Chemistry Reference
In-Depth Information
Fig. 31 Schematic
mechanism of the
enantiomerization of the
twisted conformation t-
D
2
via a planar transition state
p-
D
2h
1'
1
p
Z
1'
1
1'
1
t
t
Z-P
Z-M
1'
1
p
1'
1
1
1'
t
t
E-M
E-P
a
b
1'
1'
s
Z-RS'
1
1
a
Z-RR'
1'
1
1'
1
1
1'
t
t
Z-P
1'
1
t
Z-P
t
Z-M
Z-M
1'
1
1
s
Z-SR'
1'
a
Z-SS'
1'
1'
s
1
a
1
E-RR'
E-RS'
1'
1
1'
1
1
1'
t
t
1
1'
t
t
E-P
E-M
E-P
E-M
1'
1
1
1'
a
s
E-SR'
E-SS'
Fig. 32 Schematic mechanisms of the enantiomerization of the twisted conformation t-
D
2
via (a)
an
anti
-folded transition state a-
C
2h
(
y
)or(b)a
syn
-folded transition state s-
C
2v
(
x
)
double bond is reduced until the transition state, a-
C
2h
(
y
), is reached. Twisting in
the opposite direction and unfolding leads to the enantiomeric twisted conforma-
tion. The two parallel pathways have transition states with opposite folding direc-
tions of the tricyclic moieties. Transient structures along the steepest descent paths
have
C
2
(
y
) symmetry. The transition vector of a-
C
2h
(
y
) leading to the twisted
conformations has A
u
symmetry. Note that a-
C
2h
(
y
) has also been considered as a
transition state for the inversion of the
syn
-folded conformation (Fig.
28b
). How-
ever, this would require a transition vector with B
u
symmetry. Alternatively, a-
C
2h
(
y
) may also be a second-order saddle point or a local minimum and intermediate in
one or both of these two dynamic processes (see Sects.
4.3.4
and
4.3.5
). Note also
the analogy to the mechanism of conformational inversion of the
anti
-folded
conformation via a twisted transition state (Fig.
24a
), in which the roles of minima
and transition states are interchanged.
In the second enantiomerization mechanism (Fig.
32b
), the two moieties of the
twisted conformation are folded in the same direction (
syn
) while the twist of the
