Asymmetric induction in the preparation of helical receptor-anion complexes: Ion-pair formation with chiral cations

Article abstract of DOI:10.1002/anie.201202196

Asymmetry through ion pairing: Upon addition of chloride and bromide ions, as chiral ammonium salts, to solutions of pyrrole-based π-conjugated linear oligomers, helical structures form with asymmetric induction, which is guided by the formation of diaste

Full text of DOI:10.1002/anie.201202196

Angewandte
Chemie
DOI: 10.1002/anie.201202196
Supramolecular Chemistry
Asymmetric Induction in the Preparation of Helical Receptor–Anion
Complexes: Ion-Pair Formation with Chiral Cations**
Yohei Haketa, Yuya Bando, Kazuto Takaishi, Masanobu Uchiyama, Atsuya Muranaka,
Masanobu Naito, Hiroshi Shibaguchi, Tsuyoshi Kawai, and Hiromitsu Maeda*
A number of helical structures have been reported.[1]
Foldamers form helical structures in response to chemical
stimuli such as neutral molecules,[2] metal cations,[3] and
anions.[4] The ability to prepare enantiomerically enriched
helical foldamers is crucial for applying helical structures to
functional materials with chiroptical properties.[5] One strat-
egy for preparing enantiomerically enriched helices is the
direct attachment of chiral moieties to the foldamers.[1] In
addition, the introduction of a chiral guest species can also
induce the preferential formation of one diastereomer of the
resulting complex through specific noncovalent interactions
between the guest and the host system.[4] Electrostatic
interactions between oppositely charged species can occur
in the absence of specific interactions.[6] Therefore, a challeng-
ing way to make a compound fold into an enantiomerically
pure chiral structure is to use electrostatic interactions
between an achiral ion and an enantiomerically pure chiral
counterion. In fact, chiral anions have been used for the
preparation of enantiomerically pure metal helicates.[7,8]
Conversely, the association of chiral cations with helix-
forming compounds that contain receptor and anionic
moieties[9] has led to the formation of enantiomerically pure
helical structures.[10] The chiroptical properties of receptor–
anion helical complexes that form through hydrogen bonding
can be difficult to examine because they can undergo more
facile interconversion between enantiomeric helical forms
compared to metal-based helices that form through coordi-
nation bonds. This fast interconversion is not a problem when
one diastereomer of an ion pair consisting of a helical
receptor–anion complexes and chiral counter cations is more
stable than the other because then only one enantiomeric
helix structure predominates in solution.
p-Conjugated molecules that form helical structures in the
presence of anions include boron complexes of 1,3-dipyrrolyl-
1,3-propanediones. These complexes, an example being 1a
(Scheme 1a),[11] bind anions through dynamic conformational
changes involving rotation of the bond between the carbonyl
group and the pyrrole moiety, thus resulting in helical
oligomers (e.g., 2a and 2b, Scheme 1b).[11b,c,e] These helical
oligomers were observed in the solid state and were
comprised of alternately stacking negatively and positively
charged species, that is, oligomer–anion complexes and
counter cations, respectively.[11c,12] Anion complexes of the
receptor-containing oligomers could be formed in enantio-
merically enriched state in solution through ion pairing with
optically active cations. In this paper, we report the prepa-
ration of enantiomerically enriched anionic helices that form
electrostatic interactions with chiral counter cations; we also
describe the chiroptical properties of these helices such as
their circularly polarized luminescence (CPL).[11d,13]
[*] Dr. Y. Haketa, Y. Bando, Prof. Dr. H. Maeda
College of Pharmaceutical Sciences, Ritsumeikan University
Kusatsu 525–8577 (Japan)
E-mail: maedahir@ph.ritsumei.ac.jp
Dr. K. Takaishi, Prof. Dr. M. Uchiyama, Dr. A. Muranaka
Advanced Elements Chemistry Research Team, RIKEN-ASI
Wako 351–0198 (Japan)
Dr. K. Takaishi
Faculty of Science and Technology, Seikei University
Musashino 180–8633 (Japan)
Prof. Dr. M. Uchiyama
Graduate School of Pharmaceutical Sciences
The University of Tokyo
Tokyo 113–0033 (Japan)
Dr. A. Muranaka, Prof. Dr. M. Naito
PRESTO (Japan) Science and Technology Agency (JST)
Kawaguchi 332–0012 (Japan)
Prof. Dr. M. Naito, H. Shibaguchi, Prof. Dr. T. Kawai
Graduate School of Materials Science
Nara Institute of Science and Technology (NAIST)
Ikoma 630–0192 (Japan)
Prof. Dr. M. Naito
National Institute for Materials Science (NIMS)
Tsukuba 305–4047 (Japan)
[**] This work was supported by PRESTO/JST (2007–2011), Grants-in-
Aid for Young Scientists (B) (No. 21750155) and (A) (No.
