Synthesis of helical polyisocyanides bearing aza-crown ether groups as pendant

Article abstract of DOI:10.1002/pola.28919

We, herein, present a novel synthesis of responsive helical poly(aryl isocyanide)s bearing aza-crown ethers as pendant groups. Chiral aryl isocyanide monomers bearing an aza-crown ether as a pendant were designed and synthesized, piror to polymerization u

Full text of DOI:10.1002/pola.28919

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Synthesis of Helical Polyisocyanides Bearing Aza-Crown Ether
Groups as Pendant
Naoya Kanbayashi , Shingo Tokuhara, Tomoko Sekine, Yuki Kataoka
Taka-aki Okamura , Kiyotaka Onitsuka
,
Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
Correspondence to: N. Kanbayashi (E-mail: naokou@chem.sci.osaka-u.ac.jp)
Received 14 October 2017; accepted 19 November 2017; published online 00 Month 2017
DOI: 10.1002/pola.28919
ABSTRACT: We, herein, present a novel synthesis of responsive
helical poly(aryl isocyanide)s bearing aza-crown ethers as pen-
dant groups. Chiral aryl isocyanide monomers bearing an aza-
crown ether as a pendant were designed and synthesized, piror
to polymerization using a Pd-Pt m-ethynediyl complex as an ini-
tiator to give the corresponding polymers in good yield. The
resulting polyisocyanides adopted a stable helical structure in
solution, as confirmed by circular dichroism spectroscopic
analysis. In addition, the polymers were soluble in various sol-
vents. Furthemore, the addition of suitable alkali metal ions to
the crown ether of the sidechain on the helical polyisocyanide
to form host-gest complexes resulted in deformation of the
helix due to electrostatic repulsion, and these phenomena
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depended on the size of metal cations.
2017 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 00,
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KEYWORDS: chiral; crown ether; helical polymer; molecular recog-
nition; polyisocyanides; synthesis
functional groups have been synthesized.39–41 For example, the
polymerization was successful using a monomer bearing a
bulky tetraphenyl porphyrin as a substituent,39 and the porphy-
rin pendants were regularly arranged to form stacked columns
through the helical main chain of the polyisocyanides. Addition-
ally, intramolecular energy transfer from zinc-porphyrin to the
free-base-porphyrins has also been observed.42 As a result,
novel properties that were not observed for simple monomers
would be expected upon the expansion of this system.
INTRODUCTION Chiral helical polymers have received a great
deal of attention in polymer chemistry, as naturally occurring
polymers often form sophisticated helical structures. Such
structures act as foundations for the construction of higher-
ordered structures that often determine the characteristic
functions of biomacromolecules. In artificial polymers, the
precise synthesis of helical polymers has also been reported
for the preparation of potential functional materials.1–5 To
date, various types of helical polymers have been synthes-
ised,6–9 for use in chiral catalysts,10–12 and molecular recog-
nition materials as well as in biomimetic applications.
Although these poly (aryl isocyanide)s conventionally form
stable one-handed helical structures in solution, it is possible
to alter their conformation by derivatization of the side chain.
Indeed, we previously presented the synthesis of chiral poly(i-
socyanide)s bearing ferrocenyl pendants [Scheme 1(a)], which
exhibited response towards oxidation of ferrocenyl groups
through transformation from a stable helical structure to a
disordered one through electronic stimulus of the ferrocenium
cation.41 Moreover, the conformational changes of polyisocya-
nides using external stimuli can be potentially applied not
only in the area of stimulus-responsive polymers, but also in
the synthesis of new type of novel types of polymers through
the combination with conventional polyisocyanide functionali-
ties. However, example of stimuli-responsive poly-isocyanides
accompanied by structural change are limited due to the
In this context, polyisocyanides consist of an sp2-hybridised
carbon main chain and imino side groups, which can adopt a
stable helical conformation in solution when bulky substituents
are present.13,14 Indeed, optically active helical polyisocyanides
have been synthesized using chiral monomers and chiral initia-
tors,15–24 and have been widely applied in many fields.25–31 In
addition, we previously developed a living polymerization of
aryl isocyanide using a Pd-Pt m-ethynediyl complex (I)32 as an
initiator,33,34 where polyisocyanides exhibiting predominantly
one-handed helical structures were produced by polymeriza-
tion of the isocyanide monomers with chiral groups.35–38 As
this polymerization system was applicable to a range of sub-
stituents, helical poly(aryl isocyanide)s bearing various
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Responsive poly(arylisocyanode)s using (a) oxidation and (b) complexation with metal cation. [Color figure can be
viewed at wileyonlinelibrary.com]
difficulty in synthesizing the polymer.42 Thus, we herein pre-
sent the synthesis of one-handed helical poly(aryl isocyanide)s
bearing aza-crown ether groups as stimuli-responsive pend-
ants. Furthermore, we demonstrate the response of the helical
conformation of these novel chiral polymers to the addition of
specific alkali metal ions [Scheme 1(b)].
