Complexation-induced inversion of helicity by an organic guest in a dynamic molecular propeller based on a tristerephthalamide host with a two-layer structure

Article abstract of DOI:10.1039/c4cc00323c

A tristerephthalamide host exhibited two helical geometries with (M)- and (P)-helicity, respectively, in terms of the twisting direction of a two-layer structure, and the helical preference switched upon complexation with a ditopic guest. In both uncomplexed and complexed states, the intramolecular transmission of chirality was responsible for the control of helicity. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc00323c

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Complexation-induced inversion of helicity by an organic guest
in a dynamic molecular propeller based on a tristerephthalamide
host with a two-layer structure
A dynamic molecular propeller provides two helical geometries
with M- and P-helicity, respectively, in terms of the twisting
direction of a two-layer structure. A preference for a particular
sense of dynamic helicity in a state is switched in the other state,
accompanied by a change in conformation from a nonpropeller
form to a propeller form. A molecule takes on a different shape
from itself in the mirror.
See Ryo Katoono,
Takanori Suzuki et al.,
Chem. Commun., 2014, 50, 5438.
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Complexation-induced inversion of helicity by an
organic guest in a dynamic molecular propeller
based on a tristerephthalamide host with a
two-layer structure†
Cite this: Chem. Commun., 2014,
50, 5438
Received 15th January 2014,
Accepted 28th January 2014
Ryo Katoono,* Kenshu Fujiwara and Takanori Suzuki*
DOI: 10.1039/c4cc00323c
www.rsc.org/chemcomm
A tristerephthalamide host exhibited two helical geometries with
(M)- and (P)-helicity, respectively, in terms of the twisting direction
of a two-layer structure, and the helical preference switched upon
complexation with a ditopic guest. In both uncomplexed and
complexed states, the intramolecular transmission of chirality was
responsible for the control of helicity.
A dynamic molecular propeller is an attractive motif for studies on
transmission of chirality1,2 and inversion of helicity,3–7 since a
propeller-shaped conformation is helically chiral and dynamically
interconvertible to the antipodal conformer, similar to polymeric
helices.8 A preference for a particular sense of dynamic helicity is a
result of the intra-1 or intermolecular2 transmission of point
chirality. When point chirality is transferred to dynamic helicity,
two conformers with (P)- or (M)-helicity are no longer enantiomers,
but rather are diastereomers. One of them is more stable and
favored over the other due to the transmission of chirality. It is
important that we understand how the helical preference switches
Fig. 1 Chemical structures of hosts 1, chiral ditopic guests 2,12a and
substructures 3 and 4.
in response to a change in the environmental conditions, such as molecule originally prefers a particular sense of (P)- or (M)-helicity
the solvent,3 temperature,4 electric charge,5 or the addition of a due to the intramolecular transmission of chirality, and this pre-
guest.6,7 In the development of an asymmetric catalyst,3d,9 the ability ference changes in a newly generated helical structure upon com-
to switch helical chirality enabled the selective production of either plexation (complexation-induced change in conformation and inversion
enantiomer from a single catalyst.3d Furthermore, it might be of helicity).7 Two planes of 1,3,5-triethynylbenzene arranged one
challenging for an artificial system to undergo a change in con- above the other provide the potential for creating helical chirality in
formation from a helical form to other helical form(s) as well as terms of the twisting direction (Scheme 1a).10 We denote the two
inversion of helicity in response to external stimuli.
conformers as uppercase P and M, even though they are enantio-
We used a dynamic molecular propeller to study the inversion of mers or diastereomers. To fix the two planes, we used a threefold
helicity accompanied by a change in conformation upon complexa- terephthalamide. In a syn-form terephthalamide unit, disrotatory
tion with a guest, and designed tristerephthalamide hosts 1 with a twisting of the two amide groups leads to a nonhelical conformation
two-layer structure (Fig. 1). This is different from the above- with a mirror plane in the unit, and conrotatory twisting allows the
described simple switching of the helical preference of a diastereo- unit to adopt helical conformations that can undergo interconver-
meric pair through complexation with a guest.6 Instead, a helical sion about the 2-fold axis of symmetry (Scheme 1b).11 We denote the
two helical conformations in the terephthalamide unit as lowercase
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810,
p and m, in addition to the global helicity of P/M. In the case where a
nonhelical conformation (n) is energetically favored over helical
forms (p/m), we can anticipate that threefold conformational switch-
Japan. E-mail: katoono@sci.hokudai.ac.jp, tak@sci.hokudai.ac.jp;
Fax: +81 11 706-2714; Tel: +81 11 706-3396
† Electronic supplementary information (ESI) available: NMR, UV and CD spec-
ing from a nonhelical conformation (n) to a helical conformation
(p/m) would result in the generation of propeller chirality in the
troscopic data, and experimental details of preparation of a new compound. See
DOI: 10.1039/c4cc00323c
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Fig. 2 Energy-minimized structures for a model (R,R,R)-1a0: (a) Mnnn
(rel. 0 kJ molꢀ1) and (b) Pppp (+7.9 kJ molꢀ1), obtained by a conformational
search using the MacroModel software (v9.9 Monte Carlo Multiple Mini-
mum method, MMFF*, nonsolvated, 50 000 steps).