Chiral p-conjugated cations are suitable candidates for
inducing asymmetry in helix formation owing to their ability
to form interactions with p-conjugated receptor–anion com-
plexes. Therefore, we focused on the chiral binaphthylammo-
nium Clꢀ and Brꢀ salts, RR·X and SS·X (X = Cl and Br)
(Scheme 1c), which Ooi, Kameda, and Maruoka reported as
being efficient phase-transfer catalysts in enantioselective
reactions.[14] The formation of 1:1 receptor–anion complexes
in solution can be followed by analyzing electronic spectra.
Upon the addition of RR·Cl (1.5 equivalents) to 2b in CH2Cl2
(1 mm) at 20 and ꢀ708C, the UV/Vis absorption bands
associated with 2b at 514 and 523 nm decreased and those at
23685032) from the MEXT, and Ritsumeikan R-GIRO project (2008–
2013). We thank Prof. Takashi Ooi, Nagoya University, for the
valuable suggestion for the preparation of chiral cations, Prof.
Shuichi Hiraoka, the University of Tokyo, for the valuable discussion
for the conversion of helical structures, Prof. Ryo Kitahara and
Soichiro Kitazawa, Ritsumeikan University, for analysis of EXSY, the
RIKEN Integrated Cluster of Clusters (RICC) for the computer
resources, and Prof. Hitoshi Tamiaki and Prof. Tadashi Mizoguchi,
Ritsumeikan University, for various measurements. Y.H. and Y.B.
thank JSPS for a Research Fellowship for Young Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202196.
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Scheme 1. a) Anion-binding mode of anion receptor 1a, b) anion
receptor dimers 2a and 2b, and c) binaphthylammonium halides RR·X
and SS·X (X=Cl and Br).
Figure 1. a) CD spectra (top) and UV/Vis spectra (bottom): 2b in
CH2Cl2 (1ꢀ10ꢀ3 m) at 208C (broken black line) and at ꢀ708C (solid
black line); 2b in CH2Cl2 (1ꢀ10ꢀ3 m) in the presence of RR·Cl
(1.5 equiv), a mixture which forms 2b·Clꢀ···RR+, at 208C (red line) and
at ꢀ708C (blue line). Photographs of solutions of 2b·Clꢀ···RR+ at 20
and ꢀ708C under visible light are given in the inset. b) Schematic
models of a receptor–anion complex (as an M-type helix) in ion-pair
forms and as a free ion (the representation of the ion-pair forms are
schematic and do not represent the true form exactly).
