrium be-
tween the thiophene S-oxide 1 and the correspond-
ing intermediate 2 exists in this reaction only below
מ60ЊC and that the structure of the intermediate 2
is a symmetrical one. The intermediate 2 was also
characterized by 19F and 13C NMR spectroscopy at
מ80ЊC. These NMR spectra also show that the inter-
mediate 2 is a symmetrical molecule with one signal
(מ78.1 ppm) in the 19F NMR spectrum and a set of
five peaks in the 13C NMR spectrum. These results
indicate that the intermediate 2 is probably an un-
stable sulfurane having two trifluoromethylcarboxyl
ligands [6]. Furthermore, the kinetic control of the
reaction depends sensitively on the concentration of
anhydride. When the thiophene S-oxide 1 was
treated with more than 3.0 equiv. of trifluoroacetic
anhydride at מ80ЊC in dry THF-d8, the NMR signals
of 1 disappeared completely and only those of the
corresponding intermediate 2 remained.
RESULTS AND DISCUSSION
First, we examined the reaction of the kinetically sta-
bilized 2,5-bis(trimethylsilyl)thiophene S-oxide (1)
with trifluoroacetic anhydride, as shown in Scheme
1. The thiophene S-oxide 1 was treated with 1.5
equiv. of trifluoroacetic anhydride at low tempera-
ture (below מ60ЊC) in THF-d8, and the 1H chemical
shifts of both of the 3,4-protons on the thiophene
ring and the protons of the trimethylsilyl groups
were monitored by variable low temperature NMR
spectroscopy. New signals appeared at low field (7.60
ppm (3,4-H) and at 0.37 ppm (Me3Si)) compared
with those of the substrate 1 (6.97 ppm (3,4-H) and
0.31 ppm (Me3Si)) at מ60ЊC. By elevating the tem-
perature, the NMR signal intensities of a new inter-
mediate 2 increased and those of the substrate 1 de-
creased, while, by lowering the temperature, the
NMR observation was the opposite, as shown in Fig-
1
Based on the H NMR spectral data observed at
various temperatures, the equilibrium constants (K)
between the thiophene S-oxide 1 and the corre-

446 Sato, Zhang, and Furukawa
monitored in detail by variable low temperature
NMR spectroscopy. When the temperature of the so-
lution was slowly elevated from מ60 to מ35ЊC, the
signals due to both 1 and 2 disappeared gradually
and one set of an AB quartet of two protons on the
thiophene ring at 7.04 and 7.07 ppm (J ס 7.8 Hz)
and two singlets of trimethylsilyl groups at 0.32 and
0.53 ppm appeared. By elevating the temperature to
room temperature, the 1H NMR spectrum shows one
set of an AB quartet peak of two protons on the thi-
ophene ring at 7.02 and 7.07 ppm (J ס 8.0 Hz) and
a singlet of 9 protons of one trimethylsilyl group at
0.31 ppm. These results indicate that this Pummerer-
like reaction proceeded via two intermediates. These
final signals did not change in a closed system, even
at room temperature. The intermediate 4 in the final
solution was decomposed gradually by water con-
tained in the solution or was reacted with additional
water to conver it to 3-thiolene-2-one 3. We propose
the mechanism of the Pummerer-like reaction to be
that shown in Scheme 2 on the basis of the afore-
mentioned results.
The mechanism for the present reactions may
proceed initially by the Pummerer-like reaction of
the intermediate 2 by formation of intermediates 4
and 5, and then the hydrolysis of intermediate 5
could proceed. Furthermore, the reaction was
quenched with D2O or H218O in order to determine
the source of the 5-protons and the oxygen of the
carbonyl group in 3. 18O-Uptake was not observed,
while D atoms were found at the 5-position of 4.
These results indicate that water is not the source of
the oxygen atom but is the source of the protons in
3. Thus, the source of theoxygen is the original car-
boxyl group in 3.
