Facile synthesis and X-ray structure of alkoxy-functionalized dibenzo[fg,op]naphthacenes

Article abstract of DOI:10.1021/ol034626g

1,3-Bis(2-bromophenyl)-2,5-diphenylbenzenes are readily available by the condensation of phenylacetates with the corresponding pyrylium salts and undergo a palladium-catalyzed dehydrohalogenation to give functionalized dibenzo[fg,op]naphthacenes.

Full text of DOI:10.1021/ol034626g

ORGANIC
LETTERS
2003
Vol. 5, No. 15
2587-2589
Facile Synthesis and X-ray Structure of
Alkoxy-Functionalized
Dibenzo[fg,op]naphthacenes
,†
Xiao Hong Cheng,† Sigurd Ho1ger,* and Dieter Fenske‡
Polymer-Institut, UniVersita¨t Karlsruhe, Hertzstrasse 16, 76187 Karlsruhe, Germany,
and Institut fu¨r Anorganische Chemie, UniVersita¨t Karlsruhe, Engesserstrasse 15,
76128 Karlsruhe, Germany
hoeger@chemie.uni-karlsruhe.de
Received April 11, 2003
ABSTRACT
1,3-Bis(2-bromophenyl)-2,5-diphenylbenzenes are readily available by the condensation of phenylacetates with the corresponding pyrylium
salts and undergo a palladium-catalyzed dehydrohalogenation to give functionalized dibenzo[fg,op]naphthacenes.
Polycyclic aromatic hydrocarbons (PAHs) played a major
role in the development of industrial organic chemistry.1
During the last years, the interest in these compounds was
renewed because of their electronic and optoelectronic
properties.2 In general, columnar packing motives with a
strong π-π overlap are found only in materials with a
minimum size of the polycyclic aromatic backbone. To keep
these structures tractable solubilizing side chains have to be
attached to the aromatics. Therefore, methods which are mild
enough to prepare the polycyclic backbone in the presence
of the side chains as well as strategies for the simple synthesis
of the noncondensed precursors are of high interest both for
the extension of the existing synthetic toolkit and for the
preparation of functional materials.3 Recently, we have
explored the synthetic potential of the transformation of
2,4,6-triarylpyrylium salts to the corresponding 1,2,3,5-
tetraarylbenzenes by the condensation with sodium phenyl-
acetate.4 We could show that this reaction is compatible with
the presence of halogens in the compounds and could use
the resulting functionalized 4,4′′-diiodo-m-terphenyls for the
synthesis of shape-persistent macrocycles.5
† Polymer-Institut.
‡ Institut fu¨r Anorganische Chemie.
(1) . (a) Clar, E. Polycyclic Hydrocarbons; Academic Press: New York,
1964. (b) Zander, M. Polycyclische Aromaten; Teubner: Stuttgart, Germany,
1995. (c) Harvey, R. G. Polycyclic Aromatic Hydrocarbons; Wiley-VCH:
New York, 1997.
(2) (a) Adam, D.; Schuhmacher, P.; Simmerer, J.; Ha¨ussling, L.;
Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994,
371, 141. (b) Schmidt-Mende, L.; Fechtenkotter, A.; Mu¨llen, K.; Moons,
E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (c) Percec,
V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.;
Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-
W.; Hudson, S. D.; Duan, H. Nature 2002, 417, 384.
This led us to the idea that functionalized 1,2,3-triaryl-
benzenes containing properly located bromo or iodo groups
might undergo a transition metal-catalyzed dehydrohaloge-
(3) Hagen, S.; Hopf, H. Top. Curr. Chem. 1998, 196, 45.
(4) Zimmermann, T.; Fischer, G. W. J. Prakt. Chem. 1987, 329, 975.
(5) Ho¨ger, S.; Rosselli, S.; Ramminger, A.-D.; Enkelmann, V. Org. Lett.
2002, 4, 4269.
10.1021/ol034626g CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/21/2003

