Formation of cyclic and polymeric structures from Zwitterions

Article abstract of DOI:10.1002/chem.201300704

Covalent linkages: Zwitterion-based covalent linkages were observed in cyclic oligomers and in dispersed polymers based on monomers. Addition of acids and bases promoted transitions between monomeric species and the oligomeric and/or polymeric states. Exc

Full text of DOI:10.1002/chem.201300704

DOI: 10.1002/chem.201300704
Formation of Cyclic and Polymeric Structures from Zwitterions
Harunobu Komatsu,[a] Wataru Oi,[a] Kazumasa Naritani,[a] Kazuto Takaishi,[b, c]
Masanobu Uchiyama,[b, d] Atsuya Muranaka,[b, e] and Hiromitsu Maeda*[a]
Covalent linkages that connect atoms are fundamental in-
teractions that produce molecules. The formation and cleav-
age of covalent bonds by external stimuli can control the
states of molecules, such as monomers, oligomers, and poly-
mers. Based on this concept, dynamic covalent polymers
have been fabricated by using various exchangeable-bonding
strategies, such as those that have been seen in the Diels–
Alder reaction and in disulfides.[1,2] A potential strategy for
constructing covalent linkages is the nucleophilic attack of
an anionic site at a cationic site; in this case, an equilibrium
between a covalent bond and an ion-pair state would be ob-
served. Of various cationic units, a trityl cation is a suitable
motif that should undergo a nucleophilic attack by anionic
species after appropriate modifications of the substituents.[3]
In contrast, the triazatriangulenium (TATA) cation[4] (an
analog of the trityl cation) prefers a planar structure, which
prevents the facile formation of covalent linkages with
anionic sites.[5] Therefore, zwitterionic species consisting of a
fairly flexible trityl cation and an anionic unit would form
covalent polymers depending on conditions.
zwitterions 1b, 2b, and 3b, respectively. Introduction of
electron-donating dimethylamino moieties to the trityl aryl
rings, as was observed in malachite green, is necessary to
obtain stable cationic sites that can be attacked nucleophili-
cally by anionic species under the appropriate conditions.[4,7]
A phenol moiety exhibiting a pKa value of 10.0 was selected
as the precursor for the deprotonated anionic site that
would be suitable for covalent-bond formation with the
center carbon of the trityl unit. In addition, introduction of
According to a literature procedure, trityl–phenol cationic
À
species 1a+, 2a+, and 3a+ were prepared as BF4 salts in 20,
19, and 23% yield, respectively, by the Suzuki coupling reac-
tion of iodo-substituted trityl derivatives with hydroxyphe-
nylboronic acids followed by treatment with HBF4À (Fig-
À
ure 1a).[6] Cationic monomer salts 1a+·BF4 , 2a+·BF4 , and
3a+·BF4 can be considered as precursors of the expected
À
[a] Dr. H. Komatsu, Dr. W. Oi, K. Naritani, Prof. Dr. H. Maeda
College of Pharmaceutical Sciences
Ritsumeikan University, Kusatsu 525–8577 (Japan)
Fax: (+81)77-561-2659
E-mail: maedahir@ph.ritsumei.ac.jp
[b] Dr. K. Takaishi, Prof. Dr. M. Uchiyama, Dr. A. Muranaka
Advanced Elements Chemistry Research Team
RIKEN-ASI, Wako 351–0198 (Japan)
[c] Dr. K. Takaishi
Faculty of Science and Technology
Seikei University, Musashino 180–8633 (Japan)
[d] Prof. Dr. M. Uchiyama
Graduate School of Pharmaceutical Sciences
The University of Tokyo, Tokyo 113–0033 (Japan)
[e] Dr. A. Muranaka
PRESTO, Japan Science and Technology Agency (JST)
Kawaguchi 332–0012 (Japan)
Figure 1. a) Trityl–phenol cationic species 1a+, 2a+, and 3a+ and b) rep-
resentative example of the molecular structure changes of 1a+ as result
of the addition of base (TPAOH) and acid (TFA).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300704.
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Chem. Eur. J. 2013, 19, 6956 – 6960

COMMUNICATION
at least one meta linkage between the anionic and cationic
sites was required to prevent the formation of less reactive
quinoidal structures in derivatives with only para and ortho
linkages.
