Corannulene-Based Coordination Cage with Helical Bias

Article abstract of DOI:10.1021/acs.joc.7b02709

We report here the first corannulene-based molecular cage, constructed via metal-induced self-assembly of corannulene-based ligands. In sharp contrast to those assembled via the planar π-conjugated analogues of corannulene, at ambient and elevated temperatures, the molecular cage exists as an ensemble of four stereoisomers (two pairs of enantiomers), all of which possess a D₅-symmetric (regardless of the counteranions) and inherently helical structure. Decreasing the temperature shifts the equilibrium between different pairs of enantiomers. At low temperature, only one pair of enantiomers is present. Helical bias for the cage could be efficiently achieved by inducing asymmetry with enantiopure anions. When nonenantiopure anions are used, the asymmetry induction abides by the majority rule , i.e., the major enantiomer of the chiral anions controls the bias of helical sense of the cages.

Full text of DOI:10.1021/acs.joc.7b02709

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Article
A Corannulene-based Coordination Cage with Helical Bias
Fu Huang, Lishuang Ma, Yanke Che, Hua Jiang, Xuebo Chen, and Ying Wang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02709 • Publication Date (Web): 27 Dec 2017
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A Corannulene-based Coordination Cage with Helical Bias
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Fu Huang†,‡,#, Lishuang Ma†,#, Yanke Che‡, Hua Jiang*†,§, Xuebo Chen*†, and Ying Wang*†
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†Key Laboratory of Theoretical and Computational Photochemistry and Key Laboratory of Radiopharmaceuticals,
Ministry of Education; College of Chemistry, Beijing Normal University, Beijing 100875, China
‡Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
§School of Chemical & Environmental Engineering, Wuyi University, Jiangmen 529020, China
Supporting Information Placeholder
ABSTRACT: We report here the first corannulene-based molecular cage, constructed via metal-induced self-assembly of
corannulene-based ligands. In sharp contrast to those assembled via the planar π-conjugated analogues of corannulene, at
ambient and elevated temperatures, the molecular cage exists as an ensemble
of four stereoisomers (two pairs of enantiomers), all of which possess a D5-
symmetric (regardless of the counter-anions) and inherently helical structure.
Decreasing the temperature shifts the equilibrium between different pairs of
enantiomers. At low temperature, only one pair of enantiomers is present. Hel-
ical bias for the cage could be efficiently achieved by inducing-asymmetry with
enantiopure anions. When non-enantiopure anions are used, the asymmetry-
induction abides by the "majority rule", i.e., the major enantiomer of the chiral
anions controls the bias of helical sense of the cages.
at 209 K),13 which provide corannulene a dynamic,
INTRODUCTION
switchable molecular chirality. By virtue of some of these
properties, especially the high reactivity and the concave
curved π-surfaces, a number of corannulene-based mo-
lecular entities, including dendrimer,14 liquid crystal,15
clefts,16 and supramolecular polymers,17 with unique
properties or functions, have been constructed for various
purposes. Nevertheless, to date, a corannulene-containing
molecular cage is, to the best of our knowledge, unprece-
dented.18
We envisioned that the distinct characteristics of
corannulene would make it an excellent candidate to con-
struct molecular cages with intriguing properties, in par-
ticular, the chirality, the conformational diversity, and
good impermeability. In this contribution, we report the
synthesis, characterization and asymmetric induction of
the first corannulene-based coordination molecular cage,
[Ag512]5+ (Figure 1), as results of our initial investigation
on such kind of cage systems.
Synthetic molecular entities having cage-like structures
and well-defined inner cavities have received great inter-
est.1 Such molecular cages can act as molecular flasks used
in catalysis,2 material purification,3 reactive intermediate
storages,4 and drug delivery.5 In particular, chiral cages6
could be employed in asymmetric catalysis and enantio-
meric resolution. Self-assembly with high-symmetry
building blocks containing a concave chemical surface is a
general, highly-efficient synthetic strategy that gives rise
to molecular cages.7 Besides, this is also one of main ap-
proaches for constructing chiral cages, in which chirality
of the cage arises from curvature of its molecular scaf-
fold.8 There are only a few known concave building blocks
for cage constructions, with a majority adopting either a
conical or subulate geometry.7,9 As such, new building
blocks with smaller curvature that could potentially ex-
tend the geometric diversity and applications of such
constructed cages are desirable.
Corannulene is a bowl-shaped polycyclic aromatic hy-
drocarbons that has C5v symmetry, with curvature mim-
icking that of fullerene[60].10 Due to its special structural
geometry, corannulene is endowed with many intriguing
properties, such as a curved π-surface, a large dipole mo-
ment,11 inherent charity,8 intricate redox behavior,12 and
high reactivity10b. More interestingly, corannulene dynam-
ically undergoes a very rapid bowl-to-bowl inversion in
solution due to the low inversion barrier (10.2 kcal mol–1
RESULTS AND DISCUSSION
Design, Synthesis and Characterization. The pre-
cursor of the cage, ligand 1, was designed to possess a
corannulene skeleton with five pyridine units appended.
