Hückel-Aromaten

Ever since Kekultf's intuitive assignment of the hexagon structure to benzene, organic chemists have been captivated by the idea that there might exist other cyclically conjugated hydrocarbons besides bemcne exhibiting aromatic properties. Efforts to clarify this question experimentally have remained inclusive until fairly recently due to the lack of proper synthetic methods. It was thus left to the molecular orbital theory of unsaturated systems developed by E. Hückel to give an answer. According to this theory, aromaticity is not confined to the 6 JI-electron system, but is gener­ ally to be expected for cyclically conjugated systems with (411 -1 2) JI­ electrons, provided that they possess planar or near-planar ring skeletons. Hückel's predictions have inspired over the last twenty years numcrous experimental investigations, which in brilliant verification of the theory have led to the discovery of a host of nonbenzenoid aromatic compounds. A few years ago a consideration of molccular models made us conclude that the classical aromatic hydrocarbons naphthalcne, anthracene, naphtha­ ce ne etc. may be transformed into approximately planar (4 n+ 2) JI-clcctron systems by the basically simple device of bridging. As will be delineated in this lecture, an entirely new class of aromatic compounds - tcrmcd "Hückel-Aromatics" in honour of Iiiickel-- has emergcd from this conccpt.