Chemistry Reference
In-Depth Information
a
b
Fig. 14 Structures of (a) antiaromatic pentalene [
81
-
87
] and (b) dibenzo[
a,e
]pentalene [
88
]
Path (a):
O
R
1)
RMgX
56a
:R=aryl,alkyl
56b
:R=H
2) H
+
R
O
Path (b): PCl
5
1) Br
2
2) CH
3
CO
2
Ag
Cl
Cl
Zn
Cl
Cl
Scheme 17 Original synthetic routes to (
a
) 5,10-disubstituted dibenzo[
a,e
]pentalene 56a [
89
],
and (
b
) unsubstituted dibenzo[
a,e
]pentalene 56b [
90
]
Analogous to the relationship between cyclobutadiene and [N]phenylenes, it is
possible to synthesize stable ladder structures by fusing aryl rings to a pentalene core
resulting in diaryl[
a,e
]pentalene (DAP, Fig.
14b
)[
88
]. The first example of a 5,10-
substituted dibenzo[
a,e
]pentalene (56a) was reported in 1912 [
89
], while the parent
hydrocarbon (56b) was not synthesized until 1952 (Scheme
17
)[
90
]. Both synthetic
routes relied on the diketone shown in Scheme
17
as the starting material and did not
provide much versatility beyond substitution at the C5/10 position. Increasing interest
in PAHs as materials for optoelectronic applications during the last decade has caused
a resurgence in research on the synthesis and properties of DAPs.
3.1 Recent Synthetic Developments in Diaryl[a,e]pentalenes
Several different syntheses of DAPs have been reported since 2008. Seeking to
investigate the reactivity of strained alkynes towards nucleophiles, Orita, Otera, and
coworkers discovered an efficient method for synthesizing unsymmetrical dibenzo
[
a,e
]pentalenes [
91
]. A nucleophile, such as an alkyl or aryllithium, adds across one
triple bond of 5,6,11,12-tetradehydrodibenzo[
a,e
]cyclooctene (57), generating a
vinylic anion (Scheme
18
a). The anion can then add across the remaining triple
bond and is quenched with an electrophile (“E”). Yields were high in each case,
varying both the nucleophile and the electrophile, ranging from 57% to 76%. Using
iodine as the electrophile resulted in vinyl iodide 58a, which could then be used
in metal-catalyzed cross-coupling reactions to synthesize additional derivatives
(Scheme
18
b).
