Facile preparation of helical ladder-type polymers with fused phenoxathiine rings

Article abstract of DOI:10.1246/cl.2005.164

Poly(3-chiral methylsulfinyl-1,2-phenyleneoxide) was synthesized, and the regioselective intramolecular ring-closing of the pendant sulfoxide on the polymer formed the corresponding helical ladder structure comprising of a fused phenoxathiine ring. The CD

Full text of DOI:10.1246/cl.2005.164

164
Chemistry Letters Vol.34, No.2 (2005)
Facile Preparation of Helical Ladder-type Polymers with Fused Phenoxathiine Rings
Tomokazu Iwasaki, Yoshiaki Tsukahara, and Hiroyuki Nishideꢀ
Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555
(Received November 10, 2004; CL-041346)
Poly(3-chiral methylsulfinyl-1,2-phenyleneoxide) was syn-
monomer 4-(R)11 was synthesized by the acetyl protection of 1,
the asymmetric oxidation of 2 to give 3-(R), and the subsequent
deprotection. 4-(S)11 was similarly prepared using (S,S)-diethyl
tertrate.
thesized, and the regioselective intramolecular ring-closing of
the pendant sulfoxide on the polymer formed the corresponding
helical ladder structure comprising of a fused phenoxathiine
ring. The CD of the ladder polymer gave a Cotton effect which
was caused by the chirality of the pendant group.
OR
O
n
(R) (v)
(R)
(ii)
*
*
SCH3
O
SCH3
O
A series of helical macromolecules have been studied ex-
pecting their potential applications involving chiral separation
and sensing, and liquid crystalline formation based on their pre-
cisely ordered stereostructure.1–5 Among the helical macromole-
cules, helicenes, which are helical molecules comprising of
fused aromatic rings, have been focused because of their unique
optical activity and stiff structures derived from the aromatic
fused-rings.6–8 However, many synthetic steps and tedious pro-
cedures are often required to obtain their non-racemates. We re-
port in this paper, for the first time, the synthesis of a phenoxa-
thiine-based helical ladder polymer, poly[dioxa-3,6-bis(methyl-
sulfonio)-1,2-phenylene]. The key step to form the ladder struc-
ture comprising of fused phenoxathiine ring is the one-pot
quantitative ring-closing reaction of the pendant alkyl sulfoxide
group on the aromatic polymer, which has been already reported
by us.9 In addition, a crucial factor to synthesize the helical
molecules is how to control the helicity. In this paper, the intra-
molecular ring-closing reaction of the pendant sulfoxide group
was regioselectively examined in order to induce a one-side bias
in the helicity using a chiral hydroxy sulfonium cation, as pre-
sented in Scheme 1. Our attempt to synthetically control the hel-
icity of the ladder polymer is also described.
CH3O
CH3O
OR
3-(R) R: Ac
4-(R) R: H
(iv)
5-(R)
SCH3
CH3O
1 R: H
2 R: Ac
OR
O
n
(S)
(S)
(iii)
(i)
(v)
*
*
SCH3
O
SCH3
O
CH3O
CH3O
3-(S) R: Ac
4-(S) R: H
(iv)
5-(S)
(i) Ac2O, pyridine, rt, 12 h. (ii) t-BuOOH, Ti(O-i-Pr)4, (R,R)-diethyl
tartrate, CH2Cl2, H2O, –20 °C, 24 h. (iii) t-BuOOH, Ti(O-i-Pr)4, (S,S)-
diethyl tartrate, CH2Cl2, H2O, –20 °C, 24 h. (iv) Na2CO3, CH3OH, H2O, rt,
12 h. (v) Cu(I)Cl, pyridine, nitrobenzene, rt., 24 h.
Scheme 2.
The oxidative polymerization of 4-(R) was examined, on the
basis of the previous reports regarding the polymerization of
phenol derivatives12,13 using oxidants and oxidative catalysts
such as K3[Fe(CN)6],13a Ag2O,13b PbO2,13c a copper-pyridine
complex,13d and horseradish peroxidase.13e The Cu/pyridine-
catalyzed polymerization of 4-(R) afforded poly(3-(R)-methyl-
sulfinyl-5-methoxy-1,2-phenyleneoxide) (5-(R))14 with the mo-
lecular weight, Mn of 1800 (Mw=Mn ¼ 1:2). The degree of poly-
merization of ca. 10 for 5-(R) as a polymer. It is enough to form
two pitches of the resultant helical ladder polymer (5 benzene
units in 6-(R) for 1 helical pitch).
Along with the molecular designing of the target helical lad-
der structure, the chiral sulfoxide-substituted poly(1,2-phenyl-
eneoxide) was synthesized as the precursor polymer for the fol-
lowing intramolecular ring-closing reaction (Scheme 2). Be-
cause the asymmetric oxidation10 of 4-methoxy-2-(methylthio)-
phenol 1 did not proceed because of the complexation of the
hydroxy group of 1 with titanium tetra(iso-propoxide), the chiral
The regioselective intramolecular ring-closing of the pend-
