Embedding Heteroatoms and Adjacent Pentagons in Concave Molecules

Article abstract of DOI:10.1055/s-0040-1706411

Bowl-shaped molecules, which can be viewed as fragments of fullerenes, have been extensively studied owing to their interesting fundamental properties derived from their unique geometry, and the possibilities for their further functionalization, with pote

Full text of DOI:10.1055/s-0040-1706411

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© Georg Thieme Verlag Stuttgart · New York
2020, 31, A–E
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G. Zhang
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Embedding Heteroatoms and Adjacent Pentagons in Concave
Molecules
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Gang Zhang*
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Jiangsu Co-Innovation Center for Efficient Processing
and Utilization of Forest Products, College of Chemi-
cal Engineering, Nanjing Forestry University, Nanjing,
210037, P. R. of China
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gangzhang@njfu.edu.cn
fused pentagons
embedded nitrogen
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bowl-shaped
boat-shaped
Received: 29.06.2020
Accepted after revision: 11.07.2020
Published online: 10.08.2020
DOI: 10.1055/s-0040-1706411; Art ID: St-2020-p0371-sp
Abstract Bowl-shaped molecules, which can be viewed as fragments
of fullerenes, have been extensively studied owing to their interesting
fundamental properties derived from their unique geometry, and the
possibilities for their further functionalization, with potential applica-
tions in the field of materials science. Most of the reported concave
compounds are based on corannulene or sumanene, the simplest rep-
resentative fullerene subunits possessing isolated pentagons. Synthe-
ses of concave molecules bearing adjacent pentagons have rarely been
reported. Inspired by the structure of N-heterotriangulene, the con-
struction of bowl-shaped compounds containing two fused pentagons
and a central nitrogen atom is discussed. Further surface extension of
such concave molecules can result in the formation of boat-shaped
compounds containing embedded double isolated nitrogen atoms.
Gang Zhang obtained his B.E. degree in 2004 from Nanjing Forestry
University and his M.E. degree in 2007 from Dalian University of Tech-
nology. He then moved to the Institute of Chemistry of the Chinese
Academy of Sciences and obtained his Ph.D. degree in 2011 under the
supervision of Professors Liangbing Gan and Yuliang Li. He accepted an
Alexander-von-Humboldt postdoctoral fellowship to work with Profes-
sor Michael Mastalerz in Ulm University in 2012, and went to Heidel-
berg University in 2013. In 2016, he joined Nanjing Forestry University
as a Jiangsu specially appointed professor. His research interests focus
on heteroatom-doped polycyclic aromatic hydrocarbons.
Key words acridones, aromaticity, buckybowls, conjugation, hetero-
triangulene, polycyclic aromatic hydrocarbons
Planar coronene, in which six benzene rings are angu-
larly fused around a hexagon, can be viewed as a fragment
of a single graphene layer. If the central hexagon of coro-
nene is replaced by a smaller pentagon, the resulting struc-
ture, with five fused benzene rings, becomes concave, with
the rim rising up and the central moiety sinking down. This
bowl-shaped molecule is called corannulene.1 Because
compounds of this type can be regarded as subunits of
buckminsterfullerene, they are sometimes known as
‘buckybowls’.2 Another representative buckybowl is su-
manene, which contains a central hexagon with three iso-
lated pentagons at the periphery.3 The first synthesis of cor-
annulene was reported as early as in 1966.1a After the dis-
covery of fullerene, this research area expanded rapidly.4
Investigations on buckybowls are of great significance, not
only for the development of methods for synthesizing high-
ly strained molecules, but also for studies on the funda-
mental properties of such curved structures, such as their
aromaticity and inversion processes. In combination with
some functional groups, buckybowls have been used to
synthesize novel curved surface-extended organic func-
tional materials with interesting physiochemical features.5
In addition to all-carbon-based buckybowls, heteroat-
oms have been successfully introduced into the centers or
rims of sumanene and corannulene derivatives to tune
their electronic structures in the expectation of developing
