Synthesis of a Tetrabenzotetraaza[8]circulene by a "fold-In" Oxidative Fusion Reaction

Article abstract of DOI:10.1002/anie.201505124

Tetrabenzotetraaza[8]circulene (1) has been synthesized in good yield by a "fold-in" oxidative fusion reaction of a 1,2-phenylene-bridged cyclic tetrapyrrole. X-ray diffraction analysis of 1 has revealed a planar square structure with a central cyclooctat

Full text of DOI:10.1002/anie.201505124

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505124
German Edition: DOI: 10.1002/ange.201505124
Aromaticity Hot Paper
Synthesis of a Tetrabenzotetraaza[8]circulene by a “Fold-In” Oxidative
Fusion Reaction**
Fengkun Chen, Yong Seok Hong, Soji Shimizu, Dongho Kim,* Takayuki Tanaka,* and
Atsuhiro Osuka*
Abstract: Tetrabenzotetraaza[8]circulene (1) has been synthe-
sized in good yield by a “fold-in” oxidative fusion reaction of
a 1,2-phenylene-bridged cyclic tetrapyrrole. X-ray diffraction
analysis of 1 has revealed a planar square structure with
a central cyclooctatetraene (COT) core that shows little
alternation of the bond lengths. Despite these structural
features, 1 shows aromatic-like character, such as sharp
absorption bands, high fluorescence quantum yields (F =
F
0
.55 in THF), and a single exponential fluorescence decay
with t = 3.8 ns. These observations indicate a dominant con-
F
tribution of an [8]radialene-like p conjugation and hence
aromatic character of the local aromatic segments in 1.
H
etero[8]circulenes with O, N, S, and Se atoms have been
attracting considerable interest in light of their enforced
planar structures and presumed 8p antiaromaticity at the
cyclooctatetraene (COT) center as well as their potential
[1–5]
applications for OLEDs and OFETs.
We also became
interested in tetrabenzotetraaza[8]circulene (1) since it cor-
responds to the central moiety of porphyrin sheet 3, which
displays remarkably strong antiaromatic character owing to
Porphyrin sheet 3 was synthesized by oxidative fusion of
meso-meso-linked 5,10-porphyrin cyclic tetramer 2. This
transformation can be regarded as a “fold-in” oxidative
fusion reaction. While “fold-in” approaches have been shown
to be effective for the synthesis of bowl-shaped fused
[
6]
its planar COTunit. We envisioned that tetraaza[8]circulene
may be a benchmark molecule to assess the roles of the
1
porphyrin segments in the antiaromatic character of 3. In
addition, compound 1 is an interesting molecule on its own,
since tetraaza[8]circulene has remained unexplored to date.
[
7]
aromatic molecules, they have not been employed for the
synthesis of heterocirculenes. Since the discovery of tetraoxa-
[
8]circulene in 1968, the synthesis of hetero[8]circulenes has
long relied on classical acid-catalyzed condensation reactions
[
*] Dr. F. Chen, Prof. Dr. T. Tanaka, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: taka@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
[2,3]
of benzoquinones.
Herein, we report an effective synthesis
of 1 by a “fold-in” oxidative fusion reaction of a cyclic
tetrapyrrolic precursor 5.
[8]
First, 1,2-di(pyrrol-2-yl)benzene (4) was brominated at
the 5,5’-positions and the resulting dibromide was directly
Y. S. Hong, Prof. Dr. D. Kim
[9]
coupled with 1,2-di(pinacolatoboryl)benzene under palla-
Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
dium-catalyzed conditions. Cyclic tetrapyrrole 5 was obtained
in 11% yield after repetitive purifications by column chro-
matography on silica gel (Scheme 1). The structure of 5 has
been confirmed by X-ray diffraction analysis (Figure 1a). In
the solid state, 5 takes on a twisted figure-eight conformation
E-mail: dongho@yonsei.ac.kr
Dr. S. Shimizu
Department of Applied Chemistry, Graduate School of Engineering
Kyushu University
Fukuoka 819-0395 (Japan)
with an average dihedral angle between the pyrrole rings and
1
1
,2-phenylene bridges of 30.28. The H NMR spectrum of 5 in
[
**] The work at Kyoto was supported by JSPS KAKENHI (Grant
Numbers 25220802, 25620031, and 20705446). The work at Yonsei
was supported by the Global Research Laboratory (GRL) Program
funded by the Ministry of Science, ICT and Future, Korea
CDCl exhibited a broad singlet corresponding to the pyrrolic
3
NH protons at 8.70 ppm, a singlet corresponding to the
pyrrolic b protons at 6.43 ppm, and two signals corresponding
to the phenylene protons at 7.41 and 7.20 ppm, thus indicating
(
2013K1A1A2A02050183). We thank Prof. Dr. Hiromitsu Maeda and
its high symmetry (D ). These spectral features remain
Dr. Yuya Bando (Ritsumeikan University) for MALDI-TOF MS
measurements.
