Chemistry Reference
In-Depth Information
Fig. 1 Structures of
(a) cyclobutadiene [
11
];
(b) linear and (c) angular [N]
phenylenes [
12
]
b
c
a
n-2
n-2
Scheme 1 Catalytic cycle
in the metal-catalyzed
[2 + 2 + 2] cycloaddition of
aryl alkynes to yield [N]
phenylenes [
12
]
R
2
R
2
R
3
M(L)
n
R
3
R
1
R
1
R
2
R
2
R
3
R
2
R
2
R
3
M(L)
n-2
M(L)
n-1
R
1
R
1
R
2
R
2
R
2
M(L)
n-1
R
3
R
3
R
1
R
2
Other classic methods to access this scaffold include the dimerization of arynes or
contraction of bridged biaryls through extrusion of a small molecule such as N
2
,
CO
2
,orCO[
14
,
15
]. Over the last several decades the synthesis has been dominated
by the metal-catalyzed [2
2] cycloaddition methodology (Scheme
1
), devel-
oped primarily by Vollhardt and coworkers [
12
]. This approach has improved
the synthesis of biphenylene as well as provided access to longer derivatives
.
It has been demonstrated both experimentally and theoretically that the
þ
2
þ
p
-bonds
in these systems tend to localize within the six-membered rings,
thereby
minimizing the 4
antiaromatic character of the cyclobutadiene linkage [
16
].
In the past the model of conjugated circuits, traditionally employed to determine
the resonance energy of PAHs, has failed to explain some of the properties observed
in [N]phenylenes [
17
]. Most significantly, the model does not account for the
increased stability of angular [N]phenylenes (Fig.
1c
) over linear [N]phenylenes
(Fig.
1b
)[
12
,
18
-
23
]. Recently, Randi´ and coworkers have sought to develop a
revised theory to determine the aromaticity of [N]phenylenes that can compensate
for this discrepancy [
17
].
The five possible Kekul
´
resonance structures of biphenylene are shown in
Fig.
2a
. According to these structures, the C-C bond connecting the two benzenoid
rings should have a bond order of 1½. In reality, this bond has been shown to have
even less double bond character than this value suggests. The authors propose
excluding the fifth and final structure that does not contain any Clar sextets
[
24
-
26
] as a contributor to the resonance hybrid. This leads to 1½ bond order for
all
p
bonds except for the connecting CC bond with a bond order of 1. This solution
appears to improve the theory for one aspect of the observed bond lengths in
biphenylene; however it does not yet account for the decreased bond alternation
within the benzenoid rings.
p
