Synthesis and Structure of a Propeller-Shaped Polycyclic Aromatic Hydrocarbon Containing Seven-Membered Rings

Article abstract of DOI:10.1021/acs.orglett.8b00477

The synthesis and structure of a C₃-symmetric propeller-shaped polycyclic aromatic hydrocarbon that bears three seven-membered rings is reported. The synthesis was accomplished in three steps from benzo[c]naphtho[2,1-p]chrysene, including a Pd-catalyzed intramolecular C-H arylation for the formation of the seven-membered rings. The combination of the helicities (P/M) of the three seven-membered-ring moieties and three [4]helicene moieties affords 24 possible conformers, and two relatively stable conformations were observed by ¹H NMR spectroscopy.

Full text of DOI:10.1021/acs.orglett.8b00477

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis and Structure of a Propeller-Shaped Polycyclic Aromatic
Hydrocarbon Containing Seven-Membered Rings
Kazuya Kawai,† Kenta Kato,† Lingqing Peng,‡ Yasutomo Segawa,*,§,† Lawrence T. Scott,‡
and Kenichiro Itami*,†,§,∥
†Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
‡Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467-3860, United States
§JST, ERATO, Itami Molecular Nanocarbon Project, Chikusa, Nagoya 464-8602, Japan
∥Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: The synthesis and structure of a C3-symmetric propeller-shaped
polycyclic aromatic hydrocarbon that bears three seven-membered rings is
reported. The synthesis was accomplished in three steps from benzo[c]naphtho-
[2,1-p]chrysene, including a Pd-catalyzed intramolecular C−H arylation for the
formation of the seven-membered rings. The combination of the helicities (P/
M) of the three seven-membered-ring moieties and three [4]helicene moieties
affords 24 possible conformers, and two relatively stable conformations were
1
observed by H NMR spectroscopy.
the difficulties associated with the formation of seven-
membered rings at late stages of the PAH synthesis. The
early stage introduction of seven-membered rings into PAHs
has been reported5 (for example, III in 2015).5a There are two
reports on the synthesis of π-extended PAHs, in which seven-
membered rings are generated at the final step. In 2013, Durola
and co-workers reported an unexpected rearrangement product
containing a seven-membered ring (IV) via the Scholl reaction6
of tris(phenanthrophenyl)benzene.7 Our group has reported
the synthesis of warped nanographene (V) via the Scholl
reaction of pentakis(2-biphenylyl)corannulene.8 However,
despite these numerous studies, a general method for the
formation of seven-membered rings in PAHs remains elusive.9
To investigate the chemistry of negatively curved PAHs, such
methods for the formation of seven-membered rings are highly
desirable.
olycyclic aromatic hydrocarbons (PAHs) that bear seven-
Pmembered rings have garnered interest on account of their
dynamic behavior, electronic properties, and packing modes
derived from their negatively curved structures.1 Pioneering
studies in this research area include the synthesis of
hexa[7]circulene (I, Figure 1) by Jessup and co-workers in
19752 and the synthesis of [7]circulene (II) by Yamamoto and
co-workers in 1983.3 Although several related molecules have
since been reported,4 the synthesis of negatively curved PAHs
had not been investigated until very recently, probably due to
Herein, we report the synthesis and structure of a C3-
symmetric propeller-shaped PAH that bears three seven-
membered rings and three [4]helicene moieties (1, Figure 2).
This is the first example of the formation of a seven-membered
ring in a PAH by intramolecular C−H arylation. A single-crystal
X-ray diffraction analysis of 1 revealed a propeller-shaped
structure and a gear-like dimeric packing structure. Two
conformations of 1 were observed in solution at room
temperature, and these interconverted upon heating to 140
°C. A density functional theory (DFT) study revealed the
structure of the two conformations of 1 and the interconversion
pathway between them.
Figure 1. Previously reported PAHs containing seven-membered
rings.
Received: February 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00477
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Organic Letters
Letter
formation of seven-membered rings by cyclization, a simplified
starting material 6 was subjected to the same cyclization
conditions as a model reaction. In contrast to the reaction of 4,
only a product that contains a six-membered ring (7) was
obtained in 79% yield, and the seven-membered ring
compound 8 was not observed (Scheme 1b). Considering
these results, we conclude that the unusual formation of a
seven-membered ring in 4 must be more favored than the
formation of a six-membered ring, probably due to the steric
repulsion between the [6]helicene moieties in the intermediates
that would lead to 5.
The threefold symmetric structure of 1 was unambiguously
determined by X-ray crystallography. Single crystals of 1
suitable for X-ray crystallography were obtained from a solution
of 1 in CS2. Two equivalents of CS2 per 1 were incorporated.
As shown in Figure 3a, 1 exhibits a C3-symmetric propeller-
Figure 2. Structure and components of 1.
