Benzene-Fused Azacorannulene Bearing an Internal Nitrogen Atom

Article abstract of DOI:10.1002/anie.201502599

A novel nitrogen-doped corannulene derivative, 8-tert-butyl-6b²-azapentabenzo[bc,ef,hi,kl,no]corannulene, was synthesized by 1,3-dipolar cycloaddition of a polycyclic aromatic azomethine ylide with a diarylethyne and subsequent palladium-cataly

Full text of DOI:10.1002/anie.201502599

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502599
German Edition: DOI: 10.1002/ange.201502599
Corannulenes
Benzene-Fused Azacorannulene Bearing an Internal Nitrogen Atom**
Shingo Ito,* Yuki Tokimaru, and Kyoko Nozaki*
Abstract: A novel nitrogen-doped corannulene derivative, 8-
tert-butyl-6b2-azapentabenzo[bc,ef,hi,kl,no]corannulene, was
synthesized by 1,3-dipolar cycloaddition of a polycyclic aro-
matic azomethine ylide with a diarylethyne and subsequent
palladium-catalyzed intramolecular cyclization. This molecule
represents the first example of a corannulene derivative bearing
an internal heteroatom, and exhibits unique structural and
physical properties caused by the introduction of the nitrogen
Figure 1. Corannulene and azacorannulenes.
atom and extended p-conjugation, as compared to the parent
corannulene.
a trigonal nitrogen atom leads to the formation of cationic
species, which seem to be rather unstable (Figure 1). There-
fore, an effective method to fabricate a stable neutral species
would be the fusion of five benzene rings to form 6b2-
azapentabenzo[bc,ef,hi,kl,no]corannulene,[13] which is the
target of this study.
B
owl-shaped polycyclic aromatic molecules such as C5v-
symmetric corannulene[1] and C3v-symmetric sumanene[2,3]
have attracted significant attention owing to their unique
properties and partial fullerene- and carbon-nanotube-like
structures. Synthetic chemists have, therefore, devoted sub-
stantial effort toward the synthesis and derivatization of bowl-
shaped polyaromatic molecules. However, few studies have
focused on the synthesis of heteroatom-doped derivatives,
although doping of heteroatoms into polycyclic aromatic
hydrocarbons is a powerful method to alter their intrinsic
properties, including band structures and electrocatalytic
activity.[4,5] To the best of our knowledge, the only reported
examples of such heteroatom-doped derivatives include
heteroatom-doped sumanenes, which contain nitrogen,[6]
silicon,[7,8] and sulfur[9] atoms in their peripheral positions.
Heteroatom-doped corannulene derivatives are more chal-
lenging to synthesize and are elusive compounds, although
theoretical calculations[10] and attempts toward their synthe-
sis[1b,11] have been reported. The lack of heteroatom-doped
corannulene derivatives prompted us to pursue their syn-
thesis. We decided to use nitrogen as the heteroatom, because
it is a second-row element, the same as carbon. Because of the
difference in the valences of carbon and nitrogen atoms, the
simple replacement of a trigonal carbon atom at either the
hub (2a1) or spoke (2a) position[12] of corannulene with
Recently our group,[14] as well as Feng and Mꢀllen,[15]
independently developed the 1,3-dipolar cycloaddition of the
polycyclic aromatic azomethine ylide 1 (Scheme 1) with
Scheme 1. Synthesis of 8-tert-butyl-6b2-azapentabenzo[bc,ef,hi,kl,no]cor-
annulenes (3a) from the polycyclic aromatic azomethine ylide 1.
diarylethynes and subsequent oxidative dehydrogenation to
afford fused 1,2,3,4,5-pentaarylpyrroles. We envisioned that
subsequent cyclization would lead to the formation of the 6b2-
azapentabenzo[bc,ef,hi,kl,no]corannulenes 3. Herein we dis-
close the first bottom-up synthesis of 8-tert-butyl-6b2-azapen-
tabenzo[bc,ef,hi,kl,no]corannulene (3a), which is a novel
bowl-shaped polycyclic aromatic molecule bearing a nitrogen
atom in an internal position, by the palladium-catalyzed
intramolecular cyclization of the fused 1,2,3,4,5-pentaarylpyr-
role 2 as the key reaction.
The synthetic scheme is shown in Scheme 1. For the
successful cyclization of partially fused 1,2,3,4,5-pentaaryl-
pyrroles, it is necessary to introduce some functionality which
enables carbon–carbon bond formation between neighboring
aryl groups in 2. Toward this end, we focused on the
palladium-catalyzed intramolecular arylative cyclization of
a trichlorinated compound, previously employed in the
[*] Dr. S. Ito, Y. Tokimaru, Prof. K. Nozaki
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: ito_shingo@chembio.t.u-tokyo.ac.jp
nozaki@chembio.t.u-tokyo.ac.jp
[**] This work was partially supported by the Japan Society for the
Promotion of Science (JSPS) and Japan Science and Technology
Agency (JST). Financial support from the Yazaki Memorial Foun-
dation for Science and Technology is gratefully acknowledged. We
thank U. Takeda, Dr. K. Harano, and Prof. E. Nakamura (UTokyo) for
HPLC separation, R. Mogaki, Dr. Y. Itoh, and Prof. T. Aida (UTokyo)
for fluorescence analysis, and H. Waragai (UTokyo) for quantum
yield measurement.
synthesis
of
diindeno[1,2,3,4-defg;1’,2’,3’,4’-mnop]chry-
senes.[16] Thus, 2,2’,6-trichlorodiphenylethyne was used as
a coupling partner for the 1,3-dipolar cycloaddition of 1. The
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502599.
