Facile synthesis of Janus "double-concave" tribenzo[a,g,m] coronenes

Article abstract of DOI:10.1021/jo0701029

(Chemical Equation Presented)Hexabutoxytribenzo[a,g,m]coronenes have been prepared in three steps from readily accessible hexabutoxytriphenylene. Single-crystal X-ray diffraction analysis reveals that these molecules can adopt an extraordinary "double-con

Full text of DOI:10.1021/jo0701029

Facile Synthesis of Janus “Double-Concave”
Tribenzo[a,g,m]coronenes†
Zhongtao Li,‡ Nigel T. Lucas,§ Zhaohui Wang,*,‡ and
Daoben Zhu*,‡
Beijing National Laboratory for Molecular Sciences, Key
Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100080, People’s Republic of
China, and School of Chemistry, The UniVersity of Sydney,
NSW 2006, Australia
FIGURE 1. Structures of known highly symmetric large PAHs with
different substitutents in their bay regions.
wangzhaohui@iccas.ac.cn
which could form the basis for supramolecular self-assembled
systems5 and even self-healing electronic materials.6
ReceiVed January 17, 2007
To date, most highly nonplanar fused PAHs (e.g., spheres,
bowls, tubes, onions) have been synthesized in extreme condi-
tions such as flash vacuum pyrolysis (FVP). Difficulties in scale-
up, low yields, and lack of functional group tolerance are the
main problems that have been challenging to overcome, although
a number of milder solution-phase synthetic routes have been
investigated with some success.7
An extremely twisted graphene molecule, namely per-
methoxylated hexa-peri-hexabenzocoronene 1 with a remarkable
“double-concave” conformation, has recently been reported.5a
The combination of a rigid “double-concave” aromatic core with
18 flexible methoxy groups at the periphery render 1 an ideal
model compound for supramolecular host-guest chemistry.
Herein we describe our endeavors to design a new series of
Janus-type “double-concave” graphene molecules based on
tribenzo[a,g,m]coronene 2 with lower symmetry compared with
the D6h hexa-peri-hexabenzocoronenes. Steric congestion in the
bay regions is expected to induce a unique conformation with
the likelihood of two different concave faces that can be further
differentiated through appropriate functionalization.
Hexabutoxytribenzo[a,g,m]coronenes have been prepared in
three steps from readily accessible hexabutoxytriphenylene.
Single-crystal X-ray diffraction analysis reveals that these
molecules can adopt an extraordinary “double-concave”
conformation that makes them ideal hosts for the binding of
different guest molecules at each face.
Polycyclic aromatic hydrocarbons (PAHs), which can be
regarded as molecular subunits of graphite, have attracted
increasing interest because of their potential application as
functional materials.1 Their planarity is often deemed to be their
most significant geometric characteristic;2 however, curved
PAHs, which represent a fascinating class of molecules, have
drawn persistent attention from many organic chemists,3 in
particular in view of the total synthesis of fullerenes.4 Nonplanar
PAHs may force aromatic rings to adopt unusual intermolecular
contacts that are typically unavailable to planar molecules and
A synthetic route toward 1,2,7,8,13,14-hexabutoxytribenzo-
[a,g,m]coronene 6a is shown in Scheme 1. Readily available
hexabutoxytriphenylene 38 is brominated in dichloromethane
at room temperature to give 1,4,8-tribromo-2,3,6,7,10,11-
hexabutoxytriphenylene in moderate yield.9 Palladium-catalyzed
(5) (a) Wang, Z.; Do¨tz, F.; Enkelmann, V.; Mu¨llen, K. Angew. Chem.,
Int. Ed. 2005, 44, 1247. (b) Mizyed, S.; Georghiou, P.; Bancu, M.; Cuadra,
B.; Rai, A. K.; Cheng, P.; Scott, L. T. J. Am. Chem. Soc. 2001, 123, 12770.
