Chemistry Reference
In-Depth Information
Fig. 15 X-Ray crystal structures of (a) 66a,(b) 68a,(c) 70b, and (d) selected bond lengths.
Crystal structure data from [
94
,
95
] was gathered from the Cambridge Structural Database (
CSD
)
aromaticity (Fig.
15
). The calculated NICS values for unsubstituted 56b are
NICS
6.23 for the benzene
rings, which is approximately one-third of the value for pentalene and one-half of
the value for benzene, respectively [
94
]. The difference in length of the bond fusing
the five- and six-membered rings (bond “a”) is notably shorter in 68a, and longer in
70a, than in 66a [
95
]. In 68a, the shorter length indicates decreased bond alternation
and therefore increased 8
¼þ
5.88 for the pentalene fragment and NICS
¼
-electron character in the pentalene unit. Considering the
resonance structures shown in Scheme
20
b, 68a has decreased antiaromatic char-
acter but this structure negates the stabilization of a Clar sextet in the terminal ring.
For 70a it is possible to imagine a resonance structure that benefits from both
decreased double bond character of bond “a” and a Clar sextet in the terminal ring
as drawn in Scheme
20
c.
Independently from the work reported above, Tilley and coworkers developed a
related synthetic methodology using palladium as the catalyst [
97
]. The yields
reported using these conditions were generally higher than those reported by
Kawase and coworkers (Scheme
21
a). Additionally, it was demonstrated that the
Pd-catalyzed conditions can be used to efficiently synthesize other pentalene-
containing compounds di[2,3]-thieno[
a,e
]pentalene 76 and hexaaryl-pentalene 78.
The following year Tilley and coworkers reported a method for synthesizing
PAHs containing multiple pentalene units [
98
]. The synthesis of 81 is shown in
Scheme
22
as an example of the methodology also used to synthesize PAHs 82-84.
Bromostilbene derivatives such as 79 were synthesized by Horner-Wadsworth-
Emmons reaction of the appropriately substituted xylene. Heck conditions were
then used to couple 79 with diphenylacetylene to give
s
-indacene 80. Oxidative ring
closure using FeCl
3
then yielded the desired PAH. This reaction sequence allowed
access to PAHs with a variety of topologies (Fig.
16
).
p