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reaction was carried out using an optimized procedure,[17] and
it generated 2 in 62% yield. Subsequent intramolecular
cyclization of 2 was performed in the presence of [Pd-
(PCy3)2Cl2] and a stoichiometric amount of cesium carbonate
(Cs2CO3) in N,N-dimethylacetamide (DMA) to 3a in 14%
yield.
The structure of 3a was confirmed by nuclear magnetic
resonance (NMR) spectroscopy and high-resolution mass
spectrometry (HRMS). The 1H NMR spectra of 3a in
[D6]benzene revealed the presence of seven different aro-
matic protons: one singlet, four doublets, and two triplets,
thus indicating that 3a possesses Cs symmetry. The HRMS of
3a exhibited an m/z value of 493.1848, which corresponds to
an ion mass of C38H23N (m/z: 493.1830). Finally, single crystals
of 3a that were suitable for X-ray analysis were obtained by
recrystallization of 3a from dichloromethane and diethyl
ether. The X-ray crystallographic analysis, shown in Figure 2,
structure along the a axis. The stacking distance is 3.69 ꢁ,
which is slightly shorter than that of sumanene (3.84 ꢁ).
Notably, corannulene does not adopt a concave-to-convex p-
stacking structure because of dominant CH–p interactions.[19]
Each 3a molecule in the column is shifted by about 80–838
because the tert-butyl moiety in 3a occupies the vacant space
generated by insertion of the pseudocircular column of 3a
into a square of the unit cell. The neighboring columns are
oriented toward opposite directions: this phenomenon was
also observed in hemifullerene (C36H12),[20] some corannulene
derivatives,[21] and monooxosumanene,[22] but not observed in
the parent corannulene[19] or sumanene.[2] It is worth noting
that the aromatic signals of 3a in the 1H NMR spectrum
shifted slightly upfield with increased concentration of 3a.[17]
This phenomenon suggests that some self-aggregation of 3a
occurs in the solution state. The association constant deter-
mined by using an indefinite noncooperative (isodesmic)
model[23] at 258C was (5.0 ꢀ 0.4)mꢁ1 [17]
.
This concentration
dependence is commonly observed for disklike polycyclic
aromatic compounds,[24] but to the best of our knowledge, no
report exists detailing the phenomenon in bowl-shaped
polycyclic aromatic molecules.
The photophysical properties of 3a were investigated, as
shown in Figure 3. A solution of 3a in dichloromethane (2.8 ꢂ
Figure 3. UV/Vis absorption spectrum (blue line) and fluorescence
spectrum excited at l=320 nm (red line) of 3a in CH2Cl2 solution
(2.8ꢀ10ꢁ6 m), along with the oscillator strengths (green bars) obtained
by TD-DFT calculations at the B3LYP/6-311+G(2d,p) level of theory.
Inset photograph shows a solution of 3a under irradiation with
a l=365 nm lamp.
Figure 2. a) X-ray structure of 3a with thermal ellipsoids of 50%
probability. Hydrogen atoms in both views and tert-butyl groups in
side views are omitted for clarity. b) Packing structure of 3a.
revealed a Cs-symmetrical bowl-shaped structure consisting
of 34 sp2-carbon atoms and 1 sp2-nitrogen atom.[18] The bowl
depths, defined as the perpendicular distance from the center
of the hub pyrrole ring to the parallel planes containing
carbon atoms C8, C14, and C20, are 1.38, 1.56, and 1.73 ꢁ,
respectively (Figure 2a). The bowl is deeper than that of
corannulene (0.87 ꢁ)[1] and that of sumanene (1.11 ꢁ).[2,3]
The lengths of the bonds comprising the five peripheral
benzene rings (C5-C10, C11-C16, C17-C22, C23-C28, and
C29-C34) range between 1.38 to 1.43 ꢁ, and are within the
common range of carbon–carbon bond lengths in benzene
rings. The lengths of the bridging bonds, C6-C34, C10-C12,
C16-C18, C22-C24, and C28-C30, are within 1.49–1.51 ꢁ,
which is similar to that of carbon–carbon single bonds. This
trend could be explained by the major contribution of the
bond alternation pattern consisting of one pyrrole ring and
five Clar-type benzene rings. As shown in Figure 2b, 3a
exhibited a one-directional concave-to-convex p-stacking
10ꢁ6 m) exhibited bright yellow color and showed weak and
broad absorption bands at l = 300–500 nm with maximum
wavelengths (labs) at 360, 395, 445, and 466 nm.[25] These
observed values are longer than those of corannulene[26] and
multibenzocorannulenes,[27] and comparable to those of
multiindenocorannulenes.[28] Another remarkable feature of
3a is its fluorescence. Specifically, 3a exhibited blue/green
fluorescence with maximum emission wavelengths (lem) at
490 nm and a shoulder band at l = 518 nm with a quantum
yield of FF = 0.24 (2.8 ꢂ 10ꢁ6 m).[29] Corannulene is known to
show weaker fluorescence with a quantum yield of FF =
0.024–0.070[26b,30] in solution. Therefore, the introduction of
the nitrogen atom into corannulene and extended p-con-
jugation derived from peripheral pentabenzo moieties sig-
nificantly alters its optical properties.
