Hexa-peri-hexabenzocoronenes by efficient oxidative cyclodehydrogenation: The role of the oligophenylene precursors

Article abstract of DOI:10.1021/ol053043z

Oligophenylene precursors based on 1,3,5-tris-(2′-biphenyl)ylbenzene (4a) and 1,4-bis-(2′-biphenyl)yl-2,5-diphenylbenzene (5a) were prepared and utilized for efficient hexabenzocoronene (HBC) synthesis by cyclodehydrogenations. Parent HBC 6a was efficiently synthesized from the 1,3,5-tris-(2′-biphenyl)ylbenzene precursor, and novel D_3h symmetrical HBCs were prepared from 1,3,5-tris-(2′-biphenyl)ylbenzenes with various substitution types. For the preparation of a tert-butyl containing HBC 7 with D_2h symmetry, a two-step cyclodehydrogenation was required because of changes in the spin density distribution.

Full text of DOI:10.1021/ol053043z

ORGANIC
LETTERS
2006
Vol. 8, No. 6
1145-1148
Hexa-peri-hexabenzocoronenes by
Efficient Oxidative
Cyclodehydrogenation: The Role of the
Oligophenylene Precursors
Xinliang Feng, Jishan Wu, Volk Enkelmann, and Klaus Mu1llen*
Max-Planck Institut fu¨r Polymerforschung, Ackermannweg 10, 55128, Mainz, Germany
muellen@mpip-mainz.mpg.de
Received December 15, 2005
ABSTRACT
Oligophenylene precursors based on 1,3,5-tris-(2
and utilized for efficient hexabenzocoronene (HBC) synthesis by cyclodehydrogenations. Parent HBC 6a was efficiently synthesized from the
1,3,5-tris-(2 -biphenyl)ylbenzene precursor, and novel D3h symmetrical HBCs were prepared from 1,3,5-tris-(2 -biphenyl)ylbenzenes with various
′-biphenyl)ylbenzene (4a) and 1,4-bis-(2′-biphenyl)yl-2,5-diphenylbenzene (5a) were prepared
′
′
substitution types. For the preparation of a tert-butyl containing HBC 7 with D2h symmetry, a two-step cyclodehydrogenation was required
because of changes in the spin density distribution.
Polycyclic aromatic hydrocarbons (PAHs) have continuously
attracted the interest of organic chemists and materials
scientists because of their large π-systems and strong
π-stacking that have led to one-dimensional charge-
transporting channels.1 Pioneering syntheses of PAHs were
performed by Scholl and Clar et al. under harsh conditions.2
Well-defined HBCs and their higher homologues show high
thermal and chemical stability and serve as useful organic
semiconductors in electronic and optoelectronic devices.3
However, previous synthetic methods have afforded HBCs
(2) (a) Polycyclic Hydrocarbons; Clar, E., Ed.; Academic Press: New
York, 1964; Vol. I/II. (b) The Aromatic Sextet; Clar, E., Ed.; Wiley-VCH:
London, 1972. (c) Scholl, R.; Seer, C.; Weitzenbo¨k, R. Chem. Ber. 1910,
43, 2202-2209. (d) Scholl, R.; Seer, C. Liebigs Ann. Chem. 1912, 394,
111-123. (e) Scholl, R.; Seer, C. Chem. Ber. 1922, 55, 330-341. (f) Clar,
E.; Stewart, D. G. J. Am. Chem. Soc. 1953, 75, 2667-2672. (g) Clar, E.;
Schmidt, W. Tetrahedron 1979, 35, 2673-2680.
(1) (a) Grimsdale, A. C.; Mu¨llen, K. Angew. Chem., Int. Ed. 2005, 44,
5592-5629. (b) van de Crass, A. M.; Stutzmann, N.; Bunk, O.; Nielsen,
M. M.; Watson, M. D.; Mu¨llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend,
R. H. AdV. Mater. 2003, 15, 495-499. (c) Schmidt-Mende, L.; Fechtenko¨ter,
A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. Science 2001,
293, 1119-1122.
10.1021/ol053043z CCC: $33.50
© 2006 American Chemical Society
Published on Web 02/23/2006

