An S-oxide of 6-tert-butyl-6-phenylpentathiane. Structure in the crystalline state and in solution and thermal decomposition

Article abstract of DOI:10.1016/S0040-4039(01)00383-5

Oxidation of 6-tert-butyl-6-phenylpentathiane with trifluoroperacetic acid gave the pentathiane 3-oxide mainly as the monooxide. The 3-oxide takes a twist conformation both in the crystalline state and in solution. Thermal decomposition of the 3-oxide in the presence of 2,3-dimethyl-1,3-butadiene yielded both S₂O- and S₂-transferred products.

Full text of DOI:10.1016/S0040-4039(01)00383-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3117–3119
An S-oxide of 6-tert-butyl-6-phenylpentathiane. Structure in the
crystalline state and in solution and thermal decomposition
Akihiko Ishii,* Hideaki Oshida and Juzo Nakayama*
Department of Chemistry, Faculty of Science, Saitama University, Urawa, Saitama 338-8570, Japan
Received 25 December 2000; revised 8 February 2001; accepted 2 March 2001
Abstract—Oxidation of 6-tert-butyl-6-phenylpentathiane with trifluoroperacetic acid gave the pentathiane 3-oxide mainly as the
monooxide. The 3-oxide takes a twist conformation both in the crystalline state and in solution. Thermal decomposition of the
3-oxide in the presence of 2,3-dimethyl-1,3-butadiene yielded both S2O- and S2-transferred products. © 2001 Elsevier Science Ltd.
All rights reserved.
Oxidation of cyclic polysulfides has been drawing much
attention not only for the fundamental interest in the
regio- and stereochemistries1–4 but also for the potential
of the resulting oxides as a precursor of reactive sulfur
species.2,4 It has been reported that oxidation of cyclic
trisulfides 12 and 23 with MCPBA took place at the 1
and 2 positions with low regioselectivity, which is in
contrast to the highly regioselective MCPBA-oxidation
of acyclic tri- and tetrasulfides.5 Recently, we disclosed
that oxidation of 5-(1-adamantyl)-5-tert-butyltetra-
thiolane with dimethyldioxirane took place first at the
2-position and then at the 3-position to give the 2,3-
dioxide 3.4 The 2,3-dioxide 3 is isolable as crystals at
room temperature but decomposed to the dithiirane
1-oxide 4 and S2O in solution at temperatures higher
than −10°C. This result prompted us to investigate
oxidation of a higher analog of tetrathiolane, pentathi-
ane 5.6
integral ratio of the mixture showed the formation of
monooxide 6 in 41% yield along with unreacted 5
(30%), thioketone S-oxide 7 (1%), and several uniden-
tified compounds. This indicates that 6 was formed at
least in 58% yield based on the consumed pentathiane
5. On a preparative scale, 5 was oxidized with 2 molar
equiv. of CF3CO3H and, after having been evaporated
to dryness, the mixture was washed with hexane, and
then recrystallized from hexane–CH2Cl2 to give 6 in
25% isolated yield.8 The monooxide 6 was fairly stable
at room temperature. The structure of 6 was finally
established by X-ray crystallography to be the 3-oxide,
which takes a twist conformation with the oxygen atom
cis to the tert-butyl group (Fig. 1). Meanwhile, the
starting pentathiane 5 takes a chair conformation in the
crystalline state.9 The structure of S6O was reported to
take a chair conformation with the oxygen atom occu-
pying the axial position.10
1
S
1
S
O
2
S
S
O
S S
1
S
S
S
t-Bu
Ph
1-Ad
t-Bu
S
S
1-Ad
t-Bu
S
S
2
S
S
2
S S
S
S
S
3
O
S
S
3
4
3
1
2
3
4
5
Pentathiane 56,7 was oxidized with CF3CO3H, prepared
Conformations of 5 and 6 in solution (CD2Cl2) were
investigated by low-temperature 1H NMR spec-
troscopy. The singlet (l 1.07) of the tert-butyl group of
in situ from (CF3CO)2O (5.3 molar equiv.) and H2O2
1
(1.4 molar equiv.), in CH2Cl2 at −20°C. The H NMR
Keywords: pentathiane; pentathiane 3-oxide; oxidation; structure; thermolysis.
* Corresponding authors. Tel.: +81-48-858-3394; fax: +81-48-858-3700; e-mail: ishiiaki@chem.saitama-u.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00383-5

