Bicyclo hexenyl cations homoconjugation

In the 4q case, 6 is not an important contributor to the ground state description of the properties of either 4 or 5. However, with 4 there are alternative modes of homoconjugation possible that involve the external cyclopropane bonds30,32,37 38. This is shown in Scheme 2 for the bicyclo[3.1.0]hexenyl cation. This alternative mode of conjugation of a cyclopropane in a 4q situation, an option not available to the parent 4q antiaromatic unsaturated ring systems39, leads to a fundamentally different set of properties and reactions of these systems as compared to the potentially homoaromatic 4q + 2 cases. 

In summary, the bicyclo[3.1.0]hexenyl cations clearly show that homoconjugation is an important factor in determining the chemistry and properties of these cationic systems. The properties of the bicyclo[3.1. Ojhexenyl cations are sharply different from those of the homotropenylium and cyclopropenium ions, reinforcing the designation of the latter two cations as being examples of homoaromatic systems. 

Bicyclo[2.1.0]pentene, 116, can be considered to be the prototype of a neutral homoantiaromatic molecule. The types of structural and bonding effects found for this molecule parallel in many respects those found for the bicyclo[3.1.0]hexenyl cation reported above. Further studies on both of these 4q systems will likely be rewarding in terms of fully understanding the nature of cyclopropyl homoconjugation and homoantiaromaticity. 

FIGURE 14. CC bond orders n and bond ellipticities a of cyclopropyl homoconjugated molecules (a) norcaradiene, (b) bicyclo[2.1.0]pentene, (c) bicyclo[3.1.0]hexenyl cation. On the right, the preferred mode of electron delocalization is indicated by dashed lines. Also given is the number of delocalized electrons as calculated from topological bond orders. See text... 

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