Homoantiaromaticity

We are not distinguishing between through-space, through-bond, or simply steric mechanisms, nor discussing concepts such as homoantiaromaticity or any other prefixed or hyphenated aromaticity phenomena as explanations for stabilization or destabilization of any of the aforementioned species. 

The energetics consequence of homoaromaticity and homoantiaromaticity are much more readily documented in ions than in neutrals see, for example,... 

Cyclopropyl homoconjugation, homoaromaticity and homoantiaromaticity— Theoretical aspects and analysis... 

Following Section III, there will be a section (Section IV) in which the basic requirements for an ab initio investigation of homoconjugated molecules are sketched. As an illustrative example, the ab initio investigation of the homotropenylium cation will be described in detail where special emphasis is laid on an assessment of those molecular properties that are a direct reflection of the homoaromatic character of the molecule. The section will close by deriving detailed defini tions and requirements for homoaromaticity and homoantiaromaticity that are adjusted to the more recent developments in the field. 

FIGURE 5. From bond and no-bond homoconjugation to bond and no-bond homoaromaticity or homoantiaromaticity. In each case the number of cyclically delocalized electrons (4q or 4q + 2) is given... 

In the case of homoantiaromaticity, characterization is more difficult. Antiaromaticity describes a situation in which electron delocalization leads to destabilization. Clearly, if through-space interactions would close a cyclic system to form an antiaromatic electron ensemble, the molecule would adopt another conformation that would help to avoid antiaromatic electron delocalization. Of course, steric factors may enforce through-space interactions as in 40 (Figure 5). However, simple deformations of the molecule can reduce through-space interactions to an insignificant level. 

Formally, no-bond homoantiaromaticity cannot be ruled out. However, de facto it will not play any major role in determining the chemistry of 4[Pg.361]

We conclude that each case of potential bond or, more specifically, cyclopropyl homoantiaromaticity has to be considered separately. Detailed investigations have to clarify whether homoantiaromaticity is of any chemical relevance or whether the molecule has reorganized into a non-homoaromatic electronic structure. 

Hehre66 69 and independently Jorgensen72,73 pointed out that the Mobius and Hiickel description of homoconjugated molecules (Figure 9) is consistent with the assumed homoaromtic and homoantiaromatic character of these compounds. However, it was also realized that in the general case such a classification might not be sufficient to describe subtle differences in orbital interactions, which determine the homo(anti)aromatic character of a molecule. 

In the potentially homoantiaromatic molecules of Figure 11, electron delocalization occurs along the periphery of a bicyclic system, involving in this way Aq + 2 rather than 4 q electrons. Since, however, the corresponding orbital system is of Mobius rather than Hiickel type (Figure 9), delocalization of 4q + 2 electrons leads to overall destabilization rather than stabilization. 

Obviously, the density description suggests that homoantiaromatic molecules prefer Mobius 4q + 2 electron systems rather than Htickel 4q systems. This is in line with the PMO analyses of Hehre66,69 and Jorgensen72-73 (see Section III.C). 

It has also been shown that surface delocalization can adopt a preferential direction if the cyclopropyl group interacts with a -conjugated system. There are basically two directions of surface delocalization as indicated in Scheme 9 for vinylcyclopropane (20) and divinylcyclopropane 47. In homoaromatic molecules, surface delocalization is perpendicular to the 1,3-bond while in homoantiaromatic molecules it is parallel to the 1,3-bond27-85. 

A final correction is needed because the vinylcyclopropane units in 27 or 10 do not adopt the stable trans forms but less stable gauche forms. This leads to a reduction of the homoconjugative resonance energy by another 2 kcal mol-1. The final resonance energy is 9.7 kcal moT1, clearly indicating the homoantiaromatic character of 27. 

In a similar way, a homoantiaromatic system formed by bond (cyclopropyl) homoconjugation can be described. There is, however, one major difference between homoaromatic and homoantiaromatic systems (observed in the case of cyclopropyl homoconjugation) that separates homoaromaticity from aromaticity. While aromaticity and antiaromaticity involve different numbers of electrons (4q + 2 or Aq), homoaromaticity and homoantiaromaticity both involve Aq + 2 electrons but differ with regard to the delocalization modes of these electrons, which are best described by the direction of surface delocalization in a three-membered ring (Scheme 15) ... 

For a homoantiaromatic system, surface delocalization in the cyclopropyl ring is parallel to the bridging bond, thus forming a Mobius antiaromatic electron ensemble delocalized along the periphery of the bi(poly)cyclic ring system. 

SCHEME 15. Surface delocalization in homoaromatic and homoantiaromatic molecules. Major axes of bond ellipticities are indicated by arrows the direction of surface delocalization in the three-membered ring is given by a bold arrow... 

The resonance energy of a homoaromatic molecule is < -2 kcalmol 1 and that of a homoantiaromatic molecule > 2 kcalmoT1. Typical values are between 2 and 10 kcal-moT1 indicating that homoconjugative stabilization or destabilization is normally just a matter of a few kcalmol1. 

Contrary to bond homoantiaromaticity, very little is known about no-bond homoantiaromaticity123. In a potentially no-bond homoantiaromatic molecule, there is often the... 

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