475 and 481 nm increased, owing to the formation of the 1:1
complex 2b·Clꢀ (Figure 1a); these spectral changes were
previously observed when Clꢀ, in the form of an achiral
tetrabutylammonium (TBA) salt, was added to 2b.[11c,15]
Furthermore, when chloride was added in the form of its
RR+ salt, CD signals associated with 2b·Clꢀ that appeared at
535 nm (negative peak) and 480 nm (positive peak) were
larger when the CD spectrum was acquired at ꢀ708C than
when acquired at 208C (Figure 1a), suggesting asymmetric
formation of helical structures through diastereoselective ion-
pair formation. The Cotton effects in the CD spectra, which
are consistent with the formation of a helical structure, are
associated with an excitonic interaction between the two
receptor monomer units connected by an m-phenylene link-
age. Time-dependent density functional theory (TD-DFT)
calculations suggest that in the presence of RR+, 2b·Clꢀ
should exist predominantly as the M-type helical structure.[16]
The CD anisotropy factors gabs of 2b·Clꢀ and 2b·Brꢀ in the
presence of RR+ (1.5 equivalents) in CH2Cl2 (1 mm) at ꢀ708C
were estimated to be 1.1 ꢀ 10ꢀ2 and 1.2 ꢀ 10ꢀ2, respectively.[17]
Under the same conditions, Clꢀ and Brꢀ complexes of 2a
showed gabs values of 2.4 ꢀ 10ꢀ3 and 1.4 ꢀ 10ꢀ3, respectively,
which are smaller than those of 2b, thus suggesting that 2b
will lead to materials and compounds with better chiroptical
properties. These gabs values show significant correlation with
the ratios of M- to P-type helical structures in solution,
wherein the helical receptor–anion complexes exist as ion
pairs, which include contacted, solvent-shared, and solvent-
separated ion pairs, as well as free ions (Figure 1b).[6] The
ratio of M- to P-type helical structures depend on various
factors such as the nature of the charged species, the solvent,
temperature, and concentration. In fact, the addition of excess
amounts of chiral halide salt to dilute solutions of 2b
(0.05 mm) provided increased CD signals, which allowed us
to estimate that the Kion-pair values for the ion-pair formation
for both 2b·Clꢀ and 2b·Brꢀ with RR+ at ꢀ708C, are 4 ꢀ 103
and 7 ꢀ 103 mꢀ1, respectively.
1H NMR analysis afforded various insights into the
solution-state anion-binding and ion-pair modes of the
receptor oligomers. Upon the addition of 1.5 equivalents of
TBACl to a solution of 2b in CD2Cl2 (1 mm) at ꢀ508C,
1H NMR signals at 9.58 (Ha), 9.49 (Hb), 7.62 (Hc), 7.52 (Hd),
and 6.43 (He) ppm decreased in intensity with concurrent
appearance of signals at 11.23 (Ha), 10.21 (Hb), 8.55 (Hc),
7.69 (He), and 7.24 (Hd) ppm, suggesting the formation of
a stable 1:1 2b·Clꢀ·RR+ complex (spectra i and ii, Figure 2a
and Figure 2b); additionally, the protons of b-ethyl CH2
moieties show diastereotopic coupling because of the slow
interconversion between helical enantiomers.[11c] On the other
hand, upon the addition of 1.5 equivalents of RR·Cl to
a solution of 2b in CD2Cl2 at ꢀ508C,[18] 1H NMR signals of
RR·Cl decreased in intensity with the concurrent appearance
of new signals at 11.08, 10.71, 10.22, and 9.88 ppm, which were
assigned as the pyrrole NH groups, new signals at 8.47 and
8.14 ppm, which were assigned to Hc, new signals at 7.63 and
7.51 ppm, which were assigned to He, and new signals at 7.25
and 6.98 ppm, which were assigned to Hd. These sets of
signals, an upfield-shifted set and downfield-shifted set (blue
and red signals, respectively; Figure 2a, iii) are consistent with
the presence of two types of 1:1 complexes in a ratio of 1:0.62.
