Chemistry Reference
In-Depth Information
twisted (
t
)
anti
-folded (
a
)
syn
-folded (
s
)
folded (
f
)
Fig. 2 Schematic representation of the fundamental BAE conformations
The overall conformations of BAEs may be visualized in a qualitative way as
shown in Fig.
2
. The most important conformations are the twisted conformation
(t), the
anti
-folded conformation (a), the
syn
-folded conformation (s), and the
folded conformation (f). The folded conformation is found in BAEs with different
bridges X
6¼
Y and characterized by one strongly folded tricyclic moiety, while the
other moiety remains (almost) planar. A typical example is the folded
fluorenylidene-xanthene (5) (see Sect.
2.3
). The schematic representations are
derived from Newman projections along the C
9
¼
C
9
0
double bond. However, the
lines represent the conformations of the tricyclic moieties rather than the direction
of the bonds attached to the central double bond (as in the original Newman
projection). The schematic representations illustrate the degrees of twist and fold-
ing in a BAE conformation. Note that this necessarily is a simplified picture, which
cannot adequately represent all the important features of the 3D structures.
O
5
The structural framework of bistricyclic aromatic enes is eminently suited to
stabilize several non-planar conformations. Bistricyclic aromatic enes are an attrac-
tive field for the study of non-planar ground state conformations, conformational
equilibria, the interrelations of color and conformation, functional molecular mate-
rials, and dynamic stereochemistry [
1
,
2
,
26
,
27
].
BAEs may be also viewed as tetrabenzo[
m
,
n
]fulvalenes (aka tetrabenzo-
fulvalenes) or as doubly bridged tetraarylethylenes with bridges X and Y. They
can be classified as
homomerous
bistricyclic enes (1,X
¼
Y) and
heteromerous
bistricyclic enes (1,X
Y) [
28
]. The topic of bistricyclic aromatic enes has
previously been reviewed [
1
-
3
,
27
,
29
,
30
].
6¼