(c) Petrukhina, M. A.; Andreini, K. W.; Peng, L.; Scott, L. T. Angew. Chem.,
Int. Ed. 2004, 43, 5477. (d) Sakurai, H.; Daiko, T.; Sakane, H.; Amaya, T.;
Hirao, T. J. Am. Chem. Soc. 2005, 127, 11580.
† Dedicated to Prof. Dr. Klaus Mu¨llen on the occasion of his 60th birthday.
‡ Chinese Academy of Sciences.
§ The University of Sydney.
(1) (a) Clar, E. Polycyclic Hydrocarbons; Academic Press: London, UK,
1964. For recent reviews: (b) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen,
K. Chem. ReV. 2001, 101, 1267. (c) Grimsdale, A. C.; Mu¨llen, K. Angew.
Chem., Int. Ed. 2005, 44, 5592.
(2) (a) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004,
104, 4891. (b) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34,
359.
(3) See examples: (a) Duong, H. M.; Bendikov, M.; Steiger, D.; Zhang,
Q.; Sonmez, G.; Yamada, J.; Wudl, F. Org. Lett. 2003, 5, 4433. (b) Shibata,
K.; Kulkarni, A. A.; Ho, D. M.; Pascal, R. A., Jr. J. Am. Chem. Soc. 1994,
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Science 2001, 294, 828. (b) Scott, L. T.; Boorum, M. M.; McMahon, B. J.;
Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A. Science 2002,
295, 1500. (c) Scott, L. T. Pure Appl. Chem. 1996, 68, 291.
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M. L.; Nuckolls, C. Angew. Chem., Int. Ed. 2005, 44, 7390.
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2002, 4, 3135. (b) Kim, D.; Petersen, J. L.; Wang, K. K. Org. Lett. 2006,
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Tetrahedron 1965, 21, 3229. (b) Markovitsi, D.; Germain, A.; Millie, P.;
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10.1021/jo0701029 CCC: $37.00 © 2007 American Chemical Society
Published on Web 04/17/2007
J. Org. Chem. 2007, 72, 3917-3920
3917

SCHEME 1
FIGURE 2. UV-vis absorption spectra of 6a (solid line) and 6b
(dashed line) in chloroform (2.0 × 10-6 M).
Suzuki-Miyaura cross-coupling10 of 4 with phenylboronic acid
affords the triphenyl-substituted hexabutoxytriphenylene 5a as
white solid in 60% yield. The key step in the synthesis is
oxidative cyclodehydrogenation of 5a with ferric chloride
followed by reductive workup with methanol,11 to give the
desired product 6a as a yellow solid in 60% yield.
Tribenzo[a,g,m]coronene 2 was first reported in 1968,12
synthesized from o,p,o,p,o,p-hexaphenylene.13 However, a poor
yield, long synthetic route, and difficulties in selective modifica-
tion limited its investigation further. In contrast to the vanishing
solubility of parent molecule 2, hexabutoxy-substituted tribenzo-
[a,g,m]coronene 6a is soluble in common organic solvents such
as THF and chloroform. The 1H NMR spectrum of 6a in CDCl3
displays six signals, assigned to the two core proton environ-
ments and the three methylene and methyl proton environments
of the butyl chains. A room temperature optical absorption
spectrum of 6a in chloroform is shown in Figure 2. The
absorption spectrum of 6a shows a maximum at 351 nm, which
is bathochromically shifted about 72 nm compared to triphen-
ylene 3 (279 nm) and consistent with a more extended π-system,
albeit twisted. The relatively small change in absorption
maximum in going from 3 to 6a, despite the large increase in
the size of the π-system, is typical of all-benzenoid PAHs.14
The emission spectrum of 6a in CHCl3 exhibits a Stokes shift
of 119 nm (λmax ) 470 nm, Φf ) 0.06, see the Supporting
Information).
