Critical Role of Achiral Guest Molecules in Planar Chirality Inversion of Alanine-Appended Pillar[5]arenes

Article abstract of DOI:10.1021/acs.orglett.9b00277

Planar chirality inversion of pillar[5]arenes bearing d- or l-alanine substituents on both rims was investigated upon addition of guest molecules having pyridinium or imidazole moieties and long alkyl chains. The d- and l-alanine-substituted pillar[5]arenes exhibited pS and pR planar chirality, respectively. However, this planar chirality was inverted upon inclusion of certain achiral molecules, comprising pyridinium or imidazole moieties and long alkyl chains with terminal hydroxyl or methyl groups.

Full text of DOI:10.1021/acs.orglett.9b00277

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Critical Role of Achiral Guest Molecules in Planar Chirality Inversion
of Alanine-Appended Pillar[5]arenes
Jaehyeon Park,† Yeonweon Choi,*,‡ Shim Sung Lee,† and Jong Hwa Jung*,†
†Department of Chemistry and Research Institute of Natural Sciences Gyeongsang National University, Jinju 52828, Korea
‡Accident Prevention and Assessment Division 2, National Institute of Chemical Safety, Daejeon 34111, Korea
S
* Supporting Information
ABSTRACT: Planar chirality inversion of pillar[5]arenes
bearing D- or L-alanine substituents on both rims was
investigated upon addition of guest molecules having
pyridinium or imidazole moieties and long alkyl chains. The
D- and L-alanine-substituted pillar[5]arenes exhibited pS and
pR planar chirality, respectively. However, this planar chirality
was inverted upon inclusion of certain achiral molecules,
comprising pyridinium or imidazole moieties and long alkyl
chains with terminal hydroxyl or methyl groups.
hirality inversion systems have attracted intense research
interest owing to their importance for the development
length of alkyl chain moiety, and their interactions with the
pillar[5]arene host on the inversion of planar chirality are still
not sufficiently understood.29
C
of useful chiral materials such as those employed in chemical
sensors, enantioselective catalysts, chirality switches, chiral
optoelectronics, and memory devices with specific function-
alities that involve chiral recognition and sensing.1−7
Furthermore, macrocyclic compounds functionalized with
chiral moieties can be used in the construction of biomimetic
supramolecular systems.8,9
Since they were first reported by Ogoshi et al. in 2008,
pillar[n]arenes, which are macrocycles composed of 5 to 10
repeating hydroquinone units connected by methylene bridges
at their para-positions, have garnered intense interest as a new
class calixarene-like cavitands.10 Their unique structures and
tunability, which is possible through variation of the functional
groups attached to the rims, have seen them employed in the
development of many novel materials, including liquid
crystals,11 supramolecular polymers,12,13 and nanoparticles.14,15
The repeating hydroquinone units endow pillar[n]arenes with
unique symmetrical pillar architectures that are distinct from
the basket-shaped architectures of calixarenes.
Crystals of pillar[5]arenes exist in racemic forms, and certain
substituents, when attached to the nonbridging hydroquinone
phenolic oxygen atoms, can endow them with planar chirality
(pR or pS).16−19 This planar chirality can be controlled by
external stimuli such as temperature or solvent.20−26
Furthermore, pillar[5]arene molecules have hydrophobic
internal cavities, allowing them to entrap solvent molecules
or form host−guest complexes by inclusion of a guest
molecule, which can also affect the planar chirality of the
host pillar[5]arene.27,28 However, although inversion of the
planar chirality of pillar[5]arenes upon inclusion of guest
molecules has been reported in several papers, the specific
effects of the structural properties of guest molecules, such as
ionic state, dipole moment, hydrogen bonding capacity, or
Herein, we investigated the influence of structure of guest
molecules on the planar chirality change of pillar[5]arene. For
the investigation, we varied terminal functional groups or alkyl
chain length of guest molecules. D- and L-alanine-bearing
pillar[5]arenes were synthesized as host molecules, which have
asymmetric carbons and hydrogen bonding sites and exhibit
planar chirality and used them to form inclusion complexes
with a range of guest molecules. The results were used to
elucidate how the structure of the guest molecule and its
intermolecular interactions with the pillar[5]arene effect the
inversion of its planar chirality. Four types of linear chain
compounds with different alkyl chain lengths and different
terminal functional groups were synthesized for use as guest
molecules. The alanine-substituted pillar[5]arenes exhibit
different planar chirality inversion behaviors depending on
the alkyl chain length of the guest molecule. To the best of our
knowledge, there was no report that describes the relation
between planar chirality inversion and the alkyl chain length of
guest molecule.
