Homoaromaticity general

Homoaromaticity is a term used to describe systems in which a stabilized cyclic conjugated system is formed by bypassing one saturated atom. The resulting stabilization would, in general, be expected to be reduced because of poorer overlap of the orbitals. The properties of several such cationic species, however, suggest that substantial stabilization does exist. The cyclooctatrienyl cation is an example ... 

The general conclusion from the foregoing is that the semibullvalene nucleus provides the most promising candidate for neutral homoaromaticity. 

In addition, one can classify homoaromaticity with regard to the number of electrons involved, i.e. 2, 6,10 or, in general, 4q + 2 electrons. 

The experimental assessment of homoaromaticity is often based on working definitions of homoaromaticity that are influenced by the context of the experimental measurements and available reference data. Such a working definition may be incompatible with other definitions, be limited to a small set of related compounds and frequently be rather vague with regard to a general understanding of homoaromaticity. However, it can be useful to... 

In the following, rather than discussing the many definitions of homoconjugation or homoaromaticity that have been expressed by various experimentalists, we compare the few theoretically based attempts to derive a more general definition of homoaromaticity. 

In view of the vague knowledge of bonding in the sixties, we consider it more appropriate to stress Winstein s general understanding of homoaromaticity, which covered both bond and no-bond homoaromaticity14. Winstein s requirements for homoaromaticity can be listed as follows ... 

This is a quantitative definition of homoaromaticity that is generally applicable and helps to specify exactly the point Rb in Figure 3, at which cyclopropyl homoconjugation starts. However, this definition is much more stringent than Winstein s definition because it excludes all those systems with 1,3-interactions that do not lead to a bond path (no-bond homoaromaticity). Hence, it describes homoaromaticity only for the case of cyclopropyl homoconjugation. For example, Kraka and Cremer have used this approach to describe cyclopropyl homoconjugation in norcaradiene (10)27 54. 

Bond homoaromaticity is literally identical with cyclopropyl homoaromaticity since no examples involving cyclobutyl or other rings are reported in the literature. Nevertheless, it is advisable to define bond homoaromaticity in a general way that leaves open the question whether there is any bond homoaromaticity beyond cyclopropyl homoaromaticity. 

Although the structural elements supporting cyclopropyl homoaromaticity and nobond homoaromaticity are now generally understood, it is not clear under what conditions a homoconjugated molecule will prefer to occupy a single minimum or to adopt classical forms connected by a valence tautomeric equilibrium. Of course, one can explain that the norcaradiene/cycloheptatriene system is characterized by a valence tautomeric equilibrium while the homotropenylium cation possesses a single minimum PES. This has simply... 

In general, there is no strong evidence to support homoaromatic formulations of the structures of any of these systems. There are indications from PE spectroscopy of some degree of interaction between the unsaturated fragments of these molecules. However, as we have pointed out, PE spectroscopy as a technique has limited value in probing homoaromaticity. Magnetic evidence has either not been examined in detail in most systems or, where chemical shifts have been examined, is not definitive. Thermochemical... 

Homoaromatic delocalization in 133 was initially invoked in order to account for its enhanced stability and NMR properties273. However, this explanation was challenged in 1981 by two different groups. On the basis of calculations Grutzner and Jorgensen24 (MINDO/3) as well as Mayr, Schleyer and colleagues273 (MNDO and STO-3G) concluded independently that the properties of 133 could be accounted without resort to homoaromatic delocalization. Moreover, they also stated more generally that homoaromatic stabilization was not expected to be an important phenomenon in anions. 

There are other anions for which the claim of homoaromatic delocalization has been made. Work on these systems is relatively old and has been reviewed extensively14 21. Overall, it is not clear there are any good examples of anions which are homoaromatic. Perhaps, with futher work, 133 will be demonstrated to be an example however, it is clear that homoaromatic delocalization is not generally going to be an important phenomenon in carbanions. 

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