Studies toward the Synthesis of Hepta- peri -heptabenzo-[7]circulene

Article abstract of DOI:10.1055/s-0035-1562510

In this letter we report the synthesis and characterization of two new heptagon-embedded nonplanar π-conjugated molecules, which can be regarded as synthetic precursors of hepta-peri-heptabenzo-[7]circulene, an interesting but unknown polycyclic arene wit

Full text of DOI:10.1055/s-0035-1562510

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–D
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X. Yang, Q. Miao
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Studies toward the Synthesis of Hepta-peri-heptabenzo-[7]circu-
lene
Xuejin Yanga
Qian Miao*a,b
O
O
OMe
MeO
a Department of Chemistry, The Chinese University
of Hong Kong, Shatin, New Territories, Hong Kong,
P. R. of China
5 steps
O
MeO
OMe
MeO
OMe
miaoqian@cuhk.edu.hk
b Center of Novel Functional Molecules, The Chinese
University of Hong Kong, Shatin, New Territories,
Hong Kong, P. R. of China
MeO
MeO
OMe
MeO
OMe
OMe
Received: 25.04.2016
Accepted after revision: 31.05.2016
Published online: 05.07.2016
dle-shaped π-conjugated molecule as shown by the energy-
minimized model in Figure 1 (c). Detailed below are our ef-
forts toward the synthesis of 2a and its methoxylated deriv-
ative 2b.
DOI: 10.1055/s-0035-1562510; Art ID: st-2016-w0291-c
Abstract In this letter we report the synthesis and characterization of
two new heptagon-embedded nonplanar π-conjugated molecules,
which can be regarded as synthetic precursors of hepta-peri-heptaben-
zo-[7]circulene, an interesting but unknown polycyclic arene with nega-
tive curvature.
Key words polycyclic arenes, negatively curved π molecules, oxidative
cyclodehydrogenation
The presence of nonhexagonal rings is a common cause
of curvature in polycyclic arenes1 and allotropes of trivalent
(sp2) carbon. This can be easily demonstrated with coran-
nulene and [7]circulene, which are analogues of coronene
containing a pentagon and a heptagon, respectively (Figure
1, a). Unlike coronene having a planar π backbone, corannu-
lene is an aromatic bowl with positive curvature, while
[7]circulene2 is an aromatic saddle with negative curvature.
[7]Circulene and larger heptagon-embedded curved poly-
cyclic arenes3–5 can serve as segments, model compounds,
and, in principle, synthetic precursors for interesting car-
bon nanomaterials with negatively curved surfaces,6 such
as toroidal carbon nanotubes7,8 and schwarzites.9,10 peri-
Annellation of benzene rings to coronene leads to hexa-
peri-hexabenzocoronene (HBC), a well-known planar poly-
cyclic arene.11,12 Similarly, peri-annellation of benzene rings
to [7]circulene leads to hepta-peri-heptabenzo-[7]circulene
(1), a saddle-shaped analogue of HBC as shown in Figure 1
(b). However, unlike HBC, 1 has an odd number of π elec-
trons because it has [7]circulene peri-fused with seven ben-
zene rings. Therefore 1 is an open-shell molecule with the
radical delocalized in the π backbone. We consider 2a (Fig-
ure 1, c), the ketone derivative of 1, as a synthetic precursor
to it. With the same polycyclic framework as 1, 2a is a sad-
Figure 1 (a) Structures of corannulene, coronene and [7]circulene; (b)
structures of HBC and 1; (c) structures of 2a,b and molecular model of
2a as optimized at the B3LYP level of DFT with the 6-31G(d,p) basis set
(for clarification, the radical in 1 is shown as localized on the central sev-
en-membered ring rather than delocalized in the whole molecule).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

