Chemistry Reference
In-Depth Information
The conformational, dynamic, and spectroscopic properties may be designed
and fine-tuned, e.g., by variation of the bridging groups X and Y, the overcrowding
in the fjord regions, extensions of the aromatic system, or modifications of the
general BAE structure 1, based on the fundamental understanding of the structure-
property relationships (SPR).
The symmetry analysis presented in Sect.
4
allows deeper insight into the
conformations, chirality, and the mechanisms of the dynamic stereochemistry.
The objective of the present work was to evaluate the conformational spaces and
the dynamic stereochemistry of overcrowded bistricyclic aromatic enes (1), apply-
ing fundamental symmetry considerations.
The conformations of BAEs may be described by the predominant mode(s) of
out-of-plane deformation. The following conformational types were defined: planar
(p), orthogonally twisted (t
⊥
), twisted (t),
anti
-folded (a),
syn
-folded (s), and folded
conformations with one planar (or nearly planar) tricyclic moiety (f) (Sect.
4.1.2
).
In addition, combined modes were considered. The definitions of these conforma-
tional types are empirical and qualitative. An exact classification scheme may be
derived from the point group symmetries (Sect.
4.1.3
).
Combination of the qualitative description by conformational types with the
classification according to point groups allows one to derive possible mechanisms
for dynamic stereochemical processes in unsubstituted homomerous BAEs based
on the molecular symmetry group formalism [
246
,
247
] and the symmetry theorems
for transition states [
268
,
270
]. The principle of maximum symmetry [
252
] allows
one to predict mechanistic scenarios for the enantiomerizations, conformational
inversions,
E
,
Z
-isomerizations, and interconversions of conformations, based on
plausible assumptions about the relative stability of the conformational types. The
exact symmetry analysis for unsubstituted homomerous BAEs may be generalized
to less symmetric systems following the notion of slightly distorted skeleta [
260
]
and the concept of pseudo-symmetry for substituted systems [
287
].
Hopefully, the present symmetry analysis of the conformational spaces of BAEs
will serve as a basis for future experimental and computational studies of the
dynamic stereochemistry of BAEs, with special emphasis on the transition states
for interconversions of conformers,
E
,
Z
-isomerizations, and enantiomerizations.
Prior knowledge of the symmetry of the transition states, as derived in Sect.
4.3
,
is highly instrumental in finding their geometries, as in many cases the geometry
optimization reduces to a simple symmetry constrained energy minimization rather
than a much more demanding transition state search.
The rapidly increasing understanding of the fundamental principles underlying
thermochromism, photochromism, piezochromism, and electrochromism in BAEs
and of related molecular switches and motors will hopefully lead to useful appli-
cations in the future. Many fields in modern chemical science have certainly
profited from the quest towards elucidating the fascinating enigmas of BAEs and
related aromatic compounds.