An iso-butoxy side chain was introduced into each pyri-
dine ring at the meta position to pyridine-N for better
solubility while posing no interference with the ligand-
metal coordination. Molecular cages were expected to be
formed through the self-assembly of 1 induced by
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(Figure S1);18 and once complexed, the pyridyl units on
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each ligand tilt in one of two possible directions, giving
rise to a left- or right-handed propeller-shaped geometry
(M or P helicity). However, considering the geometric
restraints imposed by the Ag+ linear coordination, only
two pair of enantiomers of [Ag512]5+, P,M,P and M,P,M,
and P,P,P and M,M,M, can be expected (Figure 1), in
which the designations of the chirality of substituted
corannulene are based on the general rules for molecular
entities possessing axial chirality, such as foldamers, but
with the additional rule that the bowl-shape corannulene
opens up (Figure S1 and S2, SI). Nevertheless, this will be
still in sharp contrast to that of traditional AgxL2-type
cages,6a,19a, 20 for which there is only one steady structure
or one pair of enantiomers presented in the systems. Giv-
en the dynamic molecular chirality of the corannulene
moiety, interconversion between the enantiomeric con-
formers of [Ag512]5+ is theoretically achieved by cleavage of
coordination bonds, followed by the bowl-to-bowl inver-
sion of corannulene and the re-association of the cage
complexes, while the P,M,P⇆P,P,P and M,P,M⇆M,M,M
conversions could be readily realized through twist of the
cage.19a
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DFT (B3LYP/CPCM-UAHF) energy minimized models
of [Ag512]5+ provide a realistic illustration of the helical,
D5-symmetric cage geometry of all the four conformers
and an enantiomeric relationship between each pair of
enantiomers (Table S7 and S8, SI). In the cages, five Ag+
cations are held between two helically-arranged ligands 1
facing each other, with the coordination angles of ca. 169°
in the cases of P,M,P/M,P,M and ca. 172° in those of
P,P,P/M,M,M. Due to the curvature of corranulene, the
different arrangement patterns will give different torsion
angles between the two corannulene units (the two
corranulene units are offset by 22~23° and ca. 6° for
P,M,P/M,P,M and P,P,P/M,M,M conformers, respectively).
As a result, despite of possessing almost the same
molecular volume, compared to P,P,P/M,M,M conformers,
the P,M,P/M,P,M ones, in which the pyridyl groups tilt in
the opposite direction to the helical pattern of the
substitution on corranulene to balance the influence of
the curvature of corranulene, are 11 kcal mol–1 lower in
energy. Ion-pairing with triflate anions gives a slight
change in geometry of the cage due to the produced steric
interaction for all conformations, which further gives rise
to a decrease in the energy gap between P,M,P/M,P,M
and P,P,P/M,M,M conformers (Table S7, SI). Overall, the
DFT calculations predicted that the energy level of
Figure 1．Structures of corannulene, the corannulene-based
ligand 1, and mixture of four [Ag512]·[OTf]5 cage conformers
(denoted [mix-Ag512]·[OTf]5) studied herein.
Scheme 1. Synthesis of ligand 1.a
O
N
O
OH
O
B
(i)
(ii)
Br
Br
N
N
O
2
3
(iv)
1
Cl
Cl
Cl
(iii)
Cl
Cl
4
a
Reagents and conditions: (i) 1-iodo-2-methylpropane,
K2CO3, DMF, 60 °C, 8 h (65%); (ii) bis(pinacolato) diboron,
Pd(dppf)2Cl2, KOAc, DMSO, 80 °C, 8 h (80%); (iii) ICl, DCM,
from –78 °C to rt, 82 h (50%); (iv) Pd(PPh3)4, K3PO4, ben-
zene/ethanol/water, 100 °C, 6 d (50%).
[(P,M,P/M,P,M)-Ag512]·[OTf]5 lies 6 ̴
9 kcal mol–1 lower in
energy than that of [(P,P,P/M,M,M)-Ag512]·[OTf]5, de-
pendent on the location of counter triflate anions (Table
S7, SI).
metal cations favoring linear coordination geometry
such as Ag+ cations.6a,19
1
Synthetic routes for ligand 1 was outlined in Scheme 1.
Ligand 1 was synthesized in three steps by O-alkylation of
3-bromo-5-hydroxypyridine with alkyl iodide, borylation
with bis(pinacolato)diboron, and a Suzuki cross-coupling
reaction with 1,3,5,7,9-pentachlorocorannulene. The
structure of 1 was confirmed by NMR and high-resolution
mass spectra (see the Supporting Information (SI)).
Due to the chirality and the bowl-shape of corannulene,
the ligand 1 itself exists as a pair of M/P enantiomers
The H NMR spectra of solutions of 2:5 mixtures of 1
and several metal salts, such as [Cu(CH3CN)4] (PF6), in
common or mixed solvents exhibited complicated or no
signals, even after the samples were heated. However,
1
with Ag(OTf) as the salt, the H NMR spectrum of the
mixture in 93:7 (v/v) CDCl3/CD3CN clearly showed a set
of intense, well-resolved signals that is assigned to the
racemic of [(P,M,P)/(M,P,M)-Ag512]·[OTf]5 (Figure 2b and
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silver/triflate ions during the analysis (Figure S21 and Ta-
1
ble S3, SI).
2
3
P,M,P/M,P,M⇆P,P,P/M,M,M
Conversion
and
between
Asymmetry-induction.
Interconversion
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P,M,P/M,P,M and P,P,P/M,M,M conformers were clearly
indicated by the variable-temperature (VT) 1H NMR spec-
tra of the cage in 93:7 (v/v) CDCl3/CD3CN (Figure S5, SI).
