Angewandte
Communications
Chemie
Cycloaddition Reactions
Abstract: A novel strategy for the synthesis of partially
from relatively simple substrates under mild reaction
[
9]
saturated acene derivatives has been developed based on
conditions, and they have been successfully applied to the
I
[10]
a Au -catalyzed cyclization of 1,7-enynes. This method
synthesis of functionalized aromatic frameworks. We have
provides straightforward access to stable polycyclic products
featuring the backbone of the acene series, up to nonacene.
reported the cyclization of 1,6- and 1,7-enynes bearing an aryl
substituent at the alkyne terminus in the presence of gold(I),
respectively affording naphthalene and polyhydrogenated
anthracene derivatives through formal [4+2] cycloaddi-
A
cenes—a class of polycyclic aromatic hydrocarbons
PAHs) made up of n linearly fused benzene rings—have
[
11]
(
tions.
The cyclizations of certain 1,7-enynes, bearing an
been extensively studied in recent years on account of their
aryl group bonded to the alkene, also afford polyhydrogen-
ated anthracenes in the presence of gold(I) at high temper-
atures. Herein, we report ready access to stable function-
alized higher hydroacenes 1 through the gold(I)-catalyzed
cyclization of suitable 1,7-enynes in which the alkene is part of
an enol ether function.
distinctive electronic properties, which make them attractive
[
1]
[12]
candidates for use in molecular electronics. However, the
application of the higher acenes (n ꢀ 6) as functional
materials is limited by the rapid decrease of both solubility
[
2]
and stability as the number of annealed rings grows.
Although syntheses of the parent acenes have been reported
up to and including nonacene (n = 9), the low stability of the
higher acenes makes isolation a formidable challenge, requir-
We envisioned that the gold(I)-catalyzed cyclization of
the 1,7-enynes that result from a palladium-catalyzed
Sonogashira cross-coupling between an aryl iodide and key
precursors 2, would afford hydroacenes 1 upon aromatization
by elimination of a molecule of methanol (Scheme 1). By
combining these two robust and broad scope metal-catalyzed
methods, a wide variety of linear hydroacenes 1 could in
principle be obtained by annulation of a wide range of readily
available aryl iodides.
[
3]
ing inert matrices. One of the most common strategies to
curb the intrinsic photo-instability of the higher acenes is the
[4]
attachment of suitable stabilizing substituents, predomi-
nantly bulky groups close to the most reactive central rings.
Nevertheless, many of these derivatives still suffer from
decomposition, even in dilute solution. Another approach to
circumvent acene instability is the use of protecting groups
that reduce their reactivity, allowing for long-term storage as
well as imparting additional solubility, before deprotection
[
3,5]
reveals the acene. In this regard, partially saturated acenes,
which have been extensively employed as direct precursors of
[
3c,6]
the corresponding fully conjugated acenes,
can be consid-
[
7]
ered to be “hydrogen-protected”, exhibiting improved
solubilities and excellent stabilities. The synthesis of partially
hydrogenated acene derivatives has been achieved by direct
Scheme 1. Conceptual approach to hydroacenes.
[
7,8]
reduction of the corresponding acenes or quinones.
How-
ever, these methods often require harsh conditions and are
prone to produce regioisomeric mixtures. Therefore, general
methods to selectively obtain partially saturated acenes still
remain elusive.
Cyclizations of enynes catalyzed by gold(I) complexes
have emerged over the last decade as one of the most
powerful tools to construct complex polycyclic architectures
The simplest 1,7-enyne 3a, which was assembled from
iodobenzene and 2a, was chosen as the model substrate to
explore the gold(I)-catalyzed cyclization to form
5,12-dihydrotetracene (1a; Table 1). The cyclization of 3a
was first examined at 258C in the presence of cationic gold(I)
complexes A–C (10 mol%), spanning a wide range of
electrophilicity. Gratifyingly, all three gold complexes
successfully delivered the desired dihydrotetracene 1a as
the major product. The most electrophilic catalyst C caused
the concomitant formation of a rearrangement byproduct in
approximately 15% yield, as determined by NMR spectros-
copy, whereas the only identifiable byproduct in the reactions
with complexes A and B was tetracycle 1a’, which is an
[
*] R. Dorel, Dr. P. R. McGonigal, Prof. Dr. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Barcelona Institute of Science and Technology
Av. Paꢀsos Catalans 16, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
Prof. Dr. A. M. Echavarren
Departament de Quꢁmica Orgꢂnica i Analꢁtica
Universitat Rovira i Virgili
[13]
intermediate during the formation of 1a. Thus, commer-
I
cially available JohnPhos–Au catalyst A was selected for
further optimization. Lowering the catalyst loading to
C/ Marcel·lꢁ Domingo s/n, 43007 Tarragona (Spain)
5
mol% (Table 1, entry 4) increased the amount of 1a’ that
remained, even when extended reaction times were
employed. Complete consumption of 1a’ was achieved by
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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