Chemistry Reference
In-Depth Information
O
O
O
O
Al
2
O
3
Air
+
O
O
83%
96%
O
O
O
17
19
18
20
18
82%
O
O
O
O
O
1)KOH
2
)HOC
l
O
74%
O
O
O
O
O
21
22
Scheme 6 Synthesis of polyacene 22 via Diels-Alder [
25
]
Although milder conditions have been developed for the selective synthesis of
PAHs since then, most of the methods developed during this pioneering work are
still used to the present day. Moreover, progress in analytical methods has greatly
enhanced the characterization of PAHs. The following sections illustrate more
recent syntheses of PAHs, highlighting the advancement in organic chemistry
based on the early roots described above.
2.2 Syntheses via the Diels-Alder Reaction
The Diels-Alder reaction represents one of the most efficient methods for assem-
bling six-membered carbon cycles. As this structural element is the characteristic of
graphene-type structures, Diels-Alder reactions have been applied successfully for
the synthesis of PAHs such as acene and coronene.
Various oligoacenes have been synthesized using quinone derivatives as the
dienophile in Diels-Alder reactions. One example is the synthesis of a series of
epoxypolyacenequinones 19-21 by Miller et al., via repetitive Diels-Alder/oxida-
tion reactions with appropriate bis-dienes and quinones. The ether-bridge could be
cleaved to give the desired acene 22 with up to 11 linearly annulated rings
(Scheme
6
)[
25
]. The final aromatization, however, has not been reported.
Another example of a Diels-Alder reaction of benzoquinone 24 with a
substituted cyclopentadienone 23 is the synthesis of a molecular ribbon (Scheme
7
)
[
26
]. The compound is twisted in two directions and does not adopt a single
longitudinal twist as originally intended. Again, no route for the aromatization has
been described.
Arynes are also highly suitable dienophiles for the synthesis of PAHs via
Diels-Alder reactions [
27
,
28
]. For example, Kitamura et al. reported a new
hypervalent iodide precursor 27 for the in situ generation of naphthyne, which
can be trapped in a Diels-Alder reaction to form PAHs, such as 28 and 29
(Scheme
8
)[
29
].
