Homo aromaticity

As stated at the beginning of this Introduction, (homo)aromaticity refers to a special (thermodynamic) stability relative to some hypothetical reference state. It is therefore most attractive to use a thermochemical discriminator for the designation of homoaromaticity. However, such thermochemical methods suffer the same disadvantages when applied to homoaromaticity as they do in the case of aromaticity (see for example Garratt, 1986 Storer and Houk, 1992). There have been several recent studies using the heats of hydrogenation of potential homoaromatics in an attempt to classify these species (vide infra). Due, in the main, to the hypothetical nature of the localized model reference states there is some debate regarding these results (see Dewar and Holder, 1989 Storer and Houk, 1992). 

Experimental log k2 values were correlated with Brown para-localization energies, Dewar reactivity numbers, Herndon structure count ratios, Hess-Schaad resonance energy differences, indices of free valence, and second-order perturbation stabilization energies. The latter are based on Fukui s frontier orbital theory [67] which classifies the Diels-Alder reaction of benzenoid hydrocarbons with maleic anhydride as mainly HOMO (aromatic hydrocarbon)-LUMO (maleic anhydride) controlled. However, the corresponding orbital interaction energy given by... 

In the same way, Winstein has introduced the notion of homo-aromaticity which is, in fact, more closely synonymous to n space conjugation than to delocalization along the ring system. It had been observed for many years — by means of some specific techniques which were classically supposed to measure aromaticity - that molecules like 5.6 diphenyl-naphthacene (or rubrene) ), cyclooctatetraene or bisperitetraphenyl-naphthalene have the specific behaviour of aromatic compounds although a ir delocalization along the chemical bonds of these molecules seems geometrically quite strictly forbidden. 

TWO-ELECTRON AROMATIC AND HOMO AROMATIC IONS Cyclopropenylium ion (CsH ) (38)... 

K. H. Allinger. N.L. Resurrection of neutral tris-homo-aromaticity [Review], J. Org. Chem. 2002. 67. 6599-6611. 

The crystal structme of the complex, 7.4, has been determined and shows that the ligand may be regarded either as a combination of 2- and 3-electron, 7.4, ligands or as a S-electron homo-aromatic ligand, 7.S. Examples of n x 2-electron ligands are given in Figure 32, p 166. 

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