The synthesis of 1 was achieved in two steps from 2, which
was prepared using a previously reported procedure.10 A Suzuki
coupling reaction of 2 with 2-boryl-2′-chlorobiphenyl (3)
catalyzed by Pd(OAc)2 and PPh3 afforded 4 (68%; Scheme 1a),
Scheme 1. (a) Synthesis of 1; (b) Model Reaction for the
Formation of Six- Or Seven-Membered Rings by a Direct
a
Intramolecular Coupling
Figure 3. (a) ORTEP drawing of 1 with thermal ellipsoids set to 50%
probability; all hydrogen atoms and solvent molecules are omitted for
clarity. (b) Dihedral angle between the outer (blue) and the inner
(red) benzene rings. (c, d) Packing structure of 1.
shaped structure, in which three seven-membered-ring moieties
are twisted in the same chiral sense. The twisting angle between
the outer benzene rings (blue in Figure 3b) and the central
benzene ring (red in Figure 3b) is 75.6°. In the packing
structure (Figure 3c, d), the enantiomers of 1 (green and gray)
are meshed with each other to form a gear-like dimeric
structure, and the dimers are aligned two-dimensionally on the
ab plane.
a
In contrast to the simple C3-symmmetric structure of 1 in the
Abbreviations: Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl,
1
DMAc = N,N-dimethylacetamide.
crystalline state, the H NMR spectrum of 1 in deuterated
1,1,2,2-tetrachloroethane at room temperature showed a ca. 3:2
mixture of symmetric and relatively unsymmetric components,
which fused to a single component upon heating to 140 °C. To
gain insight into the conformational isomers of 1, a density
functional theory (DFT) study was carried out at the B3LYP/6-
31G(d) level of theory. Similar to previously reported multiple
helicenes,13 compound 1 has many possible conformations as
local minima. The combinations of the helicity (P/M) of the
three [4]helicene moieties and the three seven-membered ring
moieties of 1 (Figure 4a) afford 12 pairs of enantiomers, i.e. 24
isomers (A−L and A*−L* where X and X* are the pair of
which was subsequently subjected to a catalytic intramolecular
C−H arylation reaction using PdCl2(PCy3)2, Cs2CO3, and
pivalic acid11 to afford 1 in 26% yield. The threefold cyclization
product 1 was observed only when pivalic acid was added. To
the best of our knowledge, this is the first example of the
formation of seven-membered rings in PAHs by a Pd-catalyzed
direct coupling reaction.12 Although we initially expected the
formation of three six-membered rings from the cyclization of
4, the corresponding product 5 was not observed under the
reaction conditions. To gain insight into the generality of the
B
DOI: 10.1021/acs.orglett.8b00477
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Letter
Figure 4. (a) Description of the helicities of [4]helicene and seven-membered-ring moieties. (b) All possible conformers of 1 (A−L) with relative
Gibbs free energy (ΔG in kcal·mol−1). (c) Favored and disfavored combinations of the [4]helicene moieties. All calculations were performed at the
B3LYP/6-31G(d) level of theory.
enantiomers) in total. We optimized the structures of 12
conformations (A−L), and the calculated Gibbs free energy
values (kcal·mol−1) relative to that of the most stable
conformer A (and A*), which is the conformer observed in
the crystalline state, are summarized in Figure 4b. The relative
energies clearly increase with increasing number of the
“disfavored” conformation (Figure 4c): zero [A (0.0), B
(0.5)], one [C (4.3), D (4.3), E (6.5), F (6.6)], two, [G (8.4),
H (8.8), I (9.9), J (12.2)], and three [K (13.5), L (not
obtained)]. Conformer L could not be obtained as a local
minimum, presumably due to the instability arising from three
disfavored moieties.14 Considering the low difference in energy
between A and B (0.5 kcal·mol−1), B should be the minor
1
conformer observed in the H NMR spectrum of 1 at room
temperature. An extensive DFT study revealed the lowest
interconversion pathway between A and B, i.e., A → TSAC → C
→ TSBC → B, wherein TSAC and TSBC correspond to the
transition states (TSs) of the inversion of the helicities of the
seven-membered ring and [4]helicene moiety, respectively, and
the energy barriers (18.7 kcal·mol−1) are similar to each TS
Figure 5. Thermodynamic behavior of 1 (a) and 9 (b). Energy values
value of the pristine unit. Similarly, the racemization barrier of
were calculated at the B3LYP/6-31G(d) level of theory.
A was estimated to be 19.3 kcal·mol−1,15 which implies that
chiral separation and isolation of 1 should be difficult. The
thermodynamic behavior of 1 and the pristine benzo[c]-
naphtho[2,1-p]chrysene (9) are summarized in Figure 5. In
contrast to 1, the minor conformer of 9 (9b) could not be
1
observed in the H NMR spectrum, and this difference was
ascribed to the energy barrier of interconversion between the
PPP conformation (A or 9a) and the PPM conformation (B or
9b). Given that the inversion of the axial chirality of the
biphenyl moiety (TSAC) represents the rate-determining step, it
seems feasible to consider the three biphenyl moieties in 1 as
“stoppers,” which delay the rate of interconversion between A
and B, and, as a result, B can be observed. This result is
important especially for stabilizing the helicity of helicene
moieties.
