Hollow sphere formation from a three-dimensional structure composed of an adamantane-based cage

Article abstract of DOI:10.1021/jo500989c

An adamantane-based cage with a three-dimensional (3D) framework was synthesized by the copper-mediated acetylene coupling reaction, in which two 1,3,5-triethynyladamantane units were linked by phenyldiacetylenic bridges possessing ester groups. X-ray cry

Full text of DOI:10.1021/jo500989c

Note
pubs.acs.org/joc
Hollow Sphere Formation from a Three-Dimensional Structure
Composed of an Adamantane-Based Cage
Masahide Tominaga,*,† Kazuaki Ohara,† Kentaro Yamaguchi,† and Isao Azumaya*,‡
†Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki Kagawa 769-2193, Japan
‡Faculty of Pharmaceutical Sciences, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
S
* Supporting Information
ABSTRACT: An adamantane-based cage with a three-dimensional (3D) framework was synthesized by the copper-mediated
acetylene coupling reaction, in which two 1,3,5-triethynyladamantane units were linked by phenyldiacetylenic bridges possessing
ester groups. X-ray crystallography revealed that the cage has an internal space and accommodates a solvent molecule, which
afforded molecular networks through CH/O interactions between cage molecules. Furthermore, the cage containing six ester
groups spontaneously self-assembled into hollow spherical aggregates with an average diameter of 230 nm in a mixed solvent.
ollow three-dimensional (3D) structures have attracted
significant interest because of their potential in guest
workers.17 Recently, we demonstrated the synthesis and crystal
structure of a series of adamantane-based molecules with
multiple branching and macrocyclic frameworks, which
provided network structures with cavities and one-dimensional
channels.18 Trisubstituted adamantanes are especially versatile
building blocks for the construction of novel 3D frameworks
because their bulky skeletons allow them to form internal
spaces. Therefore, we designed an adamantane-based cage (1)
in which two 1,3,5-triethynyladamantane units were linked by
three phenyldiacetylenic bridges bearing ester groups. In this
paper, we report the construction and structural analysis of a
nanosized synthetic cage. Furthermore, a molecular cage with
ester groups at its periphery spontaneously assembled into
uniform hollow spherical aggregates with a multilayer
membrane, vesicle-like aggregates, in a mixed solvent.
An adamantane-based cage with a cavity was designed and
synthesized according to Scheme 1. We selected triethynyla-
damantane as a trisubstituted adamantane derivative because it
has proven to be a versatile scaffold.19 Triethynyladamantane
can be prepared in two steps from adamantane according to the
routes of Malik and co-workers.20 We chose 3-iodo-5-
[(trimethylsilanyl)ethynyl]benzoic acid methyl ester as the
bridge moiety and prepared it by the method established by
Moore et al. and Keana et al.21 A Sonogashira coupling reaction
between triethynyladamantane and 3-iodo-5-
[(trimethylsilanyl)ethynyl]benzoic acid methyl ester afforded
H
inclusion, the stabilization of reactive intermediates, and specific
chemical transformations.1−4 A number of 3D structures
including cages, capsules, tubes, and others have been produced
using directional noncovalent interactions such as hydrogen
bonds and metal coordination in addition to the construction of
covalently bound 3D molecules.5−7 A wide variety of molecular
cages have been used as receptors for guest molecules and also
as building blocks for the self-assembled nanostructures of
higher hierarchy in the fields of supramolecular chemistry and
materials science.8−10 In particular, vesicles as unique and basic
spherical objects have received attention because of their
practical applications as light-harvesting systems and in drug
delivery.11−13 To construct these vesicular aggregates, numer-
ous synthetic amphiphiles have been designed as surfactants
with polar head groups and hydrophobic tails. Recently, large π-
conjugated aromatic molecules have been transformed into
innovative amphiphilic molecules, which turn into vesicles with
desirable and tunable sizes, stabilities, and functions because of
their π-stacking ability in aqueous and polar organic solutions.