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rings (Figure 5b). This trend is opposite to that observed with
corannulene, in which the central five-membered ring exhibits
anti-aromatic characteristics and the six-membered rings
exhibit aromatic characteristics.[32]
An important feature of bowl-shaped molecules is their
bowl-to-bowl inversion behavior. DFT calculations using the
B3LYP function revealed activation energies ranging from
15.3 to 17.0 kcalmolꢁ1, depending on which basis set was
utilized.[33] The calculated barriers are higher than those of
corannulene (experimental value: 10–11 kcalmolꢁ1[34] and
theoretical value: 10.4 kcalmolꢁ1[35]), but lower than those of
sumanene (experimental values: 19.6–20.4 kcalmolꢁ1 [2a,36]
and theoretical values: 16.8–19.1 kcalmolꢁ1 [37]). The higher
barrier than that of corannulene could be attributed to the
presence of peripheral pentabenzo moieties, which retard
bond shrinkage required for the transition state in the bowl-
to-bowl inversion. This data is in good accordance with the
general trend that deeper bowl depths typically lead to high
activation energies for bowl-to-bowl inversions.[10,38]
Figure 4. Kohn–Sham molecular orbitals (from HOMOꢁ1 to
LUMO+1) of 3b calculated at the B3LYP/6-311+G(2d,p) level of
theory.
In summary, we developed the first bottom-up synthesis of
8-tert-butyl-6b2-azapentabenzo[bc,ef,hi,kl,no]corannulene
(3a) by 1,3-dipolar cycloaddition of the azomethine ylide
1 and subsequent palladium-catalyzed intramolecular cycli-
zation of the fused 1,2,3,4,5-pentaarylpyrrole 2. The advan-
To understand the observed photophysical properties,
theoretical calculations were performed for 3b, in which the
tert-butyl group of 3a is replaced by a hydrogen atom, using
density functional theory (DFT) at the B3LYP/6-311 + G- tages of azacorannulene over parent corannulene include:
(2d,p) level of theory (Figure 4). The bond lengths and angles
of the calculated bowl structure are in good agreement with
those of 3a, as determined by X-ray crystallographic analy-
sis.[17] The lowest unoccupied molecular orbital (LUMO;
ꢁ1.95 eV) and LUMO + 1 (ꢁ1.94 eV) are similar, which
differs from those of corannulene which has doubly generated
LUMOs.[31] The lone pair on the nitrogen atom contributes to
p-conjugation in HOMOꢁ1 and LUMO + 1. Time-depen-
dent (TD) DFT calculations using the same method revealed
that the transition energies and oscillator strengths show good
agreement with the observed values. The calculated bands at
l = 462 and 449 nm could be assigned to the transitions of
HOMO!LUMO and HOMO!LUMO + 1, respectively.
The strongest absorption band at l = 395 nm (Figure 3)
could be attributed to the HOMOꢁ1!LUMO/LUMO + 1
transitions. The aromaticity of 3b was evaluated by nucleus-
independent chemical shift (NICS) analysis (Figure 5a).
1) the unique structure with Cs symmetry because of the
introduction of a trigonal nitrogen atom, 2) formation of one-
directional bowl-in-bowl p-stacking in the solid state, and
3) efficient fluorescence with longer emission wavelengths
resulting from extended p-conjugation of five fused benzene
rings. Detailed analyses and derivatization of the benzene-
fused azacorannulene are currently in progress in our
laboratory and will be reported in due course.
Keywords: azomethine ylide · corannulenes · polycycles ·
supramolecular chemistry · synthetic methods
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Received: March 20, 2015
Published online: && &&, &&&&
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Corannulenes
S. Ito,* Y. Tokimaru,
K. Nozaki*
&&&& — &&&&
Benzene-Fused Azacorannulene Bearing
an Internal Nitrogen Atom
Bowled over: A novel benzene-fused
azacorannulene (left) was synthesized by
the 1,3-dipolar cycloaddition of a polycyc-
lic aromatic azomethine ylide with a di-
arylethyne and subsequent palladium-
catalyzed intramolecular cyclization. This
molecule represents the first example of
a heteroatom-doped corannulene deriva-
tive and in the solid state bowl-in-bowl
columnar p-stacking is observed (right).
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
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