These complexes could be diastereomeric ion pairs consisting
of the M and P helices (2bM·Clꢀ and 2bP·Clꢀ, respectively)
interacting with RR+ (Figure 1b and Figure 2c, top two
structures). It should be noted that the chemical shifts
associated with each set of signals, represent an average of
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the de value; lowering the temperature leads to the formation
of more tightly associated ion pairs and an increase in the
de value; in fact, the de value of 2b·Clꢀ···RR+ in CD2Cl2
(1 mm) increased to 29% when the temperature was lowered
to ꢀ708C, as observed in the increase of the CD signals. The
nature of the anions bound in the helical structure can also
control the de value; for example, when the anion is changed
from chloride to bromide, as in 2b·Brꢀ···RR+, the de value, as
measured at ꢀ708C, increases to 43% in favor of the M-type
helix.[19] The higher Kion-pair value associated with the mixture
of 2b·Brꢀ and RR+ is one of the factors that explain the higher
de value. When 2b in CH2Cl2 (1 mm) is treated with 1.5 equiv-
alents of chiral salt at ꢀ708C, the resulting Kion pair values
represent 74% and 82% conversion into 2b·Clꢀ···RR+ and
2b·Brꢀ···RR+, respectively, suggesting that the ideal de values
based on mixtures of just ion pairs, rather than that associated
with mixtures of ion pairs and free ions, were estimated as
being 40% and 52%, respectively. The selectivity in helical
direction in CD2Cl2 may be correlated with the nature of the
helical structure. Furthermore, solvents can control the
de values. When 2b in CD2Cl2/octane-d18 (1:1, 1 mm) is
treated with 1.5 equivalents of chiral salt at ꢀ708C, the
resulting ion pairs, 2b·Clꢀ···RR+ and 2b·Brꢀ···RR+, were
formed with a higher but similar de value of 70% together
with gabs values of 1.9 ꢀ 10ꢀ2 and 2.3 ꢀ 10ꢀ2, respectively; this
increase in de value is presumably due to increased stabiliza-
tion of the ion pairs in less polar solvent. Considering the ideal
de values in CH2Cl2, diastereoselectivity in the formation of
the helical structures can be controlled not only by the degree
of ion-pair formation but also by the state of the ion pairs, that
is, whether they are tightly bound or relaxed, for example.
Furthermore, the ROESY spectrum of a sample of
2b·Clꢀ···RR+ in CD2Cl2 (1 mm) at ꢀ508C showed a correla-
tion between the signals associated with the M- and P-type
helices, thus suggesting interconversion between M- and
P-type helices. When 2b (1 mm), is treated with the chiral salt
(1.5 equivalents) in CD2Cl2 the interconversion between
M- and P-type helices in 2b·Clꢀ is slower than that in
2b·Brꢀ, as suggested by their coalescence temperatures,
which are approximately 20 and ꢀ308C, respectively. In
both cases, the interconversion between helices is faster than
the anion binding/folding and unfolding/anion releasing
processes. For the transition of the helix from M-type to P-
type in 2b·Clꢀ···RR+ at ꢀ508C, EXSY[20] experiments pro-
vided a rate constant kM!P of 3.8 sꢀ1. In contrast to the ion
pairs derived from 2b, those derived from 2a, either as Clꢀ or
Brꢀ complexes, did not show two sets of signals associated
with diastereomers in the 1H NMR spectrum, which was
acquired at ꢀ508C, owing to fast interconversion between the
helical structures; this fast interconversion is probably due to
the absence of terminal phenyl moieties.
Figure 2. a) 1H NMR spectra of i) 2b, ii) 2b with 1.5 equiv of TBACl,
and iii) 2b with mixtures of RR·Cl and SS·Cl in various ratios (1.5 equiv
in total; ratios of salts RR·Cl:SS·Cl=10:0, 8:2, 5:5, 2:8, and 0:10 from
top to bottom) in CD2Cl2 (1 mm) at ꢀ508C (the signals that are
indicated by an asterisk in iii are derived from the binaphthyl unit of
RR+). b) A schematic representation of 2bM·Clꢀ. c) Schematic models
illustrating the interconversion between M- and P-type helices (hori-
zontal equilibria) and where the ion pairs undergo cation exchange
(vertical equilibria).
the shifts of the respective ion pair (2bM·Clꢀ···RR+ and
2bP·Clꢀ···RR+) and the corresponding free ion (2bM·Clꢀ and
2bP·Clꢀ). The more stable diastereomer, 2bM·Clꢀ···RR+, has
signals that are in a more upfield region of the spectrum (blue
signals; Figure 2a, iii) owing to the greater shielding effect
imparted by the chiral cation, which contains aromatic
moieties. However, it is evident from the averaged NMR
signals ascribable to several ion-pair modes in each diaste-
reomer that the specific ion-pair modes of 2b·Clꢀ and RR+
cannot exist for a long time.