ant sulfoxide group on 5-(R) proceeded by just the addition of
triflic acid (without any further treatment) to yield the ring-
closed 6-(R) (Scheme 3).15 For the obtained sample of 6-(R),
CH3
S
A
A
CH3
CH3
S
O
O
CH3
H A
HO
HO
S
O
S
O
1
O
the H NMR signal at 2.99 ppm ascribed to the methylene unit
O
adjacent to the sulfur atom of the parent polymer 5-(R) com-
pletely disappeared and it was shifted to down-field (3.24 ppm)
due to the electron-withdrawing effect of the formed sulfonio
group. The signal attributed to the phenyl ring was also shifted
to down-field (8.31 ppm) for 6-(R) and the integration value of
the proton (1H) agreed with the calculated one based on the
fused-ring structure of 6-(R). The UV–vis absorption maximum
and shoulder of 6-(R) bathochromically shifted in comparison
with those of the pre-polymer 5-(R) to support the extended ꢀ-
Intermediate
CH3
CH3
A
A
S
S
O
H2O
O
Scheme 1.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.2 (2005)
165
tacked oneside of the neighboring phenylene ring. The bridged
chiral sulfonio group located on the oneside of the phenylene
ring to yield the helical ladder structure with one-sided bias. This
synthetic route is expected to be applicable for the preparation of
other sulfur-containing helical ladder polymers.
TfO
CH3
CH3
S
S
TfO
CH3O
O
O
O
O
CF3SO3H
CF3SO3H
5-(R)
5-(S)
6-(R)
6-(S)
OCH3
CH3
TfO
CH3
S
O
S
S
This work was partially supported by Grant-in-Aid for the
Scientific Research and the COE Research Programs ‘‘Molecular
Nano-Engineering’’ and ‘‘Practical Nano-Chemistry’’ from
MEXT, Japan.
TfO
CH3O
TfO
CH3 OCH3
6-(R)
6-(S)
References and Notes
Scheme 3.
1
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conjugation or the ladder structure of 6-(R). The (S)-enantiom-
ers, 5-(S)14 and 6-(S),15 were also synthesized according to the
same procedure of the corresponding 5-(R) and 6-(R), respec-
tively.
2
Circular dichroism (CD) of the monomer, 4-(R) and 4-(S),
exhibited the peak at 239 nm ascribed to the chirality of the
pendant sulfoxide, which also appeared in the CD of the precur-
sors, 5-(R) and 5-(S). However, no Cotton effect was observed
beyond the wavelength of 270 nm. On the other hand, 6-(R)
and 6-(S) showed the CD extremum at 246 nm and the Cotton
effect between 278–350 nm (Figure 1). The former CD extrem-
um was attributed to the chiral sulfonio bridge and the latter
broad one corresponded to the ꢀ–ꢀꢀ transition in the UV–vis ab-
sorption, suggesting the formation of a helical structure. 6-(R)
and 6-(S) had a symmetrical CD profile in the positive and neg-
ative sides, which was not influenced by the temperature in the
range of 20–50 ꢁC. These results indicated that the helicity of
the ladder polymer was caused by the chirality of the monomer.
In other words, the helicity of the ladder polymer 6 could be con-
trolled by the enantiotropy of the diethyl tartrate during the
asymmetric oxidation of the monomer.
3
4
5
6
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8
9
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20
20
11 4-(R): ½ꢁꢂ
+87ꢁ(c 1.00, CHCl3). 4-(S): ½ꢁꢂ
ꢃ84ꢁ (c
D
D
1.00, CHCl3).
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Figure 1. UV–vis and CD spectra of 0.1 mM 6-(R) (solid line)
and 6-(S) (dashed line) CH3CN solution.
Kobayashi, Macromolecules, 29, 8702 (1996).
14 5-(R): Yield 72%. Mn ¼ 1800 (Mw=Mn ¼ 1:2). ½ꢁꢂ
20
D
In summary, a novel helical ladder polymer comprising of
fused phenoxathiine ring 6 was synthesized through the regiose-
lective intramolecular ring-closing of the pendant chiral sulfox-
ide in the poly(1,2-phenyleneoxide) derivative. The mechanism
of the helical formation with one-sided bias could be described
as follows (Scheme 1). The pendant chiral sulfoxide of 5 was
protonated by the superacid to generate the chiral hydroxy sulfo-
nium cation as an intermediate. The cation electrophilically at-
+128ꢁ (c 1.00, CHCl3). UV–vis (CH3CN, 0.1 mM):
ꢂmax ¼ 307 nm, ꢂshoulder ¼ 372 nm). 5-(S): Yield 77%.
20
Mn ¼ 1800 (Mw=Mn ¼ 1:2). ½ꢁꢂ
ꢃ124ꢁ (c 1.00, CHCl3).
D
15 6-(R) and 6-(S): Yield: 97%. 1H NMR (DMSO-d6, 500 MHz;
ppm): ꢃ 8.31 (s, 1H), 3.91 (s, 3H), 3.24 (s, 3H). IR (KBr,
cmꢃ1): 1268, 774 (ꢄC-F), 1259 (ꢄC-O-C), 1304, 1148 (ꢄSO ).
2
UV–vis (CH3CN, 0.1 mM): ꢂmax ¼ 321 nm, ꢂshoulder
¼
406 nm.
Published on the web (Advance View) December 25, 2004; DOI 10.1246/cl.2005.164