Chemistry Reference
In-Depth Information
Table 26 Conformations and point groups of
Z
- and
E
-disubstituted homomerous BAEs
Z-Disubstituted
E-Disubstituted
Planar
p
Z
-
C
2v
(
y
), p
Z
-
C
s
(
yz
)
p
E
-
C
2h
(
x
), p
E
-
C
s
(
yz
)
Twisted
ta
Z-RPR
0
-
C
2
(
y
)
ts
E-RR
0
-
C
2
(
x
)
anti
-Folded
at
Z-RPR
0
-
C
2
(
y
)
a
E-RS
0
-
C
i
syn
-Folded
s
Z-RS
0
-
C
s
(
xy
)
st
E-RR
0
-
C
2
(
x
)
ft
Z-RPR
0
-
C
1
a
ft
Z-RPS
0
-
C
1
b
ft
E-RPS
0
-
C
1
a
ft
E-RPR
0
-
C
1
b
Unequally folded/twisted
a
The relative direction of folding of the two moieties is
anti
b
The relative direction of folding of the two moieties is
syn
In disubstituted heteromerous BAEs all non-planar conformations necessarily
have
C
1
symmetry. In monosubstituted BAEs, all non-planar conformations
have
C
1
symmetry, with the exception of the orthogonally twisted conformation
t
⊥
-
C
s
(
d
). Planar conformations of disubstituted heteromerous BAEs and
monosubstituted BAEs have
C
s
symmetry (cf. Table
4
).
Because of the reduced symmetry, an analysis based on the molecular symmetry
groups of heteromerous and/or (di)substituted BAEs would yield substantially less
information as compared to the above analysis of unsubstituted homomerous BAEs.
Therefore, it is not developed here. In many cases, a qualitative approach may be
more promising. A substitution of a homomerous BAE, e.g., in 2- and 2
0
-positions,
should lead only to small changes in energy and geometry. Such a situation has
been termed 'slightly distorted skeleta' [
260
]. Mislow et al. introduced the concept
of pseudo-symmetry for substituted systems [
287
]. All conformations should
remain similar (in energy as well as in geometry) to those of the unsubstituted
parent system. In particular, the mechanisms of the conformational isomerizations
and the corresponding transition states should remain similar. Thus considerably
more information may be gained by studying the dynamic stereochemistry based on
the molecular symmetry group of the unsubstituted parent system and considering
the substitution in a second step as small “perturbation” causing minor changes in
the energies and geometries. Likewise, in heteromerous BAEs the dynamic stereo-
chemistry may be considered similar to that of homomerous BAEs, as long as the
steric and conformational properties of the two moieties are not too different. Such
an approximate application of symmetry [
268
] assuming an analogous dynamic
stereochemistry will yield more powerful predictions.
5 Conclusions
Bistricyclic aromatic enes (BAEs) and the related polycyclic systems are a class of
molecular materials that displays a rich variety of conformations, dynamic stereo-
chemistry and switchable chirality, color, and spectroscopic properties. This is due
to the a subtle interplay of the inherent preference for planarity of aromatic systems
and the competing necessity of non-planarity due to intramolecular overcrowding
in the fjord regions built into the general molecular structure of BAEs (1).