The diastereoselective formation of the ion pair between
2b·Clꢀ and the chiral cation depends on the configuration of
the cation, that is, whether it is RR+ or SS+; of course, the ion
1
pairs of 2b·Clꢀ with each cation afford identical H NMR
The ratio (0.62:0.38) of M- and P-type helical structures at
ꢀ508C represents a diastereomeric excess (de) value of 24%.
Similar to the gabs values, the de values are also affected by the
experimental conditions. Temperature can be used to control
spectra, which exhibit the same ratio of diastereomers and the
same chemical shifts (Figure 2a, iii, 10:0 and 0:10). The slow
interconversion between the diasteromeric M- and P-type
helical structures (top and bottom horizontal equilibria,
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Figure 2c) explains the observation of two independent sets
of signals in the H NMR spectrum. On the other hand, the
anion complexes with relatively high de values. Notably, CPL
is induced through ion-pair formation involving helical
charged species and counter ions in solution.
1
1H NMR spectrum of a mixture of 2b, RR·Cl, and SS·Cl
showed interesting behavior, which was dependent on the
ratios of the chiral cations. Upon addition of a 1:1 mixture of
RR·Cl and SS·Cl (1.5 equivalents in total) to a solution of 2b
in CD2Cl2 (1 mm) at ꢀ508C, a single set of signals, assigned as
a 1:1 complex, was observed. Furthermore, when the ratio of
enantiomeric cations was increased or decreased from 1:1, the
set of 1H NMR signals split into two sets of signals with
changes in the relative integrations of the signals associated
with the diastereomeric ion pairs (Figure 2a, iii). This
observation suggested that the presence of unequal amount
of the chiral cations, RR+ and SS+, leads to different amounts
of 2bM·Clꢀ···XX+ and 2bP·Clꢀ···XX+ (XX = RR and SS),
which consist of both ion pairs and free ions. In addition, the
1H NMR spectra are consistent with fast cation exchange
between 2bM·Clꢀ···RR+ and 2bM·Clꢀ···SS+ and between
2bP·Clꢀ···RR+ and 2bP·Clꢀ···SS+ (left and right perpendicular
equilibria, Figure 2c). That is, cation exchange between
diastereomeric ion pairs is faster than racemization of the
helix, a flipping process that involves inversion of the pyrrole
moieties.
In summary, we have herein described the first example of
solution-state asymmetric induction in the formation of
helical p-conjugated-receptor–anion complexes, which form
electrostatic interactions with chiral p-conjugated cations.
The asymmetric induction in the formation of the helical
structures led to enhanced CD signals together with an
increase in CPL. This fascinating observation can be attrib-
uted to the characteristic properties of electronically neutral
anion-responsive molecules that form helical anion com-
plexes. Further oligomerization should afford enantiomeri-
cally enriched macromolecular systems comprised of anionic
helical structures and appropriate counter cations, which form
ion pairs.[22] Further investigation into more functional
materials based on these types of ion pairs is currently
underway.
Received: March 20, 2012
Revised: June 1, 2012
Published online: && &&, &&&&
Keywords: anion binding · chirality · chiroptical properties ·
The ion pairs composed of 2b·Clꢀ or 2b·Brꢀ helices are
highly fluorescent. When a solution of either 2b·Clꢀ or 2b·Brꢀ
in the presence of RR+ (1.5 equivalents) in CH2Cl2 (1 mm) at
ꢀ708C was excited with light at 483 nm, which was the
isosbestic point in the corresponding UV/Vis absorption
spectrum, the resulting fluorescence spectrum exhibited
a maximum emission wavelength (lem) of 559 nm (Figure 3
bottom).[21] The ion pairs formed from 2b·Xꢀ (X = Cl and Br)
and the chiral cations exhibit CPL.[13] In fact, ion-pair-induced
CPL was observed in solutions of 2b·Clꢀ···RR+ and
2b·Brꢀ···RR+ (Figure 3 top), under the conditions described
above, and the CPL anisotropy factors glum were 8.4 ꢀ 10ꢀ3 and
1.3 ꢀ 10ꢀ2, respectively. Furthermore, the glum values increased
to 1.8 ꢀ 10ꢀ2 and 2.1 ꢀ 10ꢀ2, respectively, when the ion pairs
were formed in CH2Cl2/octane (1:1, 1 mm) at ꢀ708C. On the
other hand, smaller glum values were observed at 208C. As was
observed above in the context of gabs values, larger glum values
were also obtained under conditions that engendered helical
.
