Homoaromatic cations

The concept of homoaromaticity29 1001-1006 was advanced by Winstein in 1960. It represented a challenge to experimental and theoretical chemists alike. - The question of homoaromatic overlap has been mainly studied in six-Jt-electron Hiickeloid systems,29 1001 1006 although several two-jt-electron homoaromatic systems have been discovered subsequently.230 1006 1010-1013 

Other examples of monohomoaromatic cations are the 6ji homotropylium ion 179 of Pettit1020-1022 and Winstein1023 1025 and the previously discussed 2jt homoaromatic ll-methyltricyclo[4.4.1.01,6]undec-ll-yl cation 46.163 

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Just as the unusual stability and reactivity of benzene are placed into their proper context by comparison with cyclobutadiene and cyclooctatetraene39, the 4 -electron homo-logues of benzene, it is instructive to compare the formally homoantiaromatic bicyclo [3.1.0]hexenyl/cyclohexadieny 1 cation systems with the homocyclopropenium and homo-tropenylium ions (Scheme 14). Such a comparison not only puts in context the properties of the latter two homoaromatic cations, but also reveals a different mode of cyclopropyl conjugation that occurs in the 4 -electron systems. 

Participation by Double Bonds. Homoallylic and Homoaromatic Cations... 

Fig. 8.8. Electron density contours for C( 1 )-f (3) bridging in homoaromatic cations (a) cyclobutenylium ion (b) cyclooctatrienyUum ion. Reproduced from J. Phys. Org. Chem., 6, 445 (1993).

In contrast to the homoaromatic cations 6 and 7, MO calculations fail to reveal substantial stabilization of the anion 8. There does not seem to be any diamagnetic ring current associated with the anion. The weight of the evidence seems to be against significant homoaromatic stabilization in 8. 

All the homoallylic ions discussed above belong to homoaromatic cations. The homoallylic nonhomoaromatic ions with the p-orbitals overlap the developing cation centre and only the y-olefine carbon have been postulate as intermediates in solvolysis and electrophilic addition but until recently could not be obtained in a stable form though siKh attemirts have been made. Thus, Hart and Kuzuya have tried to obtain a stable homoallylic ion 522 from dienol 521, however the latter interacted with HSO F to form the aUytk ion 523. 

Perhaps the simplest example of a homoaromatic cation is the ion C4H5", 11. 

SbFj has also been found useful in the preparation of homoaromatic cations. For example, the simplest 2ir monohomoaromatic cations, the homocyclopropenyl cations, can be prepared from corresponding 3-halocyclobutenes in SbFj (eq 31). ... 

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