Homoaromaticity

Homoaromaticity in organic chemistry is found in conjugated cyclic systems that are able to skip a part of the ring as opposed to regular aromaticity. Like ordinary aromatic compounds, homoaromatic compounds are more stable than what would be expected based on conjugation alone. Homoaromaticity as a concept was developed by Saul Winstein ^{[1] [2]} and first observed in a 3-bicyclo[3.1.0]hexyl cation (scheme 1).

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The solvolysis reaction was found to be much faster when the tosyl leaving group was positioned equatorial and not axial in the ring facilitating anchimeric assistance of the cyclopropane sigma bond to the non-classical ion. The positive charge in this ion is delocalized over three carbon atoms containing 2 pi-electrons obeying Hückel's rule. A total of three methylene groups are excluded from the conjugated system and therefore the ion is a trishomoaromat.

Another example of a homoaromatic system is the 6 electron homotropylium ion (scheme 2).

The bridged Bicyclo[3.2.1]octa-3,6-dien-2-yl cation depicted in scheme 3 is bishomoantiaromic because it contains 4 electrons making it antiaromatic ^[3]. The cation is prepared at low temperatures in a superacidic solvent consisting of fluorosulfuric acid and sulfuryl chloride fluoride and it can recombine with a methoxy anion as a 50/50 mixture of isomers. Antoaromatic behaviour in this system is evidenced from NMR analysis and reaction rates in solvolysis experiments.

References

1. ^  The tris-homocyclopropenyl cation S. Winstein, Joseph Sonnenberg, Louis De Vries J. Am. Chem. Soc.; 1959; 81(24); 6523-6524. Abstract
2. ^  Homo-aromatic structures S. Winstein J. Am. Chem. Soc.; 1959; 81(24); 6524-6525. Abstract
3. ^  Bicyclo[3.2.1]octa-3,6-dien-2-yl Cation: A Bishomoantiaromate Heinrich Volz and Jung-Hyu Shin J. Org. Chem.; 2006; 71(6) pp 2220 - 2226 Abstract