Aryne-mediated syntheses of structurally related acene derivatives

Article abstract of DOI:10.1039/c003422n

Cycloaddition reactions of tetrakis(hexyloxy)-substituted 2,3-triphenylyne (2,3-didehydrotriphenylene) provide straightforward access to three new cata-condensed polyarenes characterized by the presence of 5, 8 and 11 fused benzene rings. The Royal Societ

Full text of DOI:10.1039/c003422n

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Aryne-mediated syntheses of structurally related acene derivatives†‡
Diego Rodr´ıguez-Lojo, Diego Pen˜a,* Dolores Pe´rez* and Enrique Guitia´n
Received 19th February 2010, Accepted 11th June 2010
First published as an Advance Article on the web 23rd June 2010
DOI: 10.1039/c003422n
Cycloaddition reactions of tetrakis(hexyloxy)-substituted
2,3-triphenylyne
(2,3-didehydrotriphenylene)
provide
straightforward access to three new cata-condensed
polyarenes characterized by the presence of 5, 8 and 11
fused benzene rings.
Large polycyclic aromatic hydrocarbons (PAHs) characterized by
the fusion of various benzene rings are prominent among the most
interesting materials studied in the fields of molecular electronics,
supramolecular chemistry and materials science.1 The recent
entry onto the scientific scene of graphene,2 the 2D counterpart
of graphite that is structurally related to PAHs, has led to a
remarkable renaissance in the synthesis of polyarenes formed by
the fusion of a large number of benzene rings. Cycloaddition
reactions of arynes have been widely used for the preparation
of PAHs.3 In particular, the use of polycyclic arynes has allowed
the synthesis of a variety of structurally fascinating polycyclic
aromatic compounds.4
Fig. 1 Structures of triphenylyne 1 and polyarenes 2–4.
Despite the progress in computational studies it is still challeng-
ing to predict how small changes in the structure of molecules
can modify properties such as aggregation, deposition or the
solid-state packing of complex molecules, and ultimately the
macroscopic behaviour of molecular materials. In this context
it would be useful to obtain a series of molecules with gradual
structural changes in order to evaluate the influence of such
modifications in structure. After several years working on the
development of new strategies for the synthesis of PAHs,5 we
decided to prepare a small group of molecules with gradual
changes in topology, which should in turn affect properties such
as planarity and conjugation.
Scheme 1 Synthesis of substituted benzo[b]triphenylene 2.
Generation of this aryne by treatment of triflate 5 with
tetrabutylammonium fluoride (TBAF) in the presence of
commercially
available
2,3,4,5-tetraphenylcyclopenta-2,4-
Recently, our group described the use of substituted triph-
enylyne 1 (Fig. 1) to yield polyarenes with 5, 7, 9, 10 and 13
fused benzene rings by palladium-catalyzed [2+2+2] cycloaddition
reactions.5c We describe here the use of the same aryne to afford
three new cata-condensed acene derivatives, namely compounds
2, 3 and 4 with 5, 8 and 11 fused benzene rings, respectively. We
envisaged that the presence of four hexyloxy groups attached to
the polyaromatic cores of compounds 2–4 would provide enough
solubility to manipulate and to characterize these large polyarenes.
Based on our experience in this field, we chose
o-(trimethylsilyl)aryl triflate 5 (Scheme 1) as the precursor for
dienone (6) in THF afforded substituted benzo[b]triphenylene
2 in 72% yield. Presumably, intermediate 7, resulting from
the [4+2] cycloaddition of aryne 1 with dienone 6, evolves in
situ to afford polyarene 2 by cheletropic extrusion of carbon
monoxide.6 This methodology to obtain a benzo[b]triphenylene
core has been previously used by Pascal and co-workers to isolate
9,10,11,12,13,14-hexaphenylbenzo[b]triphenylene in 24% yield
by reaction of 6 with 1,4-diphenyl-2,3-triphenylyne, generated
by thermal decomposition of the corresponding diazonium salt
precursor.4a
Similarly, Diels–Alder cycloaddition of triphenylyne 1 with
cyclopentadienone 8 (phencyclone), followed by CO extrusion6
led to substituted tetrabenzo[a,c,j,l]tetracene 3 with 8 fused
benzene rings in 55% yield (Scheme 2). Notably, Pascal and
co-workers succeeded in preparing highly twisted 9,10,19,20-
tetraphenyltetrabenzo[a,c,j,l]tetracene in 1–2% yield by reaction
of 8 with 1,4-diphenyl-2,3-triphenylyne.4a Curiously, while the
tetracene derivative isolated by them decomposes slowly in so-
lution, polyarene 3 proved to be stable under ambient conditions.
6,7,10,11-tetrakis(hexyloxy)-2,3-didehydrotriphenylene
(1).5c
Departamento de Qu´ımica Orga´nica, Universidad de Santiago de Com-
postela, 15782 Santiago de Compostela, Spain. E-mail: diego.pena@
usc.es, dolores.perez@usc.es; Fax: +34 981591014
† This article is part of an Organic & Biomolecular Chemistry web theme