Upon the addition of aqueous tetrapropylammonium hy-
droxide (TPAOH, up to 1 equiv) to a CH3CN solution of
À
1a+·BF4 (1.5mm), the deep green color due to the trityl
moiety of 1a+ was gradually reduced to almost colorless,
and turbidity was observed. This result initially suggested
the formation of a covalent bond between the trityl carbon
and the phenoxide (phenolate) oxygen as has been seen in
oligomer and/or polymer states 1bn (n represents nmer(s),
Figure 1b, i). In this case, along with the solution containing
oligomeric species, precipitates were formed due to the for-
mation of polymers 1 bpoly. The pale green color in the pre-
cipitates can be explained by the trace amounts of terminal
trityl units in the polymers. Further addition of TPAOH
(1 equiv) resulted in a pale orange solution (Figure 1b, ii).
The UV/Vis absorption spectrum of 1a+ in CH3CN (1.5 mm)
showed the disappearance of bands at l=450 and 622 nm
and a concomitant increase in the band at 263 nm with the
addition of one equivalent of TPAOH (Figure 2). These
À
Figure 2. UV/Vis absorption spectral changes of 1a+·BF4 in CH3CN
(1.5 mm) upon the addition of TPAOH and corresponding photographs
of chromatic changes under visible light (inset). The precipitates were ob-
served (as shown in the photograph of 1.0 equiv).
Figure 3. a) ESI-TOF-MS at a positive mode of soluble species 1bn as an
CH3CN solution (1.5mm for the starting monomer) prepared from 1a+
spectral changes were consistent with the quenching of the
trityl-cation moiety by the base, resulting in the transforma-
tion of 1a+ to 1cÀ through 1bn as oligomer and polymer
structures, which seemed predominant in the equilibrium
with zwitterion 1b. Similar trends were observed in the UV/
Vis absorption spectral changes ascribable to the formation
À
·BF4 by the addition of 1.0 equiv of TPAOH, b) 1H NMR spectral
À
changes of 1a+·BF4 in CD3CN (1.5 mm) upon the addition of TPAOH
and TFA, and c) corresponding states with proton labels in a Scheme
along with the optimized structure of a cyclic trimer 1b3 with a C3 sym-
metry at B3LYP/6-31GACTHNUTRGNEU(GN d,p).
À
of oligomers/polymers 2bn and 3bn from 2a+·BF4 and 3a+
À
·BF4 , respectively.
ionic monomers are of course electronically neutral and
cannot be detected by ESI-MS. However, under the appro-
priate, but unusual conditions of significantly higher concen-
trations (>1 mm), the cyclic structures can interact with the
TPA cation, which was added as the counterionic species of
The oligomer soluble in CH3CN as a main species was de-
duced to be a cyclic trimer 1b3 by ESI-TOF-MS, exhibiting
a distinct signal ascribable to the cationic complex with
TPA+ (Figure 3a). Cyclic species derived from three zwitter-
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H. Maeda et al.
the hydroxide, thus allowing the detection by ESI-MS. That
is, the ionization was achieved by the complexation with tet-
raalkylammonium cations. On the other hand, the cyclic
oligomers as cationic species by complexation with TPA
were not detected from diluted solutions. In addition, dis-
persed polymers were also fabricated as precipitates along
with cyclic oligomers that were soluble in CH3CN, suggest-
ing that the role of TPA as a template seems less effective.
Evidence for the formation of 1b3 derived from the zwit-
of TPAOH (Figure 2b) were correlated with the model
structure of 1b3 at the B3LYP/6-31GACTHNUTRGNEUNG
(d,p) level[8] (Fig-
ure 3c). In contrast, model structures of a cyclic dimer 1b2
and a linear dimer as a polymer segment were not consistent
with the observed symmetry features. By using ESI-TOF-
MS, the soluble oligomers of 2bn and 3bn were also assigned
as trimers 2b3 and 3b3, respectively, as main products solu-
1
ble in CH3CN solutions. Base- and acid-responsive H NMR
spectral changes similar to those of 1bn were observed in
the formation and cleavage of 2bn and 3bn. Furthermore,
1H NMR integrals based on a TPA cation provided the pre-