Figure 1 shows a schematic illustration of planar chirality
inversion as well as the chemical structures of the alanine-
substituted pillar[5]arenes and the guest molecules employed.
A carboxylic acid substituted pillar[5]arene, which was
synthesized according to a previously reported procedure,27,30
was chlorinated and reacted with L-alanine tert-butyl ester
hydrochloride or D-alanine tert-butyl ester hydrochloride to
produce L3 and D3, respectively. L3 and D3 were then
acidized and treated with NH4OH in a mixed solution of
Received: January 22, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00277
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 1. (A) Chemical Structures of L-ala-PA, D-ala-PA, Py-11-OH, Im-11-OH, PCC, and Py-6-OH. (B) Schematic illustration of planar chirality
inversion in an L-alanine-substituted pillar[5]arene upon inclusion of different guest molecules.
methanol and water to provide the L-alanine-substituted
pillar[5]arene (L-ala-PA) and D-alanine-substituted pillar[5]-
arene (D-ala-PA), respectively (Scheme S1).
measurement at 25 °C. D-ala-PA also obtained a single peak by
HPLC measurement (Figure S6B), indicating that D-ala-PA
was also enantiopure as pS (pS, pS, pS, pS, pS) planar chirality
as shown in CD observation. In addition, the H1 proton in the
aromatic region of the NMR spectrum appeared as a singlet at
6.89 ppm (Figure S7), indicating that L-ala-PA and D-ala-PA
are enantiomerically pure in water.18,21,34 The singlet of the
aromatic proton of L-ala-PA was not split even at 278 K,
indicating that L-ala-PA has only pR chirality in water even at
low temperature (Figure S8). Thus, incorporation of the rim-
mounted chiral substituents endows the pillar[5]arene with
planar chirality. On the other hand, the NMR spectra of L-ala-
PA in DMF, DMSO, and ACN showed a split peak for the
aromatic proton, suggesting that L-ala-PA has both pS and pR
configurations in such solvents (Figures S9 and S10).
