Henry Rzepa's blog

Henry and I have worked together more many years – today he mailed me about the latest entry in his blog. He’s wondering whether blogs are a way of recording scientific ideas – which used to be published in letters. Anyway here’s a link

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

In 1988, Wilke (DOI: 10.1002/anie.198801851) reported molecule 1

A 24-annulene. Click on image for model.

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 but conventional wisdom has it that it avoids the antiaromaticity by adopting a highly non-planar tub shape and hence 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! …..