Chiral Organic Cages with a Triple-Stranded Helical Structure Derived from Helicene

Article abstract of DOI:10.1021/jacs.7b13512

We report the use of helicene with an intrinsic helical molecular structure to prepare covalent organic cages via imine condensation. The organic cages revealed a [3+2]-type architecture containing a triple-stranded helical structure with three helicene units arranged in a propeller-like fashion with the framework integrally twisted. Such structural chirality was retained upon dissolution in organic solvents, as indicated by a strong diastereotopy effect in proton NMR and unique Cotton effects in circular dichroism spectra. Further study on chiral adsorption showed that the chiral organic cages possess considerable enantioselectivity toward a series of aromatic racemates.

Full text of DOI:10.1021/jacs.7b13512

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Communication
Chiral Organic Cages with A Triple-Stranded
Helical Structure Derived from Helicene
Abaid Ullah Malik, Fuwei Gan, Chengshuo Shen, Na Yu,
Ruibin Wang, Jeanne Crassous, Mouhai Shu, and Huibin Qiu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13512 • Publication Date (Web): 09 Feb 2018
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Journal of the American Chemical Society
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Chiral Organic Cages with A Triple-Stranded Helical Structure
Derived from Helicene
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‡,#
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Abaid Ullah Malik, Fuwei Gan, Chengshuo Shen, * Na Yu, Ruibin Wang, Jeanne
§
†,
†,‡,
Crassous, Mouhai Shu, * Huibin Qiu
*
†
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China.
‡
School of Physical Science and Technology, ShanghaiTech University, Shanghai, 201210, China.
§
Institut des Sciences Chimiques de Rennes, UMR 6226, Campus de Beaulieu, CNRS-Université de Rennes 1, 35042
Rennes Cedex, France.
|
|
Instrumental Analysis Center, Shanghai Jiao Tong University, Shanghai, 200240, China.
Supporting Information Placeholder
ABSTRACT: We report the use of helicene with an
intrinsic helical molecular structure to prepare cova-
lent organic cages via imine condensation. The organic
cages revealed a [3+2] type architecture containing a
triple-stranded helical structure with three helicene
units arranged in a propeller-like fashion and the
framework integrally twisted. Such structural chirality
was retained upon dissolution in organic solvents as
indicated by a strong diastereotopy effect in proton
NMR and unique Cotton effects in circular dichroism
spectra. Further study on chiral adsorption showed
that the chiral organic cages possess considerable en-
antioselectivity towards a series of aromatic racemates.
Helicenes are a type of molecules possessing a non-
planar helical structure composed by ortho-fused aro-
9
matic rings (total number ≥ 4). Owing to their unique
helical π-conjugated system, helicenes present compel-
10
ling chiroptical properties. The helical geometry to-
gether with the distinctive optical performance make
helicene a promising candidate in asymmetric cataly-
11
12
13
sis, chiral recognition, chiroptical switches, circu-
14
larly polarized light devices, etc. Herein, we demon-
strate a synthetic route to prepare covalent organic
cages using ditopic aldehyde-substituted helicene de-
rivatives as chiral building blocks. Covalent linking of
three helicene molecules with two triamine-
functionalized vertices via imine condensation led to
the formation of chiral organic cages with a novel tri-
ple helix structure. We also demonstrate the potential
application of such chiral organic cages in the separa-
tion of chiral molecules utilizing the structural chirali-
ty derived from the aligned helicene units and the
twisted framework.
