Corannulene

Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings. It is of scientific interest because it is a geodesic polyarene and can be considered a fragment of buckminsterfullerene. Due to this connection and also its bowl shape, corannulene is also known as a buckybowl. Corannulene exhibits a bowl-to-bowl inversion with an inversion barrier of 10.2 kcal/mol (42.7 kJ/mol) at −64 °C.

Several synthetic routes exist to corannulene. Flash vacuum pyrolysis techniques generally have low chemical yields but one organic synthesis exists based on solution chemistry consisting of a nucleophilic displacement–elimination reaction of an octabromide with potassium hydroxide:


 * [[Image:Corannulene synthesis.gif|none]]

The bromine substituents are removed with an excess of n-butyllithium.

The observed aromaticity for this compound is explained with a so-called annulene-within-anannulene model. According to this model corannulene is made up of an aromatic 6 electron cyclopentadienyl anion surrounded by an aromatic 14 electron annulenyl cation. Corannulenes react with alkali metals in a series of one-electron reductions. The corannulene dianion is antiaromatic and tetraanion is again aromatic. With lithium as reducing agent two tetraanions form a supramolecular dimer with two bowls stacked into each other (convex-convex) and with 4 lithium ions in between and 2 pairs above and below the stack.

In one cyclopenta[bc]corannulene a concave - concave aggregate is observed by NMR spectroscopy with 2 C–Li–C bonds connecting the tetraanions.


 * [[Image:Cyclopenta-bc-corannulene.png|left|100px|Cyclopenta[bc]corannulene]]