A facile synthesis of elusive alkoxy-substituted hexa-peri- hexabenzocoronene

Article abstract of DOI:10.1021/ol8020429

(Chemical Equation Presented) Oxidative cyclodehydrogenation of hexakis(4-alkoxyphenyl)benzene produces a quantitative yield of an indenofluorene derivative rather than the expected alkoxy-substituted hexa-peri-hexabenzocoronene (HBC). The structure of the unexpected indenofluorene was established by X-ray crystallography. The mechanistic considerations for the formation of the indenofluorene derivative led us to devise an alternative synthesis of elusive alkoxy-substituted HBC - a potentially important, disk-shaped structure for the preparation of liquid crystalline materials for practical applications in the emerging areas of molecular electronics and nanotechnology.

Full text of DOI:10.1021/ol8020429

ORGANIC
LETTERS
2008
Vol. 10, No. 22
5139-5142
A Facile Synthesis of Elusive
Alkoxy-Substituted
Hexa-peri-hexabenzocoronene
Shriya H. Wadumethrige and Rajendra Rathore*
Department of Chemistry, Marquette UniVersity,
P.O. Box 1881, Milwaukee, Wisconsin 53201
rajendra.rathore@marquette.edu
Received September 1, 2008
ABSTRACT
Oxidative cyclodehydrogenation of hexakis(4-alkoxyphenyl)benzene produces a quantitative yield of an indenofluorene derivative rather than
the expected alkoxy-substituted hexa-peri-hexabenzocoronene (HBC). The structure of the unexpected indenofluorene was established by
X-ray crystallography. The mechanistic considerations for the formation of the indenofluorene derivative led us to devise an alternative synthesis
of elusive alkoxy-substituted HBCsa potentially important, disk-shaped structure for the preparation of liquid crystalline materials for practical
applications in the emerging areas of molecular electronics and nanotechnology.
The study of polycyclic aromatic hydrocarbons (PAH),
such as triphenylenes (TP) and hexa-peri-hexabenzocoro-
nenes (HBC), has attracted considerable attention1 since these
materials hold promise for applications in the emerging areas
of molecular electronics and nanotechnology.2
It is noteworthy that hexaalkoxytriphenylenes (1) have
been extensively explored as building blocks for the prepara-
tion of liquid crystalline materials, owing to their disk-shaped
structure which allows efficient columnar packing.3 Colum-
nar discotic liquid crystals have received considerable
attention as functional materials for applications such as
(2) (a) Introduction to Molecular Electronics; Petty, M. C., Bryce, M. R.,
Bloor, D., Eds.; Oxford University Press: New York, 1995. (b) Maiya, B. G.;
Ramasarma, T. Curr. Sci. 2001, 80, 1523–1530. (c) Gross, M.; Muller, D. C.;
Nothofer, H.-G.; Scherf, U.; Neher, D.; Brauchle, C.; Meerholz, K. Nature
2003, 405, 661–665. (d) Organic Electronics; Klauk, H., Ed.; Wiley-VCH:
Weinheim, 2006.
(3) (a) Mori, H. Proc. SPIE-Int. Soc. Opt. Eng. 2006, 6135 (Liquid
Crystal Materials, Devices, and Applications XI), 613503/1613503/8. (b)
Tschierske, C. J. Mater. Chem. 2001, 11, 2647–2671. (c) Goodby, J. W.;
Saez, I. M.; Cowling, S. J.; Gortz, V.; Draper, M.; Hall, A. W.; Sia, S.;
Cosquer, G.; Lee, S.-E.; Raynes, E. P. Angew. Chem., Int. Ed. 2008, 47,
2754–2787.
(1) (a) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99,
1747–1785. (b) Watson, M. D.; Fechtenkotter, A.; Mu¨llen, K. Chem. ReV.
2001, 101, 1267–1300. (c) Chebny, V. J.; Gwengo, C.; Gardinier, J. R.;
Rathore, R. Tetrahedron Lett. 2008, 49, 4869–4872.
