Chemistry Reference
In-Depth Information
t-
D
2
intermediates. In Figs.
35
and
36
, enantiomeric versions of the same confor-
mation are on opposite sides.
E
,
Z
-isomerizations are not facilitated by the above
two conformational isomerization mechanisms.
4.3.5
Interconversion of the Twisted and
syn
-Folded Conformations
The permutation-inversion operators of the twisted conformation t-
D
2
, the
syn
-
folded conformation s-
C
2v
(
x
), and the largest common subgroup are shown in
Table
21
.
The permutation-inversion operator corresponding to the point groups of the
twisted and the
syn
-folded conformation,
D
2
and
C
2v
(
x
) have the common
permutation-inversion operators
E
and (18
0
)(81
0
)(99
0
), corresponding to the point
group
C
2
(
x
). The two possibilities for a transition state for the conformational
isomerization of the twisted and
syn
-folded conformations are st-
C
2
(
x
) (or ts-
C
2
(
x
)) and ft-
C
1
, as shown in Table
22
.
Conformations with a
C
2
axis parallel to
x
may combine twisting and
syn
-
folding: ts-
C
2
(
x
)orst-
C
2
(
x
) depending on the dominant out-of-plane deformation
mode (Table
4
). For simplicity, only st-
C
2
(
x
) is mentioned in this discussion
(it may be replaced by ts-
C
2
(
x
) in each case). There are
n
TS
¼
8 versions of the
st-
C
2
(
x
) transition state, four with
Z
-configuration (in a labeled atom sense), and
four with
E
-configuration. These four
Z
- and
E
-transition states interconvert the
corresponding
Z
- and
E
-versions of t-
D
2
and of s-
C
2v
(
x
) as schematically outlined
in Fig.
37
.
The lower symmetry ft-
C
1
transition state has no symmetry and thus the two
moieties may be folded to a different degree. There are
n
TS
¼
16 versions of this
transition state, 8
Z
-versions, and 8
E
-versions. The mechanism for the intercon-
versions via the
C
1
transition state may be derived from Fig.
37
by replacing each
transition state by two distinct transition states with different degrees of folding of
the two moieties. This breaks the
C
2
(
x
) symmetry along the pathways and leads to
doubling of all pathways as shown in Fig.
38
.
In the labels identifying the versions of the ft-
C
1
transition states, underlining
indicates the more folded moiety. Each two versions corresponding to parallel
pathways differ only in the relative degrees of folding of the two moieties.
Comparing the two mechanisms via the st-
C
2
(
x
) and via the ft-
C
1
transition
states, in the former, the two moieties move in a
C
2
symmetric, synchronous way,
while in the latter, the two moieties move independently, giving rise to parallel
pathways because either the first or the second moiety may show a higher degree of
folding.
In addition to interconverting twisted and
syn
-folded conformations, both mech-
anisms may also serve as a two-step enantiomerization mechanism for the twisted
