Ortho-methylated tribenzotriquinacenes - Paving the way to curved carbon networks

Article abstract of DOI:10.1039/c1cc14703j

The synthesis of sterically crowded tribenzotriquinacenes with complete and partial methylation of the ortho-positions has been achieved using the double cyclodehydration strategy. This leads to a twisted tribenzotriquinacene core and enables further func

Full text of DOI:10.1039/c1cc14703j

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COMMUNICATION
Ortho-methylated tribenzotriquinacenes—paving the way to curved
carbon networkswz
Yvonne Kirchwehm,a Alexander Damme,b Thomas Kupfer,b Holger Braunschweigbc and
Anke Krueger*ac
Received 30th July 2011, Accepted 21st September 2011
DOI: 10.1039/c1cc14703j
The synthesis of sterically crowded tribenzotriquinacenes with
complete and partial methylation of the ortho-positions has
been achieved using the double cyclodehydration strategy.
This leads to a twisted tribenzotriquinacene core and enables
further functionalization for the future synthesis of curved model
compounds for defective carbon networks.
Graphene and its unique properties have recently spurred a
huge increase in interest.1 Not only electronic applications of
as-grown graphene materials but also the controlled modification
of their chemical and electronic properties by chemical functio-
nalization have been discussed in detail.2
One of the major issues of its structure is the existence of
lattice defects related to the insertion of non-hexagonal rings,
i.e. pentagons and heptagons.3 These do not only induce a
local curvature, such as a bowl shaped indentation, in the
otherwise flat network but also have a major impact on the
electronic and magnetic properties of the material.4 Kaiser
et al. have recently published the direct imaging of such defects
using STEM, showing the existence of fused pentagonal
defects in pentalene-like structures as well as the existence of
heptagons in the graphene lattice.5 Furthermore, these defects
play a role in the formation of new classes of two-dimensional
materials such as amorphous sp2 carbon.6
A model for non-aromatic bowl shaped defects in graphene
including fused pentagons and heptagons has been proposed
by Tellenbroker and Kuck.7 The structure of a respective
Fig. 1 The structures of a defective carbon flake (1) and related
¨
defective graphene flake would be derived from a suitably
model compounds (2 and 3).
embedded tribenzotriquinacene (TBTQ) 2a after its dehydro-
defined molecules such as 3 where the pre-shaped bays of the
genation (Fig. 1). It would be highly desirable to synthesize
TBTQ core are bridged with 1,2-vinylidene units in order to
study the resulting properties of such a ‘‘faulty graphene flake’’
a Institute for Organic Chemistry, Julius-Maximilians University,
Am Hubland, Wurzburg, Germany.
¨
fragment. However, the synthesis cannot be adapted from the
formation of flat and defect-free nanographene fragments,
E-mail: krueger@chemie.uni-wuerzburg.de; Fax: +49 931 318 4606;
Tel: +49 931 3185334
which can be obtained using e.g. methods reported by Mullen
¨
and coworkers.8 The synthesis of the model compound 3 has
rather to be adapted from the stepwise formation of the
tribenzotriquinacene unit as reported by Kuck9 and requires
inevitably the partial or complete substitution of the hydrogen
atoms in the ortho-positions of the aromatic rings.
b Institute for Inorganic Chemistry, Julius-Maximilian University,
Am Hubland, Wurzburg, Germany
¨
c Rontgen Research Center, Julius-Maximilian University,
¨
Am Hubland, Wurzburg, Germany
¨
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Experimental
procedures, analytical and crystallographic data. CCDC 837041,
837042, 837043 and 837044. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1cc14703j
However, in addition to the derivatization at the three
bridgehead positions, so far only meta-substituted derivatives
of 2 have been reported as sterical hindrance impedes the
c
1502 Chem. Commun., 2012, 48, 1502–1504
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derivatization of the tribenzotriquinacene in the ortho-position
by subsequent aromatic substitution.7,10 Kuck discussed the
challenging nature of the required transformations in 2006.11
Here we describe the synthesis of such sterically crowded
tribenzotriquinacenes carrying up to six methyl groups in the
ortho-positions of the benzene rings based on the introduction
of the methyl substituents at the beginning of the synthetic
scheme.