In addition to the use of amphiphilic polymers14 and disk- or
rodlike molecules,15 vesicular structures derived from amphi-
philic macrocyclic molecules such as calixarenes, cyclodextrins,
cucurbiturils, and cyclophanes have been reported.16 In
contrast, the preparation of synthetic molecular cages with
3D frameworks for direct self-assembly into spherical
architectures has been limited. The reverse-vesicle organization
of hydrogen-bonded arylamide-derived hexaammonium capsu-
les in apolar solvents has been described by Li and co-
Received: May 6, 2014
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of the Adamantane-Based Cage 1
Figure 1. (a) Side view and (b) top view of the crystal structure of
cage 1 as a space-filling model. Hexane and mesitylene molecules are
omitted for clarity.
of adamantane in a neighboring cage molecule and in the
crystalline lattice (Figure 2). Two different types of channels
Figure 2. Packing diagram of crystal 1. View of the network structure
viewed along the a axis. Hexane and mesitylene molecules are omitted
for clarity.
were thus formed along the a axis, and these were composed of
the cavities of the cages and the void spaces between the cages.
The hexane and mesitylene molecules were accommodated
within the internal spaces.
2 in 41% yield. Desilylation (93%) followed by Hay acetylene
coupling provided cage 1 in 12% yield. Adamantane-based cage
1
1 was characterized by H and 13C NMR spectroscopy, and
only one signal for the methine proton and one signal for the
carbon of the adamantane parts were found, respectively,
indicating that the product was synthetic cage 1. The mass
spectroscopy data of cage 1 are in agreement with the
structures presented.
Molecular cage 1 dissolved in tetrahydrofuran, chloroform,
and benzene, whereas it was only slightly soluble in methanol
and hexane. The aggregation behavior of the molecular cage
was examined in a polar mixed solvent of methanol and
tetrahydrofuran (1:2, v/v) using dynamic light scattering
(DLS), field-emission scanning electron microscopy (FE-
SEM), and transmission electron microscopy (TEM). Cage 1
was dissolved in tetrahydrofuran (6.7 × 10−4 M) at 40 °C and
allowed to stand for 5 days at 25 °C. DLS measurements
revealed a low scattering intensity, indicating that no large
aggregates were formed in solution. In the mixed solvent, cage
1 showed aggregation behavior with a narrow size distribution,
and the diameter of the spherical assemblies was estimated to
be 230 nm (Figure S1, Supporting Information). The
generation of cage-based spheres was also confirmed by the
FE-SEM experiment, and spherical shapes with a uniform
diameter in the range of 150−300 nm were obtained (Figure
3a). The SEM image indicated that the resultant aggregates are
robust and stable because they retain their spherical shape
without collapsing despite being dried on a solid surface. Most
of the vesicles built using various amphiphiles have been found
The structure of cage 1 was unambiguously determined by
single-crystal X-ray analysis (Figure 1). Single crystals of cage 1
were obtained by the vapor diffusion of hexane into a
chloroform/mesitylene solution of 1. X-ray crystallographic
analysis revealed that the compound crystallized in the P-1
space group with one molecule of 1, one molecule of hexane,
and four molecules of mesitylene in the asymmetric unit. The
crystal structure of 1 exhibited a cagelike shape with an internal
space where the distance between the two bridgehead protons
in the two adamantane moieties was 19.8 Å. The distances
between the methyl carbons in the ester groups were 15.85−
19.93 Å. The acetylene parts deviate slightly from linearity with
CC−C angles of 177.0−179.7°. Each cage molecule was
packed into a 3D network architecture through CH/O
interactions between ester oxygen atoms and phenyl ring
protons, methyl protons of ester groups, or methylene protons
B
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allow for the exploitation of host−guest systems and specific
delivery. Furthermore, the cages possess ester groups, which
allow for the incorporation of additional functional molecules at
the periphery of the cages. The linking of a variety of functional
groups to the six ester groups resulted in six functional groups
being incorporated into the spherical assemblies. The design
and self-assembly of cage molecules functionalized with
biofunctional groups such as sugars and peptides is the subject
of our current studies.
Figure 3. Hollow spherical assemblies prepared in a mixed solvent
with adamantane-based cage 1 (6.7 × 10−4 M): (a) SEM and (b) TEM
micrographs.