4h
unchanged even at À808C (Figures S3 and S4). From the
lower symmetry structure in the solid state, it is concluded
that 5 is in dynamic conformational equilibrium in solution.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505124.
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signals at 130.4, 124.4, 122.6, 121.9, and 114.0 ppm, again
highlighting its D -symmetric structure.
4h
Single crystals of 1 were obtained by slow diffusion of
hexane into a solution of 1 in a mixture of acetone and THF.
X-ray diffraction analysis showed the structure to be a per-
fectly planar square with a small mean-plane deviation value
of 0.025 Š (Figure 2a). All the pyrrole rings point outward,
and undergo hydrogen-bonding interactions with the carbon-
Scheme 1. Synthesis of 5. Bpin=4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl, dba=dibenzylideneacetone, NBS=N-bromosuccinimide.
Figure 1. a) X-ray crystal structure of 5. The thermal ellipsoids were
scaled to 50% probability. b) Dynamic equilibrium of 5.
Despite its formal [24]porphyrin(2.2.2.2) expanded-porphy-
rin-like structure, 5 is non-aromatic, as judged from its
1
[10]
H NMR and UV/Vis spectra.
In the next step, the intramolecular oxidative fusion
reaction of 5 was examined (Scheme 2). After extensive
optimization of the conditions, we were delighted to find that
tetrabenzotetraaza[8]circulene (1) was cleanly synthesized in
Figure 2. a) X-ray crystal structure of 1. The thermal ellipsoids were
scaled to 50% probability. Acetone molecules coordinating to NH
groups by hydrogen bonding are shown. b) Bond lengths and NICS(0)
values of reference molecules calculated at the B3LYP/6-311+G(d,p)
level.
yl oxygen atoms of acetone molecules. The eight CÀC bond
lengths constructing the central COT are similar, in the range
1
.428–1.438 Š, thus indicating no significant alternation of the
bond length. Importantly, these bond lengths are longer than
those of the optimized D8h COT (ca. 1.40 Š). On the other
hand, the C ÀC bond lengths in the pyrrole moieties are
Scheme 2. Oxidative fold-in fusion reaction of 5 to afford 1.
DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone, Tf=trifluorometha-
nesulfonyl.
a
b
distinctly shorter (1.393–1.400 Š). These structural features
underline the importance of the contribution of [8]radialene-
like conjugation. Single crystals of 1 were also obtained by
diffusion of hexane into a solution of 1 in THF, 1,4-dioxane, or
DMSO. Although the crystal-packing structures are different,
the structures of the molecular frameworks and the hydrogen-
bonding interactions of the four pyrrole rings with solvent
molecules are nearly the same (see the Supporting Informa-
tion).
[11]
a high yield of 96% by oxidation of 5 with DDQ/Sc(OTf)3.
Compound 1 is poorly soluble in CHCl , toluene, and other
3
nonpolar solvents, but soluble in THF and DMSO. The
1
H NMR spectrum of 1 in [D ]THF exhibited a signal at
8
1
1.77 ppm corresponding to the pyrrolic NH protons and two
signals at 8.83 and 7.76 ppm corresponding to the phenylene
1
3
protons. The C NMR spectrum of 1 showed five sharp
1
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[12]
[15]
Nuclear independent chemical shift (NICS) values at
the center of each of the ring segments A, B, C, and D were
calculated with the GIAO method at the B3LYP/6-311 +
G(d,p) level to be + 6.82, À7.52, À9.66, and À7.68 ppm,
respectively (Figure 2a). The calculated positive NICS(0)
value in ring A may be regarded as an indication of its
antiaromaticity being derived from COT, as discussed for
omatic porphyrinoids are not detected for 1. The Stokes
À1
shift is only 175 cm , thus indicating a rigid structure in the
excited state. In line with these absorption and emission
features, TD-DFT calculations have indicated intense degen-
erate bands with f = 0.22 at 399 nm to be ascribed to HOMO–
LUMO and HOMO-1–LUMO transitions. To gain further
insight into the frontier orbital diagrams, the magnetic
circular dichroism (MCD) spectrum of 1 was measured in
THF. The spectrum exhibits a positive-to-negative sign
sequence (negative Faraday A term) corresponding to the
main absorption at 416 nm, which indicates that the absorp-
tion bands consist of transitions from degenerate orbitals
(HOMO and HOMO-1) to a nondegenerate orbital (LUMO;
Figure 3b). Overall, these characteristics of 1 suggest an
aromatic-like character despite the presence of the central
planar COT. On the basis of frontier MO profiles it is
considered that the electronic properties of 1 stem from
peripheral aromatic segments (Figure S7-6).