The creation of architectures beyond a simple mole-
cule has emerged as a main topic in a wide range of
1
fields. In this context, organic cages have recently at-
tracted much attention due to their unique hollow mo-
lecular structures and prominent applications in catal-
2
ysis, separation, sensing, gas storage, etc. The synthe-
The cage construction started from the synthesis of
enantiopure M- and P-4,13-diethynyl[6]helicene 1 via
photocyclization, followed by chiral resolution by
sis of organic cages, despite of a few cases using irre-
3
versible bond formation, normally involves reversible
4
15
linkage chemistries such as imine condensation, me-
HPLC (See Supporting Information for details). The
5
6
tathesis, and boronic ester formation, to avoid dy-
namically trapped random conjunctions. On the other
hand, the structure and function of organic cages high-
ly rely on the chemistry and geometry of building
enantiomers were further functionalized with iodo-
substituted aldehyde 2 through Sonogashira coupling
(Scheme 1), yielding precursors M-3 and P-3, respec-
tively. X-ray diffraction of a single crystal of P-3 reveals
7
1
6
blocks. Up to now, the utilization of relatively com-
ʌʌ ľ
a Δ geometry with six benzene rings orthogonally
fused in a right-handed helical fashion and two ben-
zaldehyde units pointing to opposite directions (Figure
S3-2). Subsequently, commercially available tritopic
amine unit tris(2-aminoethyl)amine 4 was reacted with
3 in a 2:3 molar ratio under catalytic amount of tri-
fluoroacetic acid in dichloromethane. The imine con-
densation between the amine and aldehyde groups led
plex molecules in cage synthesis remains a challenge
and only a few systems have been reported which em-
ployed for example porphyrin or hemicarcerand deriv-
8
atives. Consequently, it is highly desirable to develop
new molecular building blocks to further enrich the
organic cage family.
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Page 2 of 6
to the formation of yellow solids in a yield of 67-69%,
which were dissolvable (>1 mg/mL) in organic solvents
such as CH Cl , CHCl , and diethyl ether.
formations resolved a pseudo D symmetry with a C3
axis going through the center of two amine moieties
3
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and three C axes perpendicularly across the C axis
2
2
3
2
3
and passing through the center of each helicene (Fig-
ure 1b). The crystal packing of P,P,P-5 was mirror-
image from that of M,M,M-5, and likewise, the unit cell
Electrospray ionization mass spectrometry (ESI-MS)
recorded well-resolved peaks at m/z = 1184.0593 and
a
b
1
184.0559 for the materials derived from P- and M-
contained 2/3 of P,P,P-5 and 1/3 of P,P,P-5 .
helicenes, respectively (Figures S2-18 and S2-19), indi-
cating the formation of a [3+2] cage (calculated m/z =
By further scrutinizing the crystal structure, we
found that the skeleton of 5 adopted a structurally chi-
ral conformation where three lateral arms were twisted
1
184.0542, z = 2) with two trisimide caps linked by
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three helicene units (Scheme 1). This was further veri-
fied by NMR spectra (Figures S2-7 to S2-13). The disap-
pearance of the aldehyde signal at 9.95 ppm and the
a
b
into a triple helix. In both M,M,M-5 and M,M,M-5 the
ʌľ
helicene units were aligned in a Λ propeller-like fash-
1
emergence of the aldimine signal at 7.69 ppm in the H
ion with the ends pointing inwards the cage cavity,
forming a open cave that can be accessible from three
directions (Figure 1a). Based on this propeller, the
three lateral arms turned clockwise around 120° from
NMR spectrum confirmed the complete imine conden-
sation (Figure S2-7). Besides, the absence of obviously
1
13
split or broadened signals in the H and C NMR spec-
tra (Figures S2-7 and S2-8) eliminated the possibility of
any polymeric products, and also indicated that cage 5
may possess a highly symmetric structure.
bottom to top along the pseudo C axis, resulting in a
3
left-handed triple helix. On the contrary, right-handed
a
b
triple helix can be observed in P,P,P-5 and P,P,P-5 .
Such structural chirality was thought to be derived
from the intrinsic chiral feature of helicene (especially
the large dihedral angle between two terminal aro-
matic rings).
Scheme 1. Synthetic route of organic cages using
helicene derivatives as building blocks.
The absolute structure was subsequently resolved by
X-ray crystallography. Pale-yellow single crystals were
obtained by layering a methanol solution of amine 4
onto the dichloromethane solutions of M-3 or P-3 with
catalytic amount of trifluoroacetic acid. Enantiopure
cages 5 were thus synthesized in situ in a crystal form,
and the crystals were quickly investigated by X-ray
diffraction since they could easily effloresce. The visu-
alized results revealed the presence of the expected
a
Figure 1. (a) Crystal structures of organic cages M,M,M-5
a
and P,P,P-5 (H atoms are omitted) (b) Schematic represen-
tations of M,M,M-5 and P,P,P-5 illustrating the D symmetry
3
and the triple-stranded helical structure.