With the increase of the temperature, the equilibrium
shifted toward P,P,P/M,M,M conformers. On the contrary,
decreasing the temperature gave rise to more conformers
being lower in energy. At low temperature such as 253 K,
only one set of signals corresponding to P,M,P/M,P,M
species could be observed (Figure 2c).
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A
series of the equilibrium constants for the
P,M,P/M,P,M⇆P,P,P/M,M,M interconversion at different
temperatures were calculated from the molar ratio of two
pairs of stereoisomers (Table S1, SI). From these constants,
the standard enthalpy of reaction, ΔH = –10.1 ± 0.4 kcal
mol–1, and the entropy of reaction, ΔS = –30.3 ± 1.3 cal
mol–1 K–1, were derived from the Eyring plots. At 298 K,
the Gibbs free energy of reaction, ΔG, was calculated to be
~ –1.1 kcal mol–1, the absolute value of which is much low-
er than the DFT-calculated energy gap between
P,M,P/M,P,M and P,P,P/M,M,M conformations of the cage.
This might be ascribed to the steric interaction between
the cage and the encapsulated solvent molecules, which
would inevitably present in real solution but had ignored
in the DFT calculations.24
Figure 2. 1H NMR spectra (600 MHz, 93:7 (v/v)
CDCl3/CD3CN) of ligand and cages complexes. (a) ligand 1 (2
mM, 298 K); (b–c) cage [mix-Ag512]·[OTf]5 (1 mM) ((b) at 298
K, (c) at 253 K); (d–e) [mix-Ag512]·[OTf]5 (1 mM, 253 K) in the
presence of (d) 10 equiv of Δ-TRISPHAT anions and (e) 10
equiv of Λ-TRISPHAT anions and D2O (0.4% (v/v) accounted
to CDCl3/ CD3CN mixture). Cage [mix-Ag512]·[OTf]5 was pre-
pared by mixing 1 with 2.5 equiv of Ag(OTf). Resonances
corresponding to P,P,P and M,M,M isomers were labelled in
light blue dots (Figure 2b).
Figure S3, SI) based on the results of DFT calculations.21
Another set of signals with much weaker intensities can
also be observed. We attributed it to the P,P,P/M,M,M
We next explored the possibility of obtaining enantio-
merically enriched [Ag512]5+ cages at low temperature via
asymmetric induction using noncovalent chiral auxiliaries
such as Δ/Λ-tris(tetra-chlorobenzenediolato)phosphate(v)
(Δ/Λ-TRISPHAT) anions (Figure 3a).19a,25 The DFT calcula-
tions suggested that (P,M,P)- and (M,P,M)-[Ag512]5+ cage
still exhibited a helical, D5-symmetric geometry when
ion-pairing with TRISPHAT anions (Figure 3b,c and Ta-
ble S8). In the cage, the two ligands are offset by ~22°.
Charity of the cage is induced through a steroselective
interaction between two aromatic rings of the anions and
the pyridyl groups on the cage (Figure 3b).
1
isomers.21 The assignments of the H NMR signals of lig-
and 1 and [(P,M,P)/(M,P,M)-Ag512]·[OTf]5 were performed
1
1
based on the corresponding H−1H NOESY, H−1H COSY,
1H−13C HSQC, and H−13C HMBC experiments (see Figure
1
S7−S15, SI).22 Relative to that of 1, the pyridine protons
those are located in the inner cavity of the cage, Hc, shift-
ed far to the upfield, as a response to the strong shielding
of the cage. The other two pyridyl protons, Hb and Hd,
downshifted due to the deshielding of the introduced
metal cation. Notably, upon complexation, the methylene
protons on the iso-butyl side chains, He, split into two
sets, which is an important indicator of the formation of
cage species in which the substituted pyridyl groups are
constrained so that the protons directed inward and out-
ward of the cage are inequivalent.23
The asymmetry-inducing of chiral TRISPHAT anions
were first investigated by circular dichroism (CD) spec-
troscopy. In the experiments, enantiopure TRISPHAT
anions were added to the solution of [mix-Ag512]·[OTf]5
(50 μM) in 93:7 (v/v) CHCl3/CH3CN; and the changes
were recorded at 253 K. [nBu4N][Δ-TRISPHAT] and
[nBu3NH][Λ-TRISPHAT] were used as the TRISPHAT
sources. Despite having a slight difference in structure of
the countercations, these two compounds exhibit a per-
fect enantiomeric relationship to each other (Figure S22
and Figure S23, SI). The CD spectra associated with
[Ag512]5+ cages themselves were obtained after subtraction
of the corresponding TRISPHAT contributions.
In the absence of chiral anions, the cages exhibited no
CD signals, as expected for a racemic mixture. With the
addition of [nBu4N][Δ-TRISPHAT], the signals appeared
(Figure 4), indicative of the induction of helical bias
through diastereoselective Ag+·[TRISPHAT]− ion-pair
formation. The spectrum showed a main positive peak
2D DOSY experiments indicated that the two sets of
signals shown in Figure 2b belonged to species having
almost the same diffusion coefficient (Figure S18 and S19,
SI), in accordance with relationship in size between four
cage species shown in Figure 1. The calculated diffusion
coefficient is (3.3~3.4) × 10–10 m2/s, smaller than that of
ligand 1 in the same solvent mixture, 4.6 × 10–10 m2/s. Ac-
cording to the Stokes–Einstein equation, the ratio of the-
se two values is largely consistent with the volume ratio
between the complex cages and the ligands (ca. 2.5, Table
S2, SI).