temperature was kept at 50 8C. IR spectra were measured on
Shimadzu IR Prestage 21. CD and UV–vis spectra were measured
on JASCO J-720WO and Shimadzu UV 3100PC, respectively.
Materials
1-Aza-15-crown-5 and 1-aza-18-crown-6 were commercially
available. Pt-Pd dinuclear complex was prepared according
to the literature.32
EXPERIMENTAL
General
Synthesis of 1a
1H and 13C NMR spectra were recorded on JEOL GSX-400 and
JEOL ECA-500 spectrometers. HR-MS measurement was carried
out on PerkinElmer/Sciex Q-Star mass spectrometer. Polymeri-
zation progress was checked by size exclusion chromatography
(SEC) equipped with TOSOH a-M using Shimadzu LC-6AD and
Shimadzu SPD-10A UV–vis detector. DMF was used for eluting
solvent, the flow velocity was configured to 0.8 mL/min, and the
(S)-2-(Benzyloxycarbonylamino)-1-(1,4,7,10-tetraoxa-13-
azacyclopentadecan-13-yl)propan-1-one (2a)
To a dry DMF solution (2 mL) of benzyloxycarbonyl-L-alanine
(0.20 g, 0.91 mmol) and benzo-aza-15-crown-5 (0.20 g, 0.91
mmol) was added 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-meth-
ylmorpholinium chloride (DMT-MM) (0.416 g, 1.37 mmol) at
room temperature. After the reaction mixture was stirred for
2
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3 h, the suspended solution was filtrated. The filtrate was added
to water and extracted with CH2Cl2 three times. The organic
phase was washed with brine, then dried over Na2SO4, and con-
centrated. The residue was purified by alumina chromatography
(CH2Cl2/MeOH5 20: 1) to give yellow oil (0.317 g, 82%).1H
NMR (CDCl3, 500 MHz): d 7.35–7.31 (m, 5H, Ph), 5.61 (d, 1H,
J 5 7.8 Hz, NAH), 5.11 (d, 1H, J 5 12.3 Hz, CH2Ph), 5.08 (d, 1H,
J 5 12.3 Hz, CH2Ph), 4.74 (quart, 1H, J 5 6.7 Hz, NCHCO)
3.98–3.46 (m, 20H, azacrown), 1.34 (d, 3H, J 5 6.7 Hz, CHCH3).
C
21H29N3NaO6 ([M 1 Na]1): m/z 442.1954, Found: m/z
442.1959.
Synthesis of 1b
(S)-2-(Benzyloxycarbonylamino)-1-(1,4,7,10,13-pentaoxa-16-
azacyclooctadecan-16-yl)propan-1-one (2b). To a dry DMF
solution (10 mL) of benzyloxycarbonyl-L-alanine (2.20 g,
9.83 mmol) and 1-aza-18-crown-6 (2.59 g, 9.83 mmol) was
added DMT-MM (3.26 g, 11.8 mmol) at room temperature.
After the reaction mixture was stirred for 6 h, the sus-
pended solution was filtrated. The filtrate was added to
water and extracted with CH2Cl2 three times. The organic
phase was washed with brine, then dried over Na2SO4, and
concentrated. The residue was purified by alumina chroma-
tography (CH2Cl2/MeOH 5 20:1) to give yellow oil (3.74 g,
90%).1H NMR (CDCl3) d: 7.35–7.29 (m, 5H, Ph), 5.61 (d,
1H, J 5 7.8 Hz, NAH), 5.11 (d, 1H, J 5 12.1 Hz, CH2Ph), 5.07
(d, 1H, J 5 12.1 Hz, CH2Ph), 4.74 (quart, 1H, J 5 7.1 Hz,
NCHCO), 3.75–3.50 (m, 24H, azacrown), 1.35 (d, 3H, J 5 6.7
Hz, CHCH3).