10.1021/ol8020429 CCC: $40.75
Published on Web 10/30/2008
2008 American Chemical Society

oxidative cyclization may allow the preparation of the long-
sought hexaalkoxy-HBC 2 (Scheme 1). However, we now
describe that the oxidative cyclization of 3 unfortunately does
not produce quinone 4 but rather an unexpected indeno[1,2-
b]fluorene derivative (7), whose structure is confirmed by
X-ray crystallography. Mechanistic consideration for the
formation of indenofluorene 7 via oxidative cyclodehydro-
genation of 3 now leads us to an alternative route to access
the elusive 2. The details of these preliminary findings are
described herein.
Thus, addition of a solution of FeCl3 in nitromethane to a
solution of 3 in dichloromethane at ∼0 °C produces a green
solution which was stirred for 1 h while a slow stream of
argon was bubbled through the solution to remove gaseous
hydrochloric acid, which was formed in the reaction. The
standard workup7 afforded a pale yellow solid in quantitative
yield. The 1H/13C NMR analysis of the above solid suggested
that it is a single compound that has lost two alkyl groups
and two “H” atoms. As such, the preliminary spectral analysis
was in accord with the phenanthroquinone-like structure 4;
however, further experimentation8 and X-ray crystallography
showed that it is indenofluorene 7 (i.e., eq 1). Note that the
thermal ellipsoids for the ORTEP representation of 7 (R )
CH3), in eq 1, are shown in 50% probability, and hydrogens
are omitted for the sake of clarity.
photovoltaic solar cells, light emitting diodes, and field effect
transistors.4 It can be envisioned that a corresponding HBC
analogue (2), possessing a disk of relatively large size, may
prove to be superior for the preparation of liquid crystalline
materials.3,4 Unfortunately, the synthesis of the parent
hexaalkoxy HBC (2) has thus far eluded chemists.5 Mu¨llen
and co-workers have reported6 that attempted preparation
of 2 from hexakis(4-alkoxyphenyl)benzene via oxidative
cyclodehydrogenation led to a quantitative yield of a phenan-
throquinone 4, i.e., Scheme 1.
Scheme 1. Postulated Synthesis of Hexaalkoxy-HBC 2
It was conjectured that a reduction of quinone 4 to the
corresponding hydroquinone 5 followed by O-alkylations and
(4) (a) Nelson, J. Science 2001, 293, 1059–1060. (b) Hassheider, T.;
Benning, S. A.; Lauhof, M. W.; Kitzerow, H. S.; Bock, H.; Watson, M. D.;
Mu¨llen, K. Mol. Cryst. Liq. Cryst. 2004, 413, 461–472. (c) Oukachmih,
M.; Destruel, P.; Seguy, I.; Ablart, G.; Jolinat, P.; Archambeau, S.; Mabiala,
M.; Fouet, S.; Bock, H. Sol. Energy Mater. Sol. Cells 2005, 85, 535–543.
(d) Bayer, A.; Zimmermann, S.; Wendorff, J. H. Mol. Cryst. Liq. Cryst.
2003, 396, 1–22. (e) Katsuhara, M.; Aoyagi, I.; Nakajima, H.; Mori, T.;
Kambayashi, T.; Ofuji, M.; Takanishi, Y.; Ishikawa, K.; Takezoe, H.;
Hosono, H. Synth. Met. 2005, 149, 219–223.
Moreover, both 3a (R ) methyl) and 3b (R ) n-hexyl)
formed the corresponding indenofluorenes (7a and 7b) upon
oxidative cyclodehydrogenation as judged by the similarity
1
of their H/13C NMR spectra (Figure S1) and by mass
spectrometry (see the Supporting Information for the details).
(5) See: Zang, Q.; Prins, P.; Jones, S. C.; Barlow, S.; Kondo, T.; An,
Z.; Siebbeles, L. D.; Marder, S. R. Org. Lett. 2005, 7, 5019–5022.