Additional evidence for the formation of the cage com-
plexes was provided by ESI-HRMS, which showed a peak
for [Ag512·(OTf)4]1+ at m/z = 3127.3357 and a series of other
intense peaks resulting from partial loss or addition of
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(M,P,M)-[Ag512]5+ should exist predominantly over the
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P,M,P ones under such condition (Figure 4). As expected,
with [nBu3NH][Λ-TRISPHAT], the induced CD spectrum
showed a mirror image as well as the same g values as
those in the case of Δ-TRISPHAT (Figure 4 and Table S4,
SI). Besides, cage (P,M,P)-[Ag512]5+ prevails in this case,
according to the results of TD-DFT calculations (Figure 4).
It is noteworthy that the TD-DFT-calculated ECD spectra
of [(P,P,P)-Ag512]5+ and [(M,M,M)-Ag512]5+ do not agree
well with that of experimental ones (Figure S49), confirm-
ing the absence of P,P,P/M,M,M conformers in our
asymmetric induction experiments.
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To get deeper insights into the asymmetry-induction of
the chiral anions, we next carried out the H NMR titra-
1
tion experiments. Upon addition of [nBu4N][Δ-TRISPHAT]
to the racemic mixture (1 mM) in 93:7 (v/v) CDCl3/CD3CN
at 253 K, a new set of signals mainly corresponding to
[(M,P,M)-Ag512]·[Δ-TRISPHAT]5 appeared (Figure 2d and
Figure S32, SI), in which the signal of protons those in
proximity to the counter anions, Hd, shifted upfield dra-
matically owing to the greater shielding effect imparted
by the chiral anions. Clearly, the ion-association of
Ag+·[Δ-TRISPHAT]− ion-pair is much stronger than that
of Ag+·(OTf)−, giving that the new cage complexes equili-
brate slowly on the NMR time scale with all previous spe-
cies, including [mix-Ag512]·[OTf]5 and the solvated [mix-
Figure 3. (a) Structures of Δ/Λ-TRISPHAT anions. (b-c) DFT
(B3LYP/CPCM-UAHF) energy minimized structure of
[(M,P,M)-Ag512]·[Δ-TRISPHAT]5 ((b) Ball (for cations) and
stick diagrams; (c) space-filling representation); one C5 and
five C2 axes presented in [Ag512]5+ architecture are shown;
other TRISPHAT anions were deleted for clarity.
Ag512]·[OTf]5. The [(M,P,M)/(P,M,P)-Ag512]·[OTf]5
⇆
[(M,P,M)-Ag512]·[Δ-TRISPHAT]5 reaction involves five-
steps of ion-exchange. Assuming that the ion-exchange
process for each step is completely independent from the
others, the equilibrium constant for the ion-exchange
[(1/5)·mix-Ag512]+·[OTf]–
+
[Δ-TRISPHAT]–
⇆
[(1/5)·(M,P,M)-Ag512]+·[Δ-TRISPHAT]– + [OTf]– was calcu-
lated to be 2.3 ± 0.5 (Table S6, SI). The stronger ion-
pairing with Δ-TRISPHAT anions may probably be at-
tributed to the formation of π–π and/or halogen–π26 in-
teractions between TRISPHAT anion and the two pyridyl
groups bridged by Ag+ cation. On the other hand, the fact
that only one set of signals could be observed indicates
that the cage complexes interconvert rapidly between the
P,M,P and M,P,M conformers on the NMR timescale.19a
Ascribed to the enantiomeric nature, addition of
[nBu3NH][Λ-TRISPHAT] gave rise to very similar results
as those in the case of Δ-TRISPHAT (Figure 2e and Figure
S33, SI), except that less amount (5 equiv) of TRISPHAT
was needed to saturate the anion exchange compared to
that in the case of [nBu4N][Δ-TRISPHAT] (8 equiv). The
equilibrium constant for [(1/5)·mix-Ag512]+·[OTf]– + [Λ-
Figure 4. Solid lines: CD spectra (93:7 (v/v) CDCl3/CD3CN,
253 K) of [mix-Ag512]·[OTf]5 (50 μM) in the presence of 10
equiv of [nBu4N][Δ-TRISPHAT] and that of [nBu3NH][Λ-
TRISPHAT]. Contributions from TRISPHAT anions were
subtracted; main fractions of the cage isomers are indicated.
Dash dot lines: TD-DFT calculated CD spectra of the isolated
cages [(M,P,M)-Ag512]5+ and [(P,M,P)-Ag512]5+ using the
B3LYP//LANL2DZ/6-31G(d) and the CPCM model for 93:7
(v/v) CDCl3/CD3CN (further details are described in Section
7, SI). The calculated spectra are vertically scaled.
TRISPHAT]–
⇆ +
[(1/5)·(P,M,P)-Ag512]+·[Λ-TRISPHAT]–
[OTf]– was calculated to be 10.5 ± 3.1 (Table S6, SI). This
indicates that the use of TRISPHAT anion as a nBu3N+H
salt leads to the formation of a more tightly associated
Ag+·[TRISPHAT]− ion-pair compared to that as a nBu4N+
one, probably due to that nBu3N+H is more strongly solv-
ated under the studied condition.
1
at 290 nm, accompanying two shoulder-peaks at 265 and
326 nm, respectively, and a main negative one at 370 nm.