Scheme 1
Mnnn (rel. 0 kJ molꢀ1) with two layers twisted in an (M)-helical
whole molecule (ppp/mmm). This can be induced by supplying
sufficient energy for transformation, e.g. by complexation with a
hydrogen-bonding ditopic guest at the two amide carbonyls.11a,c,12
Helicity biasing would be enabled by the attachment of point
chirality to the nitrogen atom(s) and/or by complexation with a
chiral ditopic guest through the intra- or intermolecular transmis-
sion of point chirality to dynamic helicity (P/M and/or p/m).11,12 We
envisioned that point chirality associated with a host could be
intramolecularly transferred to dynamic helicity in different ways
in response to an uncomplexed (Pnnn/Mnnn) or complexed state
(Pppp/Mmmm). Thus, we attached point chirality (R) to an amide
nitrogen as an internal chirality [(R,R,R)-1a] and used a chiral ditopic
guest12 such as (R,R)-2 or (S,S)-2 to induce the host to transform
(Fig. 1). In this communication, we report the successful design of a
dynamic molecular propeller in which the helical sense was switched
upon complexation (Scheme 2). The host preferred a particular sense
of P or M due to the intramolecular transmission of point chirality in
the absence of a guest, and this helical preference changed in a
complexed state (inversion of helicity). During complexation, the host
underwent conformational switching from a nonpropeller form
(nnn) to a propeller form (ppp) (helicity generation and biasing).
We prepared tristerephthalamide hosts 1a and b through a
threefold condensation reaction of a trianiline and a tricarboxylic
acid, both of which were derived from 1,3,5-triethynylbenzene13
(Scheme S1, ESI†). A conformational search for a model (R,R,R)-1a0
[X = Me, Y = (R)-C*HMe(cHex)] predicted several energy-minimized
structures. The most energetically-minimized conformation was
manner, and each terephthalamide unit adopted a nonhelical form
(Fig. 2a). A propeller arrangement was found for Pppp (+7.9 kJ molꢀ1
)
(Fig. 2b) as well as Mmmm (+14.2 kJ molꢀ1), and in both conforma-
tions, all six blades were twisted in a particular direction. It should be
noted that Pppp was favored over Mmmm, since point chirality (R)
preferred p-conformations with dynamic helicity in a terephthal-
amide unit. These predictions led us to expect that M-helicity would
change to P-helicity when the host forms a complex with a ditopic
guest at the two amide carbonyls to adopt a p-helical form
(Scheme 2). The energy diagrams obtained for 1a0 and 1b0 are shown
in Fig. S1 (ESI†).
In the 1H NMR spectrum of (R,R,R)-1a, we observed a single set
of averaged resonances assigned to C3 symmetry, which was main-
tained while the temperature was lowered to 223 K. During the VT
measurements (223–293 K), we found a significant change in the
chemical shift for the aromatic protons of a 1,3,5-triethynylbenzene
unit (Fig. S2a, ESI†). We considered the change to be the result of
the increased contribution of energetically higher conformers
involved in the equilibrium at elevated temperatures, since a similar
change was also observed for 1b (Fig. S2b, ESI†).
We first investigated the complexation of 1a with a ditopic guest 2
by monitoring complexation-induced changes in the chemical shift
1
by H NMR spectroscopy, and spectra were measured in CDCl3 at
ambient temperature. We confirmed that the host and guest formed
a 1 : 3 complex by a significant upfield shift for both the phenylene
protons of a terephthalamide unit in 1 and the phenylene protons in
2 (Fig. S3, ESI†). In addition, we found that the aromatic protons of a
1,3,5-triethynylbenzene unit were shifted downfield with an increase
in guest equivalents (Fig. S3a, ESI†). This change indicated a
conformational switching of 1a from the most stable conformation
to some other conformation(s) and corresponded to the above-
mentioned change with an increase in temperature (Fig. S2a, ESI†).