(6) (a) Weiss, K.; Beernink, G.; Do¨tz, F.; Birkner, A.; Mu¨llen, K.; Wo¨ll,
C. H. Angew. Chem., Int. Ed. 1999, 38, 3748–3752. (b) Dou, X.; Yang, X.;
Bodell, G. J.; Wagner, M.; Enkelmann, V.; Mu¨llen, K. Org. Lett. 2007, 9,
2485–2488. (c) Feng, X.; Pisula, W.; Takase, M.; Dou, X.; Enkelmann,
V.; Wagner, M.; Ding, N.; Mu¨llen, K. Chem. Mater. 2008, 20, 2872–2874.
(7) Rathore, R.; Burns, C. L. J. Org. Chem. 2003, 68, 4071–4074.
(8) A hydrogenation of 7b (4) in ethyl acetate using Pd/C as a catalyst
did not afford the corresponding hydroquinone 5 but a reduced indenof-
luorene derivative containing cyclohexanone moieties; see the Supporting
Information.
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Org. Lett., Vol. 10, No. 22, 2008

The transformation in eq 1 can be carried out using a
9
variety of 1-e- oxidants such as NO+SbCl6 , PhI(O-
COCF3)2,10 DDQ-TFA,11a and FeCl3 as well as electro-
chemical oxidation.
-
Scheme 3. Synthesis of Hexaalkoxy HBC 2
The oxidative transformation in eq 1 can be reconciled
according to an ECECC mechanism (Scheme 2)11,12 based
Scheme 2. Postulated Mechanism for the Formation of 7
intramolecular C-C bond formation to yield a distonic
radical cation (I2).14 It is the distonic nature of this rearranged
radical cation which facilitates the removal of a second
electron at a much lower potential15 and leads to the
formation of a dication (I3).14b The loss of a proton and an
alkyl group produces the fluorene derivative (I5), which in
turn undergoes a similar sequence of ECECC events to
produce indenofluorene 7. Note that indenofluorene 7 does
not suffer further oxidative transformations owing to the fact
that it undergoes oxidation (Eox ) 1.18 V vs SCE) at a
relatively low potential to its cation radical which is stable
under the reaction conditions. (See Figures S2 and S3 in the
Supporting Information for a cyclic voltammogram and the
cation radical spectrum of 7, respectively.)
From the mechanistic considerations in Scheme 2, it was
clear that an alternative C-C bond-forming sequence was
desired to access HBC 2. It was conjectured that if some of
the C-C bonds were preformed in a new precursor of HBC
2, it should be feasible to produce 2 rather than indenofluo-
rene 7.16
on the spectroscopic and structural data. Thus, one-electron
oxidation of 3 generates its radical cation where a single
charge is stabilized via quinoidal distortion in such a way
that the radical character is largest at the p-carbon of the
anisyl group (see I1).13 The radical cation I1 undergoes an
(9) Rathore, R.; Abdelwahed, S. H.; Guzei, I. A. J. Am. Chem. Soc.
2004, 126, 13582–13583
(10) Takada, T.; Arisawa, M.; Gyoten, M.; Hamada, R.; Tohma, H.;
Kita, Y. J. Org. Chem. 1998, 63, 7698–7706
.
.
(11) For a discussion of the arenium ion versus cation-radical mecha-
nisms in various organic transfromations, see: (a) Rathore, R.; Kochi, J. K.
Acta Chem. Scand. 1998, 52, 114–130. (b) Rathore, R.; Zhu, C.-J.;
Lindeman, S. V.; Kochi, J. K. J. Chem. Soc., Perkin Trans. 2 2000, 1837–
1840. (c) Rathore, R.; Wygand, U.; Kochi, J. K. J. Org. Chem. 1996, 61,
5426–5436
.