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X. Yang, Q. Miao
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O
OMe
O
O
O
OH
Ph
Ph
p-MeOC6H4Li
BBr3
toluene
reflux
+
THF
CH2Cl2
Ph
Ph
4
5a
3 (90%)
7a (70%)
Scheme 1 Synthesis and reaction of 3
To synthesize 2a, we envisioned compound 3 (Scheme
1) as a precursor, which, in principle, can be converted into
2a by oxidative cyclodehydrogenation. As shown in Scheme 1,
Diels–Alder reaction of diphenylcyclopropenone 4 and
phencyclone 5a followed by ring opening and decarbonyla-
tion yielded ketone 6,13 which reacted with p-methoxyphe-
nyl lithium resulting in 3. However, attempted oxidative
cyclodehydrogenation of 3 with DDQ/MeSO3H,14 DDQ/
CF3SO3H,3 or FeCl3 yielded complicated mixtures, from
which 2a was not obtained. One possible reaction of 3 here
is elimination of the hydroxyl group and the methyl group
under the acidic conditions converting the 4′-methoxyphe-
nyl moiety into a quinonoid structure, which possibly
makes the whole molecule less reactive toward oxidation in
the Scholl reaction. In agreement with this hypothesis,
treatment of 3 with BBr3, a Lewis acid without oxidizing re-
agents, led to compound 7a with a quinonoid structure.
In order to facilitate the cyclodehydrogenation reaction,
we introduced four methoxy groups to 6a leading to 6b
(Scheme 2), which has the methoxy groups ortho and para
to the reaction sites. This strategy was used earlier by us to
activate cyclization at the desired position for the synthesis
of heptagon-embedded polycyclic arenes,4,15 as inspired by
the report that the Scholl reaction formed C–C bonds pref-
erably ortho or para to activating substituents.16 As shown
in Scheme 2, 6b was synthesized from the Diels–Alder reac-
tion of 4 and methoxylated phencyclone 5b, the latter of
which was prepared by double aldol condensation of 1,3-di-
arylacetone 815 with 9,10-phenanthrenequinone 9. Oxida-
tive cyclodehydrogenation of 6b using FeCl3 as both Lewis
MeO
OMe
OMe
OMe
O
O
O
MeO
8
OMe
OMe
OMe
MeO
OMe
4, toluene
reflux
+
O
O
FeCl3
KOH, MeOH
reflux
CH2Cl2, MeNO2
MeO
OMe
O
9
6b (64%)
5b (80%)
OMe
MeO
MeO
OMe
10 (75%)
iPrO
O
OiPr
O
OH
MeO
MeO
OMe
OMe
MgBr
FeCl3
AlCl3
OMe
MeO
6b
CH2Cl2, MeNO2
THF, reflux
CH2Cl2
MeO
MeO
OMe
OMe
MeO
OMe
11 (90 %)
7b (53 %)
12 (60%)
Scheme 2 Synthesis of 10 and 12
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

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acid and oxidant resulted in the ring-closed product 10 in a
good yield.17 Ketone 10 appeared almost inert toward nuc-
leophilic addition by phenyllithium or phenylmagnesium
bromide because of the steric hindrance as found from the
crystal structure. Therefore, 6b was treated with p-isoproy-
loxyphenylmagnesium bromide yielding 11. Here the Gri-
gnard reagent had an isopropyloxy group instead of a me-
thoxy group so that the isoproyl group could be selectively
removed in the next step. Treatment of 11 with AlCl3 led to
7b with the four methoxy groups unchanged. Similar to the
reaction of 6b, FeCl3-mediated oxidative cyclodehydrogena-
tion of 7b yielded 12 with incomplete ring closure. Treat-
ment of 12 with DDQ/MeSO3H (or CF3SO3H) resulted in
complicated mixtures instead of formation of 2b, although
acid can in principle catalyze the ring-closing reactions in
12 and subsequent oxidation can lead to 2b (Scheme S1 in
the Supporting Information). Furthermore, 12 appeared
also unreactive toward Fujiwara–Moritani oxidative cou-
pling when it was treated with Pd(OAc)2 and oxidants.18
The poor reactivity of 12 toward the attempted ring-closing
reactions may be attributed to the large distance (3.51 Å)
between the potential reacting sites as found from the en-
ergy-minimized model (Figure S2 in the Supporting Infor-
mation).
Single crystals of 10 as grown from solution in toluene
were selected for X-ray crystallography. It is found that the
unit cell of this crystal contains crystallized solvent (tolu-
ene) molecules, and 10 has a saddle-shaped backbone as
shown in Figure 2 (a). Having a bond length in the range of
1.44–1.48 Å, the C–C bonds shown in light blue in Figure 2
(b) resemble C–C single bonds between sp2–sp2 carbons,
which have a typical bond length of 1.45–1.48 Å depending
on the degree of conjugation.19 In comparison to these
bonds, the other C–C bonds as shown in grey are 1.36–1.41
Å long, which is close to the typical length of C–C aromatic
bonds in arenes (1.38–1.40 Å).19 This is in agreement with
the structure of 10 having six aromatic sextets at the pe-
riphery.
Figure 2 Crystal structure of 10·toluene: (a) a unit cell with toluene
shown with blue-capped sticks and hydrogen atoms removed for clari-
ty; (b) top view a molecule of 10 (carbon and oxygen atoms in 10 are
shown as gray and red ellipsoids at the 50% probability level, and hydro-
gen atoms are shown as white balls).
In summary, two new heptagon-embedded nonplanar
π-conjugated molecules 10 and 12 were synthesized, which
can be considered as synthetic precursors of hepta-peri-
heptabenzo-[7]circulene. However, synthesis of 2b from 10
and 12 has not succeeded yet. Further studies on synthesis
of 2a,b are in progress in our laboratory.
Acknowledgment
Both 10 and 12 are soluble in common organic solvents.
The yellow solution of 10 exhibits orange fluorescence
when irradiated with UV light. In contrast, the orange solu-
tion of 12 appears almost nonfluorescent. As shown in Fig-
ure S4 in the Supporting Information, the UV-vis absorp-
tion spectrum of 12 is similar to that of 10 with a red shift
of the absorption edge, in agreement with the extended π
conjugation. In the test window of cyclic voltammetry (Fig-
ure S5 and S6 in the Supporting Information), 10 exhibits
two quasi-reversible oxidation waves with half-wave oxida-
tion potentials of 0.43 V and 0.62 V vs. ferrocenium/ferro-
cene (Fc+/Fc), while 12 exhibits two quasi-reversible oxida-
tion waves with half-wave oxidation potentials of 0.38 V
and 0.59 V vs. Fc+/Fc and one quasi-reversible reduction
wave with half-wave reduction potential of –1.55 V vs.
Fc+/Fc.
We thank Ms. Hoi Shan Chan (the Chinese University of Hong Kong)
for the single-crystal crystallography. This work was supported by the
Research Grants Council of Hong Kong (CRF C4030-14G) and the Chi-
nese University of Hong Kong (the Faculty Strategic Development
Scheme from the Faculty of Science and the One-Off Funding for Re-
search from Research Committee).
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1562510.
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References and Notes
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

