Neutral Homoaromaticity

Fig. 5. [5]Pericyclines with conceived cyclic homoconjugation and neutral homoaromaticity...

All this is in line with the most recent finding for triquinacene, for which the direct determination of its AHy(g) from its experimentally measured heat of combustion finally corroborated the results of the most advanced computational studies that triquinacene is not homoaromatic [35]. Evidently, heat of combustion measurements should also be carried out for some representative [nlpericyclines to finally settle the quest for their neutral homoaromaticity. 

Monohomoaromatic neutral species 296 Bishomoaromatic neutral systems 299 Trishomoaromatic neutral systems 308 Higher homoaromatic neutral systems 311 Homoaromaticity in the bridged annulenes 312 Other neutral homoaromatic systems 313... 

Wenkert et al. (1973) proposed, on the basis of l3C NMR evidence, that elassovalene [82] was in fact the first example of a neutral homoaromatic molecule [82a]. Subsequent studies (Vogel et al., 1973 Gunther et al., 1973 ... 

This is perhaps the area where there is the most optimism of attaining the elusive goal of neutral homoaromaticity. It has been suggested that semibullvalene [83], which undergoes degenerate Cope rearrangements through a homoaromatic transition state [83b] (2) with extremely low... 

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

A selective and brief survey of some of the more recent studies follows. Askani et al. (1984), using isotope perturbation methods (Siehl, 1987), showed that both the dimethyl [92] and the tetracarbomethoxy [93] semibullvalenes are Cope systems. In a related study, mild perturbation of the symmetrical semibullvalenes [94] and [95] is achieved by replacing one of the methyl groups with an ethyl group [94a] and [95a]. Again these systems were shown to be rapidly equilibrating tautomers and not neutral homoaromatics (Gompper et al., 1985). 

In response to the prediction by Dewar et al. (1971) that the azasemibull-valenes [100] and [101] should be neutral homoaromatics, the Mullen group prepared the diazasemibullvalene [102]. Although the Cope rearrangement of [102] is more facile than in the all-carbon analogue (replace N with CH in... 

Numerous semiempirical studies have been made of semibullvalene and a wide range of derivatives (see Williams and Kurtz, 1988 Dewar and Jie, 1988, and references cited therein). Agreement with experimental data, where available, is excellent. Several target molecules are predicted to exhibit the elusive goal of neutral homoaromaticity which provides an enticing challenge for synthetic efforts. 

Perhaps the facet of most current interest is that of neutral homoaromatic-... 

There has been considerable controversy over whether there are any existing examples of neutral homoaromatic systems223. Perhaps as a result of the difficulties in this area, a large amount of work has been reported224 1. ... 

In conclusion, the energy, geometry and magnetic properties of 105 are fully consistent with it being classified as a neutral homoaromatic molecule with a small, but significant, resonance energy of 4.5 kcalmol1. 

In summing up this section on neutral homoaromatic compounds we point out that a considerable number of neutral molecules have been identified as benefiting from homo-conjugative electron delocalization. These include cycloheptatriene as well as several bridged derivatives of these molecules. We anticipate that further work on these systems and the related homoantiaromatic bicyclo[2.1. OJpentene will prove rewarding. 

The bishomoaromatic neutral systems are of particular interest. Evidence for the importance of neutral homoaromatic delocalization appears to exist solely with certain substituted semibullvalenes. In terms of the latter systems the best candidates for experimental work appear to be 126 and 127. 

There are no neutral molecules with trishomoaromatic character. This is not surprising, given the small size of the resonance energies associated with neutral homoaromatic molecules and the magnitude of the strain effects associated with a potential trishomoaromatic system. 

A theoretical study was carried out to evaluate some bicyclic heterocyclic systems related to the bicyclo[3.2.11-octane skeleton for evidence of neutral homoaromaticity <2005JOC1998>. Included in the study were phosphines 17 and 18. Consideration of a series of factors including NICS values, diamagnetic susceptibility exaltations, and stabilization energies led to the conclusion that, while bicyclic compound 17 was nonhomoaromatic, the analogue 18 displayed evidence of homoaromaticity. 

The major confusion as to the existence of neutral homoaromatic systems results from the fact that homoaromatic resonance energies, contrary to aromatic resonance energies, are normally less than 10 kcalmoF 225,226 general it is difficult to separate homoaromatic... 

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