Chemistry Reference
In-Depth Information
Table 18 Possible point groups and conformations of the transition state for
E
,
Z
-isomerization of
the twisted conformation t-
D
2
Group of permutation-inversion operators
n
TS
Cp
a
b
TS
h
TS
{
E
, (18)(1
0
8
0
), (11
0
88
0
)(99
0
)*, (18
0
81
0
)(99
0
)*} t-
D
2
4 4 1 2
D
2
A
{
E
, (18)(1
0
8
0
)} t-
C
2
(
z
) 2 8 1 4
C
2
(
z
) A
{
E
, (11
0
)(88
0
)(99
0
)} ta-
C
2
(
y
) 2 814
C
2
(
y
) A
{
E
, (18
0
)(81
0
)(99
0
)} ts-
C
2
(
x
) 2 814
C
2
(
x
) A
{
E
} ft-
C
1
1 16 1 8
C
1
A
a
Point group symmetry along pathway from transition state to reactant or product, i.e., maximum
common subgroup of transition state and reactant or product
b
Symmetry species of the mode of the transition vector (using the conventional setting of the
transition state point group [
279
])
Fig. 34 Counterclockwise
rotation of the second
moiety of t
Z-P
to give t
E-P
1
1'
1
1'
^
M
p
Z
t
1'
1
1
1'
1
1'
t
Z-P
t
t
Z-M
E-P
this self-inverse automerization process cannot be symmetry elements of the
transition state. The point group of the reactant and product,
D
2
, and its subgroups
are the only possible candidates for a transition state of the
E
,
Z
-isomerization
without inversion of helicity.
The
E
,
Z
-isomerization of a twisted conformation without inversion of the
helicity of the central double bond requires a 180
rotation of one moiety about
the central double bond. This is not likely to occur in one step. In fact, the existence
of a stable twisted conformation may be seen as due to a compromise between two
energetically unfavorable conformations: an orthogonally twisted conformation,
which leads to loss of
-overlap across the central double bond, and a planar
(or nearly planar) conformation during passage of the overcrowded regions,
which involves high steric strain. A rotation of one moiety by 180
forces the
molecule to pass through both unfavorable conformations. This implies a double
barrier for a 180
rotation about the double bond (cf. [
30
]). Thus, the dynamic
process of
E
,
Z
-isomerization without inversion of helicity is more likely to proceed
in two consecutive steps: an inversion of helicity of the twisted conformation,
followed by an
E
,
Z
-isomerization with simultaneous inversion of helicity as
discussed in Sects.
“Enantiomerization of the Twisted Conformation”
and
“
E
,
Z
-
Isomerization with Simultaneous Inversion of Helicity of the Twisted Conforma-
tion”
, respectively. This is illustrated by the example of a counterclockwise rotation
of the second moiety of t
Z-P
to give t
E-P
shown in Fig.
34
. Note that the net effect of
two consecutive inversions of helicity is retention of helicity.
π