In the presence of 10 equiv of Δ-TRISPHAT, the apparent
CD anisotropy factor, g, of the cage was calculated to be
1.6×10–3 and 2.7×10–3 at 290 nm and 370 nm, respectively.
Time-dependent (TD) DFT calculations suggest that
An interesting question arising from above H NMR ti-
tration experiments is how the P,M,P⇆M,P,M conversion
occurs while [Ag512]5+ is ion-pairing with the chiral anions.
It is worth noting that, in solution, the Δ/Λ-TRISPHAT-
cage complexes may exist as three kinds
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and P,P,P/M,M,M ones or increase the energy level of the
1
2
transition state (wherein all the N–Ag–N linkages are par-
allel to the helix axis) 19a of the flips.
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We finally explored the asymmetry-induction with
1
non- enantiopure anions, by measuring H NMR and CD
spectra of [Ag512]5+ with the mixtures of [nBu4N][Δ-
TRISPHAT] and [nBu3NH][Λ-TRISPHAT] at a constant 10
1
times total concentrations to that of the cages. The H
7
NMR spectrum did not show obvious changes upon
changing the ratios of two chiral anions (Figure S45, SI).
The CD spectrum changed as expected (Figure 6a), but
the intensity was not linearly proportional to the enanti-
omeric excess (e.e.) of the mixture (Figure 6b). This indi-
cates that, to some extent, the major enantiomer of
TRISPHAT anions controls the bias of helical sense of
[Ag512]5+ cages. Such "majority rules" has been widely ob-
served in many supramolecular interaction systems.29
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46
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48
49
50
51
52
53
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55
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59
60
Figure 5. Schematic illustration of the interconversion be-
tween (M,P,M)-[Ag512]5+ and (P,M,P)-[Ag512]5+ cages, which
take place when the cages are loosely associated with the
chiral TRISPHAT anions (as an example, a solvent-shared
model is shown). Solvent molecule is labelled with "S".
of ion pairs,27 including tight, solvent-shared and solvent-
separated ones.28 Therefore, interconversion between the
diasteromeric P,M,P- and M,P,M-type helical cage may
possibly occurs at any above states. However, DFT calcu-
lations indicated that the energy level of the ground state
Figure 6. (a) CD spectra (93:7 (v/v) CHCl3/CH3CN, 253 K) of
[Ag512]5+ cages (50 μM) obtained from different ratios of Δ-
and Λ-TRISPHAT anions. (b) Corresponding plots of CD
spectral intensities against e.e.’s of TRISPHAT anions.
conformation
of
contacted
[(P,M,P)-Ag512]·[Δ-
TRISPHAT]5 is 13 kcal mol−1 higher in energy than that of
[(M,P,M)-Ag512]·[Δ-TRISPHAT]5 (Table S7 and Table S8,
SI). This suggests that a two way P,M,P⇆M,P,M conver-
sion in fact more likely take place when the cages are
loosely associated (including solvent-shared and solvent-
separated ion-pairing) with the chiral anion (Figure 5).
This is understandable because looser association would
give a much lower enantiomerization barrier of the com-
plexes.
CONCLUSIONS
A new supramolecular helical cage that based on the
coordination of metal cations to corannulene-based lig-
ands has been designed and synthesized. The cage exists
as an ensemble of four stereoisomers, i.e., two pairs of
enantiomers, at ambient and elevated temperatures,
which is much different from that of the traditional
AgxL2-type ones. Owing to the dynamic, switchable mo-
lecular chirality of the corannulene moiety, decreasing
the temperature gave finally the presence of only one pair
of enantiomers. Further, with the help of asymmetry-
induction using chiral TRISPHAT anions, helical sense
bias for the cage could be efficiently achieved. The finding
verifies that the corannulene moiety can indeed provide
molecular cages distinctive conformational and dynamic
characteristics compared to traditional ones. The con-
structed cage extends the applications of the bowl-shaped
polycyclic aromatic hydrocarbons and would potentially
expand the geometric diversity of artificial molecular cag-
es. We envisage that the cage might find applications in
chiral molecular recognition or preparation of functional
materials with chiroptical properties.
Increasing temperature leads to the formation of more
loosely associated ion pairs thus giving rise to a decrease
in the asymmetry-induction ability of the chiral anions.
