Corannulene

Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings, so another name for it is circulene. 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. Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings, so another name for it is circulene. 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 lower chemical yields than solution-chemistry syntheses, but offer routes to more derivatives. Corannulane was first isolated in 1966 by multistep organic synthesis. In 1971, the synthesis and properties of corannulane were reported. A flash vacuum pyrolysis method followed in 1991. One synthesis based on solution chemistry consists of a nucleophilic displacement–elimination reaction of an octabromide with potassium hydroxide: The bromine substituents are removed with an excess of n-butyllithium. A kilogram scale synthesis of corannulene has been achieved. Much effort is directed at functionalization of the corannulene ring with novel functional groups such as ethynyl groups, ether groups, thioether groups, platinum functional groups, aryl groups, phenalenyl fused and indeno extensions. and ferrocene groups. The observed aromaticity for this compound is explained with a so-called annulene-within-an-annulene model. According to this model corannulene is made up of an aromatic 6 electron cyclopentadienyl anion surrounded by an aromatic 14 electron annulenyl cation. This model was suggested by Barth and Lawton in the first synthesis of corannulene in 1966. They also suggested the trivial name 'corannulene', which is derived from the annulene-within-an-annulene model: core + annulene. However, later theoretical calculations have disputed the validity of this approximation. Corannulene can be reduced up to a tetraanion in a series of one-electron reductions. This has been performed with alkali metals, electrochemically and with bases. 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 with 4 lithium ions in between and 2 pairs above and below the stack. This self-assembly motif was applied in the organization of fullerenes. Penta-substituted fullerenes (with methyl or phenyl groups) charged with five electrons form supramolecular dimers with a complementary corannulene tetraanion bowl, 'stitched' by interstitial lithium cations. In a related system 5 lithium ions are sandwiched between two corannulene bowls In one cyclopentacorannulene a concave - concave aggregate is observed by NMR spectroscopy with 2 C–Li–C bonds connecting the tetraanions.