1
terion 1b was also provided by H NMR spectral changes of
1a+ upon the addition of TPAOH in CD3CN (1.5 mm; Fig-
ure 3b and c). The species 1a+, 1b3, and 1cÀ were mainly
observed in the presence of 0, 1.0, and 2.0 equivalents of
TPAOH, respectively, whereas the zwitterion 1b was not ob-
served in the solution. For example, the proton Hf at d=
7.85 ppm, proximal to the cationic carbon, was shifted up-
field to d=7.47 and 7.48 ppm (Hfꢀ and Hfꢀꢀ) with 1.0 and
2.0 equivalents of TPAOH, respectively, suggesting that the
cationic site disappeared upon the addition of OHÀ. Fur-
thermore, in contrast to the signal of phenol Ha of 1a+ at
d=6.92 ppm, which was shifted downfield to d=6.99 ppm
(Haꢀ of 1b3) and upfield to d=6.36 ppm (Haꢀꢀ of 1cÀ) with
1.0 and 2.0 equivalents of TPAOH, respectively, the signal
of phenol Hd of 1a+ at d=7.21 ppm was shifted upfield to
d=6.55 ppm (Hdꢀ of 1b3) and downfield to d=6.63 ppm
(Hdꢀꢀ of 1cÀ) with 1.0 and 2.0 equivalents of TPAOH, respec-
tively. These observations indicate that the phenol protons
Ha and Hd of 1a+ were deshielded and shielded, respective-
ly, in 1b3 (as Haꢀ and Hdꢀ) by aromatic rings made proximal
cipitated polymer yields of 45%, 75%, and 19% for 1 bpoly
,
2 bpoly, and 3 bpoly, respectively, in CD3CN (1.5 mm) as initial
concentrations of cationic monomers. The yields depend on
the polymerization conditions: a higher concentration at
6.0 mm for formation of 1 bpoly gave a value of 85%, suggest-
ing that greater amount of polymer was formed in solutions
with higher concentrations. Such concentration-dependent
enhancement of the yields were also observed in 2 bpoly and
3 bpoly
.
Cyclic trimers were obtained preferentially over other
cyclic oligomers, such as dimers and tetramers. As suggested
by theoretical study,[8] cyclic trimers are more stable than
the corresponding dimers and tetramers with strained con-
formations. Thus, the stabilities of cyclic trimers in solution
were examined. Addition of one equivalent of TPAOH to
À
À
the mixture of 1a+·BF4 and 2a+·BF4 (1:1, 1.5 mm in total)
at room temperature in CD3CN gave cyclic oligomers com-
prising two kinds of monomer units along with 1b3 and 2b3,
according to their relative stabilities. Furthermore, the
À
through the formation of C O covalent bonds, and that
such effects were compensated in 1cÀ (as Haꢀꢀ and Hdꢀꢀ) by
the removal of the proximal aromatic rings along with the
emergence of an anionic phenoxide moiety. Addition of
more than 1.0 equivalent of OHÀ based on 1a+ competed
with the oligomerization and polymerization of the zwitter-
ion 1b by the nucleophilic attack of OHÀ at the trityl cation-
ic site. Interestingly, 1bn, as the cyclic trimer and polymers,
were gradually transformed to the monomeric diol 1d by
hydration in the solution state with water as the solvent for
TPAOH. However, attempted dehydration of 1d to form
oligomer/polymers 1bn failed. This result suggests that the
oligomer/polymers 1bn were kinetically produced as meta-
stable states that could only be attained from a zwitterionic
state. The diol 1d was also obtained from 1cÀ by the addi-
tion of one equivalent of TFA (Figure 1b, iii), and addition
of a second equivalent of TFA gave 1a+ (Figure 1b, iv). In
contrast to the behavior in CD3CN, addition of TPAOH in
protic CD3OD solution provided the derivative substituted
by a methoxy group at the trityl carbon without the forma-
tion of 1b and 1bn.
mixing of 1b3 and 2b3, which were prepared from 1a+·BF4
À
À
and 2a+·BF4 (1.5 mm in each), respectively, in CD3CN re-
sulted in similar results with the reaction starting from the
mixture of 1a+ and 2a+; this indicates the exchange of con-
stituent monomer units in these trimers. Although further
examination was required for detailed information about
the exchange mechanism, this observation suggested charac-
teristic dynamic behaviors in these zwitterion-based covalent
polymers.
