Stereochemistry of Bistricyclic Aromatic Enes and Related Polycyclic Systems - Polyarenes - page 241

Fig. 29 Schematic
mechanism of the E , Z -
isomerization with
simultaneous inversion of
one moiety of the syn -
folded conformation s- C 2v
( x ) via an orthogonally
twisted/folded transition
state ft ⊥ - C s ( d )

1'

1'

1

1

s

E-RR'

1'

1

t ┴ t

PR

┴ PR'

1 1'

1'
MR

t

1

t

┴ MR'

┴ 1'

1'

1

1

s Z-RS' s Z-SR'

1

1'

1'
┴ PS

t

1

t

┴ PS'

1 1'

1' 1

1'

1

t

s

t

┴ MS'

┴ MS

E-SS'

4.3.3 Degenerate Isomerizations of the Twisted Conformation

The effect of the permutation-inversion operators of the molecular symmetry group
G 16 on the twisted conformation t- D 2 is shown in Fig. 30 . The conformation t Z-P is
used as the reference version (top left in Fig. 30 ). The permutation-inversion
operator is given above the schematic projections and stereochemical descriptors
referring to E -or Z -configuration and positive or negative helicity of the central
double bond below.
The permutation-inversion operators of the molecular symmetry group G 16 may
be classified according to their effect on the twisted conformation t- D 2 as shown in
Table 15 .
Three types of degenerate isomerization processes may be distinguished
according to the classification of permutation-inversion operators:
1. Enantiomerization, i.e., inversion of the helicity of the central double bond, e.g.,
t Z-P t Z-M
2. E , Z -isomerization with retention of the helicity of the central double bond, e.g.,
t Z-P t E-P
3. E , Z -isomerization with simultaneous enantiomerization, e.g., t Z-P t E-M
The four permutation-inversion operators corresponding to each of the three
types of dynamic processes are the respective cosets. Each of the three cosets of
operators corresponding to the three different types of degenerate isomerizations
contains at least one self-inverse permutation-inversion operator. Thus, all three
dynamic processes are self-inverse processes. The corresponding transition states
may have point groups including the symmetry operators of t- D 2 as well as the
permutation-inversion operators of the coset interchanging reactant and

product [ 271 ].