Interestingly, the CD intensity decreases when an achiral
molecule comprising a pyridinium headgroup and a long alkyl
tail (C11) with a terminal hydroxyl group (Py-11-OH) is
added to the aqueous solution of L-ala-PA (Figure 2A). The
degree of decrease in the CD intensity is dependent on the
amount of Py-11-OH added. Above 0.6 equiv, the positive CD
signal at 303 nm is inverted to form a negative CD signal,
indicating inversion of the planar chirality of L-ala-PA from pR
to pS. The intensity of the negative signal gradually increases
UV−vis and circular dichroism (CD) spectra of L-ala-PA in
water are shown in Figure S1, while the corresponding spectra
in acetonitrile (ACN) are given in Figure S2. L-ala-PA presents
an absorption band at ca. 290 nm, which is ascribed to π−π*
transitions in the aromatic moieties of the pillar[5]arene units,
in both water and ACN (Figures S1A and S2A). Although L-
ala-PA has asymmetric carbons in the alanine moieties, no
induced CD signal is observed in the CD spectrum measured
in ACN (Figure S2B). Conversely, in the CD spectrum of
aqueous L-ala-PA, an induced CD signal with a positive Cotton
effect is presented in the aromatic π−π* transition region,
indicating that L-ala-PA does not exist as a racemic compound
in water but instead has pR planar chirality (Figure
S1B).24,31,32 L-ala-PA possesses 10 alanine groups, which
form the strong intramolecular hydrogen-bonding interaction
between the alanine and the alanine moieties as shown in
Figure S3.33 Among four conformers for pR planar chirality,
the existence of “pS, pS, pR, pR, pR” and “pS, pS, pS, pR, pR”
conformers would be rare, due to inhibition of rotation of
dialanine-appended phenolic units. Furthermore, we obtained
the sharp CD spectrum of L-ala-PA in water, indicative of the
existence of one conformer. The shape of the CD spectrum of
L-ala-PA was the same as those in previously reported
literature.22,24 These results also support that L-ala-PA only
exists as one conformer. Thus, we believe that L-ala-PA exists
as the “pR, pR, pR, pR, pR” conformer. Furthermore, the
stereoenantiomeric alanine moieties of L-ala-PA might induce
pR planar chirality with the “pR, pR, pR, pR, pR” conformer.
Also, CD spectra of L-ala-PA in DMF and DMSO showed a
positive Cotton effect; however, their intensities were
significantly lower than that measured in water (Figure S4).
This result indicates that enantiomeric purity of L-ala-PA is
lower in DMF and DMSO than in water. Predictably, the CD
spectrum of the aqueous solution of D-ala-PA reveals a
negative Cotton effect, indicating pS planar chirality (Figure
S5).
To investigate the planar chirality of L-ala-PA and D-ala-PA,
chiral HPLC measurements were performed upon injection of
L-ala-PA onto an appropriate chiral HPLC column with
various eluents. The single peak was observed (Figure S6A),
which was attributed to the stereoenantiomeric alanine
moieties. These findings indicate that L-ala-PA was an
enantiopure, pR type (pR, pR, pR, pR, pR) as shown by CD
Figure 2. (A) CD spectra of L-ala-PA (0.1 mM) in water with various
equivalents of Py-11-OH. (B) Plot of the CD intensities at 303 nm
shown in (A). (C) CD spectra of D-ala-PA (0.1 mM) with various
equivalents of Py-11-OH in water. (D) Plot of CD intensities at 303
nm of (C).
B
DOI: 10.1021/acs.orglett.9b00277
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Letter
upon further addition of Py-11-OH, reaching equilibrium at
ca. 1.0 equiv (Figure 2B), which is clear evidence for the
formation of a 1:1 complex between L-ala-PA and Py-11-OH.
The CD inversion of L-ala-PA was due to the formation of a
complex with Py-11-OH by the hydrophobic interaction in the
cavity of L-ala-PA and the electrostatic interaction between the
pyridinium moiety and the carboxylate of L-ala-PA.
The intensity of the negative CD signal for D-ala-PA also
decreases upon addition of Py-11-OH, becoming inverted to a
positive CD signal upon addition of over 1.0 equiv of Py-11-
OH, indicating a change in planar chirality from pS to pR
(Figure 2C). This is attributed to the relatively high
concentration of complexed L-ala-PA with pR chirality,
compared with that of the uncomplexed L-ala-PA.
To investigate the binding mode between the alanine-
substituted pillar[5]arenes and Py-11-OH, the 1H NMR
spectra of Py-11-OH with various amounts of L-ala-PA in
water were measured (Figure S11). The alkyl chain protons of
Py-11-OH at 1.93, 1.44, and 0.91 ppm were remarkably shifted
to a higher field upon the addition of L-ala-PA. Furthermore,
all the peaks attributed to Py-11-OH protons were significantly
broadened, indicating the shielding of Py-11-OH upon its
inclusion in the hydrophobic cavity of L-ala-PA.35−37 As the
temperature of the aqueous solution of Py-11-OH and L-ala-
PA increases, the signals for the alkyl chain protons of Py-11-
OH become gradually shifted to lower field, indicating that Py-
11-OH is partially excluded from L-ala-PA (Figure S12).