The aforementioned structural chirality was also de-
1
[
3+2] constitution of cage 5 with two amine moieties
tected in solution. As indicated by H NMR spectros-
on the top and bottom, and connected by three heli-
cene-based lateral arms. Taking M,M,M-5 for instance,
copy (Figure S2-20), four sets of signals were found
between 4.0 and 2.4 ppm, corresponding to the twelve
the compound crystallized in a C space group with a
2
2
CH groups adjacent to the N atoms. Due to the D3
large unit cell containing six cage molecules with two
symmetry, these twelve CH groups can be classified
2
a
nearly identical conformations, namely M,M,M-5 and
into two types of identical CH groups. Interestingly,
2
b
M,M,M-5 , in a ratio of 2:1 (Figure S3-6). Both two con-
each type of CH groups gave two sets of peaks, which
2
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can be interpreted by a diastereotopy effect resulting
solutions of 7-9, filtrating the host-guest systems and
extracting the encapsulated guest molecules with di-
ethyl ether and finally examining the enantiomeric
excess (ee) by chiral HPLC (see Supporting Infor-
mation for more details).
1
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from the structural chirality of the twisted organic cage
framework. To confirm this, we prepared a single-
stranded helicene derivative 6 (Figure 2, inset) analo-
gous to one of the lateral arms of cage 5. The open
termini of 6 eliminated the restricted helical confor-
mation that existed in cage 5 and consequently, the
protons of the corresponding CH in 6 revealed only
2
two sets of signals (Figure S2-20). Therefore, it can be
concluded that cage 5 retained its structural chirality
in solution, with chirality transferring from the heli-
cene units to the entire arms through integral twisting.
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Electronic circular dichroism (ECD) spectroscopy
was further employed to investigate the chiral struc-
ture of cage 5 in solution. As shown in Figure 2, the
M,M,M-5 and P,P,P-5 cages revealed mirror-image ECD
spectra with very strong Cotton effects in the region
between 230 and 450 nm. Notably, P,P,P-5 displayed a
positive Cotton effect at 330 nm followed by a negative
one at 242 nm with Δε values of +537 and −708
−1
−1
L·mol ·cm , respectively, which can be ascribed to the
Figure 2. UV-vis and ECD spectra of M,M,M-5 (solid red
lines) and P,P,P-5 (solid green lines) and their analogues M-6
(dash orange lines) and P-6 (dash blue lines) in CH Cl (c = 2
× 10 mol·L ). The intensity of the UV-vis and ECD spectra
of M-6 and P-6 were deliberately enhanced by three times
17
characteristic nature of the P-helicene components.
However, these Cotton effects appeared more struc-
tured than the simple and smooth signals presented in
the P-6 analogue. Besides, P,P,P-5 showed a relatively
strong negative Cotton effect around 365 nm (Δε = −181
2
2
−5
−1
(
denoted by ε' and Δε', respectively) for a clearer comparison
−1
−1
with cage 5 which possesses three helicene-derived lateral
arms in one cage molecule.
L·mol ·cm ), which may be related to the twisting of
the ethynyl conjugated arms in the helical cage frame-
work (Figure 2, inset). While for P-6, the lack of struc-
tural restriction rendered the ethynyl conjugated arms
more planar (without additional induced chirality
caused by twisting, Figure 2, inset). Hence, the peak of
the Cotton effect bathochromically shifted to a longer
wavelength and the intensity dramatically decreased
Table 1. Data of Chiral Adsorption Experiments.
Entry
1
Guest
Host
ee
M,M,M-5
60% (S)
2
3
4
P,P,P-5
M,M,M-5
P,P,P-5
58% (R)
84% (S)
67% (R)
7
−
1
−1
(
Δε' = −45 L·mol ·cm at 380 nm in maximum). In
contrast to the ECD performances, cage 5 showed sig-
nificantly weaker absorption bands between 290 and
8
4
4
50 nm than for analogue 6 (Figure 2, ε = 6.2 × 10
5
M,M,M-5
P,P,P-5
23% (S)
20% (R)
−
1
−1
4
−1
−1
L·mol ·cm for cage 5 vs ε' = 12.2 × 10 L·mol ·cm for
at 360 nm). This presumably also reflected the more
OH
OH
6
6
9
extended π-conjugation in helicene P-6 than in the
more rigid P,P,P-5.