(12) The ECEC mechanism is also applicable to other (oxidative) biaryl
syntheses; see: (a) Ronlan, A.; Hammerich, O.; Parker, V. D. J. Am. Chem.
Soc. 1973, 95, 7132–7138. (b) Rathore, R.; Kochi, J. K. J. Org. Chem.
1995, 60, 7479–7490. The electron-tranfer or cation-radical mechanisms
proposed in Schemes 2 and 3 may alternatively be considered to be occurring
via carbocation intermediates. For example, an intramolecular attack of the
protonated anisyl moiety on the adjecent anisyl group will result into a
dearomatized intermediate which can then undergo oxidative aromatization.
Although we strongly favor a cation-radical mechanism for the Scholl
reactions (such as in Scheme 3), the carbocation pathways can not be easily
ruled out. Compare: (c) King, B. T.; Kroulık, J.; Robertson, C. R.; Rempala,
P.; Hilton, C. L.; Korinek, J. D.; Gortari, L. M. J. Org. Chem. 2007, 72,
Accordingly, we chose 1,3,5-tris(5,5′-di-n-hexyloxy-2-
biphenyl)benzene (10) as a precursor to HBC 2, which was
obtained by a one-pot Suzuki coupling17 of 3 equiv of
(13) Compare: Chebny, V. J.; Shukla, R.; Rathore, R. J. Phys. Chem. B
2006, 110, 13003–13006, and references cited therein.
2279–2288
.
Org. Lett., Vol. 10, No. 22, 2008
5141

The absorption and emission spectra of 2 in dichlo-
romethane (Figure 1) showed highly structured absorption
(λmax ) 380 nm, log ꢀ380 ) 4.61) and emission (λmax ) 478
and 509 nm) bands. The optical spectra of 2 were charac-
teristically similar to those observed with the alkyl-substituted
HBC derivatives.1c Excited state emission of 2 was also
found to be highly concentration dependent and is in accord
with the observations made with the alkyl-substituted HBC
derivatives.1c For example, at higher concentrations (10-6
to ∼10-4 M), a lower energy emission band centered near
554 nm, tentatively assigned as excimer-based,1c grow in
intensity at the expense of the higher energy (478 nm) band,
with an apparent bathochromic shift. A further bathochromic
shift of ∼30 nm was observed upon increasing the concen-
tration of 2 by a factor of 10 (i.e., 10-4 to ∼10-3 M). In the
solid state, the emission of 2 occurs as a broadband at λmax
) 591 nm. The observation of concentration dependent
emission of 2 is tentatively attributed to the formation of
molecular aggregates such as dimers, trimers, tetramers, and
higher-order aggregates.1c
The preliminary X-ray diffraction pattern of 2 was found
to be characteristically similar to those observed for the other
HBC derivatives19 (see Figure S6 in the Supporting Informa-
tion) which are known to display liquid crystalline behavior.20
In summary, we have demonstrated that the oxidative
cyclodehydrogenation of hexakis(4-alkoxyphenyl)benzenes
(3) produces indenofluorenes 7 in quantitative yields rather
than HBC 2, as confirmed by X-ray crystallography. This
finding led us to design an alternative (simple) synthesis of
HBC 2 from an easily synthesized 1,3,5-tris(dialkoxybiphe-
nyl)benzene 10. The structure of hexaalkoxy HBC 2 was
confirmed by NMR spectroscopy and MALDI mass spec-
trometry. Moreover, the absorption and emission character-
istics of 2 were found to be similar to those observed with
the other HBC derivatives. The ready availability of
hexaalkoxy HBC 2 should spur theoretical and experimental
interest in the exploration of its materials’ properties.
Figure 1
.