EXPERIMENTAL SECTION
■
General Information. All solvents and reagents were obtained
from commercial suppliers and were used without further purification.
1H and 13C NMR spectra were recorded with a 400 MHz NMR
to be flat on the surface under similar conditions.22
Furthermore, several spherical aggregates had holes in their
periphery, revealing a hollow center. The TEM measurements
also provided evidence for the generation of hollow spheres.
The TEM images indicated a clear contrast between the
peripheries and the centers of the spherical structures. The wall
thickness of the hollow spheres was estimated to be 25−30 nm,
which again indicates multilayer structures (Figure 3b).
Additionally, burst spherical structures were partly present
(Figure S2, Supporting Information). These findings are
consistent with the results obtained from the DLS and SEM
experiments. The cage-based spheres were relatively stable in
solution under these conditions without precipitation occur-
ring, and they were of similar shape and size after 2 weeks. In a
more polar mixed solvent of methanol and tetrahydrofuran
(1:1, v/v), larger spherical assemblies were observed by SEM
(Figure S3, Supporting Information). However, after 2 weeks
under these conditions, precipitated solids were present among
the spherical assemblies in solution. Conversely, spherical
aggregates were not present when using adamantane-based cage
1 in a mixed solvent of methanol and tetrahydrofuran (1:4, v/
v). These results indicate that the polarity in a mixed solvent
considerably influences the stability and size of the spherical
assemblies.
instrument in CDCl3 at 298 K using tetramethylsilane as an internal
standard. Mass spectra were obtained by FAB-MS or high-resolution
ESI-MS experiments. X-ray crystal structure data were collected using
a Quantum Q315 CCD area detector at the beamline of BL38B1 of
SPring-8. The structure was refined with the SHELX-97 programs.
Dynamic light-scattering experiments were performed on an instru-
ment equipped with a 4 mW He−Ne laser (633 nm wavelength) at a
fixed detector angle of 90°. SEM and TEM samples were prepared by
depositing the solution of 1 on Al foil and a carbon-coated copper grid
and dried in vacuo. SEM samples were then coated with Pt/Pd in an
ion coater for 40 s. SEM images were obtained at an accelerating
voltage of 1.5 kV on a FE-SEM. TEM measurements were performed
using a transmission electron microscope at an accelerating voltage of
120 kV.
1,3,5-Triethynyladamantane. This compound was synthesized
as described in the literature:20 1H NMR (400 MHz, CDCl3, 25 °C) δ
2.14 (s, 3H), 2.11 (t, J = 3.2 Hz, 1H), 1.95 (s, 6H), 1.78 (d, J = 3.2 Hz,
6H); 13C NMR (100 MHz, CDCl3, 25 °C) δ 90.0, 68.2, 46.2, 40.4,
29.8, 27.8.
3-Iodo-5-[(trimethylsilanyl)ethynyl]benzoic Acid Methyl
Ester. This compound was prepared as described in the literature:21
1H NMR (400 MHz, CDCl3, 25 °C) δ 8.30 (t, J = 1.6 Hz, 1H), 8.07 (t,
J = 1.2 Hz, 1H), 7.97 (t, J = 1.6 Hz, 1H), 3.92 (s, 3H), 0.25 (s, 9H);
13C NMR (100 MHz, CDCl3, 25 °C) δ 164.9, 144.3, 138.2, 132.2,
131.7, 125.4, 102.0, 97.1, 93.1, 52.5, −0.3.