helical structures · ion pairs
[1] A book and selected reviews on synthetic helical structures:
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[2] For selected examples of chiral-guest-induced one-handed
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Figure 3. CPL (top) and fluorescence (bottom) spectra (excited at
483 nm) of 2b in CH2Cl2 (1 mm) with RR·Cl salt (1.5 equiv) at 20 (red)
and ꢀ708C (blue), along with photographs of 2b·Clꢀ···RR+ at 20 and
ꢀ708C under UV365nm light (inset). These spectra, which were mea-
sured under their respective optimized conditions, are represented
using arbitrary units.
4
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[6] A book and a review on ion pairing: a) C. Reichardt, T. Welton,
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[8] a) For an example of the differentiation of two enantiomers of
cationic metal helicates in the 1H NMR spectrum through
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[15] Binding constants (Ka) of 2b with Clꢀ and Brꢀ as RR+ salts were
estimated as being 30000000 and 930000mꢀ1, respectively, based
on UV/Vis spectral changes. These values are higher than those
of the corresponding TBA salts, 12000000 and 24000mꢀ1
,
respectively (Ref. [11c]), a fact that may be attributed to the
difference in the stabilities of ion pairs comprised of halide
anions and counter cations.
[16] a) Gaussian09 (RevisionB.01), M. J. Frisch, et al. Gaussian, Inc.,
Wallingford CT, 2009; b) ZINDO calculations provided evi-
dence that longer helical pitches will afford larger CD inten-
sities; c) It is challenging to estimate exactly the arrangement of
components in the ion pairs.
[17] a) Difference in the gabs values of the anion complexes might be
attributed to the slightly different conformations of the associ-
ated species in solution (Ref. [16b]); b) The observed CD signals
can be considered to be derived from the chromophores of
excess amounts of enantiomeric helical structures in the form of
diastereomeric ion pairs.
[10] For examples of asymmetric induction in the formation of
anionic helical structures that are not receptor–anion complexes,
through the use chiral cations, see: a) H. Katagiri, T. Miyagawa,
Y. Furusho, E. Yashima, Angew. Chem. 2006, 118, 1773 – 1776;
Angew. Chem. Int. Ed. 2006, 45, 1741 – 1744; b) K. Miwa, Y.
Furusho, E. Yashima, Nat. Chem. 2010, 2, 444 – 449.
[18] Based on the estimated Ka values (Ref. [15]), > 99.9% of 2b
exists as 1:1 complexes (1 mm) in CH2Cl2 at 208C in the presence
of 1.5 equivalent of Clꢀ and Brꢀ salts.
[19] a) The exact de value of 2b·Brꢀ···RR+ at ꢀ508C could not be
determined; b) In a preliminary examination, the addition of
RR+ as an Iꢀ salt showed a faster interconversion between M-
and P-type helices as unstable 1:1 complexes.
[11] a) H. Maeda, Y. Haketa, T. Nakanishi, J. Am. Chem. Soc. 2007,
129, 13661 – 13674; b) H. Maeda, Y. Haketa, Org. Biomol. Chem.
2008, 6, 3091 – 3095; c) Y. Haketa, H. Maeda, Chem. Eur. J. 2011,
17, 1485 – 1492; d) H. Maeda, Y. Bando, K. Shimomura, I.
Yamada, M. Naito, K. Nobusawa, H. Tsumatori, T. Kawai, J.