the final double cyclodehydration step was accomplished with
H3PO4 in p-xylene at elevated temperature. Despite the sterical
influence of the two additional methyl groups Me3-TBTQ 15a
could be obtained in 16% yield. (The unsubstituted Me-TBTQ
2b is obtained with 33% yield under comparable conditions.)12
The successful synthesis of Me3-TBTQ 15a shows the feasibility
of the introduction of substituents in the ortho-positions at an
early stage of the sequence. To extend this new access to entirely
ortho-methylated TBTQs, the methylated indandiols 8 and 9 as
well as the tetramethylated benzhydrol-derivative 12 were pre-
pared according to literature procedures; starting with a Diels–
Alder reaction of 2,5-dimethylfuran 4 and maleic anhydride 5,
followed by dehydration of adduct 6 to the phthalic anhydride 7
and subsequent conversions to 8 and 9, respectively.13b,14 The
reaction of 8 and 9 with the benzhydrol-derivative 12 yielded
the 2-benzhydryl-1,3-indandiones 13b and 13c which could be
reduced to their corresponding 2-benzhydryl-1,3-indandiols 14b
and 14c.
Since it was expected that the level of steric hindrance is
increasing with the number of methyl groups attached in the
ortho-positions of the starting materials, the first synthetic
attempt was made to insert only two methyl groups. Therefore,
the originally reported synthesis of unsubstituted TBTQ 212
had to be modified (Scheme 1). The required 1,3-indandione 10
and the dimethylated benzhydrol derivative 11 were prepared
according to procedures described in the literature.13 After their
conversion to the corresponding 2-methyl-2-benzhydryl-1,3-
indandione 13a and subsequent reduction to the indandiol 14a
Scheme 1 Synthesis of tribenzotriquinacenes carrying up to six methyl groups in the ortho-positions of the aromatic rings using the
cyclodehydration step as the structure-defining transformation.
c
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Fig. 2 X-Ray structures of ortho-substituted tribenzotriquinacenes 15a–c and the rearrangement product 16. In the crystal no molecular
symmetry is observed due to steric repulsion of the adjacent methyl groups. In contrast, 1H/13C NMR spectra for 15a, 15b and 15c in solution
exhibit the typical signal patterns reflecting Cs- and C3v-symmetries, respectively. Even at low-temperature (197.3 K) no line broadening or
separation was observed.
The final double cyclodehydration step represents the
challenge in this synthetic approach. All attempts to synthesize
Me7-TBTQ 15b using a variety of conditions (i.e. H3PO4/
chlorobenzene; H3PO4/xylene; AlCl3/dichloromethane; Amberlyst
15/benzene; Amberlyte 120/toluene) were to no avail. In some
cases (Amberlyst 15/benzene; AlCl3/dichloromethane) the product
of a Wagner–Meerwein rearrangement was isolated. In contrast,
under strongly acidic conditions (HCl/HOAc), the desired
Me7-TBTQ 15b was obtained in 1% yield besides the above-
mentioned rearrangement product 16 (36%, see Fig. 2 for crystal
structure). 16 results from the formation of a tertiary carbocation
from the initially generated secondary benzylic carbocation
(Scheme S1 in the ESIz).15
tribenzotriquinacenes, which will be useful for the investiga-
tion of curved defects in graphene-derived structures and their
effect on electronic properties of defective carbon networks.
We acknowledge the financial support of this project by the
DFG (GRK 1221). YK is thankful for a PhD scholarship of
the Studienstiftung des Deutschen Volkes.
Notes and references
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110, 132; (b) S. Guo and S. Dong, Chem. Soc. Rev., 2011, 40, 2644.
2 X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey
and H. Zhang, Small, 2011, 7, 1876.
3 (a) J. Kotakoski, J. C. Meyer, S. Kurasch, D. Santos-Cottin,
U. Kaiser and A. V. Krasheninnikov, Phys. Rev. B: Condens.
Matter Mater. Phys., 2011, 83, 245420; (b) J. Ma, D. Alfe,
A. Michaelides and E. Wang, Phys. Rev. B: Condens. Matter
Mater. Phys., 2009, 80, 033407.
