Oxidative conversion of tetraaryldihydrodibenzothiepins into elemental sulfur and stable cationic dyes accompanied by dual uvvis and cd spectral changes

Article abstract of DOI:10.1246/cl.130275

Upon oxidation of the title heterocycles with dimethylamino groups, elemental sulfur is extruded to form dicationic dyes, which regenerate the heterocycles by the reaction with Na2S. The electrolysis of the heterocycles induces a drastic change in UVvis spectrum exhibiting several isosbestic points. The bayregion substituents enhance configurational stability, so the optically pure heterocycle can be obtained in terms of helicity of one-handedness, and its chromism is accompanied by a large chiroptical response as detected by CD spectroscopy.

Full text of DOI:10.1246/cl.130275

doi:10.1246/cl.130275
706
Published on the web May 28, 2013
Oxidative Conversion of Tetraaryldihydrodibenzothiepins into Elemental Sulfur
and Stable Cationic Dyes Accompanied by Dual UV-vis and CD Spectral Changes
Takanori Suzuki,*1 Takuma Kuroda,1 Hitomi Tamaoki,1 Sho Higasa,2 Ryo Katoono,1
Kenshu Fujiwara,1 Takanori Fukushima,3 and Hidetoshi Yamada2
1Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
2School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337
3Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503
(Received March 29, 2013; CL-130275; E-mail: tak@mail.sci.hokudai.ac.jp)
Upon oxidation of the title heterocycles with dimethylamino
a)
Ar Ar
groups, elemental sulfur is extruded to form dicationic dyes,
which regenerate the heterocycles by the reaction with Na2S.
The electrolysis of the heterocycles induces a drastic change in
UV-vis spectrum exhibiting several isosbestic points. The bay-
region substituents enhance configurational stability, so the
optically pure heterocycle can be obtained in terms of helicity of
one-handedness, and its chromism is accompanied by a large
chiroptical response as detected by CD spectroscopy.
Ar
2e-, "S"
S
Ar
Ar
Ar
Ar
x 2
MG+
S2-
1a
b)
Ar Ar
S
Ar Ar
Ar
X
Ar
Ar
2e-, "S"
X
X
X
X
Organosulfur compounds exhibit unique reactivities that are
absent in the oxygen congeners. For example, redox activation
of sulfur enables various transformations that are useful in
organic synthesis1 and materials chemistry.2 Oxidation of
sulfides commonly induces oxygenation of sulfur to give the
corresponding sulfoxides and sulfones (eq 1).3 On the other
hand, desulfurization accompanied by C-S bond fission is
another reaction path that is often found in the strained
thiiranes.4 The C-S bond in an aryl benzyl sulfide is cleaved
readily under electron-transfer conditions.5 Based on the facile
C-S bond mesolysis,6 we envisaged that the novel reaction
giving elemental sulfur and two cationic species would be
realized (eq 2) upon oxidation of sulfides with cation-stabilizing
substituents, such as bis(triarylmethyl) sulfides 1.
X
X X
X
X
X
X
S2-
3a,b2+
2a,b
[Ar = 4-Me2NC6H4; a: X = H; b: X = OBn ]
Scheme 1. Oxidative desulfurization of bis(triarylmethyl)
sulfides.
a pale colored powder in 67% yield. Similarly, “clamped”
¹
analogue 2a was obtained upon treatment of 3a2+(BF4
)
13 with
2
5 equiv of Na2S in CH3CN/H2O (5/1 v/v) as a white powder.
The quantitative yield of 2a was accomplished despite con-
gestion around the suflur atom by the two triarylmethyl
units.11,14 To minimize steric hindrance, 2a adopts a skewed
conformation with a twisted biphenyl skeleton as in 3a2+
(Figure S1).15 Thus, both 2a and 3a2+ are endowed with a
chiral element of helicity and axial chirality, respectively,
although interconversion between enantiomers proceeds easily
at room temperature11 (Scheme 2). Thiepin derivative 2b with
benzyloxy groups was then designed with anticipation that the
bulky bay-region substituents would increase the racemization
barrier to allow optical resolution. The M enantiomer of 2b12
was obtained in 90% yield upon Na2S-treatment of the
oxdn
oxdn
oxdn
R2S
R2SO2
R2S=O
2 R+
ð1Þ
ð2Þ
R2S
+
1/8 S8
Although parent bis(triphenylmethyl) sulfide has been
known for a century,7 its properties and reactivities were
scarcely investigated,8 and only few derivatives are known to
date.9 During the course of our studies on novel redox-based
response systems,10 we found that, upon treatment with Na2S,
biphenyl-2,2¤-diylbis[bis(4-dimethylaminophenyl)carbenium]
(3a2+) was converted to 2a,11 the first member of 5,5,7,7-
tetraaryl-5,7-dihydrodibenzo[c,e]thiepin, which is the “clamped”
analogue of bis(triarylmethyl) sulfide 1a. Here, we found that 2a
as well as newly prepared 2b undergo an oxidative desulfuriza-
tion, giving rise to elemental sulfur and the corresponding
dicationic dyes 3a2+ and 3b2+, respectively (Scheme 1); this is
the successful demonstration of the reaction illustrated in eq 2.
“Unclamped” sulfide 1a was also prepared from bis(4-dimethyl-
aminophenyl)phenylcarbenium (MG+), and its properties and
reactivities are also examined in this study.
¹
corresponding dicationic dye salt (R)-3b2+(BF4 )2,12 which
was prepared from diol (R)-4b12 derived from diester (R)-5b
(Scheme 3). The optically pure diester 5b16 was facilely
synthesized in a stereoselective manner and was therefore
chosen as a suitable starting material in this study. All these
hexabenzyloxy derivatives are configurationally stable, and
there were no signs of racemization upon transformation or
standing.
According to the voltammetric analyses,17 sulfide 1a and
thiepins 2a and 2b are strong electron donors. They undergo
irreversible electrochemical oxidation at E1/2 = +0.71, +0.57,
and +0.59 V vs. SCE, respectively (Figure S2).15 Upon treat-
ment of 1a with 3 equiv of iodine in CH2Cl2, a deep green
¹
When MG+BF4 was allowed to react with 5 equiv of Na2S
in CH3CN/H2O/CH2Cl2 (5/1/1 v/v), characteristic deep blue
¹
color of the cationic dye gradually disappeared, affording 1a12 as
powder of MG+I3 was obtained in 92% yield. Considering
Chem. Lett. 2013, 42, 706-708
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

