Chemistry Reference
In-Depth Information
Fig. 12 Schematic
representation of important
BAE conformations
p
t
t
a
s
f
^
In cases where two out-of-plane modes are combined, the order may be used to
designate the dominant mode. Thus, ta is closer to a twisted conformation, while at
is closer to an
anti
-folded conformation. The symbols au and su describe unequally
folded conformations, 'u', which are close to
anti
- and
syn
-folded conformations,
a and s, respectively. In the same spirit, symbols tau, tsu, aut, and sut may be
defined. However, in this work the shorter symbols tf and ft will be preferred for
these types of conformations. It should be noted that this classification according to
conformation types is an empirical, qualitative approach and is intuitive rather than
exact. It may be applied in a strict as well as in an approximate manner. Neverthe-
less, the conformation type symbols are very useful in describing the conformations
of overcrowded bistricyclic aromatic enes.
4.1.3 Conformations and Corresponding Point Groups
An exhaustive list of point groups that may be possible candidates for conformations
of overcrowded bistricyclic aromatic enes may be derived from an analysis of the
subgroups of the point group
D
4h
, which is isomorphous to the molecular symmetry
group of BAEs,
G
16
(see Sect.
4.2.2
). The point group
D
4h
includes the symmetry
operators of all possible BAE conformations as supergroup. The subgroups of
D
4h
may be found by analyzing the character table and identifying the subgroup associ-
ated with each symmetry species. For character tables of
D
4h
and its subgroups see
[
279
]. The results of such an analysis are summarized in Fig.
13
(adapted from
[
279
]). Lines connect subgroups and supergroups. The lines are labeled with the
symmetry species of the higher point group leading to the respective lower point
group. The point group order
h
of the groups is given at the left-hand-side. Note that
point groups including
C
4
axes do not qualify as point groups for any conformations
of BAEs since a
C
4
axis is not compatible with the topology. Thus,
D
2d
and
D
2h
are
the highest possible point groups (
h
8) of BAE conformations.
Combining the above classification schemes and descriptors with a symmetry
analysis based on the point groups which are subgroups of
D
4h
leads to a detailed
classification of the topology, substitution pattern, conformations, symmetry, and
chirality of overcrowded bistricyclic aromatic enes. The results are summarized in
Table
4
.
In Table
4
the unsubstituted homomerous and heteromerous, monosubstituted
and disubstituted classes are arranged in columns. Each of the basic confirmations,
p, t
⊥
, t, a, s, and f (Fig.
12
) is listed in a separate block. Conformational symbols
and the point group symmetry are given along with the chirality or achirality of the
conformation.
E
- and
Z
-isomers, the helicity of the central double bond (
P
/
M
), and
the absolute configuration of the (substituted) folded moieties (
R
/
S
) are indicated by
appropriate stereodescriptors. For simplicity only one enantiomer (
P
) is given in
¼