This was evidenced by the VT CD measurements, which
showed that the CD signals of TRISPHAT-paired cages
gradually decreased upon increasing temperature and
completely disappeared at 333 K (Figure S28–S31, SI). Cor-
1
responding VT H NMR investigation showed that in-
creasing temperature from 253 K produced continuous
downfield shift of proton Hd (Figure S35–S36, SI), indicat-
ing that tight Ag+ꢀ[Δ/Λ-TRISPHAT]− ion-pairing kept
turning into loose ones in this process which agrees well
with the results of VT CD measurements. Overall, the
P,M,P/M,P,M conformations in such cases were found to
be more stable than those in the case of OTf–-paired cages:
at least at 303 K, there was still only one set of signals
mainly corresponding to [(M,P,M)-Ag512]ꢀ[Δ-TRISPHAT]5
that could be observed (compared to that should be lower
than 263 K in the case of [mix-Ag512]·[OTf]5; Figure S5);
[(P,M,P)-Ag512]ꢀ[Λ-TRISPHAT]5 can even stand higher
temperature. The higher energy barriers of P,M,P/M,P,M
⇆ P,P,P/M,M,M interconversions are attributed to the
stronger Ag+ꢀ[Δ/Λ-TRISPHAT]− ion-pairing, which may
enlarge the energy gap between P,M,P/M,P,M conformers
EXPERIMENTAL SECTION
General Information. All starting chemicals were obtained
from commercial sources and used without further purifica-
tion, unless indicated otherwise. Anhydrous toluene was
dried from sodium/benzophenone and then distilled under
inert atmosphere, while anhydrous dichloromethane (DCM)
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was distilled over calcium hydride (CaH2), respectively. An-
7.35 (t, J = 2.4 Hz, 1H), 3.75 (d, J = 6.8 Hz, 2H), 2.10 (m, 1H),
1.02 (d, J = 6.8 Hz, 6H) ppm; 13C NMR (100MHz, CDCl3): δ =
155.8, 142.7, 136.6, 123.8, 120.4, 75.0, 28.2, 19.1 ppm. TOF-
HRMS-ESI (1% HCOOH in MeCN, ion type, % RA for m/z):
Calcd. for C9H13BrNO at [M+H]+: 230.0181 (100.0%), 231.0214
(9.7%), 232.0160 (97.3%, 2.6 ppm), 233.0194 (9.5%); Found:
230.0176 (100.0%, 2.2 ppm), 231.0211 (8.7%, 1.3 ppm), 232.0154
(97.9%, 2.6 ppm), 233.0185 (8.5%, 3.9 ppm).
1
2
3
4
5
6
7
8
hydrous dimethyl sulfoxide (DMSO) was obtained from
commercial sources. Corannulene was provided by Prof. Jay S.
Siegel at School of Pharmaceutical Science and Technology,
Tianjin University, Tianjin, China. [Tetrabutylammoni-
um][Δ-tris(tetrachloro-1,2-benzenediolato)phosphate(V)] (Δ-
TRISPHAT
tetra-butylammonium
salt,
[nBu4N][Δ-
TRISPHAT]) was obtained from commercial sources; the
enantiomer, [nBu3NH][Λ-TRISPHAT], was prepared accord-
ing to the procedure reported in the literature25.
3-iso-Butoxy-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)pyridine (3): We followed the previously published
procedure30 for preparation of the compound. To starting
9
Analytical TLC was carried out using tapered silica plates
with a preadsorbent zone. Crude compounds were purified
by flash column chromatography, using flash grade silica gel
and 0 – 20 psig pressure, performed at ambient pressure. Per-
deuterated solvents for NMR spectroscopy were obtained
from Cambridge Isotope Laboratories and used as received.
NMR spectra were obtained with a Bruker spectrometer (1H,
400, 500, and 600 MHz) or a JEOL Delta spectrometer (1H,
400 and 600 MHz) using chloroform-d (CDCl3), acetonitrile-
d3 or the mixture of them as solvent. The chemical shift ref-
erences were as follows: (1H) chloroform, 7.26 ppm (chloro-
form-d), (13C) chloroform-d, 77.16 ppm (chloroform-d); (1H)
acetonitrile-d3, 1.94 ppm (acetonitrile-d3), (13C) acetonitrile-d3,
118.26 ppm (acetonitrile-d3); (1H) 1,1,2,2-tetrachloroethane-d2,
6.00 ppm (1,1,2,2-tetrachloroethane-d2), (13C) 1,1,2,2-
tetrachloroethane-d2, 73.8 ppm (1,1,2,2-tetrachloroethane-d2).
(1H) tetramethylsilane (TMS), 0.00 ppm (the mixture of chlo-
roform-d and acetonitrile-d3). Typical 1D FID was subjected
to exponential multiplication with a line broadening expo-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
material
2
(2.0
g,
8.7
mmol,
1.0
equiv),
bis(pinacolato)diboron (2.43 g, 9.56 mmol, 1.1 equiv),
Pd(dppf)2Cl2 (191 mg, 0.26 mmol, 0.03 equiv) and KOAc (2.56
g, 26.1 mmol, 3 equiv) in 50 mL double-necked flask prefilled
with argon, anhydrous DMSO (Acros, 15 mL) was added. The
mixture was stirred at 80 °C for 8 h, poured into brine and
then extract with dichloromethane. The combined organic
layer was washed with brine for three times, dried over sodi-
um sulfate and concentrated on rotary evaporator to give a
black viscous crude oil. The oil was washed with n-hexane
and filtered. The filtrate was concentrated and dried in vacuo
to give a yellow oil (1.94 g, yield: 80%) which was directly
used for the next step reaction without further purification.
IR (KBr, cm–1): 2980, 2930, 2875, 1600, 1450, 1370, 1160, 1040,
850. 1H NMR (400MHz, CDCl3): δ = 8.52 (d, J =1.2 Hz, 1H),
8.35 (d, J = 3.2 Hz, 1H), 7.52 (dd, J = 1.2 Hz, 3.2 Hz, 1H), 3.77 (d,
J = 6.4 Hz, 2H), 2.10 (m, 1H), 1.35 (s, 12H), 1.02 (d, J = 7.6 Hz,
6H) ppm; 13C NMR (100MHz, CDCl3): δ = 155.2, 147.4, 140.9,
126.23, 84.3, 82.7, 28.4, 24.9, 19.2 ppm. TOF-HRMS-ESI (1%
HCOOH in MeCN, ion type, % RA for m/z): Calcd. for
C15H24BNNaO3 at [M+Na]+: 300.1747 (100.0%), 299.1783
(24.87%), 300.1780 (16.23%); Found: 300.1744 (100.0%, 1.0
ppm), 299.1788 (24.0%, 1.7 ppm), 300.1772 (16.3%, 2.7 ppm).