707
Ar
X
Ar
Ar Ar
S
Ar
X
Ar
Ar
Ar
X
X
X
X
X
X
X
X
X
X
(R)-32+
(M)-2
X = H
X = OBn
X = H
Ar
X = OBn
Ar Ar
ArAr
S
Ar
X
Ar
Ar
X
X
X
X
X
X
X
X
X
X
X
(S)-32+
(P)-2
[ Ar = 4-Me2NC6H4 ]
Figure 1. A continuous change in UV-vis spectrum upon
constant current electrochemical oxidation of 1a (5.0 © 10¹6 M)
in CH2Cl2 containing 0.05 M Bu4NBF4 (5 ¯A, every 4 min). The
newly appeared absorptions are identical to those of MG+.
Scheme 2. Configurational stability of biphenyl moiety.
Ar Ar
Ar
HO
Ar
OH
MeOCO CO2Me
BnO
OBn
BnO
OBn
OBn
ArLi
y. 90%
OBn
BnO BnO
OBn
BnO BnO
OBn
(R)-5b
(R)-4b
y. 94%
HBF4
Na2S
-
(M)-2b
(R)-3b2+(BF4 )2
y. 90%
[ Ar = 4-Me2NC6H4 ]
Scheme 3. Preparation of optically pure (M)-2b.
steric congestion around the sulfur atom,14 the S-I bond forming
process may not be an essential initiation to cause desulfuriza-
¹
tion. In fact, treatment of 1a with (4-BrC6H4)3N•+SbCl6
(2 equiv) similarly induced effective formation of cationic salt,
MG+SbCl6 (y. 93%). Unimolecular cleavage5 of C-S bond
¹
in 1a•+ would be a plausible key step to furnish MG+
(Scheme S1).15 Thus, the conversion of 1a was also induced
electrochemically. When the electrolysis of 1a was conducted in
CH2Cl2, a continuous change exhibiting several isosbestic points
was observed by UV-vis spectroscopy (Figure 1), demonstrat-
ing that the oxidation reaction of Scheme 1a is accompanied by
a chromic response with a vivid change in color from colorless
to deep blue.
When thiepin 2a was allowed to react with 3 equiv of iodine
¹
12
in CH2Cl2, a dark purple powder of 3a2+(I3
)
2
was obtained
in 88% yield. From the filtrate, elemental sulfur was also
isolated.18 When the electrolysis of 2a was conducted in
CH2Cl2, a characteristic deep blue color developed, and a
continuous change exhibiting several isosbestic points was
observed by UV-vis spectroscopy (Figure S3).15,19 Considering
that the twisted ³-systems exhibit strong circular dichroism
(CD) signals by exciton coupling mechanism,20 it is expected
that the electrochemical oxidation of (M)-2b [-ext 306 nm
(¦¾ + 30.3), 265 (+54.8), 243 (¹67.6)] would exhibit a large
change in the CD spectrum. In fact, clean conversion exhibiting
isosbestic points was observed not only in the UV-vis but also
Figure 2. Continuous changes in UV-vis (upper) and CD
(lower) spectra upon constant current electrochemical oxidation
of (M)-2b (9.1 © 10¹6 M) in CH2Cl2 containing 0.05 M
Bu4NBF4 (5 ¯A, every 8 min).
Chem. Lett. 2013, 42, 706-708
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

708
in the CD spectra (Figure 2).19 Thus, oxidative desulfurization
reaction of title heterocycles can be accompanied by electro-
chiroptical response when the biphenyl moiety in 2 and 32+ is
attached with the proper substituents (e.g., OBn) to enhance
configurational stability.
Further studies on the oxidative desulfurization of the
sulfides and related compounds with cation-stabilizing substitu-
ents are now in progress.
9
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12 Selected spectral data are given in Supporting Information
(ref 15).
13 T. Suzuki, J.-i. Nishida, T. Tsuji, Angew. Chem., Int. Ed.
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This work was performed under the Cooperative Research
Program of “Network Joint Research Center for Materials and
Devices.” This work was supported by a Grant-in-Aid for
Scientific Research on Innovative Areas: “Organic Synthesis
Based on Reaction Integration” (No. 2105) from MEXT, Japan.
References and Notes
15 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
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resulting suspension was filtered to give 3a2+(I3 )2 (1.73 g,
y. 88%) as a dark purple powder. The filtrate was treated
with P(OMe)3 to remove remaining iodine, and chromato-
graphic separation (Al2O3, ether) gave elemental sulfur
(12.9 mg), which corresponds to 31% of the calculated
amount according to eq 2.
19 The newly appeared absorptions are not identical to those of
dication 32+ (ref 15) since the C-S mesolysis of 2•+ would
afford a cationic intermediate that needs further oxidation to
cleave another C-S bond before furnishing 32+ (Scheme S1)
(ref 15).
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Chem. Lett. 2013, 42, 706-708
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/