Posts Tagged ‘anti-aromatic systems’

A molecule with an identity crisis: Aromatic or anti-aromatic?

Monday, April 13th, 2009

In 1988, Wilke[1] reported molecule 1

A [24] annulene. Click on image for model.

A 24-annulene. Click for 3D.

It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned! The reason is that a 24- annulene belongs formally to a class of molecule with 4n (n=6) π-electrons, and which makes it antiaromatic according to the (extended) Hückel rule. This is a select class of molecule, of which the first two members are cyclobutadiene and cyclo-octatetraene. The first of these is exceptionally reactive and unstable and is the archetypal anti-aromatic molecule. The second is not actually unstable, but it is reactive and conventional wisdom has it that it avoids the undesirable antiaromaticity by adopting a highly non-planar tub shape and hence instead adopts reactive non-aromaticity. Both these examples have localized double bonds, a great contrast with the molecule which sandwiches them, cyclo-hexatriene (i.e. benzene). The reason for the resurgent interest is that a number of crystalline, apparently stable, antiaromatic molecules have recently been discovered, and ostensibly, molecule 1 belongs to this select class!



  1. G. Wilke, "Contributions to Organo‐Nickel Chemistry", Angewandte Chemie International Edition in English, vol. 27, pp. 185-206, 1988.