in very low yields.4 Our group has developed an efficient
way for preparing a variety of HBCs 2 through a mild
intramolecular oxidative cyclodehydrogenation of hexaphe-
nylbenzene precursors 1 (Scheme 1).5 Several suitable
been synthesized, and functional groups such as esters at
the end of aliphatic chains can also be introduced. However,
the synthesis of HBC molecules with lower symmetry such
as D3h met with difficulties.9 In addition, the cyclodehydro-
genation of HPB precursors is sometimes limited by the
electron-rich and -poor characters of the substitutions.10 To
broaden the scope of HBC analogues, we have found the
use of other oligophenylene precursors such as 1,3,5-tris-
(2′-biphenyl)benzene (4) or 1,4-bis-(2′-biphenyl)yl-2,5-diphe-
nylbenzene (5) (Scheme 2). Although they differ from HPBs
Scheme 1. General Synthesis of HBCs by Oxidative
Cyclodehydrogenation of Hexaphenylbenzene Precursors
Scheme 2. Potential Synthesis of HBCs from Precursors 4a-c
and 5a,b
oxidant systems have been utilized for these transformations
including CuCl2/AlCl3, Cu(OTf)2/AlCl3/CS2, and FeCl3/CH3-
NO2. Through detailed studies of the oxidative cyclodehy-
drogenation, the parent hexaphenylbenzene (HPB) was found
to exclusively cyclodehydrogenize in an intramolecular
fashion. Furthermore, the intermediate phenyldibenzo[fg,ij]-
phenanthro[9,10,1,2,3-pqrst]pentaphene (3) could be sepa-
rated, indicating that the cyclodehydrogenation proceeded
in a stepwise process.6 Recently, theoretical studies have
pointed toward a radical cation mechanism for these in-
tramolecular fusion reactions. The geometry, charge, and spin
distribution of the radical cation intermediates are decisive
for the step-by-step bonding formation mechanism.7
in topology, they can undergo similar intramolecular cyclo-
dehydrogenations to afford HBC molecules. Following this
concept, here we describe an efficient synthetic method for
the preparation of known HBCs (including 6a) and unknown
derivatives such as iodo compounds 6b,c and the D3h or D2h
symmetric species 7.
As shown in Scheme 3, 1,3,5-tris-(2′-bromophenyl)-
benzene (9) was synthesized in 72% yield by trifluo-
romethanesulfonic-acid-mediated trimerization of 2-bro-
moacetophenone (8). It is worthy to note that other Lewis
acids typically used for condensation trimerizations of
substituted acetophenones such as SiCl4 and TiCl4 were
ineffective or problematic.11 Molecule 9 constitutes an
important building block for subsequent palladium-catalyzed
HPBs can be synthesized by the cyclotrimerization of
diphenylacetylene or the Diels-Alder reaction between
diphenylacetylene derivatives and tetraphenylcyclopentadi-
enones.8 D6h symmetric or unsymmetric HBC molecules with
iodo, iodophenyl, alkyl, and alkylphenyl substitutions have
(3) Watson, M. D.; Fechtenko¨ter, A.; Mu¨llen, K. Chem. ReV. 2001, 101,
1267-1300.
(4) (a) Clar, E.; Ironside, C. T. Proc. Chem. Soc. 1958, 150. (b) Clar,
E.; Ironside, C. T.; Zander, M. J. Chem. Soc. 1959, 142-147. (c) Halleux,
A.; Martin, R. H.; King, G. S. D. HelV. Chim. Acta 1958, 129, 1177-
1183. (d) Hendel, W.; Khan, Z. H.; Schmidt, W. Tetrahedron 1986, 42,
1127-1134.
(5) (a) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem.
1995, 107, 1768-1770; Angew. Chem., Int. Ed. Engl. 1995, 34, 1609-
1611. (b) Mu¨ller, M.; Ku¨bel, C.; Mu¨llen, K. Chem.-Eur. J. 1998, 4, 2099-
2109.
(6) Ku¨bel, C.; Eckhardt, K.; Enkelmann, V.; Wegner, G.; Mu¨llen, K. J.
Mater. Chem. 2000, 10, 879-886.
(7) (a) Rempala, P.; Kroulik, J.; King, B. T. J. Am. Chem. Soc. 2004,
126, 15002-15003. (b) Stefano, M. D.; Negri, F.; Carbone, P.; Mu¨llen, K.
Chem. Phys. 2005, 314, 85-99.
(8) (a) Hyatt, J. A. Org. Prep. Proced. Int. 1991, 23, 460-463. (b)
Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater. 1996, 8,
510-513. (c) Fechtenko¨tter, A.; Saalwa¨chter, K.; Harbison, M. A.; Mu¨llen,
K.; Spiess, H. W. Angew. Chem. 1999, 111, 3224-3228; Angew. Chem.,
Int. Ed. Engl. 1999, 38, 3039-3042. (d) Brand, J. D.; Ku¨bel, C.; Ito, S.;
Mu¨llen, K. Chem. Mater. 2000, 12, 1638-1647.
(9) (a) Wu, J.; Watson, M. D.; Mu¨llen, K. Angew. Chem., Int. Ed. 2003,
42, 5329-5333. (b) Wu, J.; Baumgarten, M.; Debije, M. G.; Warman, J.
M.; Mu¨llen, K. Angew. Chem., Int. Ed. 2004, 43, 5331-5335.
(10) (a) Lambert, C.; No¨ll, G. Angew. Chem., Int. Ed. Engl. 1998, 37,
2107-2110. Angew. Chem. 1998, 110, 2239-2242. (b) Lambert, C.; No¨ll,
G. Chem.-Eur. J. 2002, 8, 3467-3477.
(11) (a) Elmorsy, S. S.; Pelter, A.; Smith, K. Tetrahedron Lett. 1991,
32(33), 4175. (b) Elmorsy, S. S.; Khalil, A. G. M.; Girges, M. M.; Salama,
T. A. Tetrahedron Lett. 1997, 38(6), 1071. (c) Boorum, M. M.; Vasil’ev,
Y. V.; Drewello, T.; Scott, L. T. Science 2001, 294, 828. (d) Scott, L. T.;
Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner,
H.; de Meijere, A. Science 2002, 295, 1500. (e) Ansems, R. B. M.; Scott,
L. T. J. Am. Chem. Soc. 2000, 122, 2719-2724. (f) Scott, L. T. Angew.
Chem., Int. Ed. 2004, 43, 4994-5007.
1146
Org. Lett., Vol. 8, No. 6, 2006