(S)-2-Amino-1-(1,4,7,10-tetraoxa-13-azacyclopentadecan-
13-yl)propan-1-one (3a)
A methanol solution (20 mL) of 2a (0.597 g, 1.4 mmol) and
10% Pd/C (0.075 g, 0.070 mmol) was stirred under 0.7 MPa
atmosphere of H2 overnight at room temperature. The mix-
ture was filtrated with Celite. The solution was concentrated
under reduced pressure to give colorless oil (0.317 g, 77%).
1H NMR (CDCl3, 400MHz): d 3.90–3.35 (m, 20H, azacrown),
1.25 (d, 3H, J 5 6.7 Hz, CHCH3).
(S)-N-(1-(1,4,7,10-Tetraoxa-13-azacyclopentadecan-13-yl)-
1-oxopropan-2-yl)-4-formamidobenzamide (4a)
(S)-N-(1-(1,4,7,10,13-Pentaoxa-16-azacyclooctadecan-16-
yl)-1-oxopropan-2-yl)-4-formamidobenzamide (4b)
To a dry DMF solution (1 mL) of 3a (0.12 g, 0.41 mmol) and
p-formamidobenzoic acid14 (0.067 g, 0.41 mmol) was added
DMT-MM (0.135 g, 0.49 mmol) at room temperature. After
the reaction mixture was stirred overnight, the suspended
solution was filtrated. The filtrate was added to water and
extracted with CH2Cl2. The organic phase was washed with
brine. After dried over Na2SO4, the solution was concen-
trated to give white solid (0.135 g, 76%). 1H NMR (CDCl3,
400 MHz): d 8.78 (d, 1H, J 5 11.2 Hz, CHO, minor), 8.41 (d,
1H, J 5 1.6 Hz, CHO, major), 7.82–7.78 (m, 2H, Ar, major and
minor), 7.60 (d, 2H, J 5 8.7 Hz, Ar, major), 7.17–7.13 (m, 1H,
NAH), 7.09 (d, 2H, J 5 8.7 Hz, Ar, minor), 5.12 (quart, 1H,
J 5 6.7 Hz, NACHACO), 3.95–3.57 (m, 20H, azacrown), 1.44
(d, 3H, J 5 6.7 Hz, CHCH3). 13C NMR (CDCl3, 125 MHz): d
173.6, 165.9, 162.1, 140.5, 128.8, 128.1, 119.2, 71.2, 70.6,
70.3, 70.1, 70.0, 70.0, 69.6, 69.0, 50.0, 49.4, 45.9, 19.0.
To a dry DMF solution (10 mL) of 3b (2.25 g, 6.73 mmol)
and p-formamidobenzoic acid14 (1.11 g, 6.73 mmol) was
added DMT-MM (2.23 g, 8.08 mmol) at room temperature.
After the reaction mixture was stirred overnight, the sus-
pended solution was filtrated. The filtrate was added to
water and extracted with CH2Cl2. The organic phase was
washed with brine. After dried over Na2SO4, the solution
1
was concentrated to give white solid (2.12 g, 64%). H NMR
(CDCl3, 400 MHz): d 8.78 (d, 1H, J 5 11.3 Hz, CHO, minor),
8.42 (d, 1H, J 5 1.4 Hz, CHO, major), 7.82–7.78 (m, 2H, Ar,
major and minor), 7.61 (d, 2H, J 5 8.6 Hz, Ar, major), 7.20–
7.16 (m, 1H, NAH), 7.10 (d, 2H, J 5 8.6 Hz, Ar, minor), 5.15
(quart, 1H, J 5 7.2 Hz, NCHCO), 3.78–3.67 (m, 24H, aza-
crown), 1.45 (d, 3H, J 5 6.7 Hz, CHCH3). 13C NMR (CDCl3,
125 MHz): d 172.9, 166.5, 164.3, 135.0, 128.7, 128.3, 126.4,
71.0, 70.7, 70.7, 70.7, 70.7, 70.6, 70.5, 70.4, 69.5, 69.4, 48.8,
46.8, 46.1, 19.1.
