kekulene Aromaticity https://youtu.be/9TclqAZavmA

1. kekulene
❑ 1865: Kekule visualized the ring structure of benzene, various versions of the dream
❑ 1965: At the Kekule´ Centenial in Bonn, Germany, Staab reported the first attempts towards the synthesis
of given the name Kekulene.
❑ 1978: Staab and Diederich ,first conclusive synthesis of Kekulene
Snake biting its tail six snakes/six monkeys linking hands and tails Kekulene: large PAH with formula C48H24

2. ❑ Kekulene, in homage to August Kekule´, due to its planar, conjugated nature and D6h symmetry like benzene
(Superbenzene)
❑ kekulene: Compound-specific name and does not imply a common name for a certain class of compounds.
❑ Kekulene belongs to a class of compounds called cycloarenes.
❑ Cycloarene: Only the benzenoid rings (angular and linear annelation of benzene units)
Naming

3. Each Clar structure represents 2n Kekule structures
❑ Each Clar structure represents 2n Kekule´ structures where n is the number of aromatic sextets.
❑ Kekulene: 200 different Kekule structures, only one Clar structure with 6 aromatic sextets. It represents 26 = 64 Kekule
structures.

4. Naming
❑ IUPAC nomenclature: complicated names, not easily relate any of the structural characteristics of the compounds
❑ Staab and Diederich: First, is to indicate the number of benzene, cyclododecakisbenzene.
❑ Next indicate the annelation of these benzene units.
❑ Labelling ‘‘a’’ the bond of fusion & proceed clockwise.
❑ Linear annelations get a ‘‘d’’ designation (highlighted in red)
❑ Angular annelations receive an ‘‘e’’ designation (highlighted in green).
❑ Start with the lower letter designation
Cyclo[d.e.d.e.d.e.d.e.d.e.d.e.]dodecakisbenzene

5. Contrasting ideas of Pauling and McWeeny on p electrons movement
❑ The chemical shift of the inner protons of gave the answer.
‘Annulene-within-an-
Annulene’ (AWA) Model
❑ Do p electrons move throughout the entire system, as hypothesized by Pauling,
❑ Or do they remain localized in rings, as predicted by McWheeny and described by Clar.
(Global annulenoid
conjugation )
(Local benzenoid
conjugation)

6. Diamagnetic anisotropy
❑ Diamagnetic anisotropy is an interesting phenomenon that is especially evident in aromatic molecules
Q. Kekulene is shown below with 1H-NMR shifts.Discuss potential aromaticity of Kekulene and what
NMR data tells about the nature of actual molecule?
Deshielding region of quasi-
localized benzenoid subunits

7. ❑ Pauling’s model suggested that kekulene should behave as concentric annulenes and that the inner protons
ought to be strongly shielded, as in [18]annulene.
❑ Inner protons are deshielded and resonate at d>7 ppm. This deshielding demonstrates that electrons do not
move freely about the entire molecule, but are instead localized into individual rings, just as in benzene.
❑ Clar structures, not Kekule structures, should be used to represent bonding in polycyclic aromatic molecules.
Global annulenoid conjugation versus local benzenoid conjugation debate
❑ AWA model fails for regular [N]-circulenes (that is, conjugated systems in which N hexagons are tessellated around a
central N-membered ring
McWeeny electron
delocalization theory is the
valid one, not the Pauling
theory.

8. Superaromaticity
❑ Initial speculation that kekulene might be superaromatic.
❑ Superaromatic: Idea that macrocyclic conjugation in large, cyclic PAHs leads to an increased
stabilization of the molecules.
❑ Computational studies suggest that cycloarenes gain little, so there is no enhanced stabilization.
❑ Superconjugation resulting from the cyclic array of benzene rings does not yield appreciable
superaromatic energetic effects.
❑ Despite its 48 π-electrons neither bond length equalization nor any appreciable extra stabilization
energy is perceived that would justify to consider it as a superaromatic molecule