Scheme 3
Suzuki, Stille, and Negishi coupling reactions and in our
prolongation of the reaction time up to 23 h. The MALDI-
TOF mass spectrum of the obtained insoluble, yellow powder
showed only a single species with a molecular ionic peak at
905, suggesting that only six hydrogen atoms had been
removed. The poorly soluble compound 11 was then reacted
with trimethylsilylacetylene under Hagihara-Sonogashira
conditions to give a single soluble product 12, whose
structure could be confirmed by single-crystal structure
analysis.13 The crystal structure (Scheme 3, inset) clearly
shows that the biphenyl group of 12 is highly twisted out of
the central plane and has a dimeric oblique packing mode
(for details in a CIF file, see Supporting Information).
Therefore, structure 11 was ascribed to the insoluble yellow
powder where one of the biphenyl units had not formed new
C-C bonds during the reactions. This result further supports
a stepwise cyclodehydrogenation mechanism proceeding
from 4a-c to 6a-c. We believe the reason that 4c can only
be partially closed is due to the steric hindrance induced by
the twist of the biphenyl units and the poor solubility of the
product.
study led to various 1,3,5-tris-(2′-biphenyl)ylbenzene struc-
tures. Suzuki coupling of 9 with phenylboronic acid, 3-tri-
methylsilyl-1-phenylboronic acid, and 4-trimethylsilyl-1-
phenylboronic acid afforded 4a, 10a, and 10b in 86%, 90%,
and 93% yields, respectively. Comparatively, it should be
noted that 10a was previously synthesized by a three-step
procedure in only 48% yield.9 The trimethylsilyl groups in
10a and 10b were then converted into iodo groups by
treatment with iodine monochloride and gave 4b and 4c in
92% and 93% yields, respectively.
The intramolecular oxidative cyclodehydrogenation reac-
tions were then performed on compounds 4a-c using FeCl3
as an oxidant. The parent HBC 6a was obtained as yellow
powder in 80% yield from compound 4. The MALDI-TOF
mass spectra of the product revealed that no chlorination had
occurred during this cyclodehydrogenation process, even after
prolonged reaction times up to several hours. This result is
important because chlorination was frequently observed with
HPB cyclodehydrogenations5b,12 and suggests that the present
synthesis of parent HBC is superior. It also appears that the
topology of the precursor plays an important role for
cyclodehydrogenation. Likewise, compound 6b can be
prepared from 4b under similar conditions and provides a
useful building block for further functionalization.9 Interest-
ingly, the similar cyclodehydrogenation reaction of 4c did
not afford completely planarized HBC 6c, even after
Encouraged by the efficient cyclodehydrogenation of 1,3,5-
tris-(2′-biphenyl)ylbenzene precursors, we revisited a previ-
ously failed reaction, namely the cyclodehydrogenation of
5b to compound 7.14 In this study, we carefully controlled
the quantity of oxidant and the reaction time. Interestingly,
(13) Crystals suitable for X-ray structure analysis were obtained by slow
evaporation of a CH2Cl2 solution of compounds 12 and 13 at room
temperature. Crystal structure determination was carried out on a KCCD
diffractometer with graphite-monochromated Mo KR irradiation. The
structure was solved by direct methods (SHELXS-97).
(14) Wu, J.; Gherghel, L.; Watson, M. D.; Li, J.; Wang, Z.; Simpson,
C. D.; Kolb, U.; Mu¨llen, K. Macromolecules 2003, 36, 7082-7089.
(12) (a) Do¨tz, F.; Brand, J. D.; Ito, S.; Gherghel, L.; Mu¨llen, K. J. Am.
Chem. Soc. 2000, 122, 7707-7717. (b) Simpson, C. D.; Brand, J. D.;
Berresheim, A. J.; Przybilla, L.; Ra¨der, H. J.; Mu¨llen, K. Chem.-Eur. J.
2002, 8(6), 1424-1429.
Org. Lett., Vol. 8, No. 6, 2006
1147