(S)-N-(1-(1,4,7,10-Tetraoxa-13-azacyclopentadecan-13-yl)-
1-oxopropan-2-yl)-4-isocyanobenzamide (1a)
To a dry CH2Cl2 solution (18 mL) of 4a (0.191 g, 0.44 mmol)
was added dry Et3N (0.5 mL, 3.6 mmol) and triphosgene
(0.194 g, 0.65 mmol) with cooling in ice bath, and the reac-
tion mixture was stirred for 1 h at room temperature under
Ar atomosphere. After quenched with water, the solution
was washed with NaHCO3 aq and NaCl aq, successively. The
organic layer was dried over Na2SO4, and concentrated. The
residue was purified by alumina chromatography (CH2Cl2/
MeOH 5 200:1) to give brown oil (0.138 g, 76%). 1H NMR
(CDCl3, 400 MHz): d 7.85 (d, 2H, J 5 8.7 Hz, Ar), 7.43 (d, 2H,
J 5 8.7 Hz, Ar), 7.22 (d, 1H, J 5 7.0 Hz, NH), 5.11 (quint, 1H,
J 5 7.0 Hz, NCHCO), 3.95–3.45 (m, 20H, azacrown), 1.45 (d,
3H, J 5 7.0 Hz, CHCH3). 13C NMR (CDCl3, 125 MHz): d
173.16, 166.69, 164.51, 135.22, 129.00, 128.54, 126.66,
71.66, 70.89, 70.60, 70.42, 70.28, 70.25, 69.79, 69.27, 50.12,
49.60, 46.41, 19.39. IR: 3296 cm21 (NAH), 2123 cm21
(CBN), 1651, 1633 cm21 (C@O). HRMS (ESI): Calcd for
(S)-N-(1-(1,4,7,10,13-Pentaoxa-16-azacyclooctadecan-16-
yl)-1-oxopropan-2-yl)-4-isocyanobenzamide (1b)
To a dry CH2Cl2 solution (15 mL) of 4b (0.214 g, 0.43
mmol) was added dry Et3N (0.15 mL, 1.1 mmol) and tri-
phosgene (0.061 g, 0.22 mmol) with cooling over ice bath,
and the reaction mixture was stirred for 1 h at room temper-
ature under Ar atomosphere. After quenched with water, the
solution was washed with NaHCO3 aq and NaCl aq, succes-
sively. The organic layer was dried over Na2SO4, and concen-
trated. The residue was purified by alumina chromatography
(CH2Cl2/MeOH 5 200:1) to give brown oil (0.169 g, 85%).
1H NMR (CDCl3, 400 MHz): d 7.85 (d, 2H, J 5 8.5 Hz, Ar),
7.43 (d, 2H, J 5 8.5 Hz, Ar), 7.21–7.23(m, 1H, J 5 7.0 Hz, NH),
5.15 (quint, 1H, J 5 7.0 Hz, NCHCO), 3.80–3.55 (m, 24H, aza-
crown), 1.47 (d, 3H, J 5 7.0 Hz, CHCH3). 13C NMR (CDCl3,
125 MHz): d 172.9, 166.5, 164.3, 135.0, 128.7, 128.3, 126.4,
71.0, 70.7, 70.7, 70.7, 70.7, 70.6, 70.5, 70.5, 69.5, 69.4, 53.4,
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for each sample. DE (mol L21 cm21) was calculated using
the equation:
DE5 ð4plog e3hobs3M0Þ =ð180l31000cÞ
where hobs is the measured ellipticity in millidegrees, while
M0 is the molecular weight or molecular weight per unit in
the polymer, l is path length in 10 mm, and c is the concen-
tration of the sample.
Complexation of Poly-1 with Metal Cations
An acetonitrile solution of poly-1 (1.95 3 1026 M) and
metal tetraphenyl borate (1.95 3 1025 M) was heated at 50
8C in a sealed quartz cell with 10 mm optical path length
and the spectra change was monitored by the UV and CD
spectra.
