Homoaromatic systems, neutral

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... 

We start with an examination of some examples of acyclic systems in which there is evidence or the possibility of cyclopropyl homoconjugation. We then move on to a broader examination of homoaromatic systems, treating cationic, neutral and anionic systems in separate sections. The results of experimental work and theoretical examinations are integrated so as to provide a cohesive overview of each system. In order to limit the size of the chapter, we refrain from reviewing in detail systems such as the bridged annulenes and radical species. The chapter concludes with a reflective section that seeks to draw together theory with experiment and point out new directions for future work. 

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

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... 

Dissolving 3-aryl(alkyl)-1,2,4,5-tetrazines (21) in liquid ammonia or primary aliphatic amines at — 35°C to — 40°C, followed by addition of potassium permanganate, gives 6-alkylamino-3-aryl-(alkyl)-1,2,4,5-tetrazines (23) in reasonable to excellent yields. This shows that, under the reaction conditions, addition of A-nucleophiles to the tetrazine ring must have occurred (Scheme 1) <8 JHC123>. The presumption of a 1,6-dihydro intermediate (22) rests on the experimental result that tetrazines can be transformed by sodium borohydride into the isolable 1,6-dihydro-1,2,4,5-tetrazines, which can be considered as neutral homoaromatic systems <8UOC2i38>. 

Haddon (1977) reinvestigated this system theoretically and determined that the 1,6 (homoaromatic) interaction was smaller than in the neutral bridged annulenes (vide infra), but nevertheless important. It would seem that, even at the experimentally measured large 1-6 distance, the potential for homoaromatic interaction is realized. 

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). 

Hirsch s rule has more limited applicability than the Hiickel rule. However, the 2(n +1)2 concept has been used very successfully to interpret relative fullerene stabilities [66], and to suggest new systems, including neutral and charged non-fullerene carbon [67] and homoaromatic cages [68]. All these species have large NICS values in their centers and satisfy other criteria of aromaticity. 

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

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. 

However, homoaromatic stabilisation appears to be absent in neutral systems. Homobenzene (cycloheptatriene) 1.23 and trishomobenzene (triquinacene) 1.26, even though transannular overlap looks feasible, show no aromatic properties. In both cases, the conventional structures 1.23 and 1.24, and 1.26 and 1.27 are lower in energy than the homoaromatic structures 1.25 and 1.28, which appear to be close to the transition structures for the interconversion. 

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