Scheme 4. Oxidation of 5b
cyclodehydrogenation of 5b by treatment with FeCl3 in
typical cyclodehydrogenation conditions. The new synthetic
method starting from 1,3,5-tris-(2′-bromophenyl)benzene
offers opportunities for synthesizing novel HBC derivatives,
especially D3h symmetric HBCs that can otherwise not be
obtained from typical hexaphenylbenzene precursors. The
attachment of tert-butyl groups helps to change the spin
density distribution and therefore requires a two-step pro-
cedure to prepare its HBC analogues. The partially cyclo-
dehydrogenized products can be separated and characterized,
further suggesting a stepwise ring-closing mechanism includ-
ing radical cations.
dichloromethane over 30 min and up to 5 h only yielded a
partially closed intermediate 13, which was again supported
by single-crystal analysis (Scheme 4).15 Alternatively, the
analogue species, 1,4-bis(2′-biphenyl)yl-2,5-diphenylbenzene
(5a), afforded complex mixtures under the same oxidations.
Comparing these two examples, we then ascribed that the
tert-butyl groups in precursor 5b stabilize the intermediate
cation radical during the dehydrogenation process. This
stabilizing effect in turn does not allow further cyclization
and complete planarization to form HBC 7. The stable cation
radical of 13 was quenched by adding methanol to afford
13 in 68% yield. To support our hypothesis of cation radical
stabilization, compound 13 was then further submitted to the
oxidative cyclodehydrogenation by using 12 equiv of FeCl3
for 30 min. Amazingly, completely closed HBC molecule 7
was obtained in quantitative yield and without any chlorina-
tion! Therefore, two independent cyclodehydrogenations
must be performed to clearly transform 5b into 7.
Acknowledgment. This work was supported by EU
project NAIMO (NMP4-CT-2004-500355). We thank Dr.
L. Zhi and Dr. K. N. Plunkett for their assistance in the
preparation of the manuscript.
Supporting Information Available: Full experimental
details, the MALDI-TOF mass spectra of compounds 7 and
11, and crystallographic information files (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
In conclusion, parent HBC 6a was successfully synthesized
from the 1,3,5-tris-(2′-biphenyl)ylbenzene precursor using
(15) For crystal growth, see ref 13.
OL053043Z
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Org. Lett., Vol. 8, No. 6, 2006