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Homoaromaticity
Homoaromaticity in organic chemistry refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability.
The concept of homoaromaticity was pioneered by Saul Winstein in 1959, prompted by his studies of the “tris-homocyclopropenyl” cation.〔Winstein, S. "Homo-Aromatic Structures." ''J. Am. Chem. Soc.'' 1959, ''81,'' 6523.〕 Since the publication of Winstein's paper, much research has been devoted to understanding and classifying these molecules, which represent an additional “class” of aromatic molecules included under the continuously broadening definition of aromaticity. To date, homoaromatic compounds are known to exist as cationic and anionic species, and some studies support the existence of neutral homoaromatic molecules, though these are less common.〔Freeman, P. K. "Neutral Homoaromaticity in Some Neutral Heterocycles." ''J. Org. Chem.'' 2005, ''70,'' 1998. 〕 The 'homotropylium' cation (C8H9+) is perhaps the best studied example of a homoaromatic compound.
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