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A. Ishii et al. / Tetrahedron Letters 42 (2001) 3117–3119
ene-d8 at 183 K, the tert-butyl appeared at l 0.48 (br s,
6H) and 1.18 (br s, 3H), and the aromatic protons at l
6.44 (m, 1H), 6.65 (m, 1H), 6.76 (m, 2H), and 7.23 (m,
1H).
Calculations12 on 6 at the B3LYP/6-31G* level includ-
ing solvent effects (the IPCM model13) showed that a
twist form of 6 (twist-6) is more stable than a chair
form of 6 (chair-6) in CH2Cl2 (m 9.08) and toluene (m
2.39) by 0.668 and 0.194 kcal mol−1, respectively,
whereas the twist-6 is less stable than the chair-6 by
0.264 kcal mol−1 in the gas phase.14 Calculated dipole
moments of twist-6 and chair-6 in CH2Cl2 were 6.5577
and 4.4009, respectively. It is reasonable that twist-6
with a larger dipole moment is more favorable in
solution than chair-6. Incidentally, a chair form of 5 is
5.9 and 4.4 kcal mol−1 more stable than a twist form of
5 in the gas phase and in CH2Cl2, respectively, at the
B3LYP/6-31G* level.
Figure 1. (a) ORTEP drawing of the 3-oxide 6 (50% ellip-
soidal probability) and (b) the molecular structure viewed
from another direction.
pentathiane 5 decoalesced at temperatures lower than
207 K and changed to two singlets at l 0.87 (6H) and
1.47 (3H) at 183 K. At low temperatures down to
183 K, the aromatic protons of 5 appeared as measured
at room temperature [l 7.48 (t, 1H), 7.56 (t, 2H),
and 7.66 (d, 2H)]. These observations are consis-
tent with the chair conformation of 5 with a mirror
plane (CS symmetry). The ring inversion of the unsub-
stituted pentathiane at high temperatures has been
reported.11
The stereochemistry 6 shows that the electrophilic
attack of CF3CO3H took place mainly at the least
hindered 3-position from the axial side of a chair form
of 5 to give chair-6, followed by a conformation change
to twist-6. At the 3-position of 5, a molecular orbital
derived from lone pair electrons of the sulfur atoms,
which is the HOMO at the B3LYP/6-31G* level,
spreads in larger extent to the tert-butyl side than the
phenyl side as depicted at the lower left, supporting the
present stereoselectivity.
On the other hand, the tert-butyl group of 6 decoa-
lesced at temperatures lower than 195 K to become two
broad singlets at l 1.05 (6H) and 1.63 (3H), and the
five aromatic protons8 appeared at l 7.16 (br s,
1H), 7.40 (m, 3H), and 7.81 (d, 1H) at 183 K. The
latter indicates the nonequivalency of the aroma-
tic protons to each other. Assuming that two methyls
of the tert-butyl overlapped at l 1.05, these observa-
tions are in harmony with the twist conformation of 6
[C1 symmetry, see Fig. 1(b)]. When measured in tolu-
Pentathiane 3-oxide 6 gradually decomposed in solu-
tion at room temperature to give the corresponding
thioketone 8 and ketone 9. The thermal decomposition
of 6 in the presence of 2,3-dimethyl-1,3-butadiene
yielded three products 10–12 along with thioketone 8
and its derivatives, 5, 9, 13, and 14, where yields of the
products were determined by using an internal stan-
dard. The formation of 10 indicates the generation of
O
S S
S S
S
S
S
S
+
+
6
CDCl3, refl., 29 h
1 mol equiv
10
0.47
11
0.31
12
0.15 mol equiv
S
S S
(S)n
S S
S
t-Bu
Ph
t-Bu
t-Bu
Ph
+
+
+
+
t-Bu(Ph)C=X
6
Ph
S
X = S (8, 21%)
X = O (9, 32%)
6%
13 (24%)
n = 1 (5, 1%)
n = 2 (14, 3%)

A. Ishii et al. / Tetrahedron Letters 42 (2001) 3117–3119
3119
S2O,4,15 while dihydrodithiin 11 and tetrasulfide 12 are
known as the products of the reaction of the butadiene
and S2-transfer reagents.16 Thus, 6 might transfer both
S2O and S2 to 1,3-butadienes.
Science DIP3000 diffractometer with
a
graphite-
monochromater. The data reduction was made by the
maXus program system; 2871 unique reflections; 1915
reflections [I]2|(I)] were used for refinement (210
parameters). The final R1=0.049, wR2=0.064, and
GOF=1.585; max./min. residual electron density=0.72/−
−3
,
,
Acknowledgements
0.38 e A . Selected bond lengths (A) and angles (°):
C1–S1, 1.845(3); S1–S2, 2.008(1); S2–S3, 2.182(1); S3–S4,
2.157(2); S4–S5, 1.999(1); S5–C1, 1.887(3); C1–C2,
1.525(4); C1–C3, 1.585(4); S3–O1, 1.432(4); S2–S1–C1,
102.8(2); S1–S2–S3, 104.7(1); S2–S3–S4, 96.9(1); S2–S3–
O1, 113.2(2); S4–S3–O1, 102.7(2); S3–S4–S5, 103.8(1);
S4–S5–C1, 105.7(2), S1–C1–S5, 107.6(2), C2–C1–C3,
113.3(3).
This work was supported by a Grant-in-Aid for Scien-
tific Research (No. 90193242) from the Ministry of
Education, Science, Sports and Culture, Japan.
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