novel properties (Figure 1).6 Sumanene derivatives with up
to three heteroatoms in rim positions have been reported.
Buckybowls containing one embedded nitrogen atom are
limited to corannulene derivatives. A hydrazine-embedded
buckybowl bearing two nitrogen atoms has been reported.6f
Another report describes the on-surface synthesis of a
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bowl-shaped molecule containing two fused pyrrole rings.7
However, there had been no satisfactory answer to the
question of how to embed larger numbers of isolated het-
eroatoms into a buckybowl through a scalable synthesis.
course, strategies used to stabilize non-IPR fullerenes can
be used to improve the stability of pentagon-fused bucky-
bowls. However, the reduced conjugation through sp3-
bonding carbon atoms and the sensitivity of pentalene or-
ganometallics might limit the range of their potential appli-
cations in materials science.10 Novel subunits are required
to stabilize such pentagon-fused concave molecules.
We recently found that it is possible to construct a sta-
ble bowl-shaped compound with a central nitrogen atom
and two adjacent pentagons.11 This work was inspired by
the planar carbonyl-bridged triphenylamine, N-heterotri-
angulene. The formation of two C–C bonds after the remov-
al of two carbonyl groups from N-heterotriangulene gives a
concave molecule with two fused pentagons. The synthesis
of the target concave compound was performed with 2-
chloro-9H-carbazole (3) as the starting material (Scheme
1). 2-Chloro-9H-carbazole (3) was synthesized by a Fischer
indole synthesis followed by aromatization. A benzoate es-
ter group was introduced onto the amino position of carba-
zole, and subsequent intramolecular cyclization furnished
the acridone–carbazole hybrid 5. An attempt to close the
remaining fjord directly was unsuccessful. Consequently,
the carbonyl group was reduced to a methylene group, and
subsequent palladium-catalyzed intramolecular C–C bond
formation afforded the concave compound 7. The methy-
lene group was then oxidized back to a carbonyl to give
compound 8. All the reactions gave good yields.
Compound 8 was further characterized by single-crystal
X-ray diffraction, which confirmed that it has a bowl-
shaped structure with a depth of 1.5 Å (Figure 2). The mole-
cules pack in a concave-convex manner through – inter-
action in the crystals. Theoretical calculations confirmed
that compound 7 is bowl-shaped with a depth of 1.5 Å,
comparable to that of 8. The contributions of inner atoms to
the curvatures of compounds 7 and 8 were estimated by -
orbital axis vector (POAV) analysis. The POAV angles of the
nitrogen atoms were in the range 15.9–16.5°, which were
much larger than those of other carbon atoms, indicating
Figure 1 Representative structures of reported heteroatom-doped
buckybowls. Color code: gray, carbon; white, hydrogen; blue, nitrogen.
Note that most widely studied buckybowls can be
viewed as subunits of fullerenes that obey the isolated pen-
tagon rule (IPR). In the field of fullerene chemistry, more
than 100 non-IPR fullerenes bearing fused pentagons have
been reported during the past 20 years.8 Fullerenes con-
taining double adjacent pentagons show increased local
strain; consequently, non-IPR fullerenes are unstable. The
reported non-IPR fullerenes are either endofullerenes,
which are stabilized by encapsulated metal clusters
through the principle of local aromaticity, or exohedral
substituted fullerenes in which sp2 carbon atoms are con-
verted into the sp3 state to release strain.9 Concave mole-
cules containing fused pentagons that can be viewed as
fragments mapped from non-IPR fullerenes are rarely re-
ported.7 Two pentagons sharing one bond (pentalene) con-
tain eight -electrons, which does not comply with the
Hückel rule, and therefore destabilizes the molecule. Of
CO2Me
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Cl
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Cl
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Scheme 1 Synthesis of the bowl-shaped compound 8. Reagents and conditions: (a) (2-chlorophenyl)hydrazine hydrochloride, HOAc, TFA, reflux, 80%;
(b) p-chloranil, xylene, reflux, 73%; (c) methyl 2-iodobenzoate, CuI, K2CO3, o-dichlorobenzene, 180 °C, 82%; (d) NaOH, EtOH–H2O, reflux, HCl, then PPA,
180 °C, 73% for 2 steps; (e) BH3, THF, reflux, 99%; (f) Pd(OAc)2, Cy3PHBF4, K2CO3, N,N-dimethylacetamide, 170 °C, 72%; (g) Na2Cr2O7, HOAc, reflux, 95%.