FIGURE 3. Molecular structure of 6a as determined by single-crystal
X-ray structure analysis.
through significantly longer interbenzenoid C-C bonds (1.44-
1.48 Å) when compared with the intrabenzenoid C-C bonds
(1.37-1.43 Å). The six carbon atoms of the central benzene
ring (C31-C36) are coplanar to within 0.018 Å. Steric
congestion between hydrogen atoms and oxygens in the bay
positions forces the three unsubstituted benzene rings (C4-C9,
C14-19, C24-C29) to bend 13.0-20.3° out of the plane of
the central ring and all to the same side. Similarly, the three
butoxy-substituted rings all bend toward the opposite side of
the central ring by 8.6 - 9.6°, thereby giving 6a a remarkable
Janus-type “double-concave” conformation. The molecules pack
as dimer pairs which exhibit close offset face-to-face contacts
between one of the unsubstituted outer benzo rings (C14-C19)
and its symmetry equivalent molecule; the inter-ring separation
is 3.28 Å. This Janus-type conformation provides a large cavity
on one side and a smaller cavity in the other side that are
expected to be geometrically complementary to guest molecules
of different size and shape.
Crystals of 6a suitable for single-crystal X-ray diffraction
structure analysis were obtained by slow evaporation of a
solution of 6a in acetone at room temperature. The molecular
core is found to be markedly nonplanar as a consequence of
the presence of pronounced peri-interactions (Figure 3). Further
confirmation of the all-benzenoid character of 6a is provided
As a further step toward the functionalization of this Janus-
type “double-concave” graphene, analogues with 6-fold halo-
genation at the three outer benzene rings have been investigated.
The introduction of six electron-withdrawing halogens is
expected to not only have an influence on the electronic
properties of the PAH, but also provide the possibility of
nucleophilic substitution (such as hexasulfuration) of the
aromatic core.
(10) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b)
Tellenbro¨ker, J.; Kuck, D. Angew. Chem., Int. Ed. 1999, 38, 919.
(11) (a) Boden, N.; Bushby, R. J.; Cammidge, A. N. J. Chem. Soc., Chem.
Commun. 1994, 465. (b) Wang, Z.; Watson, M. D.; Wu, J.; Mu¨llen, K.
Chem. Commun. 2004, 336.
(12) Sofer, H.; Polansky, O. E.; Derfling, G. Monatsh. Chem. 1968, 99,
1879.
(13) Fujioka, Y. Bull. Chem. Soc. Jpn. 1984, 57, 3439. Fujioka, Y. Bull.
Chem. Soc. Jpn. 1985, 58, 481.
(14) Clar, E. The Aromatic Sextet; Wiley: London, UK, 1972.
Hexachlorotribenzo[a,g,m]coronene 6b has been prepared
following the synthetic route shown in Scheme 2. Suzuki-
Miyaura cross-coupling of commercially available 3,4-dichlo-
rophenylboronic acid with 4 affords 5b bearing six halogens
(white solid, 60% yield). Following the critical final cyclode-
3918 J. Org. Chem., Vol. 72, No. 10, 2007

SCHEME 2
hydrogenation step with ferric chloride, 1,2,7,8,13,14-hexabu-
tyoxy-4,5,10,11,16,17-hexachlorotribenzo[a,g,m]coronene 6b
was obtained as a yellow solid in 37% yield. It should be noted
that oxidative cyclodehydrogenation with ferric chloride is
sometimes limited by the electronic character of the substitu-
ents.16 Typically, phenyl rings bearing electron-withdrawing
groups are less active in the cyclodehydrogenation reaction.17
Thus, the successful cyclodehydrogenation of 5b to afford the
desired product is particularly significant. The solution 1H NMR
spectrum displays five proton signals, one less than for 6a, and
is also consistent with the absence of conformational rigidity
in solution (there is no change in the number of signals down
to 233 K; see the Supporting Information). The 233 K 1H NMR
spectrum does, however, show broadening of the signals and a
shift to lower frequency, indicative of greater face-to-face self-
association at this lower temperature. The optical absorption
maximum of 6b (Figure 2) shows that incorporation of 6
chlorine atoms induces a small bathochromic shift of 10 nm
compared with parent molecule 6a. This red-shift is likely due
to a combination of increased supramolecular interaction and a
small degree of charge-transfer character between the donor and
acceptor groups at the periphery. The emission spectrum of 6b
in CHCl3 exhibits a Stokes shift of 112 nm (λmax ) 473 nm, Φf
) 0.045; see the Supporting Information).
constitute the core of the tribenzo[a,g,m]coronene (cf. only one
ring overlap in the structure of 6a). The chlorine atoms seem
to play a significant role in supramolecular bonding interactions.