RESULTS AND DISCUSSION
Crown ethers are cyclic oligo(ethylene oxide)s, that can form
stable complexes with alkali metals or organic cations
through ionic and dipole-dipole interactions.43 The combina-
tion of helical polymers, such as poly(phenyl acetylene), and
crown ethers has also been reported for the preparation of
potential functional materials.44–49 In our case, if crown
ethers can be stably positioned based on a chiral helical
structure, novel responsive chiral polymer could be create.
We began to our study by designing the chiral isocyanide
monomers bearing aza-crown ether groups (1a and 1b in
Scheme 2). As indicated synthetic route shown in Scheme 1,
N-benzyloxycarbonyl-L-alanine was employed as the chiral
source for condensation with two different sized aza-crown
ethers in the presence with 4-(4,6-dimethoxy-1,3,5-triazin-2-
yl)-4-methylmorpholinium chloride (DMT-MM) as the cou-
pling reagent. The benzyloxycarbonyl (Z) groups of the pro-
tected amine on products 2a and 2b were deprotected in
the presence of a Pd/C catalyst under 0.7 MPa H2 to yield
chiral amines 3a and 3b. Subsequent, condensation with p-
formamidobenzoic acid in the presence of DMT-MM and
dehydration of 4a and 4b using triphosgene in the presence
of triethylamine to yield desired monomers 1a and 1b.
SCHEME 2 Synthesis of isocyanide monomer 1.
48.8, 46.8, 46.1. IR: 3296 cm21 (NAH), 2123 cm21 (CBN),
1651, 1633 cm21 (C@O). HRMS (ESI): Calcd for C23H33N3NaO7
([M 1 Na]1): m/z 486.2216, Found: m/z 486.2215.
Polymerization of 1a
To a dry THF solution (2 mL) of 1a (135 mg, 0.32 mmol)
was added Pt-Pd dinuclear complex (I) (2.8 mg, 3.2 lmol)
under Ar atmosphere. The solution was stirred under reflux
for 12 h. The solution was added to hexane to give yellow
solid (90 mg, 66%). 1H NMR (DMSO-d6): d 9.0–7.9 (broad,
1H, NH), 7.4–6.4 (broad, 2H, Ar), 4.8–6.2 (broad, 2H, Ar),
5.0–4.5 (broad, 1H, CH) 4.3–3.1 (broad, 20H, aza-crown ether
moiety), 1.47 (broad, 3H, CH3). IR (KBr, cm21): 3247 cm21
(NAH), 1629, 1604 cm21 (C@O), 1536 cm21 (C@N).
The polymerization of 1a was carried out in the presence of
Pd–Pt m-ethynediyl complex (I) based on previously reported
conditions; [1a] 5 0.05 M, [1a]/[I] 5 100, in THF at 55 8C.36
However, after allowing the reaction to proceed 4 days, the
monomer was not completely consumed (77% conv.) and
both polymeric and oligomeric product were formed, and
the further polymerization did not proceeded.50.The concen-
tration of 1a was increased (0.15 M) and attempting the
polymerization reaction under same conditions (see Table 1,
entry 1), polymerization proceeded smoothly with 92%
monomer conversion after 12 h to give the desired polymer
with a unimodal profile, as determined by SEC in DMF (see
Supporting Information). After purification by reprecipitation
with hexane, the desired polymer (poly-1a) was obtained in
66% yield (Mw 5 10,400, Mw/Mn 5 1.26).51 The polymeriza-
tion of 1b was attempted under the conditions of entry 1 to
give the desired polymer (poly-1b) in 66% yield after
Polymerization of 1b
To a dry THF solution (2 mL) of 1a (94 mg, 0.20 mmol) was
added Pt-Pd dinuclear complex (1.76 mg, 2.0 mmol) under Ar
atmosphere. The solution was stirred under 55 8C for 12 h. The
solution was added to hexane to give yellow solid (56 mg,
1
66%). H NMR (DMSO-d6): d 9.0–7.9 (broad, 1H, NH), 7.8–6.5
(broad, 2H, Ar), 4.8–6.2 (broad, 2H, Ar), 5.0–4.5 (broad, 1H,
CH), 4.3–3.1 (broad, 24H, aza-crown ether moiety), 1.51
(broad, 3H, CH3). IR (KBr, cm21): IR (KBr, cm21): 3247 cm21
(NAH), 1629, 1605 cm21 (C@O), 1543 cm21 (C@N).