1,3,5,7,9-Pentachlorocorannulene (4). The procedure
was slightly modified from the one previously reported in the
literature.31 To a solution of iodine monochloride (ICl, Al-
drich, 21 mL, 1.0 M, 21 mmol, 13.0 equiv) in dichloromethane
at –78 °C, corannulene powder (400 mg, 1.60 mmol, 1.0 equiv)
was added under a positive pressure of argon. The resulting
purple suspension was warmed slowly to room temperature
over 10 h, then stirred at ambient temperature for an addi-
tional 72 h. The solution was poured into 50 mL of chloro-
form and washed with 5% aqueous sodium thiosulfate (20
mL × 2) and pure water (20 mL× 2). The resulting yellow
suspension was evaporated to dryness and triturated 2 times
with hexanes (20 mL) and 3 times with dichloromethane (10
mL). The resulting yellowish solid was recrystallized from
1,1,2,2-tetrachloroethane to yield a pale yellow solid (335 mg,
yield: 50%). M.p. (under air): 432–434 °C. IR (KBr, cm–1):
1610, 1420, 1300, 1175, 910, 875. 1H NMR (400 MHz, 25°C,
C2D2Cl4): δ = 7.99 (s, 5H) ppm; 13C NMR (150 MHz, C2D2Cl4,
100°C): not obtained due to its poor solubility under the
1
nent (LB) of 0.3 Hz (for H) and 1.0 Hz (for 13C). 2D NOESY
was recorded with the mixing time of 300 ms. UV-vis spectra
were obtained on Shimadzu (UV-2401PC) spectrophotometer,
using 10-mm or 1-mm path-length quartz cells. Circular di-
chroism (CD) spectra were obtained using ChirascanTM
spectropolarimeter (Applied Photophysics Ltd., United
Kingdom), equipped with a Peltier temperature controller. In
cases that temperatures lower than 253 K (–20 oC) were
needed, an add-on recirculating chiller was used. 1-mm path-
length quartz cells were used in all the CD measurements.
For Both UV-vis and CD, the mixture (93:7 (v/v)) of spectro-
photometric grade chloroform (≥99.8%) and acetonitrile
(≥99.5%) was used as the solvent. Optical rotations were
measured with Autopol III (Rudolph Research) at ambient
temperature, using ethanol as the solvent. The measure-
ments were repeated for five times; specific rotation was
calculated based on the mean value of the measured rotation
in degrees. IR spectra were recorded on FTIR Spectrometer
(IR Affinity-1) with thin KBr disk. 128 scans were acquired,
with a resolution of 2 cm−1. Exact mass spectra (EI, ESI,
MALDI) were acquired on GCT, FT-ICR spectrometer.
3-Bromo-5-iso-butoxypyridine (2): To a solution of 5-
bromopyridin-3-ol (3.0 g, 17.24 mmol, 1.0 equiv) in DMF (15
mL), 1-iodo-2-methylpropane (2.2 mL, 18.96 mmol, 1.1 equiv)
and K2CO3 (3.57 g, 25.86 mmol, 1.5 equiv) was added. The
suspension mixture was stirred at 60 °C for overnight and
then poured into pure water, then extracted with ethyl ace-
tate. The organic layer was washed with brine for more than
5 times to remove the contained DMF, dried over sodium
sulfate and concentrated in vacuo. The residue was purified
on silica gel flash column chromatography (ethyl ace-
tate/hexane, 1:15) to give the product 2 as a pale yellow oil
(2.58 g, yield: 65%). Rf = 0.6 (ethyl acetate/hexane, 1:15). IR
(KBr, cm–1): 2960, 2925, 1575, 1425, 1260, 1100, 1020, 800. 1H-
NMR (400MHz, CDCl3): δ = 8.26 (s, 1H), 8.23 (s, 1H), 7.34–
measured condition.
HRMS-MALDI-TOF: Calcd. for
C20H6Cl5 at [M+H]+: 422.8883 (100%), 424.8853 (63.9%),
420.8912 (62.58%), 423.8916 (21.63%), 425.8887 (13.83%),
421.8946 (13.54%); Found: 422.8856 (100%, 6.4 ppm),
424.8842 (63.58%, 2.6 ppm), 420.8872 (62.38%, 9.5 ppm),
423.8902 (21.57%, 3.3 ppm), 425.8895 (13.91%, 1.9 ppm),
421.8926 (13.04%, 4.7 ppm).
1,3,5,7,9-Penta-(5-iso-butoxy-3-pyridinyl)corannulene
(1): Starting material 1,3,5,7,9-penta-chlorocorannulene (4)
(100 mg, 0.24 mmol, 1.0 equiv) was taken to a Schlenk vessel
and evacuated under high vacuum at ambient temperature
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The Journal of Organic Chemistry
R. Acc. Chem. Res. 2014, 47, 2063−2073; (c) Ahmad, N.; Younus,
H. A.; Chughtai, A. H.; Verpoort, F. Chem. Soc. Rev. 2015, 44,
9−25.
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Int. Ed. 2009, 48, 3418−3438.
