Chemistry Reference
In-Depth Information
C
2). The option of rotating clockwise or counterclockwise gives rise to two
parallel pathways. The symmetry of transient structures along the pathways from
transition states to reactants and products is
C
1
. The transition vector has A”
symmetry. Note that this mechanism allows an inversion of the
anti
-folded confor-
mation in a two-step process. Enantiomeric (labeled) versions of equivalent con-
formations are on opposite sides of the center of Fig.
25
.
¼
4.3.2 Degenerate Isomerizations of the
syn
-Folded Conformation
The effects of the permutation-inversion operators on the versions of the
syn
-folded
conformation s-
C
2v
(
x
) are illustrated in Fig.
26
, using s
Z-RS
0
as reference
conformation.
A classification of the permutation-inversion operators according to their effect
on the folding directions of the two moieties and the
E
-or
Z
-configuration is
summarized in Table
12
.
The classification of the permutation-inversion operators shown in Table
12
indicates three types of dynamic processes in the
syn
-folded conformation s-
C
2v
(
x
):
1. Conformational inversion, by simultaneous inversion of the folding directions of
both moieties, e.g., s
Z-RS
0
s
Z-SR
0
2.
E
,
Z
-Isomerization with simultaneous inversion of
the first moiety, e.g.,
s
Z-RS
0
s
E-SS
0
3.
E
,
Z
-Isomerization with simultaneous inversion of the second moiety, e.g.,
s
Z-RS
0
s
E-RR
0
The four permutation-inversion operators listed in Table
12
for each of the three
dynamic processes correspond to the respective cosets. Each coset contains at least
one self-inverse operator. Thus, all three types of degenerate isomerizations of the
syn
-folded conformation are self-inverse and the corresponding transition states
may have point groups containing the symmetry operators of s-
C
2v
(
x
) and opera-
tors corresponding to the respective dynamic process.
Since the two tricyclic moieties in homomerous BAEs are equivalent, processes
2 and 3 are indistinguishable and will be discussed together as
E
,
Z
-isomerization
with simultaneous inversion of one moiety. The combined process has a connec-
tivity
C
¼
2.
Conformational Inversion of the
syn
-Folded Conformation
In the conformational inversion of the
syn
-folded conformation s-
C
2v
(
x
), the
folding direction of both moieties is reversed simultaneously. As this process is a
self-inverse automerization, the point group of the transition state may include
symmetry operators of the point group of the reactant (and product) conformations