1,3,5-Tris-[(3-(methoxycarbonyl)-5-[(trimethylsilanyl)-
ethynyl]phenyl)ethynyl]adamantane (2). A mixture of 1,3,5-
triethynyladamantane (0.47 g, 2.28 mmol), 3-iodo-5-
[(trimethylsilanyl)ethynyl]benzoic acid methyl ester (3.43 g, 9.57
mmol), Pd(PPh3)4 (0.26 g, 0.23 mmol), and CuI (43.8 mg, 0.23
mmol) in triethylamine (30 mL) was refluxed for 3 days under an
argon atmosphere. After being cooled to room temperature, the
reaction mixture was filtered and the filtrate was evaporated under
reduced pressure. The residue was dissolved in CHCl3, and then the
dark brown solution was washed with H2O and brine and dried over
Na2SO4. Evaporation of the solvent followed by silica gel column
chromatography (eluent: CHCl3/hexane = 1:4) and gel permeation
chromatography (eluent: CHCl3) afforded the title compound as a
solid (0.84 g, 0.93 mmol) in 41% yield: mp 99−100 °C; FT-IR (ATR,
cm−1) 2947, 2850, 2161, 1725, 1507, 1324, 1236, 1112, 1003, 775. 1H
NMR (400 MHz, CDCl3, 25 °C) δ 8.02 (t, J = 1.6 Hz, 3H), 7.98 (t, J
= 1.6 Hz, 3H), 7.67 (t, J = 1.6 Hz, 3H), 3.92 (s, 9H), 2.24 (br s, 1H),
2.11 (s, 6H), 1.90 (d, J = 2.0 Hz, 6H), 0.25 (s, 27H); 13C NMR (100
MHz, CDCl3, 25 °C) δ 165.8, 138.8, 132.4, 132.0, 130.5, 124.2, 123.8,
103.1, 96.9, 95.9, 79.2, 52.3, 46.3, 40.5, 30.7, 28.0, −0.2; MS (FAB, m/
z) calcd for C55H59O6Si3 (M + H+) 899.45, found 898.9. Anal. Calcd
for C55H58O6Si3: C, 73.46; H, 6.50. Found: C, 73.19; H, 6.53.
1,3,5-Tris[(3-(methoxycarbonyl)-5-[(ethynylphenyl)-
ethynyl]]adamantane (3). A mixture of 2 (0.89 g, 1.0 mmol) and
tetrabutylammonium fluoride (1.89 g, 6.0 mmol) in dried THF (30
mL) was stirred for 12 h at room temperature under an argon
atmosphere. The reaction was quenched with 1 M HCl at 0 °C, and
the mixture was extracted with chloroform. The extracts were washed
with water and brine and dried over Na2SO4. Evaporation of the
The crystal structure of cage 1 showed that molecule 1 is ca.
2.0 nm wide. We thus assume that the hollow spheres end up
with multilayer morphology by the two- and three-dimensional
packing of cage frameworks. The six polar carboxylic ester
groups are likely exposed to the polar organic solvents by being
bound to the outside of the spherical assemblies, whereas the
cage frameworks that consist of aromatic and aliphatic units are
packed into layered structures, which minimizes the interaction
with polar solvents. In contrast to more common amphiphiles
with large polar head groups and hydrophobic tails, the
adamantane-based cage 1 possesses a large nonpolar scaffold
and polar groups on its periphery.
In summary, we produced nanosized synthetic cages based
on adamantane by the macrocyclization of a trisubstituted
adamantane bearing acetylenic aromatic ring units through
copper-mediated oxidative coupling. X-ray diffraction experi-
ments of the cage revealed that the cage molecule has an
internal space and that it formed molecular networks
containing two types of channels. This was possible because
of weak noncovalent interactions in the crystalline lattices with
the solvent molecules that were accommodated within the
internal spaces. The adamantane-based cage afforded well-
defined hollow spheres with a multilayer membrane in the
organic solvents. The spherical aggregates derived from the
discrete 3D framework possess the potential properties of
liposomes and 3D hosts, and the findings in this study may
C
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solvent followed by silica gel column chromatography (eluent: CHCl3)
afforded the title compound as a solid (0.65 g, 0.93 mmol) in 93%
yield: mp 88−90 °C; FT-IR (ATR, cm−1) 3293, 2936, 2910, 2857,
J. K.; Fujita, M. Angew. Chem., Int. Ed. 2009, 48, 3418−3438.
(c) Avram, L.; Cohen, Y.; Rebek, J., Jr. Chem. Commun. 2011, 5368−
5375.