Am. Chem. Soc. 2011, 133, 9266 – 9269; e) H. Maeda, K.
Kitaguchi, Y. Haketa, Chem. Commun. 2011, 47, 9342 – 9344.
[12] Assemblies of ions based on the anion receptors: a) Y. Haketa, S.
Sasaki, N. Ohta, H. Masunaga, H. Ogawa, N. Mizuno, F. Araoka,
H. Takezoe, H. Maeda, Angew. Chem. 2010, 122, 10277 – 10281;
Angew. Chem. Int. Ed. 2010, 49, 10079 – 10083; b) H. Maeda, K.
Naritani, Y. Honsho, S. Seki, J. Am. Chem. Soc. 2011, 133, 8896 –
8899; c) B. Dong, Y. Terashima, Y. Haketa, H. Maeda, Chem.
Eur. J. 2012, 18, 3460 – 3463; d) Y. Bando, S. Sakamoto, I.
Yamada, Y. Haketa, H. Maeda, Chem. Commun. 2012, 48, 2301 –
2303.
[20] A selected review and article on EXSY spectroscopy: a) C. L.
Perrin, T. J. Dwyer, Chem. Rev. 1990, 90, 935 – 967; b) D.
Zuccaccia, L. Pirondini, R. Pinalli, E. Dalcanale, A. Macchioni,
J. Am. Chem. Soc. 2005, 127, 7025 – 7032.
[21] The emission quantum yields for 2b·Clꢀ···RR+ and 2b·Brꢀ···RR+
were 0.65 and 0.33, respectively, in CH2Cl2 (1 mm) at 208C (lex
=
479 nm).
[22] Anion receptor dimer 2c and tetramers 4a and 4b in CD2Cl2
formed 1:1, 1:2, and 1:2 single helices, respectively, in racemic
form upon the addition of TBACl. However, the addition of
RR·Cl to these oligomers afforded complicated NMR signals in
CD2Cl2 at ꢀ508C owing to the existence of several diastereo-
meric ion pairs involving the chiral cation.
[13] For selected examples of CPL-active molecules and materials,
see: a) P. M. L. Blok, H. P. J. M. Dekkers, Chem. Phys. Lett.
1989, 161, 188 – 194; b) K. E. S. Phillips, T. J. Katz, S. Jockusch,
A. J. Lovinger, N. J. Turro, J. Am. Chem. Soc. 2001, 123, 11899 –
11907; c) J. E. Field, G. Muller, J. P. Riehl, D. Venkataraman, J.
Am. Chem. Soc. 2003, 125, 11808 – 11809; d) M. Seitz, E. G.
Moore, A. J. Ingram, G. Muller, K. N. Raymond, J. Am. Chem.
Soc. 2007, 129, 15468 – 15470; e) T. Kaseyama, S. Furumi, X.
Zhang, K. Tanaka, M. Takeuchi, Angew. Chem. 2011, 123, 3768 –
3771; Angew. Chem. Int. Ed. 2011, 50, 3684 – 3687; f) J. Yuasa, T.
Ohno, K. Miyata, H. Tsumatori, Y. Hasegawa, T. Kawai, J. Am.
Chem. Soc. 2011, 133, 9892 – 9902.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Supramolecular Chemistry
Y. Haketa, Y. Bando, K. Takaishi,
M. Uchiyama, A. Muranaka, M. Naito,
H. Shibaguchi, T. Kawai,
H. Maeda*
&&&& — &&&&
Asymmetric Induction in the Preparation
of Helical Receptor–Anion Complexes:
Ion-Pair Formation with Chiral Cations
Asymmetry through ion pairing: Upon
addition of chloride and bromide ion, as
chiral ammonium salts, to solutions of
pyrrole-based p-conjugated linear oligo-
mers, helical structures form with asym-
metric induction, which is guided by the
formation of diastereomeric ion pairs
with chiral counter cations (see picture).
These ions pairs exhibit circular dichro-
ism (CD) and strong circularly polarized
luminescence (CPL) with glum values of
greater than 0.1.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!