(3) García-Simón, C.; Garcia-Borràs, M.; Gómez, L.; Parella, T.;
Osuna, S.; Juanhuix, J.; Imaz, I.; Maspoch, D.; Costas, M.; Ribas,
X. Nat. Commun. 2014, 5, 5557.
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mond, K. N. J. Am. Chem. Soc. 2006, 128, 14464−14465; (b)
Kawamichi, T.; Haneda, T.; Kawano, M.; Fujita, M. Nature, 2009,
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for 1 h. The Schlenk vessel was charged with argon. Then to
this Schlenk vessel in an argon glove bag, compound 3 (825
mg, 2.98 mmol, 12.5 equiv) and Pd(PPh3)4 (82 mg, 0.07 mmol,
0.3 equiv) was added. The mixture was evacuated under high
vacuun for 2 h and charged with argon again. To the vessel,
degassed solvents (degassed by continuous bubbling of argon
gas), benzene (8 mL), EtOH (4 mL), and aqueous K3PO4 so-
lution (2 M, 8 mL) were added. The mixture was degassed
again by freeze-pump-thaw method. (In this process, the
reaction mixture was rapidly frozen under vigorous stirring
to ensure that the solidified mixture was still phase separated,
to minimize the probability of glassware breakage.) The mix-
ture was stirred for 15 minutes at room temperature, then at
100 °C for 6 days. The mixture was cooled down to room
temperature. Most of the solvent was removed; water (10 mL)
was added and the crude product was extracted with chloro-
form (2 × 10 mL). The organic layers were concentrated un-
der reduce pressure and then redisperse into toluene (2 mL).
The insoluble solid was collected and washed with toluene (2
× 2 mL), methanol (2 × 2mL) and acetonitrile (2 × 2 mL),
affording pure product 1 as a pale yellow solid (118.5 mg, yield:
50%). M.p. = 295–297 °C. IR (KBr, cm–1): 3050, 2960, 2925,
2870, 1600, 1560, 1425, 1310, 1210, 1160, 1050, 860. 1H NMR
(400MHz, CDCl3): δ = 8.49 (d, J = 1.6 Hz, 5H), 8.40 (d, J = 2.8
Hz, 5H), 7.81 (s, 5H), 7.47 (t, J = 2.4 Hz, 5H), 3.84 (d, J = 6.4
Hz, 10H), 2.10 (m, 5H), 1.03 (d, J = 6.8 Hz, 30H) ppm; 13C
NMR (100MHz, CDCl3): δ = 155.5, 142.6, 139.2, 137.2, 135.9,
135.5, 129.4, 126.5, 122.5, 75.0, 28.4, 19.3 ppm. HRMS-FTMS (1%
HCOOH in MeCN, ion type, % RA for m/z): Calcd. for
C65H66N5O5 at [M+H]+: 996.5064 (100%)，997.5097 (70.32%),
998.5131 (24.34%); Found: 996.5073 (100%, 0.9 ppm) ，
997.5086 (83.56%, 1.1 ppm), 998.5100 (20.38%, 3.1 ppm).
1
2
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ASSOCIATED CONTENT
SUPPORTING INFORMATION
The Supporting Information is available free of charge on the
ACS Publications website.
Experimental and computational details and charac-
terization data, including spectra for NMR and CD in-
vestigation on the asymmetry induction (PDF)
AUTHOR INFORMATION
CORRESPONDING AUTHOR
* jiangh@bnu.edu.cn
* xuebochen@bnu.edu.cn
* ywang1@bnu.edu.cn
(17) Kang, J.; Miyajima, D.; Mori, T.; Inoue, Y.; Itoh, Y.; Aida, T.
Science, 2015, 347, 646−651.
(18) One notable example, reported by Miyajima and Aida,
could be regarded as a pseudo-molecular cage, see: Kang, J.;
Miyajima, D.; Itoh, Y.; Mori, T.; Tanaka, H.; Yamauchi, M.; Inoue,
Y.; Harada, S.; Aida, T. J. Am. Chem. Soc. 2014, 136, 10640−10644.
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Jiang, H. J. Org. Chem. 2016, 81, 3364−3371.
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Angew. Chem. Int. Ed. 2004, 43, 3814 –3818; (c) Lee, H. Y.; Park, J.;
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15339 – 15348.
Author Contributions
#F.H. and L.M. contributed equally to this work.
NOTES
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The National Natural Science Foundation of China
(21332008, 21572023 and 21672026) and the 973 Program
(2015CB856502) are acknowledged for financial support.
We thank Prof. Jay S. Siegel for providing the starting
material, corannulene.
REFERENCES
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(21) Notably, the assignments were carried out based on just
DFT calculations without experimental support.
1
2
3
4
(22) The signals relating to P,M,P/M,P,M conformers were
1
1
tentatively assigned based on the H−1H COSY, H−1H NOESY
1
and H−13C HSQC spectra and an assumption that conformation
of the cage only slightly influences its carbon chemical shifts
(Figure S10 − S13, SI).
5
6
7
8
9
(23) Park, S. J.; Hong, J.-I. Chem. Commun. 2001, 1554–1555.
(24) Most solvent models (including the one used in our
computational calculations) is not based on explicit solvent
molecules that can interact with solute molecules but instead
model the solvent as a polarizable continuum.
10
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(28) There might be little free ions of [Ag512]5+ presented in the
system because any presence of free cage ions will inevitably give
rise to a residual of the original cage signals.
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