Close intracolumn contacts between Cl4‚‚‚C27/28 (3.38/3.31
Å) and Cl6‚‚‚C32/33 (3.37/3.27 Å), and intercolumn hydrogen
bonds between Cl2‚‚‚H60C (2.936 Å) and Cl3‚‚‚H40C (2.80
Å), leave only Cl1 and Cl5 without bonding roles. By contrast,
FIGURE 4. Molecular structure of 6b as determined by single-crystal
X-ray structure analysis.
Crystals suitable for single-crystal X-ray structure analysis
were obtained by slow evaporation of a solution of 6b in
cyclohexane at room temperature. The solid-state molecular
structure of 6b is somewhat different from that of 6a as shown
in Figure 4. The six carbon atoms (C31-C36) of the central
benzene ring are coplanar to within 0.011 Å. However, in contast
to the alternating in- and out-of-plane twisiting around the
periphery of 6a, the core of 6b is saddle-shaped; two of the
dichlorobenzo rings bend in the same direction (21.4° and 6.6°
relative to the central ring), while the third dichlorobenzene ring
bends in the opposite direction by 8.3°. The conformation of
this third ring appears to be associated with a greater degree of
π-π interaction between PAH cores than is seen for the non-
chlorinated analogue 6a (Figure 5).
A view down the b-axis (Figure 5d) shows how the packing
of 6b is dominated by alternating stacked arrays of molecules
tilted to the column axis. Overlap between intracolumn neigh-
bors involves approximately 6 of the 10 fused rings that
(15) Data collection was carried out on a Rigaku diffractometer with a
Saturn CCD area detector. The structure was solved by direct methods in
SHELXS-97.
(16) Wang, Z.; Tomovic, Z.; Kastler, M.; Pretsch, R.; Negri, F.;
Enkelmann, V.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 7794.
(17) Zhang, Q.; Prins, P.; Jones, S. C.; Barlow, S.; Kondo, T.; An, Z.;
Siebbeles, L. D. A.; Marder, S. R. Org. Lett. 2005, 7, 5019.
FIGURE 5. Crystal structures of (a) 6a, a pair of π-π interacting
molecules, (b) 6b, a pair of π-π interacting molecules, (c) 6a,
molecular packing viewed down the c-axis, and (d) 6b, molecular
packing viewed down the b-axis.
J. Org. Chem, Vol. 72, No. 10, 2007 3919

(dd, 6H), 4.23 (t, J ) 6.67 Hz, 12H), 2.01 (m, 12H), 1.64 (m, 12H),
1.03 (t, J ) 7.3 Hz, 18H). 13C NMR (CDCl3, 100 MHz): δ 149.0,
128.7, 127.1, 126.4, 121.7, 120.6, 73.7, 32.2, 19.0, 13.6. MALDI-
TOF (m/z): calcd for C60H66O6 882.5, found 883.1. Elemental
analysis calcd for C60H66O6: C, 81.60; H, 7.53. Found: C, 81.82;
H, 7.60. Mp 121-122 °C.
only one of the oxygen atoms appears to stabilize the structure
though intermolecular interactions, namely O1‚‚‚H51B (2.54 Å).