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X. Yang, Q. Miao
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Hz, 4J = 0.8 Hz, 2 H), 7.57 (t, 3J = 7.0, 4J = 1.2 Hz, 2 H), 7.33 (t, 3J =
7.3 Hz, 4J = 1.1 Hz, 2 H), 7.18–7.20 (m, 4 H), 7.06–7.12 (m, 6 H),
6.22–6.25 (m, 6 H), 3.48 (s, 12 H). 13C NMR (100 MHz, CDCl3): δ
= 199.4, 160.5, 146.5, 142.3, 135.30, 135.27, 133.9, 130.9, 130.2,
129.9, 129.6, 128.1, 127.8, 126.9, 126.2, 122.5, 108.6, 99.9, 55.2.
HRMS (ESI+): m/z calcd for C47H36O5 [M + Na]+: 703.2455; found:
703.2468.
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(17) Synthesis of 6b
Synthesis of 10
A solution of 6b (34.0 mg, 0.0499 mmol) in anhydrous CH2Cl2
(50 mL) was bubbled with N2 for 10 min. Then a solution of
FeCl3 (243 mg, 1.50 mmol, 30 equiv) in MeNO2 (1.3 mL), which
was also bubbled with N2 for 10 min, was added to the above
solution at 0 °C dropwise. The resulting black solution was
stirred at 0 °C with a stream of N2 bubbled through it for about
15 min. The reaction was quenched by addition of 1 mL of
MeOH, and the mixture was treated with water. The aqueous
phase was extracted with CH2Cl2, and the combined organic
layers were washed with a sat. NaHCO3 aqueous solution, brine,
dried over anhydrous Na2SO4 and then concentrated under
reduced pressure. The dark red crude product was purified by
column chromatography on silica gel using CH2Cl2–hexane (1:1,
v/v) as eluent. Compound 10 (25.6 mg, 0.0381 mmol) was
obtained as orange red powder in a yield of 75%; mp >350 °C. 1H
NMR (400 MHz, CDCl3): δ = 9.51–9.54 (m, 2 H), 8.78–8.81 (m, 2
H), 8.68 (d, 3J = 7.6 Hz, 2 H), 8.64 (d, 3J = 8.0 Hz, 2 H), 7.81 (t, 3J =
7.8 Hz, 2 H), 7.63–7.66 (m, 4 H), 7.04 (s, 2 H), 4.15 (s, 6 H), 4.03
(s, 6 H). 13C NMR (100 MHz, C2D2Cl4): δ = 198.0, 158.2, 156.1,
136.6, 130.1, 129.3, 128.4, 128.2, 128.0, 127.7, 127.2, 126.9,
125.7, 125.3, 125.14, 125.05, 124.9, 122.7, 119.1, 113.1, 111.3,
97.1, 56.2, 56.1. HRMS (ESI+): m/z calcd for C47H28O5 [M + H]+:
673.2010; found: 673.2012.
A mixture of compound 5b (0.25 g, 0.50 mmol) and diphenylcy-
clopropenone (4, 0.21 g, 1.0 mmol) in toluene (12 mL) was
heated under an atmosphere of N2 at reflux for about 4 d. The
solution was concentrated under a reduced pressure, and the
crude product was purified by silica gel column chromatogra-
phy using hexane–EtOAc (4:1) as eluent yielding 0.22 g (0.32
mmol, 64%) of 6b as a white solid; mp 267.6–269.7 °C. 1H NMR
(400 MHz, CD2Cl2): δ = 8.67 (d, 3J = 8.1 Hz, 2 H), 8.06 (dd, 3J = 8.4
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D