Scheme 1. Synthesis of the Dibenzo[fg,op]naphthacenes
nation to form functionalized dibenzo[fg,op]naphthacenes.
Dibenzo[fg,op]naphthacenes were obtained in variable
yields depending on the substitution pattern of the phenyl
acetate (isolated yields of the main reaction products are
given in Scheme 1). Good yields were obtained for 4a and
moderate yields for 4b and 4c, while in the case of 5a and
5b only the formation of minor amounts of the corresponding
triphenylenes was observed. The product distribution and
yield can be explained by the assumption that after the
oxidative addition of the Pd(0) to the arylbromide the newly
formed Pd(II) species electrophilicly attacks the phenyl ring
containing the R′ and R′′ substituents.8 Alternatively, deha-
logenation leads to the dehalogenated products. The actual
outcome of the reaction depends strongly on the steric and
electronic properties of the electrophilicly attacked aryl ring.
If its electron density is reduced (as going from 4a to 5a) or
if steric crowding (5b) hinders a fast attack, the competitive
dehalogenation becomes the predominant reaction. Exchange
of the catalyst did not improve the yields. When the
cyclizations were performed with (P(o-tol)3)2PdCl2 under
otherwise identical conditions, the yield of the main product
decreased from 42% to 24% (4a) and from 7% to 5% (5a),
respectively.
So far only two procedures for the synthesis of alkoxy-
functionalized dibenzo[fg,op]naphthacenes have been de-
scribed, both of them leading to highly substituted materials
by using an oxidative dehydrogenation reaction.6 Here we
show that these aromatics with only one alkoxy group at a
defined position are available by a simple Pd(0)-catalyzed
dehydrohalogenation procedure. Key compounds in our
synthesis are the pyrylium salts 2 containing bromine at the
2 and 2′′ position. Pyrylium salts with substituents in this
position are only rarely described and initial attempts to
synthesize them directly from 2-bromo acetophenone and
the corresponding benzaldehyde failed. In contrast, accept-
able to good yields were obtained by applying a two-step
route preparing first the 1,5-diketone 1 and then performing
the cycloaromatization with chalcone as oxidant (Scheme
1). Although first attempts to prepare 1 under conventional
conditions (EtOH, aq NaOH, reflux) gave the product in only
8% yield, solvent-free conditions improved the yield dra-
matically up to 55%.7 Condensation of 2 with the corre-
sponding sodium phenyl acetates formed the aromatic
dibromides in good yields. The dehydrohalogenation was
performed by heating 3 in dimethylacetamide (DMA) in the
presence of DBU and (PPh3)2PdCl2 at 160 °C for 12 h. The
end of the reaction becomes easily visible by the precipitation
of palladium black in the former slightly tan solution.
4a was crystallized from benzene/CHCl3 to afford slightly
yellow crystals suitable for X-ray investigation (Figure 1).9
The nearly perfectly planar polycyclic aromatics form infinite
stacks which are arranged in a herringbone-like motif. The
(6) (a) Musgrave, O. C.; Webster, C. J. Chem. Commun. 1969, 712. (b)
Kumar, S.; Naidu, J. J.; Rao, D. S. S. J. Mater. Chem. 2002, 12, 1335.
(7) Cave, G. W. V.; Raston, C. L.; Scott, J. L. Chem. Commun. 2001,
2159.
(8) (a) Mart´ın-Matute, B.; Mateo, C.; Ca´rdenas, D. J.; Echavarren, A.
M. Chem. Eur. J. 2001, 7, 2341. (b) Hennings, D. D.; Iwasa, S.; Rawal, V.
H. J. Org. Chem. 1997, 62, 2. (c) Echavararren, A. M.; Go´mez-Lor, B.;
Gonza´les, J. J.; de Frustos, OÄ . Synlett 2003, 585.
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Org. Lett., Vol. 5, No. 15, 2003

