Chemistry Reference
In-Depth Information
Table 25 Conformations and point groups of heteromerous BAEs
Conformations Point groups
Planar p-
C
2v
(
z
) p-
C
s
(
yz
)
Orthogonally twisted t
⊥
-
C
2v
(
d
) t
⊥
-
C
s
(
d
)
a
Twisted t
P
-
C
2
(
z
)
Unequally folded au-
C
s
(
xz
) su-
C
s
(
xz
) f-
C
s
(
xz
)
b
Unequally folded/twisted ft-
C
1
b
a
The reflection planes of
C
2v
, and
C
s
in this orientation are the planes
x
y
b
Unequally folded conformations may have
anti
-folded,
syn
-folded, or mono-folded character
¼
y
and/or
x
¼
minima always have distinct barriers for enantiomerization and for
E
,
Z
-isomeriza-
tion. On the other hand, in BAEs with
anti
-folded global minima, the interconver-
sion of the
anti
-folded and twisted conformations necessary before an
E
,
Z
-
isomerization may also serve as a mechanism for conformational inversion. In the
case where the transition state for the
anti
-folded to twisted conformational isom-
erization [at-
C
2
(
y
)] is higher than the transition state for
E
,
Z
-isomerization [t
⊥
-
D
2d
], the former is the highest transition state for both processes. Such a common
barrier was observed in several BAEs with central six-membered rings [
73
,
170
,
187
,
188
].
4.4 Symmetry of Heteromerous BAEs and Disubstituted
Homomerous BAEs
In the heteromerous BAEs there is no symmetry relation between the two moieties
due their different constitution. This drastically reduces the number of symmetry
operators and available point groups. The possible conformations and their point
groups are listed in Table
25
(cf. Table
4
).
Note that there is no point group distinctly defining an
anti
-folded or
syn
-folded
conformation. All pure folded conformations (not twisted) have the point group
C
s
(
xz
), irrespective of the relative direction of folding of the two moieties.
In
E
- and
Z
-disubstituted homomerous BAEs, the left and right side of the
tricyclic moieties are different and thus cannot be symmetry related. The available
conformations and point groups are listed in Table
26
. Their number is smaller than
in unsubstituted homomerous BAEs (cf. Table
4
).
Note that there is no symmetry defined orthogonally twisted conformation. A
conformation with planar tricyclic moieties twisted by 90
about the central double
bond would have
C
2
symmetry. There are no symmetry constraints on the planarity
of the tricyclic moieties and on the twist of the double bond of such a
C
2
symmetric
conformation. On the contrary, there is a continuum of twisted/
anti
-folded confor-
mations of the
Z
-isomer (ta
Z-RPR
0
-
C
2
(
y
) and at
Z-RPR
0
-
C
2
(
y
)) and a continuum of
twisted/
syn
-folded conformations of the
E
-isomer (ts
E-RR
0
-
C
2
(
x
) and st
E-RR
0
-
C
2
(
x
)). However, a
E-RS
0
-
C
i
, and s
Z-RS
0
-
C
s
(
xy
) still have unique point groups.