(Left) UV-vis absorption spectrum of 1.75 × 10-5 M 2
in CH2Cl2 at 22 °C. (Right) Concentration-dependent emission
spectra of 2 in CH2Cl2 at 22 °C and in the solid state.
boronic acid, derived from 5,5′-dihexyloxy-2-bromobiphenyl
(9) with 1 equiv of 1,3,5-tribromobenzene in the presence
of Pd(0) catalyst. The desired brombiphenyl 9, in turn, was
obtained by a one-pot Suzuki coupling of 3-bromohexy-
loxybenzene followed by a bromination using NBS in
acetonitrile in excellent yield (see Scheme 3). Indeed, when
10 (R ) n-hexyl) was subjected to an oxidative cyclodehy-
drogenation using FeCl3 in a mixture of dichloromethane-
nitromethane (3:1), it yielded a readily soluble hexahexyloxy-
HBC 2 as a yellow-orange solid in nearly quantitative yield
(see Scheme 3).
The molecular structure of HBC 2 was established by the
18
simplicity of its H/13C NMR spectra and was further
confirmed by MALDI mass spectrometry (see Figure S4,
Supporting Information). Note that a calculated isotope
distribution for mass ion of HBC 2 matches the prediction
quite well (see Figure S4 in the Supporting Information).
As detailed in Scheme 3, the mechanism for conversion
of 10 to HBC 2 simply followed a standard ECEC mecha-
nism applicable to other (oxidative) biaryl syntheses.12,14
Thus, a coupling of an anisyl-type cation radical (I6) with
the central benzene ring produces a distonic cation radical
(I7) which undergoes a ready loss of an electron (I8) followed
by two proton (I9) to form a biaryl-type bond. Multiple
repetitions of the ECEC sequence finally produce HBC 2
(Scheme 3).
1
Acknowledgment. We thank the National Science Foun-
dation (CAREER Award) for financial support and Sergey
V. Lindeman (Marquette University) for X-ray crystal-
lography.
With the hexaalkoxy HBC 2 at hand, we examined its
optical characteristics in dichloromethane at 22 °C as follows.
Supporting Information Available: Preparation and
(14) (a) The distonic radical cations have ample literature precedent in
the syntheses of a variety of biaryls; see: Hammerich, O.; Parker, V. D.
AdV. Phys. Org. Chem. 1984, 20, 55–190, and references cited therein. (b)
For the feasibility of the dicationic interemediates I3 and I8 in Schemes
2and 3, see: Rathore, R.; Magueres, P. L.; Lindeman, S. V.; Kochi, J. K.
Angew. Chem., Int. Ed. Engl. 2000, 39, 809–812.
1
spectral data for various compounds in Schemes 1-3, H/
13C NMR spectra, cyclic voltammogram of 7b and electronic
spectrum of its cation radical, and the X-ray structural data
for 7a. This material is available free of charge via the
Internet at http://pubs.acs.org.
(15) Cyclohexadienyl-type radicals are known to undergo oxidation at
∼0 V (vs SCE); see: Haddon, R. C.; Wudl, F.; Kaplan, M. L.; Marshall,
J. H.; Cais, R. E.; Bramwell, F. B. J. Am. Chem. Soc. 1978, 100, 7629–
7633.
OL8020429
(16) Compare: Feng, X.; Wu, J.; Enkelmann, V.; Mu¨llen, K. Org. Lett.
2006, 8, 1145–1148.
(17) Anderson, N. G.; Maddaford, S. P.; Keay, B. A. J. Org. Chem.
1996, 61, 9556–9559.
(19) Fechtenko¨tter, A.; Saalwa¨chter, K.; Harbison, M. A.; Mu¨llen, K.;
Spiess, H. W. Angew. Chem., Int. Ed. 1999, 38, 3039–3041.
(20) A detailed evaluation of the liquid crystalline properties of 2 and
its derivatives with varying chain lengths will be undertaken, and the results
will be reported in due course.
(18) Owing to the large size of HBC 2, the signals in its 1H and 13C
NMR spectra were broad at 22 °C but, however, were sharpened at higher
temperatures (90 °C); see Figure S5 in the Supporting Information.
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