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In-Depth Information
t
t
t
┴
E-M
Z-P
P
a
a
s
E-RS'
s
Z-SS'
E-SS'
Z-RS'
s
s
a
E-RR'
Z-SR'
a
E-SR'
Z-RR'
t
M
t
t
Z-M
E-P
┴
Fig. 43 Network of isomerization pathways for homomerous BAEs with twisted,
anti
-folded, and
syn
-folded minimum energy conformations
observed in the X-ray crystal structures from the symmetry indicated above may be
due to crystal packing effects.
Planar conformations of BAEs are extremely overcrowded and thus may be
expected to have very high energy [
1
,
13
,
112
,
285
,
286
]. For this reason, planar
conformations are excluded from consideration as (local) minima or transition
states.
In the isomerization and automerization mechanisms, the option with the highest
symmetry transition state may be selected (principle of maximum symmetry, PMS)
[
252
]. Transition states with lower symmetry would lead to similar results, but the
mechanisms would be more complicated due to the parallel pathways.
Excluding the planar conformation as transition states or intermediates, the
following conformational isomerization processes may be expected:
1. t-
D
2
a-
C
2h
(
y
) via [at-
C
2
(
y
)] (cf. Sect.
4.3.4
, Fig.
35
)
2. t-
D
2
s-
C
2v
(
x
) via [st-
C
2
(
x
)] (cf. Sect.
4.3.5
, Fig.
37
)
3. a-
C
2h
(
y
) s-
C
2v
(
x
) via [f-
C
s
(
xz
)] (cf. Sect.
4.3.6
, Fig.
39
)
4.
E
,
Z
-isomerization of t-
D
2
via [t
⊥
-
D
2d
] (cf. Sect.
“
E
,
Z
-Isomerization with Simul-
taneous Inversion of Helicity of the Twisted Conformation”
, Fig.
33
)
This scenario leads to the network of isomerization pathways shown in Fig.
43
.
Conformational inversions of the
anti
- and
syn
-folded conformations and
enantiomerizations of the twisted conformations proceed in two steps via an
intermediate, using one of the three conformational isomerization mechanisms
1-3.
E
,
Z
-isomerization of the twisted conformation may proceed directly in one
step via mechanism 4. Note that simultaneously with the
E
,
Z
-isomerization, the
helicity of the central double bond (
P/M
) is inverted. For an
E
,
Z
-isomerization, the
anti
-folded and
syn
-folded conformations first have to be converted into a twisted
conformation (alternatively, an
E
,
Z
-isomerization with simultaneous interconver-
sion of
syn
- and
anti
-folded conformations may be considered; cf. Sect.
4.3.7
,
Fig.
42
). According to the network of pathways shown in Fig.
43
, twisted confor-
mations have a special role as hub for
E
,
Z
-isomerizations. BAEs with twisted global