CD and UV Spectra
UV and CD spectra were recorded on a JASCO J-720WO using
10 mm path length quartz cuvettes. Ten scans were averaged
4
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TABLE 1 Polymerization of Monomer 1 Using m-Ethynediyl Pt-
UV and CD spectra of poly-1a at 0–75 8C are shown in Fig-
ure 2(A). The intensity of the Cotton effect at 365 nm was
slightly decreased (ca. 10%) at higher temperature (50 and
75 8C), and the UV spectra were not changed. Additionally,
these transitions are completely reversible. Probably, the
phenomena was showed the conformational fluctuations of
flexible terminal moiety of the poly-1a and mainly adopted
the proposed helix conformation. Additionally, we also inves-
tigated the solvent effect of poly-1a. As shown in Figure
2(B), the CD spectra of poly-1a exhibited a noticeable
solvent effect, although UV spectra was not changed. The
intensity of the Cotton effect on 365 nm in the presence of
different solvents, it was apparent that the helical structure
of the poly(aryl isocyanide)s was altered to varying extent
(i.e., toluene < CHCl3 ꢀ THF <acetonitrile < DMF < methanol)
based on the value of Hildebrand solubility parameters of
the solvents examined [Fig. 2(C)].55 This indicates that the
solubility of solvents with the crown ethers present on the
side chains influences the helical conformation. As crown
ethers are relatively flexible in solution, their bulkiness can
be altered based on their affinity to any specific solvent.
Such transformations likely influence the helical structure
through changes in the main chain configuration (syn-anti
isomerization of the imino groups (Chart 1).26
Pd Dinuclear Complexa
Solvent Conv.b (%) Yield (%) Mw
Mw/Mn
c
c
Entry
1
1
2
1a THF
1b THF
92
80
66
66
10,400 1.26
6900 1.18
a
I (3.0 mmol), 1 (0.30 mmol) in THF, [1] 5 0.15 M, [1]/[I] 5100.
Estimated by the peak area ratio of SEC.
Estimated by SEC analysis using polystyrene standard.
b
c
purification (entry 2). Both polymers were soluble in water
and in common organic solvents, such as toluene, dichloro-
methane, THF, acetonitrile, and methanol.
C1
UV and CD Spectra
To determine the architecture of the obtained polymers, UV
absorption and circular dichroism (CD) measurements were
carried out. As shown in Figure 1, the CD spectrum of poly-
1a in CHCl3 at 25 8C exhibited a negative Cotton effect at
365 nm (DE365 5 213.6), which is characteristic of helical
chiral poly(aryl isocyanide)s and is assignable to the n-p*
transition of the imino chromophore. A similar Cotton effect
was also observed for poly-1b (CHCl3, 25 8C). The helical
senses of these poly(arylisocyanide)s were determined based
on the exciton-coupled CD of the porphyrin chromophores;40
therefore, this negative Cotton effect at 365 nm confirms
that the poly(aryl isocyanide)s adopted a left-handed helical
structure. Previously, Yashima and coworkers reported the
polymerization of phenyl isocyanide bearing an L-alanine
pendant with a long n-decyl chain using a Pt–Pd catalyst to
produce almost completely right- and left-handed helical pol-
yisocyanides with different molecular weights, where each
single-handed helical polymer was separated by facial sol-
vent fractionation.52 Based on their studies, helical-sense
excess (eeh)53,54 value were estimated to be 66% eeh and
70% eeh for poly-1a and poly-1b, respectively, which indi-
cated that these polymers contain both right- and left-
handed helices with different molecular weights. Indeed,
poly-1a and poly-1b gave relatively higher molecular weight
distributions, similar to that of Yashima’s polymer, immedi-
ately after polymerization. However, it was not possible to
separate poly-1 into its two individual helical structures.
Subsequent experiments were therefore conducted using
poly-1 without further purification.
