Homoaromaticity

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The homoaromaticity from the field of organic chemistry is found in conjugated cyclic systems that are able to skip part of the ring. They are in contrast to the regular aromaticity , which can only be found in planar conjugated systems. Just like normal aromatic compounds, homoaromatic compounds are also more stable than would be expected based on pure conjugation.

Concept of homoaromaticity

The concept of homoaromaticity was developed by Saul Winstein and was the first to be observed with a 3-bicyclo [3.1.0] hexyl cation (Fig. 1).

Fig. 1: Trishomoaromaticity

It was found that the solvolysis reaction proceeded much faster when the tosyl - leaving group in equatorial was not in the axial position of the ring, making the anchimeric support the cyclopropane bond σ for nonclassical ion was relieved. The positive charge in this ion is delocalized over three carbon atoms that have two π electrons and thus fulfill the Hückel rule . A total of three methylene groups are excluded from the conjugated system, so the ion is a trishomoaromatic.

Fig. 2: Homotropylium ion

Another example of a homoaromatic system is the homotropylium ion containing six π electrons (Fig. 2).

Bishomoantiaromaticity

The bridged bicyclo [3.2.1] octa-3,6-dien-2-yl cation shown in Figure 3 is bishomoantiaromatic in that it contains four π electrons, making it antiaromatic . The cation is prepared at low temperatures in a super acidic solvent consisting of fluorosulfonic acid and sulfuryl chloride fluoride . The cation can recombine with a methanolate anion, producing a 50/50 mixture of the isomers. The antiaromatic behavior of the system could be demonstrated by means of NMR analysis and the reaction rates in solvolysis experiments.

Fig. 3: Bishomoantiaromatic system

Web links

Individual evidence

  1. ^ S. Winstein, Joseph Sonnenberg, Louis De Vries: The tris-homocyclopropenyl cation In: Journal of the American Chemical Society . Volume 81, No. 24, 1959, doi : 10.1021 / ja01533a051 , pp. 6523-6524.
  2. ↑ S. Winstein: Homo-aromatic structures. In: Journal of the American Chemical Society. Volume 81, No. 24, 1959, doi : 10.1021 / ja01533a052 , pp. 6524-6525.
  3. ↑ Heinrich Volz, Jung-Hyu Shin: Bicyclo [3.2.1] octa-3,6-dien-2-yl Cation: A Bishomoantiaromate. In: Journal of the American Chemical Society. Volume 71, No. 6, 2006, doi : 10.1021 / jo0515125 , pp. 2220-2226.