Chemistry Reference
In-Depth Information
a
b
1
/
5
2
/
5
2
/
5
2
/
5
3
/
5
Fig. 2 (a) Kekul´ structures of biphenylene and (b) theoretical
p
-bond orders based on the above
structures [
17
]
The authors also considered the effect of the proposed reduced set of Kekul´
structures on the ring currents of biphenylene, following a structural approach in
which aromaticity is a combination of the local ring current of each conjugated
circuit in the resonance structures. This model leads to a reduction in the diamag-
netic current of benzenoid rings to ½ that of benzene, and a paramagnetic current of
⅓
the magnitude of the same reference in the four-membered rings. Therefore,
although fusion of the aromatic and antiaromatic systems results in a reduced
contribution by both, the aromatic character of these PAHs is still dominant.
The revised model also provides promising results when applied to longer [N]
phenylenes. In the case of linear (Fig.
1b
,
n
¼
3) and angular [3]phenylene (Fig.
1c
,
n
3), each have 12 and 13 Kekul
´
structures respectively. Elimination of those
that do not contain at least three Clar sextets leaves only eight possible structures
for each. The resonance energy determined from these revised structures (by
summing the positive contribution of 4
n
¼
þ
2
p
-electrons and the negative contri-
bution of 4n
-electrons) is indeed higher for angular [3]phenylene, consistent with
previous experimental and theoretical findings.
p
2.1 Recent Synthetic Developments in [N]Phenylenes
As mentioned previously, the most general route to accessing a wide variety of
linear, angular, zigzag, and branched [N]phenylenes is via the appropriate
phenylethynyl precursor and subsequent [2
2] cycloaddition. This chemis-
try is relatively well-established through work by Vollhardt and coworkers and has
been reviewed previously [
12
]; therefore this chapter will focus on developments
made in the last 10 years.
Several derivatives of bent [N]phenylenes have been reported by Vollhardt and
coworkers since 2002. Bent [4]phenylene (2), the final isomer of the [4]phenylene
family, was synthesized in 2002 (Scheme
2
a) [
27
]. The initial step in the synthesis
involved non-selective Pd-catalyzed Sonogashira coupling of 2,3-diiodobiphenylene
with
mono
-silated
o-
diethynyl benzene, followed by a second Sonogashira coupling
with trimethylsilylacetylene and subsequent deprotection to give the desired triyne
precursor 1. Cobalt-catalyzed [2
þ
2
þ
2] cyclization of 1 yielded the air-sensitive
compound 2a in 1.7% overall yield over eight steps.
An alternative synthesis, resulting in the relatively more stable bis-
(trimethylsilyl)-2 (2b) and involving double [2
þ
2
þ
þ
2
þ
2] cycloaddition, was also