In conclusion, we have presented a facile synthetic protocol
to tribenzo[a,g,m]coronenes with a highly twisted core capable
of adopting an extraordinary Janus-type “double-concave”
conformation as revealed by X-ray single crystal analysis of
6a. Symmetric incorporation of six chloro groups around the
PAH periphery has the effect of enhancing π-π interactions
in the solid state, manifested by the molecules packing in
columnar arrays with a desymmetrizing twisting of the aromatic
core. Solution characterization reflects conformational flexibility
of the core that would allow the tribenzo[a,g,m]coronenes 6 to
strongly engage in host-guest interactions, potentially with
facial selectivity. Significant face-to-face interaction in the solid-
state structures is in agreement with increased aggregation at
low temperatures in solution, as evidenced from variable-
temperature NMR measurements. Further functionalization of
this new class of twisted PAHs and an exploration of their
supramolecular properties are currently underway.
2,3,6,7,10,11-Hexabutoxy-1,4,8-tris(3,4-dichlorophenyl)triphen-
ylene 5b. A Schlenk flask was charged with 1,4,8-tribromohexabu-
toxytriphenylene 4 (1.00 g, 1.15 mmol), tetrakis(triphenylphos-
phine)palladium(0) (300 mg, 0.260 mmol), THF (75 mL), and 2
M potassium carbonate solution (25 mL) under argon, then 3,4-
dichlorophenylboronic acid (950 mg, 4.98 mmol) was added. The
mixture was heated to 85 °C with vigorous stirring for 12h, then
cooled to room temperature, and the organic phase was separated
and washed twice with water. After drying over magnesium sulfate,
the solvent was removed in vacuo. The residue was purified by
column chromatography on silica (50% petroleum ether-dichlo-
romethane) to give 5b (760 mg, 0.695 mmol, 60%) as a pale yellow
soild. 1H NMR (CDCl3, 400 MHz): δ 7.83-7.39 (m, 7H), 7.24 (s,
1H), 7.16-7.06 (m, 2H), 6.90 (s, 1H), 6.81 (s, 1H), 4.05-3.11
(m, 12H), 1.64 (m, 6H), 1.41 (m, 12H), 1.18 (m, 6H), 1.01-0.94
(m, 9H), 0.77 (t, J ) 7.2 Hz, 9H). 13C NMR (CDCl3, 75 MHz): δ
150.0, 149.8, 149.3, 146.6, 146.5, 145.9, 140.6, 140.4, 139.9, 132.9,
131.5, 131.2, 130.9, 130.6, 130.2, 129.6, 129.1, 127.9, 127.1, 124.6,
124.2, 112.8, 73.7, 73.0, 68.4, 68.2, 32.6, 32.4, 31.6, 31.5, 19.4,
14.3, 14.0. MALDI-TOF (m/z): calcd for C60H66Cl6O6 1092.3,
found 1092.4. Elemental analysis calcd for C60H66Cl6O6: C, 65.75;
H, 6.07. Found: C, 65.58; H, 6.32. Mp 76-78 °C.
Experimental Section
2,3,6,7,10,11-Hexabutoxy-1,4,8-triphenyltriphenylene 5a. A
Schlenk flask was charged with 1,4,8-tribromohexabutoxytri-
phenylene 4 (1.00 g, 1.15 mmol), tetrakis(triphenylphosphine)-
palladium(0) (300 mg, 0.260 mmol), THF (75 mL), and 2 M
potassium carbonate solution (25 mL) under argon, then phenyl-
boronic acid (600 mg, 4.91 mmol) was added. The mixture was
heated to 85 °C with vigorous stirring for 12 h, then cooled to
room temperature, and the organic phase was separated and washed
twice with water. After drying over magnesium sulfate and filtering,
the solvent was removed in vacuo. The residue was purified by
column chromatography on silica (50% petroleum ether-dichlo-
romethane) to give 5a (0.6 g, 0.67 mmol, 60%) as a white solid.