1
2164, 1724, 1590, 1440, 1320, 1235, 1223, 1112, 771; H NMR (400
(2) (a) Cram, D. J. Angew. Chem., Int. Ed. 1986, 25, 1039−1057.
MHz, CDCl3, 25 °C) δ 8.05 (t, J = 1.2 Hz, 3H), 8.02 (t, J = 1.2 Hz,
3H), 7.68 (t, J = 1.2 Hz, 3H), 3.92 (s, 9H), 3.12 (s, 3H), 2.26 (br s,
1H), 2.13 (s, 6H), 1.92 (d, J = 2.8 Hz, 6H); 13C NMR (100 MHz,
CDCl3, 25 °C) δ 165.7, 138.9, 132.9, 132.2, 130.6, 124.4, 122.8, 97.1,
81.9, 79.1, 78.5, 52.4, 46.2, 40.4, 30.7, 28.0; HRMS (ESI, m/z) calcd
for C46H34O6Na (M + Na+) 705.2248, found 705.2224.
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Adamantane-Based Cage (1). A mixture of 3 (137 mg, 0.20
mol), copper(I) chloride (198 mg, 2.00 mmol), and N,N,N′,N′-
(tetramethylethylene)diamine (TMEDA) (233 mg, 2.00 mmol) in
CH2Cl2 (80 mL, 2.5 mM) was stirred for 12 h at room temperature.
The reaction mixture was washed with water and brine, and dried over
Na2SO4. Evaporation of the solvent followed by silica gel column
chromatography (eluent: CHCl3) and gel permeation chromatography
(eluent: CHCl3) afforded the title compound (16.3 mg, 0.012 mmol)
as a solid in 12% yield: mp >300 °C dec; FT-IR (ATR, cm−1) 2968,
2870, 2160, 1740, 1506, 1490, 1318, 1260, 1112, 1001, 793; 1H NMR
(400 MHz, CDCl3, 25 °C) δ 8.05 (t, J = 1.6 Hz, 6H), 8.03 (t, J = 1.6
Hz, 6H), 7.79 (t, J = 1.6 Hz, 6H), 3.93 (s, 18H), 2.30 (br s, 2H), 2.23
(d, J = 12.8 Hz, 6H), 2.14 (d, J = 12.4 Hz, 6H), 1.94 (s, 12H); 13C
NMR (100 MHz, CDCl3, 25 °C) δ 165.5, 140.6, 132.6, 131.8, 130.9,
124.6, 122.3, 97.4, 80.3, 79.0, 74.8, 52.5, 46.8, 39.7, 30.8, 27.9; HRMS
(ESI, m/z) calcd for C92H63O12 (M + H+) 1359.4314, found
1359.4316. Crystallographic data for 1: C67H61.26O6, Mr = 962.42,
triclinic, P-1, a = 13.782(3) Å, b = 19.442(4) Å, c = 21.953(4) Å, α =
88.24(3)°, β = 76.49(3)°, γ = 86.79(3)°, V = 5710(2) Å3, Z = 4, Dc =
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and 8045 unique (Rint = 0.0544). μ = 0.070 mm−1, Tmax = 0.9958, and
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra for compounds 1−3, X-ray
crystallographic file (CIF), DLS data, and SEM and TEM
images for 1. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: tominagam@kph.bunri-u.ac.jp.
*E-mail: isao.azumaya@phar.toho-u.ac.jp.
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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This work was supported by a Grant-in-Aid for Young
Scientists (B) from the Ministry of Education, Science, Sports,
and Culture of Japan (No. 22750113). We wish to thank Prof.
Dr. T. Tokumura and Dr. T. Kurita (Tokushima Bunri
University) for their technical guidance in the DLS experiment
and Dr. T. Itoh (Center for Analytical Instrumentation, Chiba
University) for the TEM measurement. We also thank Drs. S.
Baba and K. Miura (the Japan Synchrotron Radiation Research
Institute (JASRI)) for their valuable help with data collection
for the X-ray analysis of 1. The synchrotron radiation
experiment was performed at the BL38B1 beamline at
SPring-8 with the approval of JASRI (Proposal No.
2010B1179).
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