Antihomoaromaticity

Some authors have used the term antihomoaromaticity. We think that this term may be misleading since it implies either a system that, despite homoaromaticity, is destabilized or the anti form of a homoaromatic system, which may be considered as the aromatic form itself. 

It is finally of interest to consider the solvolytic chemistry of 3,4-unsaturated and 3,4-benzo 2-bicyclo[3.1.0]hexyl systems. These are of special interest because they are sources of homologs of the antiaromatic cyclopentadienyl and inden-l-yl cations and as such should be destabilized antihomoaromatic systems. 

The bicyclo[3.1.0]hex-3-en-2-yl chloride system 72 has been studied by Berson and HastyAlthough solvolytic kinetics were not run, products and deuterium scrambling were examined. The predominance of the exo- solvolysis product may be due to an antihomoaromatic effect in which only the endo side of the p orbital at C(2) interacts with the cyclopropane ring. Solvolytic chemistry in the bicyclo[3.1.0]hex-3-en-2-yl system has also been discussed by Cueille and Jullien. ... 

It is interesting to compare the properties of the above homotropylium ions, in which the breaking of the cyclopropane ring results in a homoaromatic system, with those of the bicyclo[3,l,0]hexenyl cations in these a similar breaking would result in a 4 71-electron antihomoaromatic system similar to the cyclopentadienyl cation (cf. >). 

While the bicyclo 3,l,0]hexenyl cations have the potential possibility to be antihomoaromatic particles, the isomeric bicyclo[2,l,l]hexenyl cations related to the 7-norbomenyl ions, can be stabilized by homoallylic interaction. 

Calculations by Hiickel s method predict a greater stability for the three-centre cation 549 this is in accord with the data obtained in the acetolysis of a deuterated sample. While ion 549 is homoaromatic, ion 550 is antihomoaromatic (4 delocalized electrons). 

These differences in the rates are due to essential differences in the structure of the MCRs of these cations. For example, the hindrances to 1,2-shifts in cyclobutenyl cations relative to those in benzenonium ones are likely to be connected with these two types of ions being topologically opposite the latter are antihomoaromatic systems in the ground state (true, the overlap between the ends of the pentadienyl system and, hence, the extent of antihomoaromaticity seems to be very small. 

Haddon RC (1974) Homoaromatic, nonhomoaromatic, antihomoaromatic, and dihomoaromatic character. Tetrahedron Lett 2797-2800... 

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