Chemistry Reference
In-Depth Information
Fig. 40 Schematic
mechanisms for the
interconversion of the a-
C
2h
(
y
)
anti
-folded and s-
C
2v
(
x
)
syn
-folded conformations
via a transition state ft-
C
1
1'
s
Z-RS'
1
1'
1
1
1'
ft
Z-RPS'
ft
Z-RPS'
1
1'
1'
1
ft
Z-RMS'
ft
Z-RMS'
1'
1
1'
1
1'
1
1'
1
a
a
Z-RR'
Z-SS'
ft
Z-SMR'
ft
Z-SMR'
1'
1
1'
1
ft
Z-SPR'
1
1'
ft
Z-SPR'
s
Z-SR'
1'
s
1
E-RR'
1'
1
1
1'
ft
ft
E-RPR'
E-RPR'
1
1'
1'
1
ft
ft
E-RMR'
1'
E-RMR'
1
1
1'
1
1
a
1'
a
1'
E-RS'
E-SR'
ft
ft
E-SMS'
E-SMS'
1'
1
1
1'
ft
ft
1
E-SPS'
E-SPS'
1'
s
E-SS'
(
C
s
(
xz
) symmetry) but does not indicate whether the two moieties have the same
(
syn
) or opposite (
anti
) direction of folding.
In the lower symmetry transition state, ft-
C
1
, the molecule may also be twisted in
addition to the unequal degrees of folding. Twisting the central double bond in
opposite directions gives rise to a doubling of all pathways (
p
2) in this mech-
anism (Fig.
40
). Each transition state (and pathway) of Fig.
39
is replaced by two
transition states, one with positive (
P
) and one with negative (
M
) twist. The twist
breaks the
C
s
symmetry.
The more folded moiety of the ft-
C
1
transition state is indicated by underlining
in the labels identifying the versions. In addition to interconverting the
anti
-folded
and
syn
-folded conformations, the above two mechanisms may also serve as
two-step mechanisms for conformational inversion of the
anti
-folded and
syn
-
folded conformations.
ΒΌ