[2–4]
other heterocirculenes.
However, the observed antiaro-
maticity in 1 is not as strong as the large positive NICS(0)
[6a]
value (+ 35.71 ppm) at the center of porphyrin sheet 3.
Examination of various substructures of 1 such as D -
4h
symmetric COT, tetrapyrrole 6, and [8]radialene 7 has
revealed that the positive NICS(0) value decreases with
elongation of the CÀC bond lengths around the COT
[
13]
structure. Therefore, the moderate positive NICS(0) value
and weakened antiaromaticity at ring A of 1 may be ascribed
[14]
to its “stretched” COT segment.
Figure 3a shows the UV/Vis absorption and emission
spectra of 1 and 5. Cyclic tetrapyrrole 5 exhibits a broad
absorption band at 326 nm and a broad fluorescence in the
To investigate the excited-state properties of 1 and 5, we
measured the dynamics of the S state by using the TCSPC
1
range 400–650 nm (F = 0.27) in THF. Interestingly, solid 5
(time-correlated single photon counting) technique (Fig-
ures S8-1 to S8-9). In the case of 5, we could observe
biexponential decay profiles and two time constants were
estimated to be around 1 and 5 ns. Moreover, the ratio
between the two components is slightly different depending
on the probe wavelength used. The fast decay component
corresponding to 1 ns is too slow to be assigned as structural
relaxation dynamics. As mentioned above, however, the
various conformers of 5 that exist at both room temperature
and even À808C probably give rise to the probe wavelength
dependent biexponential decay. To gain further insight into
this feature, we measured the fluorescence decay of 5 at 77 K
to reduce structural heterogeneity. As a result, we found that
the biexponential decay of 5 becomes a single exponential
decay with a lifetime of around 5.1 ns, regardless of the probe
wavelength.
F
emits fluorescence in the range 450–700 nm with F = 0.15
F
(
Figure S6-2). Compound 1 exhibits vibronic-structured
absorption bands at 390 and 413 nm and their mirror-image
sharp emission bands at 416 and 441 nm with a high quantum
yield (F = 0.55) in THF. Importantly, weak absorption tails
F
in the lower-energy region that are characteristic of antiar-
In the case of 1, the excited-state lifetime was estimated to
be 3.8 ns, with a single exponential decay. The measured
II
excited-state lifetime of 1 is much longer than that of the Zn
porphyrin sheet (1.1 ps) measured by the transient absorption
[6a]
II
technique. Considering that the Zn porphyrin sheet shows
antiaromaticity above the COT core and nonfluorescent
behavior, the well-resolved absorption/fluorescence spectra,
high fluorescence quantum yield, and long excited-state
lifetime of 1 strongly support its overall aromatic feature
being induced by the weakened antiaromatic character of the
central COT.
Finally, the fabrication of 1 was examined. Changes in the
absorption spectrum of 1 were observed upon addition of
tetrabutylammonium fluoride (TBAF), presumably because
of deprotonation of the NH protons (Figure S6-4). On the
basis of this observation, 1 was treated with an excess of
iodobutane in the presence of sodium hydride for 24 h at
room temperature to afford tetra-N-butylated tetraaza-
[
8]circulene 8 in 77% yield (Scheme 3). This product shows
improved solubility in common solvents, such as dichloro-
methane, chloroform, acetone, and benzene, which would
promise its further application as discotic liquid-crystalline
Figure 3. a) UV/Vis absorption (solid) and fluorescence (dashed) spec-
tra of 5 (gray) and 1 (black) in THF. b) MCD (top) and UV/Vis
[2c]
(bottom) spectra of 1 and 8 in THF.
mesogens, for example.
The structure of 8 has been
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1
showed a pseudo-reversible peak at À0.01 V versus the
+
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In summary, tetrabenzotetraaza[8]circulene (1) has been
synthesized from known compounds in two steps, with
a “fold-in” intramolecular oxidative fusion reaction as the
key step. Compound 1 is a planar, square molecule that
exhibits a sharp absorption spectrum, sharp fluorescence with
a high quantum yield, and a long fluorescence lifetime. These
features, which are in sharp contrast to porphyrin sheet 3—
which exhibits a strong paratropic ring current—may come
from the local aromatic segments fused to the central COT.
Antiaromaticity in the central planar COT is considerably
weakened, probably because of the elongated CÀC bonds
1
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Keywords: aromaticity · circulenes · cyclooctatetraene ·
fused-ring systems · pyrroles
How to cite: Angew. Chem. Int. Ed. 2015, 54, 10639–10642
Angew. Chem. 2015, 127, 10785–10788
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Received: June 5, 2015
Published online: July 14, 2015
1
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