FIGURE 2 van’t Hoff plots with standard deviation of equi-
libria between 1 and 2 (top graph) and 6 and 7 (bottom graph).
Furthermore, we undertook the reaction of di-
benzothiophene S-oxide (6) with trifluoroacetic an-
hydride, as shown in Scheme 3 [8]. The equilibrium
between dibenzothiophene S-oxide (6) and the cor-
responding sulfurane intermediate 7 with trifluo-
roacetic anhydride was also observed by monitoring
sponding sulfurane intermediate 2 were calculated,
based on the second-order rate equation. By plotting
lnK against 1/T, a straight line was obtained, as
1
shown in Figure 2. The enthalpy values, DHЊ (מ5.5
with variable low-temperature H NMR spectros-
מ1
1
kcal molמ1) and free energy, DG(198)Њ (0.5 kcal mol
)
copy. Only the H NMR signals of the substrate 6
were calculated. These parameters are summarized
in Table 1.
appeared at room temperature, and only those of the
sulfurane intermediate 7 appeared at מ78ЊC. The
temperature dependence of this equilibrium state
was observed even from מ78ЊC to room temperature
as shown in Figure 3. The intermediate 7 was also
characterized by 19F and 13C NMR spectroscopy at
מ78ЊC. These spectra indicate that the intermediate
7 has a symmetrical structure. These results also in-
dicate that the intermediate 7 is an unstable sulfur-
ane compound having two trifluoromethylcarboxyl
ligands. Based on this 1H NMR technique, the equi-
librium constants (K) at the corresponding tempera-
By elevating the temperature to room tempera-
ture, the 1H NMR signals due to 1 and 2 disappeared,
and new signals due to the product appeared. The
reaction mixture was separated, and after treatment
by column chromatography, the product 3 was ob-
tained in 95% yield (Scheme 2). The compound 3
1
was identified by H, 13C NMR, and mass spectros-
copy to be the known compound, 3-thiolene-2-one
(3) [7]. In order to investigate the mechanism for the
formation of product 3, this reaction system was

The Pummerer-Like Reaction of 2,5-Bis(trimethylsilyl)thiophene S-Oxide with Trifluoroacetic Anhydride 447
TABLE 1 Equilibrium Parameters for the Reactions of Thiophene S-Oxide Derivatives with (CF3CO)2O
מ1
מ1
מ1
Reactionsa
Kb (s
)
DHЊc (kcal mol
מ5.5
)
DGЊc (kcal mol
)
A
7.48 ן 10מ2 (213 K)
1.56 ן 10מ1 (203 K)
3.94 ן 10מ1 (193 K)
6.12 ן 10מ1 (183 K)
1.13 ן 10מ1 (208 K)
2.55 ן 10מ1 (198 K)
5.07 ן 10מ1 (188 K)
0.5 (198 K)
B
1.46 ן 10מ1 (258 K)
4.53 ן 10מ1 (248 K)
9.64 ן 10מ1 (238 K)
1.97 (228 K)
2.62 ן 10מ1 (253 K)
6.40 ן 10מ1 (243 K)
1.40 (233 K)
מ10.0
0.2 (243 K)
aReaction A: reaction of 1 with 1.5 equiv (CF3CO)2O. Reaction B: reaction of 6 with 2.0 equiv (CF3CO)2O.
bAverage values.
cThese values were obtained from van’t Hoff equation, lnK ס מDHЊ/RT ם const. and מDG(T)Њ ס RTlnK.
SCHEME 2
tures were calculated. By plotting lnK against 1/T, a
good straight line was obtained as shown in Figure
2. Furthermore, the free energy and enthalpy of the
equilibrium reaction were calculated. Based on the
spectral data observed at various temperatures, the
equilibrium constants (K) between 6 and 7 were cal-
culated by use of the second-order rate equation. The
enthalpy value, DHЊ (מ10.0 kcal molמ1) and free en-
ergy, DG(243)Њ (0.2 kcal molמ1) were also calculated.
These parameters are summarized in Table 1.