4a reminiscent of that of graphite with a center-to-center
offset of about 3.8 Å.11 Remarkable is the small intermo-
lecular distance between C19 and C23 (3.29 Å), C8 and C24
(3.32 Å), and C4 and C21 (3.34 Å), all below the layer
distance in graphite (3.35 Å).12
All in all we present here a simple approach toward
alkoxy-functionalized dibenzo[fg,op]naphthacenes. Current
activities study the scope and limit of this synthetic meth-
odology as well as the incorporation of the these PAHs into
complex molecular architectures. This includes also the
reaction sequence OMe f OH f OTf that allows further
transition metal-catalyzed C-C coupling reactions.13
Acknowledgment. The authors thank the Deutsche For-
schungsgemeinschaft, the Fonds der Chemischen Industrie,
and the BMBF for support.
Supporting Information Available: Experimental pro-
cedures and characterization as well as X-ray data for 4a.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 1. Crystal structure of 4a (view of two adjacent molecules
projected onto the PAH planes showing the displacement of the
molecules in the stack).
OL034626G
attached phenyl substituent is disordered, adopting torsion
angles of 21.8° and -27.4°, respectively, relative to the
polycyclic part of the molecule. The bond lengths of the
dibenzo[fg,op]naphthacene agree with the local molecular
symmetry and are in accordance with Clar’s sextet rule.10
Figure 1 displays the normal projection of two molecules of
(10) Selected bond distances (Å): C(1)-C(2) 1.387(3), C(1)-C(22)
1.382(2), C(2)-C(3) 1.385(2), C(3)-C(4) 1.462(2), C(3)-C(23) 1.419(2),
C(4)-C(5) 1.405(2), C(4)-C(9) 1.404(2), C(5)-C(6) 1.371(3), C(6)-C(7)
1.386(3), C(7)-C(8) 1.374(3), C(8)-C(9) 1.401(2), C(9)-C(10) 1.464(2),
C(10)-C(11) 1.392(2), C(10)-C(24) 1.411(2), C(11)-C(12) 1.390(3),
C(12)-C(13) 1.388(3), C(12)-C(26) 1.484(2), C(13)-C(14) 1.383(3),
C(14)-C(15) 1.461(2), C(14)-C(24) 1.420(2), C(15)-C(16) 1.410(2),
C(15)-C(20) 1.397(2), C(16)-C(17) 1.366(3), C(17)-C(18) 1.389(3),
C(18)-C(19) 1.367(3), C(19)-C(20) 1.406(2), C(20)-C(21) 1.461(2),
C(21)-C(22) 1.390(2), C(21)-C(23) 1.415(2), C(23)-C(24) 1.431(2).
(11) Goddard, R.; Haenel, M. W.; Herndon, W. C.; Kru¨ger, C.; Zander,
M. J. Am. Chem. Soc. 1995, 117, 30.
(9) Crystal data for 4a: C32H22O, monoclinic, space group P21/c (no.
14), Z ) 4, a ) 23.903(5) Å, b ) 5.0540(10) Å, c ) 17.002(3) Å, â )
92.29(3)°, V ) 2052.3(7) Å3, Dc ) 1.367 g cm-3, µ(Mo KR) ) 0.082 mm-1
;
T ) 200 K. Data collection: Stoe-IPDSII; 2θmax ) 63.3°; 14344 reflections
and 4893 with I >2σ(I); 438 parameters (C, O anisotrope, H isotrope);
maximum residual electron density 0.24 e Å-3; R1 ) 0.062, wR2 ) 0.19.
The structure was solved and refined on F by using the SHELXS-97
program. CCDC reference number CCDC 201035.
(12) The intermolecular distance between C(25) and O(1) (4.15 Å) is to
long for a hydrogen bond that it might be responsible for a stabilization of
the packing motive. However, it exists as a short intercolumnar contact
between H(5) and O(1) (2.51 Å).
(13) Cheng, X. H.; Ho¨ger, S. In preparation.
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