Corannulene

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Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C₂₀H₁₀ ^[1]. 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 ^[2] consisting of a nucleophilic displacement–elimination reaction of an octabromide with potassium hydroxide (fig. 1).

Fig 1. solution phase corannulene synthesis

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 ^[3].

[edit] References

↑ Geodesic polyarenes with exposed concave surfaces Lawrence T. Scott, Hindy E. Bronstein, Dorin V. Preda, Ronald B. M. Ansems, Matthew S. Bratcher and Stefan Hagen Pure & Appl. Chem., Vol. 71, No. 2, pp. 209–219, 1999. Article
↑ A Simple, Large-Scale Synthesis of the Corannulene System A. Sygula and P. W. Rabideau, J. Am. Chem. Soc. 122, 6323 2000. Support info Abstract
↑ Reduction of Bowl-Shaped Hydrocarbons: Dianions and Tetraanions of Annelated Corannulenes Ivan Aprahamian, Dorin V. Preda, Mihail Bancu, Anthony P. Belanger, Tuvia Sheradsky, Lawrence T. Scott, and Mordecai Rabinovitz J. Org. Chem.; 2006; 71(1) pp 290 - 298; (Article) DOI: 10.1021/jo051949c Abstract