issue which highlights work by participants in the 2009 EuCheMS young
investigators workshop.
‡ Electronic supplementary information (ESI) available: Experimental and
spectroscopic data. See DOI: 10.1039/c003422n
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Scheme 2 Synthesis of substituted tetrabenzo[a,c,j,l]tetracene 3.
By contrast, pentahelicene 4 (with 11 fused benzene rings)
was prepared by palladium-catalyzed [2+2+2] cocycloaddition
of triphenylyne 1 and two molecules of 9,10-phenanthryne (10,
Scheme 3). In particular, treatment of triflate 5 with CsF in
the presence of an excess of the 9,10-phenanthryne precursor
9 (ratio 1 : 4) and 5 mol% of Pd2(dba)3 led to the isolation of
substituted dibenzo[f ,j]triphenyleno[2,3-s]picene 4 in 30% yield
together with hexabenzotriphenylene, the phenanthryne trimer-
ization product.5a It should be noted that this route provides
straightforward access to a hitherto unprecedented polyaromatic
core by reaction of two different short-lived intermediates (arynes
1 and 10) generated in one-pot.
Fig. 2 Absorption spectra of polyarenes 2 ( ◊ ◊ ◊ ), 3 ( ) and 4 (---) in
CH2Cl2 solution (10 mM).
bathochromic shift when compared with that of unsubstituted
tetrabenzo[a,c,j,l]tetracene (lmax = 328 nm).8
The differences arising from the changes in the aromatic core
were also evident in the emission spectra of these compounds
in DCM solution (1 mM) under excitation at the corresponding
absorption maxima (Fig. 3). While polyarenes 2 and 4 have
fluorescence maxima at 460 and 474 nm, respectively, tetrabenzote-
tracene 3 showed a red-shift in the emission maximum to 501 nm.
Notably, pentahelicene 4 shows a narrow structured band with a
shoulder at 501 nm, probably due to the structural restrictions
of its non-planar aromatic core. Moreover, bearing in mind
that unsubstituted benzo[b]triphenylene exhibited luminescence at
379 nm,9 phenyl and hexyloxy substituents attached to polyarene
2 are responsible for a large bathochromic shift (81 nm) in
the emission maximum of this compound compared with its
unsubstituted parent.
Scheme 3 Synthesis of substituted pentahelicene 4.
Compounds 3 (C70H76O4) and 4 (C70H74O4) can be considered as
partially cyclodehydrogenated derivatives of benzo[b]triphenylene
2 (C70H78O4). However, the problems in controlling the chemos-
electivity in metal- or photo-promoted cyclodehydrogenation
reactions would make these alternatives to obtain compounds 3
and 4 particularly arduous.7
Despite the fact that the differences in the molecular formulae
of acene derivatives 2–4 are very small, the changes in the
aromatic cores have a marked effect on their absorption spectra in
solution (Fig. 2). In particular, while the major absorption band of
compound 2 falls at 328 nm, polyarenes 3 and 4, with an increased
number of fused benzene rings, have red-shifted absorption
maxima at 354 and 362 nm, respectively. The UV-vis spec-
tra of substituted 10,11,12,13-hexaphenylbenzo[b]triphenylene 2
(DCM, lmax = 380, 328, 314, 302 nm) and 9,10,11,12,13,14-
hexaphenylbenzo [b]triphenylene (heptane, lmax = 376, 328, 318
nm) isolated by Pascal and co-workers,4a are very similar. By
contrast, the major absorption band of alkoxy-substituted 10,19-
diphenyltetrabenzo[a,c,j,l]tetracene 3 (lmax = 354 nm) shows a
Fig. 3 Emission spectra of polyarenes 2 ( ◊ ◊ ◊ ), 3 ( ) and 4 (---) in CH2Cl2
solution (1 mM).
In conclusion, three structurally related cata-condensed pol-
yarenes characterized by the presence of 5, 8 and 11 fused benzene
rings have been obtained by cycloaddition reactions of the same
tetrakis(hexyloxy)-substituted 2,3-triphenylyne 1. Benzotripheny-
lene 2 and tetrabenzotetracene 3 were obtained from triflate 5
by a tandem one-pot fluoride-induced aryne generation/Diels–
Alder reaction/CO cheletropic extrusion, while pentahelicene 4,
characterized by a hitherto unprecedented polyaromatic core,
was prepared by a tandem one-pot fluoride-induced aryne
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generation/Pd-catalyzed cocyclotrimerization sequence. Remark-
ably, the differences in the structures of the aromatic cores of
these closely related polyarenes became evident on comparing their
absorption and emission spectra. Studies on the supramolecular
behaviour of these acene derivatives with gradual increase of
planarity are in progress and the results will be reported in due
course.
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Acknowledgements
Financial support from the Spanish Ministry of Education and
Science (MEC, CTQ2007-63244) and the Xunta de Galicia (DX-
PCTSUG2007/090 and PGIDIT07PXIB209139PR) is gratefully
acknowledged. D. R.-L. thanks MEC for the award of an
FPU fellowship. We thank Prof. M. Mosquera (Universidade de
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