To gain insight into the effect of the seven-membered rings
on the π-conjugation system of 1, the photophysical properties
of 1 were measured as shown in Figure 6. Compound 10, the
dechlorinated derivative of 4, was chosen as a reference
compound. It was clearly seen that absorption and fluorescence
spectra of 1 were shifted to a higher wavelength region
compared with those of 10 reflecting the π-elongation effect of
the seven-membered rings of 1. The fluorescence quantum
Figure 6. UV−vis absorption (solid line) and fluorescence (broken
line) of 1 and reference compound 10.
yield of 1 (ΦF = 0.10) was higher than that of 10 (ΦF = 0.03).
DFT calculation also indicated the slightly narrower HOMO−
LUMO gap of 1 (3.43 eV) compared with that of 10 (3.58 eV;
see the Supporting Information for details), which may cause
the bathochromic shift.
C
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Letter
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In summary, we have synthesized and structurally charac-
terized a C3-symmetric propeller-shaped PAH 1 that contains
three seven-membered rings. This is the first example of the
formation of seven-membered rings in a PAH by a Pd-catalyzed
intramolecular C−H arylation reaction. A single-crystal X-ray
diffraction analysis of 1 revealed a propeller-shaped structure
and a gear-like dimeric packing structure. The optimization of
the 24 possible conformations of 1 showed that the stability of
each conformer is strongly correlated with the number of
“disfavored” local conformations. Moreover, we discovered that
the three seven-membered-ring moieties in 1 work as
stabilization units for the helicity of the [4]helicene moieties.
Further investigations into the physical properties of the
heptagon−helicene hybrid π-systems are currently in progress
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00477.
Experimental details, spectra of new compounds (PDF)
Cartesian coordinates of optimized structures (XYZ)
Accession Codes
CCDC 1823019 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: ysegawa@nagoya-u.jp (Y.S.).
*E-mail: itami@chem.nagoya-u.ac.jp (K.I.).
(13) Review: (a) Li, C.; Yang, Y.; Miao, Q. Chem. Asian J. 2018, DOI:
10.1002/asia.201800073. Representative examples of multiple carbo-
helicenes: (b) Kato, K.; Segawa, Y.; Scott, L. T.; Itami, K. Angew.
Chem., Int. Ed. 2018, 57, 1337. (c) Hosokawa, T.; Takahashi, Y.;
Matsushima, T.; Watanabe, S.; Kikkawa, S.; Azumaya, I.; Tsurusaki, A.;
Kamikawa, K. J. J. Am. Chem. Soc. 2017, 139, 18512. (d) Berezhnaia,
V.; Roy, M.; Vanthuyne, N.; Villa, M.; Naubron, J.-V.; Rodriguez, J.;
Coquerel, Y.; Gingras, M. J. J. Am. Chem. Soc. 2017, 139, 18508.
(e) Saito, H.; Uchida, A.; Watanabe, S. J. J. Org. Chem. 2017, 82, 5663.
(f) Fujikawa, T.; Segawa, Y.; Itami, K. J. J. Am. Chem. Soc. 2016, 138,
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Dichtel, W. R. Chem. Sci. 2013, 4, 3973. (i) Xiao, S.; Kang, S. J.; Wu,
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ORCID
Yasutomo Segawa: 0000-0001-6439-8546
Lawrence T. Scott: 0000-0003-3496-8506
Kenichiro Itami: 0000-0001-5227-7894
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the ERATO program from JST
(JPMJER1302 to K.I.), the Funding Program for KAKENHI
from MEXT (16K05771 to Y.S.), a grant-in-aid for Scientific
Research on Innovative Areas “π-Figuration” (17H05149 to
Y.S.), the Noguchi Institute (to Y.S.), and the US National
Science Foundation (CHE-0414066 to L.T.S.). K.K. thanks the
IGER Program in Green Natural Sciences, Nagoya University
and is grateful for a JSPS fellowship for young scientists.
Calculations were performed using the resources of the
Research Center for Computational Science, Okazaki, Japan.
ITbM is supported by the World Premier International
Research Center (WPI) Initiative, Japan.
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(14) By using a scan method, the energy value for L was estimated to
be ca. 14 kcal·mol−1 relative to that of A. See the Supporting
Information for details.
(15) An enantiomerization pathway from A to A*: A → TSAC → C
→ TSBC → B → TSBF → F → TSFB* → B* → TSB*C* → C* →
TSA*C* → A*. See the Supporting Information for details.
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DOI: 10.1021/acs.orglett.8b00477
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