Simultaneously, the intensity of the inverted CD signal
decreases, and the negative signal becomes reinverted to a
positive signal above 338 K (Figure 3). However, no change in
hydrophobic interaction in the cavity of L-ala-PA and the
electrostatic interaction between the pyridinium moiety and
the carboxylate of L-ala-PA, which resulted in pS planar
chirality. Conclusively, the hydrophobic and the electrostatic
interactions acted as driving forces for CD inversion of L-ala-
PA.38,39
To further evaluate the influence of the guest molecule
structure on the formation of the host−guest complex and the
planar chirality of the alanine-substituted pillar[5]arenes, we
performed control experiments with other achiral guest
molecules. First, the pyridinium moiety in Py-11-OH was
replaced with a neutral imidazole moiety to assess the influence
of the ionic state of the guest molecule. When the imidazole-
appended guest molecule (Im-11-OH) is added to an aqueous
1
solution of L-ala-PA, similar CD and H NMR results are
obtained, indicating that ionic state does not significantly affect
complex formation of the planar chirality of L-ala-PA. The
positive CD signal is inverted to a negative signal upon
1
addition of 0.8 equiv of Im-11-OH (Figure S14). In the H
NMR spectra, the peaks attributed to the alkyl chain of Im-11-
OH are also shifted to higher field when L-ala-PA is added
(Figure S15).
Second, a guest molecule in which the terminal hydroxyl
group is replaced with a methyl group (Py-11-Me) was
synthesized to evaluate the influence of the dipole moment of
the guest molecule and hydrogen bonding interactions with the
alanine moieties of L-ala-PA. These experiments provided
similar results to those obtained with Py-11-OH and Im-11-
OH, implying that the terminal hydroxyl group does not play a
crucial role in the inversion of the planar chirality of L-ala-PA
(Figure S16A). The positive signal in the CD spectra is
inverted upon addition of 0.6 equiv of Py-11-Me (Figure
S16B).
However, when pyridine or pyridinium chlorochromate
(PCC), neither of which has an alkyl chain, is added to an
aqueous solution of L-ala-PA, no significant changes in the CD
1
and H NMR spectra are observed (Figures S17 and S18).
These results demonstrate that the hydrophobic interactions
between the phenyl moieties of the pillar[5]arene and the alkyl
chain of the guest molecule upon the formation of an inclusion
complex are the major factors that govern inversion of the
planar chirality of alanine-substituted pillar[5]arenes.
Figure 3. (A) Temperature-dependent CD spectra of aqueous
solution of L-ala-PA (0.1 mM) and Py-11-OH (0.1 mM). (B) Plot
of CD intensities at 303 nm of (A).
Finally, we investigated the effect of the alkyl chain length of
the guest molecule on the inversion of the planar chirality of
alanine-substituted pillar[5]arenes. Accordingly, a guest
molecule in which the alkyl chain length was decreased from
C11 to C6 (Py-6-OH) was synthesized. When Py-6-OH is
added to an aqueous solution of L-ala-PA, the intensity of the
positive CD signal at 303 nm gradually decreases with the
amount of Py-6-OH added (Figure S19). However, the
positive CD signal does not become fully inverted, even upon
addition of 20 equiv of Py-6-OH (Figure S19B). Furthermore,
in the 1H NMR spectra (Figure S20), the high-field shift of the
alkyl chain proton peaks and the broadening degree of the
proton peak for Py-6-OH upon addition of L-ala-PA are
evidently smaller compared to those observed for Py-11-OH,
which may indicate that the hydrophobic interactions between
the guest molecule and the phenyl moieties of L-ala-PA upon
the formation of the inclusion complex are significantly
weakened by the shortening oAf the alkyl chain, and the
consequent decrease in hydrophobicity, of the guest molecule.
