Chemistry Reference
In-Depth Information
1'
1
1
1'
ft
RPR'
ft
┴
RMS'
┴
1'
1'
1'
1'
1
1
1
1
s
Z-RS'
s
E-RR'
a
E-RS'
a
Z-RR'
1'
1
ft
RMR'
1
1'
ft
RPS'
┴
┴
1'
1
1
1'
ft
SMR'
ft
SPS'
┴
┴
1
1
1'
1
1
1'
1'
1'
s
Z-SR'
s
E-SS'
a
E-SR'
a
Z-SS'
1'
1
1
ft
SPR'
1'
ft
SMS'
┴
┴
Fig. 42 Schematic mechanism for the interconversion of the a-
C
2h
(
y
)
anti
-folded and s-
C
2v
(
x
)
syn
-folded conformations with simultaneous
E
,
Z
-isomerization
In addition to the clockwise rotation of the second moiety relative to the first
moiety (Fig.
41
), it may also be rotated counterclockwise. Thus, there are two
pathways interconverting a
Z-RR
0
and s
E-RR
0
. The connectivity of the four versions of
the
anti
-folded conformation with the four versions of the
syn
-folded conformation
by this mechanism of conformational
isomerization with simultaneous
E
,
Z
-
isomerization is shown in Fig.
42
.
Note that the orthogonally twisted/folded conformations shown above and
below the reaction arrows in Fig.
42
correspond to a conformation half way along
the reaction coordinate (90
twist). There are
n ¼
8 versions of this conformation
with twofold symmetry (
h
2). However, this midpoint of the reaction does not
necessarily correspond to the transition state of the reaction (see above).
This dynamic process performs an
E
,
Z
-isomerization, while converting an
anti
-
folded conformation into the
syn
-folded conformation that has the same folding
directions (
R
/
S
) for both moieties. Since folding directions are not changed, it
cannot serve as a mechanism of conformational inversion. It connects each version
of the
anti
-folded conformation with one and only one
syn
-folded conformation in a
pairwise manner.
It should be noted that the peculiarities in deriving the mechanism for intercon-
version of the
anti
- and
syn
-folded conformations with simultaneous
E
,
Z
-
isomerization are due to the choice of a common orientation and reference frame-
work for
E
- and
Z
-isomers and of pure permutation-inversion operators to represent
the molecular symmetry group. Using differently oriented frameworks for
E
- and
¼