1H NMR (CDCl3, 400 MHz): δ 7.52 (d, J ) 6.0 Hz, 4H), 7.44-
7.29 (m, 11H), 7.29 (s, 1H), 7.01 (s, 1H), 6.87 (m, 1H), 3.70 (br t,
J ) 5.9 Hz, 4H), 3.49 (t, J ) 6.2 Hz, 2H), 3.22 (t, J ) 6.7 Hz,
2H), 3.17 (t, J ) 6.7 Hz, 2H), 3.10 (t, J ) 6.6 Hz, 2H), 1.63-1.56
(m, 6H), 1.43-1.35 (m, 12H), 1.08 (m, 6H), 0.96-0.83 (m, 9H),
0.71 (m, 9H). 13C NMR (CDCl3, 100 MHz): δ 149.5, 149.5, 149.1,
145.6, 145.4, 140.4, 139.9, 131.9, 131.9, 131.8, 131.2, 128.5, 128.4,
128.3, 126.7, 126.6, 126.4, 124.9, 124.3, 113.0, 112.4, 73.1, 72.3,
67.6, 67.4, 67.3, 32.2, 32.2, 31.2, 31.1, 19.00, 13.8. MALDI- TOF
(m/z): calcd for C60H72O6 888.5, found 889.1. Elemental analysis
calcd for C60H72O6: C, 81.04; H, 8.16. Found: C, 80.71; H, 8.16.
Mp 42-44 °C.
1,2,7,8,13,14-Hexabutyoxytribenzo[a,g,m]coronene 6a. To a
solution of 2,3,6,7,10,11-hexabutoxy-1,4,8-triphenyltriphenylene 5a
(700 mg, 0.788 mmol) in dichloromethane (50 mL) was added ferric
chloride (1.024 g, 6.3 mmol) in 2 mL of nitromethane. The mixture
was stirred for 20 min at rt. To quench the reaction, methanol (20
mL) was added, then 30 mL of water. The organic layer was
separated and the solvent removed in vacuo. The residue was
purified by column chromatography on silica (50% petroleum
ether-dichloromethane) and recrystallization of the major product
from hexane afforded 6a as green yellow needles (420 mg, 0.47
mmol, 60%). 1H NMR (CDCl3, 400 MHz): δ 9.77 (dd, 6H), 7.74
1,2,7,8,13,14-Hexabutyoxy-4,5,10,11,16,17-hexachlorotriben-
zo[a,g,m]coronene 6b. To a solution of 5b (500 mg, 0.458 mmol)
in dichloromethane (30 mL) was added ferric chloride (890 mg,
5.47 mmol) in 2 mL of nitromethane. The mixture was stirred for
20 min at room temperature, before being quenched with methanol
(10 mL), then 30 mL of water. The organic layer was separated
and the solvent removed in vacuo. The residue was purified by
column chromatography on silica (50% petroleum ether-dichlo-
romethane) and recrystallization of the major product form acetone
1
afforded 6b as yellow needles (180 mg, 0.173 mmol, 37%). H
NMR (CDCl3, 300 MHz): δ 9.97 (s, 6H), 4.16 (t, J ) 6.5 Hz,
12H), 2.03 (m, 12 H), 1.72 (m, 12H), 1.09 (t, J ) 7.3 Hz, 18H).
13C NMR (CDCl3, 100 MHz): δ 149.4, 131.3, 129.2, 128.4, 120.9,
120.6, 74.5, 33.0, 20.0, 14.6. MALDI-TOF (m/z): calcd for C60H60-
Cl6O6 1086.2, found 1086.2. Elemental analysis calcd for C60H60-
Cl6O6: C, 66.12; H, 5.54. Found: C, 65.91; H, 5.54. Mp 220-
221 °C.
Acknowledgment. This work was supported by the “100
Talents” program of the Chinese Academy of Sciences, and the
National Natural Science Foundation of China (20421101). The
Australian Research Council is also thanked for financial support
and the award of an Australian Postdoctoral Fellowship to
N.T.L.
Supporting Information Available: Further experimental
details, NMR spectra, and crystallographic data for compounds 6a
and 6b. This material is available free of charge via the Internet at
http://pubs.acs.org.
JO0701029
3920 J. Org. Chem., Vol. 72, No. 10, 2007