Stereochemistry of Bistricyclic Aromatic Enes and Related Polycyclic Systems - Polyarenes - page 250

Fig. 36 Schematic
mechanism for the
interconversion of the t- D 2
twisted and a- C 2h ( y ) anti -
folded conformations via a
transition state ft- C 1

1'

1

a

Z-RR'

1'

1'

1

1

ft ft Z-RMR'

Z-RPR'

1'

1'

1

1

ft ft Z-RPR'

Z-RMR'

1' 1

1

1'

1

t

t

1

Z-P

1' 1'

Z-M

ft

ft

Z-SPS'

Z-SMS'

1

1

1'

1'

1

ft ft

Z-SMS'

Z-SPS'

1'

a

Z-SS'

1'

1

a E-RS'

1'

1'

1 1

ft E-RPS' ft E-RMS'

1'

1'

1

1

ft E-RPS'

ft E-RMS'

1 1'

1

1

t

1'

t

1

1'

E-P E-M

1'

ft E-SPR'

ft E-SMR'

1

1

1'

1'

1

ft E-SMR'

ft E-SPR'

a

1'

E-SR'

In the labels identifying the versions of the transition state, underlining indicates
the more folded moiety. The two versions of the transition state corresponding to
parallel pathways, i.e., interconverting the same versions of the twisted and the anti -
folded conformation, differ only in the degree of folding of the two moieties. In
Fig. 36 , the first moiety is more folded than the second along the 'outer' pathways.
The folding degrees are interchanged in the corresponding 'inner' pathways.
In the mechanism via ta- C 2 ( y ) transition states, the two moieties move in a
synchronous manner, while in the mechanism via ft- C 1 they fold or unfold inde-
pendently, one after the other. Note that both mechanisms, via ta- C 2 ( y ) transition
states and via ft- C 1 transition states interconvert enantiomeric versions (in a labeled
atom sense) in a two-step mechanism. Thus they may also serve as an
enantiomerization mechanism for the t- D 2 conformations via a- C 2h ( y ) intermedi-
ates, or as conformational inversion mechanism for the a- C 2h ( y ) conformations via