1
We found that the H chemical shift (7.64 ppm)
of the 3,4-protons in 2 appeared at quite low field
compared with those of the known 2,5-bis(trimeth-
ylsilyl)thiophene derivatives [2,5-bis(trimethylsilyl)-
thiophene S-oxide 8 (6.74 ppm), 2.5-bis(trimethyl-
silyl)thiophene S-oxide 1 (6.82 ppm), 2,5-bis(tri-

448 Sato, Zhang, and Furukawa
the electronegative ligands on the sulfur atom in 1,8,
and 9. These results suggest that the low-field shift
of the 3,4-protons in 2 is due not only to the strong
electron-withdrawing character of the two trifluo-
roacetyl groups on the sulfur atom but also to the
new aromatic character of the thiophene ring that
can make use of a pair of electrons in the hypervalent
bond to complete a [4n ם 2] shell, as shown in Fig-
ure 5. The latter phenomenon had been predicted by
Martin et al. [9].
In the present study, we showed that the equilib-
rium between thiophene S-oxide derivatives and the
corresponding hypervalent sulfur compounds (sul-
furanes or k4-sulfanes) as intermediates could be de-
tected directly by low temperature NMR spectros-
copy in the reaction of thiophene S-oxide derivatives
with an acid anhydride, and that a Pummerer-type
reaction of the corresponding sulfurane intermedi-
SCHEME 3
methylsilyl)thiophene S-oxide (O-B)trifluoroborane
9 (7.13 ppm), and 2,5-bis(trimethylsilyl)thiophene
10 (7.31 ppm)], as shown in Figure 4. The thiophene
rings of the derivatives 1, 8, and 9 do not have aro-
matic characters due to their nonplaner structures
[3a]. Therefore, these H chemical shifts of the 3,4-
protons appeared at low field compared with that of
the thiophene S-oxide 1 in spite of the bonding of
1
FIGURE 3 VT 1H NMR spectra of dibenzothiophene S-oxide 6 with trifluoroacetic anhydride.

The Pummerer-Like Reaction of 2,5-Bis(trimethylsilyl)thiophene S-Oxide with Trifluoroacetic Anhydride 449
FIGURE 4 1H NMR chemical shits of 2,5-bis(trimethylsilyl)thiophene derivatives.
ate proceeded when the temperature was elevated to
room temperature. These investigations demon-
strate alternative reactivities of the sulfinyl group in
thiophene S-oxides that are unlike those of simple
sulfoxides.
chemical shifts of 2 were shifted to high field by el-
evating the temperature. The scale of the chemical
shift shown in Figure 1 was adapted to the NMR
spectrum at מ100ЊC.
Isolation of 3-Thiolene-2-one 3
EXPERIMENTAL
General Data
2,5-Bis(trimethylsilyl)thiophene S-oxide (1.0 mmol,
240.0 mg) was dissolved in dry THF (10 mL) con-
tained in a reactor, the solution was cooled to מ78ЊC,
and trifluoroacetic anhydride (2.0 mmol, 0.32 ml)
was added by a microsyringe. The reaction vessel
was warmed to room temperature slowly. After re-
moval of the solvent, the residue was subjected to
column chromatography on silica gel (hex-
ane:dichloromethane ס 1:1 v/v) to afford 3-thiolene-
2-one 3 (48.0 mg, 48%). 3-Thiolene-2-one 3: 1H NMR
(400 MHz, CDCl3, r.t.) d 4.15 (t, J ס 2.5 Hz, 2H), 6.41
(sext, J ס 2.0 Hz, 1H), 7.58 (quint, J ס 2.9 Hz, 1H).
13C NMR (270 MHz, CDCl3, room temperature [r.t.])
All NMR spectra were measured on a JEOL LMN-
EX-270 or a Bruker ARX-400 spectrometer. Mass
spectra were taken with a Shimazu QP-2000 and a
JEOL JMS SX 102 mass spectrometer. All solvents
and reagents were obtained from commercial
sources and were further purified by general
methods.
