Annulenes 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... 

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. 

The homoaromatic interaction in other bridged annulenes has also been examined. The dications of several bridged annulenes were prepared and also studied theoretically and by NMR spectroscopy (Mullen et al., 1987 Wallraff et al., 1988). Once again homoaromatic interactions were deemed to be most important in determining the properties of these systems. Another cationic polycyclic potential homoaromatic system was investigated by Murata and Nakasuji (1980). They concluded, from NMR studies, that homoaromaticity was unimportant in the homophenalenyl cations [66], [67] and [68], (They reached the same conclusion for the corresponding anions.)... 

In addition to the major classes of homoaromaticity considered so far, in which the cyclic conjugated array is interrupted by a saturated linkage(s), there is another class of homoaromaticity. In this class a regular cyclically conjugated system is perturbed by homoconjugation(s) [transannular interaction )] (Scott, 1986). This latter class is exemplified by the bridged annulenes. 

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. 

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. 

As typical of many other attempts to describe homoconjugative interactions with the help of bond orders, we mention here recent investigations of Williams, Kurtz and Farley ". These authors used various semi-empirical methods (MNDO, AMI, MINDO-CI, AMI-Cl) to study cycloheptatriene, l,6-methano[10]annulene, elassovalene and some other potentially homoaromatic compounds. For the 1,6 interactions in cycloheptatriene and l,6-methano[10]annulene, small bond orders <0.1 were calculated suggesting the absence of homoconjugative interactions although homoaromatic character is generally accepted in the case of the l,6-methano[10]annulene. The authors concluded from this that bond orders seem to be of no use as possible discriminators of homoconjugative interactions ". ... 

A homoaromatic structure also best explains the n.m.r. characteristics of the anion 75 (see Table 15), which was derived from 1,e-methanols 10] annulene 219>. This 10r9C system has peripheral protons which absorb from t 3.19 to 5.28 (or ca. t 2.1 to 4.2 on correction for charge density), and bridge protons which are about 2.5 ppm more shielded than the corresponding protons in 65, the compound formed on protonation of the anion. 

A potentially anti-homoaromatic species (79, see Table 16) has been generated by protonation of l,6-methano-[10]annulene at the 2-position 224>. As expected, 1,3 overlap is not significant and the compound can be formulated as a classical carbonium ion. In fact the abnormally large H(2a) to H(2a) coupling constant of —24.9 Hz indicates that the 1,3 distance has been maximized in order to avoid the expected destabilization, and thus there is no evidence for a paramagnetic ring current in this system. 

For further discussion of homoaromatic systems, see (a) McEwen, A. B., Schleyer, P. v. R. (1986). In-plane aromaticity and trishomoaromaticity a computational evaluation. Journal of Organic Chemistry, 51,4357-4368. (b) Allan, C. S. M., Rzepa, H. S. (2008). Chiral Aromaticities. A Topological Exploration of Mobius Homoaromaticity. Journal of Chemical Theory and Computation, 4, 1841-1848. (c) Allan, C. S. M., Rzepa, H. S. (2008). Chiral Aromaticities. AIM and ELF Critical Point and NICS Magnetic Analyses of Mobius-Type Aromaticity and Homoaromaticity in Lemniscular Annulenes and Hexaphyrins. Journal of Organic Chemistry, 73, 6615-6622. 

It suggests that it is not the size of the ring but the number of electrons present in it determines whether a molecule would be aromatic or antiaromatic. In fact the molecules with An+ 2) n electrons are aromatic whereas with (An, 0) n electrons are antiaromatic. Thus, benzene, cyclopropenyl cation, cyclobutadiene dication (or dianion), cyclopentadie-nyl anion, tropylium ion, cyclooctatetraene dication (or dianion), etc. possess (4 + 2) ti electrons and hence aromatic whereas cyclobutadiene, cyclopentadienyl cation, cycloheptatrienyl anion, cyclooctatetraene (non-planar) etc. have An n electrons which make them antiaromatic . Systems like [10] annulene are forced to adopt a nonplanar conformation due to transannular interaction between two hydrogen atoms and hence their aromaticity gets reduced even if they have (An + 2)n electrons. On the other hand the steric constraints in systems like cyclooctatetraene force it to adopt a tube-like non-planar conformation which in turn reduces its antiaromaticity. Various derivatives of benzene like phenol, toluene, aniline, nitrobenzene etc. are also aromatic where the benzene ring and the n sextet are preserved. In homoaromatic " systems, like cyclooctatrienyl cation, delocalization does not extend over the whole molecule. 

N.M. R. Spectra of the Quasi-Annulenes Table 15. Homoaromatic systems... 

Despite many controversial arguments regarding the definition and physical origin of aromaticity [1,15-17], the concept of aromaticity has crossed the boundary of benzenoid hydrocarbons [with (4n + 2)ji -electrons] to include heterosystems [50] like pyridine, thiophine, cations such as tropylium [12] and cyclopropenium [13], anions like cyclopentadienyl [51], organometallic systems, namely ferrocene [52], purely carbon-free systems [53,54], namely Pj, [(P5)2Ti]. The three-dimensional aromaticity of boron-based clusters [55] and of fullerenes [56], the homoaromaticity of cationic systems [57], aromaticity of triplet state annulenes [58], and pericyclic transition states [59] has enlarged the concept of aromaticity. Extension of aromaticity... 

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