In order to provide more direct evidence for the difference
in the strength of binding between L-ala-PA and guest
CD intensity upon heating is observed in the CD spectra of the
aqueous solution of L-ala-PA recorded in the absence of Py-
11-OH (Figure S13). These results demonstrate that Py-11-
OH is held strongly in the cavity of L-ala-PA owing to
hydrophobic interactions between the phenyl moieties of L-ala-
PA and the long alkyl chain of Py-11-OH, forming a host−
guest complex. The inclusion of Py-11-OH changes the planar-
chiral space of the pillar[5]arene cavity, causing inversion of
the planar chirality of L-ala-PA. The hydrophobic interactions
are weakened upon heating, leading to the exclusion of Py-11-
OH from L-ala-PA and the reinversion of its planar chirality.
Based on 1H NMR spectral changes of L-ala-PA upon addition
of Py-11-OH, the alkyl chain group of Py-11-OH was included
in the hydrophobic cavity of L-ala-PA. The cationic pyridinium
moiety of Py-11-OH was also bound to the carboxylate anions
(−COO−) of L-ala-PA by the electrostatic interaction. Thus,
CD inversion of L-ala-PA in the presence of Py-11-OH was
due to the formation of the complex with Py-11-OH by the
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Letter
molecules, association constants were measured based on CD
data (Figures 2, S14, S16, and S21).40 As expected, Py-6-OH
showed a significantly lower value compared to Py-11-OH,
indicating that the binding affinity of Py-11-OH to L-ala-PA is
much higher than that of Py-6-OH to L-ala-PA (Table S1).
These results clearly demonstrate that the inversion of the
planar chirality of the alanine-substituted pillar[5]arenes is
largely governed by the hydrophobic-interaction-induced
inclusion of guest molecules. We also determined the
association constants of L-ala-PA upon addition of Py-11-
OH at 35 and 45 °C. The association constants obtained at 35
and 45 °C were (1.17 0.01) × 105 M−1 and (4.03 0.02) ×
104 M−1, respectively (Table S1), indicating that the inclusion
of Py-11-OH in L-ala-PA easily occurred at low temperature
compared to high temperature. It was realized that the strong
hydrophobic and electrostatic interactions at low temperature
led to CD inversion relatively more easily than at high
temperature (Figure S21).
In conclusion, D- and L-alanine-substituted pillar[5]arenes,
D-ala-PA and L-ala-PA, bearing rim-mounted chiral substitu-
ents were synthesized. D-ala-PA and L-ala-PA do not exist as
racemic forms in water, instead exhibiting planar chirality (pS
for D-ala-PA, pR for L-ala-PA). This planar chirality is inverted
upon addition of guest molecules bearing linear alkyl chains.
However, the planar chirality inversion behavior is not
significantly affected by the terminal functional group of the
guest molecule, despite having different chemical properties,
such as ionic state, dipole moment, and hydrogen bonding
capacity. However, the inversion behavior is greatly influenced
by alkyl chain length. Accordingly, inversion does not occur
upon the addition of guest molecules that do not have alkyl
chain moiety and the degree of inversion is lower when the
alkyl chain of the guest molecule is shortened. Thus, these
planar chirality studies demonstrate a new strategy for the
construction of planar chirality control systems as well as
informing the systematic design of guest molecules that effect
planar chirality inversion in pillar[n]arenes.
Future Planning, Korea. In addition, this work was partially
supported by a grant from the Next-Generation BioGreen 21
Program (SSAC, Grant No. PJ013186052018), Rural Develop-
ment Administration, Korea.
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ASSOCIATED CONTENT
* Supporting Information
■
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Shim Sung Lee: 0000-0002-4638-5466
Jong Hwa Jung: 0000-0002-8936-2272
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This work was supported by the NRF (2017R1A4A1014595,
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