We then monitored the 1 : 3 complexation of (R,R,R)-1a with
(R,R)-2 by CD spectroscopy, and spectra were measured in CH2Cl2 at
293 K. We found largely positive (De +19 at 310 nm) and bisignated
Cotton effects (+19 at 280 nm and ꢀ22 at 258 nm) induced in the
absorption region14 of 1a (Fig. 3a). These complexation-induced
Cotton effects were totally different from the Cotton effects (ꢀ9.6 at
300 nm and ꢀ13 at 271 nm) obtained for (R,R,R)-1a itself through
the intramolecular transmission of chirality, which appeared
Scheme 2
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Fig. 3 (a) Continuous changes in the CD spectrum of (R,R,R)-1a (8.4 ꢁ
10ꢀ5 M) upon complexation with a ditopic guest (R,R)-2 [0 (1a only, black
line), 3, 6, and 12 equiv.]. (b) Continuous changes in the CD spectrum of 1b
(8.8 ꢁ 10ꢀ5 M) upon complexation with a ditopic guest (R,R)-2 (blue lines)
or (S,S)-2 (red lines) [3, 5, and 10 equiv.]. Molar CD values for 2 are o0.2 in
the absorption region of 400–250 nm.12a All spectra were measured in
CH2Cl2 at 293 K.
negatively throughout the whole region (Fig. 3a).16 We considered
that this difference was the result of the conformational switching
of (R,R,R)-1a upon complexation with (R,R)-2. The internal chirality
was transmitted to newly generated dynamic helicity in a 1 : 3
complex, accompanied by a supramolecular transmission of guest
chirality in a cooperative manner.17 The chiralities in 2 should have
their own preference, as shown by the fact that small but similar
Cotton effects were obtained by the complexation of 1b, which has
no internal chirality, with (R,R)-2 through the supramolecular
transmission of chirality to dynamic helicity that was generated
upon complexation (Fig. 3b). We confirmed that a mirror image was
induced by the addition of (S,S)-2. The conformational switching of
(R,R,R)-1a upon complexation was also supported by the following
experiments. When we gradually added the guest (R,R)-2 to a
solution of (R,R,R)-1a, the complexation-induced Cotton effects
and chemical shifts changed sigmoidally in CD and NMR spectra,
respectively (Fig. S7, ESI†). During the 1 : 3 complexation, the guest
bound to the host in an allosteric manner.11c,18
In conclusion, we have demonstrated a complexation-
induced inversion of helicity based on a dynamic molecular
propeller. Two helical states with an inverse helical preference
were required: the molecule prefers a particular sense of
helicity in one state (Mnnn for an uncomplexed state), and this
preference changes in the other state (Pppp for a complexed
state). In both helical states, the point chirality (R) associated
with the host was responsible for the control of helicity.
Notably, we have presented a less well-developed motif for
studies on the inversion of helicity upon complexation with an
organic guest,6d,7b,c although metal ions6a and anions6b,c,7a,d,e
have often been used as guests.
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14 The absorption maxima, measured in CH2Cl2 at ambient tempera-
ture, were 291sh. nm (loge 5.09) and 277 nm (5.13) for (R,R,R)-1a,
291 nm (loge 5.11) and 281 nm (5.11) for 1b, which were hypsochro-
mically shifted compared to those for substructures (R,R,R)-3a
[lmax 313 nm (log e 5.01), 305sh. (4.98)], 3b [313 nm (5.02), 305sh.
(4.99)] (Fig. S4, ESI†) and the parent 1,3,5-tris(phenylethynyl)-
benzene15 [305 nm (4.93) in CHCl3].
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16 Some of the Cotton effects at around 271 nm negatively increased
(De ꢀ12 at 303 K to ꢀ18 at 263 K) with a decrease in temperature to
263 K (Fig. S5a, ESI†). This change may suggest that a pair of helical
conformations, such as Mnnn and Pnnn, was in equilibrium, and the
population of each component changed at lower temperatures. Even
though a diastereomeric pair of two helical conformers with M or P
helicity was in equilibrium, their chiroptical signals might not be very
striking due to the nonhelical nature of a terephthalamide unit.11a–c
The Cotton effects observed for (R,R,R)-1a had the same appearance as
those for (R,R,R)-3a. The Cotton effects for (R,R,R)-3a were similar in
shape and triple the strength of those for (R)-4 (Fig. S5b, ESI†).
Notes and references
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1 J. Sola, S. P. Fletcher, A. Castellanos and J. Clayden, Angew. Chem., 17 Similar but attenuated Cotton effects were induced when the antipodal
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In the complex, the internal chirality (R) was predominantly transmitted
to newly-generated dynamic helicity in a competitive manner.
J. W. Canary, Z. Dai and S. Mortezaei, Compr. Chirality, 2012, 18 T. Ikeda, K. Sada, S. Shinkai and M. Takeuchi, Supramol. Chem.,
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5440 | Chem. Commun., 2014, 50, 5438--5440
This journal is ©The Royal Society of Chemistry 2014