Chemistry Reference
In-Depth Information
1.2 Scope of the Review
The central themes of the present review chapter are stereochemistry and symmetry
considerations in bistricyclic aromatic enes (BAEs). The focus is on the application
of symmetry considerations to the classification and the analysis of molecular
conformations of BAEs and to the mechanisms of the dynamic stereochemistry of
these fascinating systems.
Section
2
provides an overview of BAE structures and properties. Due to space
limitations, only a relatively limited overview of BAEs and related polycyclic
systems is presented. The overview is not intended to be comprehensive. The
quoted literature published in the last dozen years since our latest 2001 review on
BAEs [
3
] is intended to be selective, emphasizing conceptual understanding. The
overview also includes a selection of BAEs for which the crystal and molecular
structures have been published. Some variations of the BAE structure 1, including
the removal of the bridges X and/or Y, cyclization across the fjord region, and some
related polycyclic aromatic systems, are also mentioned. The term “related poly-
cyclic aromatic systems” is restricted to polycyclic aromatic enes (PAEs) that are
benz-annelated BAEs. These BAE variations are treated only briefly.
Section
3
introduces the methodology (general theory) of symmetry analysis in
stereochemistry, including a discussion of symmetry operators vs permutation-
inversion operators, the molecular symmetry group, transition state symmetry,
and practical considerations.
Section
4
addresses the symmetry features of BAEs. Most attention is devoted to
symmetry considerations of unsubstituted homomerous BAEs (1,X
Y). Due to
the highly symmetric topology, the symmetry analysis of the homomerous BAEs
gives the most comprehensive results. A mechanism for the dynamic stereochem-
istry of BAEs in terms of a network of isomerization pathways, based on symmetry
analysis and on plausible assumptions of the relative (energetic) stability of the
different conformations and transition states, is presented. A brief discussion on
symmetry in heteromerous BAEs and (di)substituted BAEs is also included. In
cases where the difference between the two bridges (X
¼
Y) and/or substitution
can be considered a minor perturbation of the conformations, considerably more
information may be gained by studying the dynamic stereochemistry based on the
molecular symmetry group of the homomerous unsubstituted parent system.
The Chapter concludes with Sect.
5
, summarizing the main results and pointing
towards some future applications.
6¼
2 Overview of BAE Structures and Properties
2.1 Geometrical Parameters
Bistricyclic aromatic enes are overcrowded in the fjord regions (Structure 1).
Intramolecular overcrowding
is a steric effect shown by aromatic structures in
