Chemistry Reference
In-Depth Information
2.6 Dynamic Stereochemistry
BAEs reveal rich dynamic stereochemistry [
3
,
30
,
181
-
183
]. The conformational
dynamics of bifluorenylidene has caught attention since the seminal work by
Sutherland et al. [
184
]. In BAEs, four types of dynamic processes were observed:
1. Rapid
E
,
Z
-isomerizations of twisted and
anti
-folded BAEs as illustrated in
Fig.
8
.
2. Enantiomerization or conformational inversion, i.e., inversion of the helicity in
twisted BAEs (Fig.
9a
), or inversions of the boat conformations of the central
rings and hence the folding of the tricyclic moieties in
anti
-folded BAEs
(Fig.
9b
).
3.
syn
-Folded to
anti
-folded conformational isomerization, s
!
a, as illustrated in
Fig.
9c
.
4. Twisted to
anti
-folded conformational
a (Fig.
9d
)as
observed in the thermal reversion of photochromism B
!
A and in electro-
chemical studies.
isomerization t
!
It should be noted that enantiomerization and conformational inversion may also
be considered in processes (1), (3), and (4).
Dynamic processes with barriers in the range of 6-26 kcal/mol have been
determined by dynamic NMR (DNMR) spectroscopy. Even higher barriers have
been determined in kinetic equilibration experiments on isolated or enriched iso-
mers. Selected barriers are summarized in Tables
2
and
3
. For more examples see
the reviews and literature cited therein [
3
,
27
,
30
,
181
-
183
].
The low barriers for
E
,
Z
-isomerizations in BAEs have been attributed to the
steric strain inherent in the non-planar conformations destabilizing the ground state
and to electronic effects stabilizing a biradical or zwitterionic orthogonally twisted
transition state [
27
,
30
,
181
-
183
,
190
]. The intrinsic barrier for rotation about an
unstrained, isolated C
0.9 kcal/mol
[
191
]. In the tetraphenylethylenes, the stabilization of a biradical transition state by
delocalization may be similar to BAEs; however, the starting conformations are less
overcrowded. Their
E
,
Z
-isomerization barriers are between 32.9 and 35.3 kcal/mol
[
192
]. Push-pull ethylenes with polar double bonds may have considerably lower
E
,
Z
-isomerization barriers [
27
,
30
].
In homomerous BAEs with bridges X,Y
¼
C bond has been determined as
E
a
¼
65.9
¼
O, S, and CH
2
, identical barriers for
conformational inversion and
E
,
Z
-isomerization were found within experimental
error (Tables
2
and
3
)[
73
,
170
,
187
,
188
]. This observation strongly advocates a
common highest transition state for both dynamic processes [
73
,
170
]
corresponding to the a
!
t interconversion, which involves a passage of the
hydrogens H
1
and H
1
0
(or H
8
and H
8
0
) in the fjord regions [
27
,
155
,
171
,
193
].
Twisted conformations serve as intermediates in both dynamic processes [
170
,
171
]. The orthogonally twisted biradical transition state for
E
,
Z
-isomerization of
the twisted conformations has a lower energy in the above homomerous BAEs and
is thus not rate determining [
170
,
172
,
193
]. On the other hand, in the folded
