Chemistry Reference
In-Depth Information
1'
1'
1'
1
1
1
ft
RPR'
at
Z-RPR'
st
E-RPR'
┴
1'
1'
1
1
a
Z-RR'
s
E-RR'
Fig. 41 Interconversion of an
anti
-folded and a
syn
-folded conformation with simultaneous
E
,
Z
-isomerization
4.3.7
Interconversion of the
anti
-Folded and
syn
-Folded Conformations
with Simultaneous
E
,
Z
-Isomerization
The point groups
C
2h
(
y
) and
C
2v
(
x
) of the
anti
-folded and the
syn
-folded conforma-
tions, respectively, both include one
C
2
axis as symmetry element. However, the axes
C
2
(
y
) and
C
2
(
x
) have a different orientation in space and correspond to different
permutation-inversion operators: (11
0
)(88
0
)(99
0
) and (18
0
)(81
0
)(99
0
), respectively.
Thus, they are distinct symmetry elements, in contrast to the mirror plane
(
xz
),
which belongs to the common subgroup (cf. Sect.
4.3.6
). However, the two permu-
tation operators may be interconverted by interchanging atom labels 1
0
and 8
0
,
indicating the possibility of a pathway converting an
anti
-folded conformation into
a
syn
-folded conformation with a simultaneous
E
,
Z
-isomerization. This process may
conserve
C
2
symmetry. Figure
41
schematically shows the conversion of a
Z-RR
0
into
s
E-RR
0
. Intermediate
C
2
symmetric
anti
-folded/twisted, folded/orthogonally twisted,
and
syn
-folded/twisted transient structures are shown above the reaction arrow.
All intermediate structures along the pathway have
C
2
symmetry, including the
transition state. Thus, the transition state is not defined by a point of higher
symmetry and does not necessarily correspond to the structure with exactly 90
twist of the central double bond. However, the orthogonally twisted and folded
structure is exceptional with respect to the stereochemical nomenclature: it is
neither
Z
nor
E
, and the borderline case between
anti
- and
syn
-folded conforma-
tions. It may be labeled ft
⊥
RPR
0
, where
P
refers to the sign of the torsion angle
˃
(C
1
-
C
9
-C
9
0
-C
1
0
) (Sect.
2.2
). In the example of Fig.
41
, the second moiety is rotated
clockwise by 180
relative to the first moiety. Note that the direction of folding of
both moieties does not change throughout the process. The degree of folding may
change in a synchronous manner, in accord with the
C
2
symmetry. In general, the
anti
-folded conformation will have a different degree of folding as compared to the
syn
-folded conformation. Intermediate structures along the pathway may have
considerably higher or lower degrees of folding than the end points. A dashed
line indicates the orientation of the
C
2
axis, which changes with the rotation of the
second moiety. Due to this change in orientation of the
C
2
axis during the process, it
cannot be labeled consistently. Also, the corresponding permutation-inversion
operator changes at the point of
E
,
Z
-isomerization.
˄