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that the pyramidal geometry of the nitrogen atom plays an
important role in releasing molecular strain. A nucleus-in-
dependent chemical shift (NICS) calculation demonstrated
that the benzene rings are aromatic and that the pyrrole
rings possess weak aromaticity due to reduced planarity.
The aromatic character of the fused pentagons is significant
in improving the molecular stability. The dihydropyridine
ring of 7 is nonaromatic, and the pyridone rings of 8 are
slightly antiaromatic. An interesting property of bowl-
shaped compounds is the bowl-to-bowl inversion process.
Computational analysis revealed that the inversion process
proceeds through a planar intermediate. The inversion bar-
riers of compounds 7 and 8 are 11.6 and 8.1 kcal/mol, re-
spectively.
The bowl surface of concave compound 8 is small. If we
consider the many directions in which 8 might be extend-
ed, it should be possible to make larger curved molecules. If
two molecules of 8 could be fused by sharing an appended
benzene ring of the acridone moiety, a nonplanar com-
pound with either a boat shape or a wavelike structure
should be obtained.12 Our synthesis of such a surface-ex-
tended compound began with the bisacridone compound 9
(Scheme 2).13 The 2- and 7-positions of the acridone moi-
eties were functionalized with tert-butyl groups, which not
only improved the solubility of the final rigid product, but
also blocked the more reactive sites, restricting the regiose-
lectivity of subsequent reactions to the 4- and 5-positions.
The synthesis was carried out through stepwise bromina-
tion and palladium-catalyzed C–Br/C–H coupling reactions.
Again, the carbonyls had to be reduced to methylene groups
before the final intramolecular C–C bond formation. The
curved compound 14 was unstable and gradually decom-
posed to a complicated mixture. Consequently, the methy-
lene groups were oxidized back to carbonyls to generate the
nanoboat 15.
Crystallographic analysis of the intermediate 11 and the
target compound 15 was crucial in the determination of the
reaction pathway and the conformation of the final prod-
uct. The structural analysis of 11 showed that the first bro-
mination of each acridone moiety occurred at the position
ortho to the amino positions of compound 10. A linear in-
dole-fused carbazole structure was formed after the double
intramolecular C–C bond formation. The structure of com-
pound 15 was confirmed to be that of a highly curved nano-
boat with a depth of 2.5 Å and a span of 9.2 Å between the
Figure 2 Top views and side views of the optimized structure of 7 (a, c)
and the crystal structure of 8 (b, d). Color code: gray, carbon; white, hy-
drogen; blue, nitrogen; red, oxygen.
O
N
O
N
O
N
O
N
Br
Br
a
c
b
Br
Br
N
N
N
N
O
O
O
O
9
10
11
12
d
O
N
N
N
N
N
Br
e
f
Br
N
O
15
14
13
Scheme 2 Synthesis of boat-shaped compound 15. Reagents and conditions: (a) Br2, CH2Cl2, r.t., 80%; (b) Pd(OAc)2, Cy3PHBF4, K2CO3, N,N-dimethylac-
etamide, 130 °C, 65%; (c) Br2, CH2Cl2, r.t., 85%; (d) BH3, THF, reflux, 98%; (e) Pd(OAc)2, Cy3PHBF4, K2CO3, N,N-dimethylacetamide, 170 °C, 66%; (f) DDQ,
CH2Cl2, 1,4-dioxane, H2O, 0 °C to r.t., 63%.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

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two carbonyl groups (Figure 3). The bottom benzene ring
was deformed due to the high strain. The POAV angles of
the inner atoms were quite close to those of compound 8,
indicating that the fusion of benzene ring to extend the
bowl surface had no influence on the geometry of the con-
cave moiety. The calculated NICS values showed a slightly
enhanced aromaticity of the pyrrole ring, whereas the pyri-
done ring was still antiaromatic. Theoretical calculations
demonstrated that the bowl-to-bowl inversion process oc-
curs in a stepwise manner with an S-shaped intermediate
rather than a planar one, and the highest barrier is 11.2
kcal/mol. The structure optimization also confirmed a boat-
shaped structure for compound 14 with a depth of 2.4 Å.
However, the stepwise inversion process requires a slightly
higher energy (15.9 kcal/mol) than that of 15. In addition to
the pentagon-fused feature of compounds 14 and 15, there
are two isolated nitrogen atoms embedded in the mole-
cules. Such a situation in heteroatom-doped concave com-
pounds is extremely rare.
Funding Information
This work was supported by the Jiangsu Specially Appointed Profes-
sor Plan.()
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