7
M-3
P-3
1% (S)
7
8
2% (R)
Previously, a few pioneering groups have demon-
strated the use of chiral organic cages to separate chi-
As shown in Table 1, cage M,M,M-5 preferentially
recognized S-7 and S-8, thus resulting in ee's of 60%
and 84%, respectively, which appeared to be promising
18
ral molecules from the racemates. In our organic cage
5
, three helicene units formed a propeller array with
their helical arms pointing inwards and surrounding
the cage cavity. This unique chiral environment, to-
gether with the triple-stranded helical skeleton, might
enhance the enantioselectivity towards chiral mole-
cules. As a demonstration, the racemates of 1-
phenylethylamine (7), 1-phenylethanol (8), 1,1'-
binaphthalene-2,2'-diol (9) (Table 1) were employed as
the guest molecules and enantioselective adsorption
experiments were conducted by treating the suspen-
sion of the enantiopure cage 5 (host) in isopropanol
in consideration of the relatively low ee's revealed by
18a,b
other chiral cages.
Similarly, P,P,P-5 showed enan-
tioselectivity towards R-7 and R-8 with ee of 58% and
7%, respectively. The enantioselective adsorption of
6
larger molecules such as 9 was less efficient probably
due to the relatively compact cavity of 5, but still re-
vealed ee values of over 20%. On the contrary, the en-
antiopure precursor 3 showed neglectable enantiose-
lectivity towards 7. These results indicated that the
enantioselective adsorption originates from the inte-
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Journal of the American Chemical Society
Page 4 of 6
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guest recognition), rather than the discrete helicene
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In conclusion, we have demonstrated the formation
of chiral organic cage using helicene as the building
block. The chiral cages exhibited a unique triple-
stranded helical architecture with three helicene units
aligned in propeller shape. As indicated by the enanti-
oselectivity towards aromatic racemates, these materi-
als are of potential application in chiral separation and
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AUTHOR INFORMATION
(
9) (a) Shen, Y.; Chen, C.-F. Chem. Rev. 2012, 112, 1463−1535.
Corresponding Author
(b) Gingras, M.; Chem. Soc. Rev. 2013, 42, 968−1006.
(10) (a) Phillips, K. E. S.; Katz, T. J.; Jockusch, S.; Lovinger,
A. J.; Turro, N. J. J. Am. Chem. Soc. 2001, 123, 11899−11907. (b)
Field, J. E.; Muller, G.; Riehl, J. P.; Venkataraman, D. J. Am.
Chem. Soc. 2003, 125, 11808−11809.
*
*
*
E-mail: hbqiu@sjtu.edu.cn
E-mail: mhshu@sjtu.edu.cn.
E-mail: shenchsh@shanghaitech.edu.cn.
(
11) (a) Takenaka, N.; Sarangthem, R. S.; Captain, B. Angew.
ORCID
Chem. Int. Ed. 2008, 47, 9708−9710. (b) Yavari, K.; Aillard, P.;
Zhang, Y.; Nuter, F.; Retailleau, P.; Voituriez, A.; Marinetti, A.
Angew. Chem. Int. Ed. 2014, 53, 861−865. (c) Sánchez, I. G.;
Šámal, M.; Nejedlý, J.; Karras, M.; Klívar, J.; Rybáček, J.;
Buděšínský, M.; Bednárová, L.; Seidlerová, B.; Stará, I. G.;
Starý, I. Chem. Commun. 2017, 53, 4370−4373.
(12) (a) An, Z.; Yamaguchi, M. Chem. Commun. 2012, 48,
7383−7385. (b) Schweinfurth, D.; Zalibera, M.; Kathan, M.;
Shen, C.; Mazzolini, M.; Trapp, N.; Crassous, J.; Gescheidt, G.;
Diederich, F. J. Am. Chem. Soc. 2014, 136, 13045−13052. (c)
Anger, E.; Iida, H.; Yamaguchi, T.; Hayashi, K.; Kumano, D.;
Crassous, J.; Vanthuyne, N.; Roussel, C.; Yashima, E. Polym.
Chem. 2014, 5, 4909−4914.
Huibin Qiu: 0000-0002-4699-6558
Mouhai Shu: 0000-0002-8556-7369
Chengshuo Shen: 0000-0003-2422-3922
Author Contributions
#
These authors contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the National Natural Sci-
ence Foundation of China (21271129, 21674062, U1632117,
(
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Chem. Soc. 2014, 136, 7972−7980. (b) Shen, C.; Loas, G.; Sre-
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