Effect of a Cationic Guest on the Helical Conformation
To investigate the effect of cationic guest on the helical con-
formation of poly-1, the addition of various metal cations
was examined. Following the addition of sodium tetraphenyl-
borate (NaBPh4) to an acetonitrile solution of poly-1a
([NaBPh4]/[crown ether of poly-1] 5 10, [NaBPh4] 5 1.4 3
1022 M), CD measurements were recorded. As shown in Fig-
ure 3(A), the initial CD spectrum was comparable intensity
to that of the original spectrum of poly-1a. However, upon
warming the solution to 50 8C, the CD spectrum slowly
changed, and the CD intensity (ht) of the Cotton effect
around 365 nm decreased with time, despite the negligible
Temperature and Solvent Dependence of CD Spectra
The temperature dependence of the CD spectra of poly-1a
was examined to investigate the stability of the polymer. The
FIGURE 1 CD and UV spectra of poly-1a (0.10 mM) and poly-
1b (0.10 mM) in CHCl3 at 25 8C. [Color figure can be viewed at
wileyonlinelibrary.com]
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FIGURE 2 CD and UV spectra of (A) temperature dependence of poly-1a (0.10 M) in CHCl3 and (B) effect of the solvent of poly-1a
(0.10 M) at 25 8C, and (C) plots of Hidebrand solubility parameter (d) dependence change of the CD intensity at 365 nm. [Color fig-
ure can be viewed at wileyonlinelibrary.com]
changes in absorption [Fig. 3(A)]. After 360 h, h360 reached
67% (h360/h0 3 100) of the initial value (h0). In contrast, in
the absence of the metal salt, the change was less significant
(h360/h0 3 100 5 91%). In addition, this phenomenon was
found to depend on the type of the cation employees [Fig.
3(B)]. More specifically, when KBPh4 or CsBPh4 were
employed, the CD intensities decreased, but to a lesser extent
than for NaBPh4 [Fig. 3(B,C)]. These results indicate that the
aza-15-crown ether groups of poly-1a can selectively complex
with sodium cations to induce dramatic changes in the CD
spectra. In addition, it appears that the helical structure of
poly-1a partially collapses due to electrostatic repulsions
between the side chains bearing the cation-aza-15-crown ether
complexes (Fig. 4). Indeed, Yashima and coworkers reported
interconversion of the helical structures of poly(aryl isocya-
nide)s via syn-anti configuration isomerization around the
C@N bonds (Chart 1). In this case, the initial helical structure
of poly-1a was in its kinetically controlled conformation fol-
lowing the polymerization, and this conformation changed to
the more thermodynamically controlled structure56,57 due to
CHART 1 Stereoregularity in polyisocyanide.
6
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FIGURE 3 (A) Change in the CD spectra of the poly-1a (0.10 mM) with NaBPh4 in acetonitrile at 25 8C. (B) Change in the CD intensity
(ht/h0 3 100) of poly-1a with NaBPh4, KBPh4, and CsBPh4 in acetonitrile at 365 nm. (C) Change in the CD spectra of poly-1a with
NaBPh4, KBPh4, and CsBPh4 after 10 days, and initial polymer (t 5 0) in acetonitrile at 25 8C. (D) Change in the CD spectra of poly-1b
with NaBPh4, KBPh4, and CsBPh4 after 10 days, and initial polymer (t 5 0) in acetonitrile at 25 8C. The spectra of (C) and (D) are nor-
malized on the basis of the CD intensity on 365 nm of initial polymer. [Color figure can be viewed at wileyonlinelibrary.com]
the both electrostatic repulsions between the metal cationic
complexes of the crown ethers and to syn-anti isomerization at
high temperatures. As the interconversion of poly-1a is slow
in the absence of cationic salts even when heat is applied [Fig.
3(B)], it is apparent that electrostatic repulsions of the side
chains is necessary to meet the required activation energy for
the conformational change.26 Similar experiments were also
conducted using poly-1b, which has a larger aza-crown ether
ring size, that is, aza-18-crown. In this case, a decrease in the
CD intensities compared to that of the initial polymer was also
observed for all cations, with the largest change being
observed for the sodium cation. These results differ from the
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ACKNOWLEDGMENTS
This work was supported by a grant-in-aid for JSPS KAKENHI
Grant number JP16K21154 (N.K) and JP21350065 (K.O), and
the Kyoto Technoscience Center. The authors thank the group
of Takahiro Sato (Osaka University) for CD measurement.
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