Reaction of 2,5-Bis(trimethylsilyl)thiophene S-
oxide 1 with Trifluoroacetic Anhydride
ם
d 39.0, 133.5, 154.0, 210.5. MS m/z 100 (M ). RI
(NaCl) 1669 (cmמ1).
2,5-Bis(trimethylsilyl)thiophene S-oxide
1
(0.1
mmol, 24.0 mg) was dissolved in dry THF-d8 (0.60
mL) contained in an NMR tube, the solution was
cooled to מ78ЊC, and trifluoroacetic anhydride (0.2
mmol, 0.032 ml) was added by a microsyringe. This
reaction was monitored by variable temperature 1H
NMR spectroscopy. The NMR spectra of the sulfur-
ane intermediate 2 could be observed over the tem-
perature range from מ100 to מ60ЊC. Sulfurane 2:
1H NMR (270 MHz, THF-d8, מ80ЊC) d 0.34 (s, 18 H,
MeH), 7.64 (s, 2 H, ThH). 13C NMR (68 MHz, THF-
d8, מ80ЊC) d מ1.28, 115.9 (q, J ס 286 Hz), 149.6,
161.0 (q, J ס 41 Hz), 161.8. 19F NMR (254 MHz,
THF-d8, מ80ЊC) d מ78.1 (relative to CFCl3). 1H NMR
FIGURE 5 The electronic structure of sulfurane 2.

450 Sato, Zhang, and Furukawa
J. C. J Am Chem Soc 1994, 116, 1880; (c) Furukawa,
N.; Zhang, S.-Z.; Sato, S.; Higaki, M. Heterocycles
1997, 44, 61; (d) Nakayama, J.; Yu, T.; Sugihara, Y.;
Ishii, A. Chem Lett 1997, 499.
The Quenching of the Pummerer-Like Reaction
by D2O or H218O
Reaction of 2,5-(bistrimethylsilyl)thiophene S-oxide
1 with trifluoroacetic anhydride was quenched by
D2O and by H218O at r.t. The aforementioned reaction
solution was warmed gradually to r.t. and D2O (2.0
mmol, 0.04 mg) or H218O (2.0 mmol, 0.04 mg) was
added by a microsyringe. Each product was identi-
fied by its 1H NMR spectra and by mass
spectroscopy.
[3] (a) Furukawa, N.; Zhang, S.-Z.; Horn, E.; Takahashi,
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Reaction of Dibenzothiophene S-Oxide 6 with
Trifluoroacetic Anhydride
The dibenzothiophene S-oxide 6 (0.05 mmol, 10.0
mg) was dissolved into dichloromethane-d2 (0.60 ml)
contained in an NMR tube. The trifluoroacetic an-
hydride (0.05 mmol, 0.008 mL) was added by a mi-
crosyringe at מ78ЊC. This reaction was monitored
[4] Otani, T.; Sugihara, Y.; Ishii, A.; Nakayama, J. Tetra-
hedron Lett 1999, 40, 5549.
1
by variable temperature H NMR spectroscopy. The
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references therein).
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5057.
NMR spectra of sulfurane intermediate 7 was ob-
served at מ60ЊC. Sulfurane 7: H NMR (270 MHz,
1
CDCl3, מ60ЊC) d 7.65 (t, J ס 8.3 Hz, 2 H, PhH), 7.82
(t, J ס 98.3 Hz, 2 H, PhH), 8.02 (d, J ס 7.7 Hz, 2 H,
PhH), 8.20 (d, J ס 7.7 Hz, 2 H, PhH) 13C NMR (68
MHz, CDCl3, מ60ЊC) d 113.2 (q, JFמC ס 286 Hz),
123.5, 125.9, 130.3, 133.8, 140.3, 159.8, 149.2 (q, JFמC
ס 48 Hz). 19F NMR (254 MHz, CDCl3, מ60ЊC) d מ
76.1 (relative to CFCl3).
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