In contrast, the driving force for such a rearrangement
should be significantly reduced when the methyl group at the
central carbon is missing. Therefore, the formation of Me6-
TBTQ 15c should be less hampered by this concurrent reaction
pathway.
4 A. B. Kaiser and V. Skakalova, Chem. Soc. Rev., 2011, 40, 3786.
5 A. Chuvilin, J. C. Meyer, G. Algara-Siller and U. Kaiser, New J.
Phys., 2009, 083019.
6 J. Kotakoski, A. V. Krasheninnikov, U. Kaiser and J. C. Meyer,
Phys. Rev. Lett., 2011, 106, 105505.
Although attempts using a variety of dehydration conditions
(i.e. Amberlyst 15/benzene; AlCl3/dichloromethane; PPA/
chlorobenzene) did not give the rearrangement product as
expected, the Me6-TBTQ 15c was only found in traces at
the maximum. Furthermore, the application of strongly
acidic conditions (HCl/HOAc) afforded the product of an
elimination–substitution sequence leading to a chlorinated
indene moiety 17 (see ESIz).
7 J. Tellenbroker and D. Kuck, Angew. Chem., Int. Ed., 1999,
¨
38, 919.
8 (a) M. Treier, C. A. Pignedoli, T. Laino, R. Rieger, K. Mullen,
¨
D. Passerone and R. Fasel, Nat. Chem., 2011, 3, 61; (b) R. Rieger and
K. Mullen, J. Phys. Org. Chem., 2010, 23, 315 and ref. cited therein.
¨
9 D. Kuck, Angew. Chem., Int. Ed. Engl., 1984, 23, 508.
10 (a) A. Schuster and D. Kuck, Angew. Chem., Int. Ed. Engl., 1991,
30, 1699; (b) D. Kuck, Top. Curr. Chem., 1998, 196, 167;
(c) D. Kuck, A. Schuster, R. A. Krause, J. Tellenbroker,
¨
¨
C. P. Exner, M. Penk, H. Bogge and A. Muller, Tetrahedron,
The successful synthesis of Me6-TBTQ 15c was achieved using
H3PO4/chlorobenzene at 120 1C in the cyclodehydration step.
Due to the reduced symmetry of the product (Fig. 2) the
solubility of 15c was markedly increased in comparison to the
rigid and highly symmetrical unsubstituted 2a and crystallization
was not successful for the isolation of 15c (as for the isolation of
15a and 15b). Only multistep chromatographic workup (column
chromatography, HPLC/MPLC) provided the desired product
in up to 6% yield,16 which is surprisingly high regarding the
maximum yield of 11% for the unsubstituted compound 2a.12
In summary, the synthesis of three ortho-methylated
tribenzotriquinacenes was achieved using the well-established
cyclodehydration strategy with suitably substituted benz-
hydrylindane-1,3-diols. The further functionalization (e.g.
oxidation and subsequent intramolecular coupling) of the title
compounds and the synthesis of benzannulated derivatives of
TBTQ will open the way to the formation of bridged
¨
2001, 57, 3587; (d) X.-P. Cao, D. Barth and D. Kuck, Eur. J. Org.
Chem., 2005, 3482; (e) D. Kuck, Pure Appl. Chem., 2005, 78, 749.
11 D. Kuck, Chem. Rev., 2006, 106, 4885.
12 D. Kuck, T. Lindenthal and A. Schuster, Chem. Ber., 1992,
125, 1449.
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J. Org. Chem., 1971, 36, 1561.
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15 The synthesis of a similar structure as 16 with H/t-BuO/t-Bu
residues instead of methyl groups has been reported by Okada
et al. via a different reaction sequence in K. Okada, H. Kawai,
K. Okubo, T. Uesugi and M. Oda, Tetrahedron Lett., 1988,
29, 2333.
16 Isolated yields vary between 3–6% depending on the scale of
synthesis due to demanding isolation processes.
c
1504 Chem. Commun., 2012, 48, 1502–1504
This journal is The Royal Society of Chemistry 2012