Precipitates of 1 bpoly, 2 bpoly, and 3 bpoly were not soluble in
various solvents due to their interpolymer interactions in
the solid state, resulting in the difficulty in the studies on
their structures in detail. Therefore, the precipitates pre-
pared by casting the suspension in CH3CN were examined
by scanning electron microscopy (SEM) by using silicon sub-
strates, and these results suggested the formation of organ-
ized structures (Figure 4). Compound 1 bpoly showed aggre-
gates of flake-like structures with submicrometer-scale
widths, whereas 2 bpoly and 3 bpoly formed partially linked
spherical particles with approximately 100–200 nm diame-
ters. The formation of the particles observed in 3 bpoly that
are smaller than those observed in 2 bpoly correlated with the
amounts of polymeric precipitates. The linkages between
cationic and anionic sites were essential for determining the
morphologies of polymeric structures. Furthermore, the pre-
cipitates of 1 bpoly, 2 bpoly, and 3 bpoly were hardly transformed
The formation of the cyclic trimer 1b3 was also supported
by diffusion-ordered spectroscopy (DOSY) in CD3CN, ex-
hibiting a diffusion constant of 7.99ꢁ10À10 m2 sÀ1, which was
slightly smaller than that of monomeric 1d (10.4ꢁ
10À10 m2 sÀ1). Furthermore, the deshielded and shielded sig-
nals of Haꢀ and Hdꢀ by the formation of the covalent linkage
in 1b3 prepared from 1a+ in the presence of one equivalent
to the cyclic oligomers in CD3CN. The precipitates of 1 bpoly
,
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Cyclic and Polymeric Structures from Zwitterions
COMMUNICATION
Acknowledgements
This work was supported by PRESTO/JST (2007–2011), Grant-in-Aid for
Young Scientists (A) (No. 23685032) from the MEXT, and Ritsumeikan
R-GIRO project (2008–2013). The authors thank Prof. Naoki Aratani,
NAIST, and Prof. Atsuhiro Osuka, Kyoto University, for ESI-TOF-MS
measurements, Dr. Takashi Nakanishi, NIMS, for SEM measurement,
Prof. Tomonori Hanasaki, Ritsumeikan University, for DSC measure-
ment, Dr. Noboru Ohta, JASRI/SPring-8, for synchrotron XRD analyses
(BL40B2), Prof. Shu Seki and Dr. Tsuneaki Sakurai, Osaka University,
for valuable discussion, the RIKEN Integrated Cluster of Clusters
(RICC) for the computer resources, and Prof. Hitoshi Tamiaki, Ritsumei-
kan University, for various measurements.
Figure 4. SEM images of a) 1 bpoly and b) 2 bpoly as precipitates prepared
À
from CH3CN (1.5 mm as initial concentrations of 1a+·BF4 and 2a+
·BF4À, respectively) on silicon substrates.
2 bpoly, and 3 bpoly showed transitions by decomposition at
137, 170, and 1808C, respectively, during heating. Synchro-
tron XRD analysis of 1 bpoly, 2 bpoly, and 3 bpoly as precipitates
and after melting showed less and no ordered organized
structures, respectively.[9]
Keywords: anions
supramolecular chemistry · zwitterions
·
cations
·
nanostructures
·
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À
of the polymers was the fairly polarized C O bond con-
structed by nucleophilic attack of the phenoxide oxygen as a
strong base on the trityl carbon as a weak acid. Therefore,
À
the C O covalent bond had a relatively basic property, as
was observed in salts consisting of weak acids and strong
bases. In fact, even though 1 bpoly as a precipitate was stable
in CD3CN, treatment of 1 bpoly with CD2Cl2 as a solvent in
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À
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lent oligomers and polymers exhibiting dynamic behaviors
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polymers. Formation of the covalent polymers based on
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merization, as was observed in supramolecular polymers,[12]
because the monomer units have two reactive sites, cationic
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monomer units can control the stabilities of the oligomeric
and polymeric structures and provide various functional ma-
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base and photoacid generators. These detailed investigations
are currently underway.
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gave 4 bpoly, 5 bpoly, and 6 bpoly as precipitates in CH3CN. The de-
tailed-study manuscript is under preparation; b) dynamic behavior
1
was also observed in the H NMR spectra of the mixture of 1b3 and
À
4b3, prepared from 1a+·BF4À and 4a+·BF4 (1.5 mm in each), respec-
tively, in CD3CN, along with ESI-TOF-MS.
[10] This behavior is correlated with the reactivity of FÀ as a tetrabu
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
[11] For preliminary experiments, thermally responsive and photorespon-
sive behaviors of the polymer structures as precipitates were exam-
ined. The details of the transition mechanism are under investiga-
tion.
[12] Supramolecular Polymers (Ed.: A. Ciferri), Marcel Dekker, New
York, Basel, 2000.
[9] a) Various derivatives of zwitterion-based polymers can be fabricat-
À
ed. For example, cationic monomers 4a+, 5a+, and 6a+ as BF4
Received: February 22, 2013
Revised: March 22, 2013
Published online: April 16, 2013
salts, which contain phenyl, biphenyl, and naphthyl moieties, respec-
tively, in the spacer of 1a+ for constructing electronic materials, also
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Chem. Eur. J. 2013, 19, 6956 – 6960