Homoaromatic systems

Si4 revealed a distorted tetrahedral configuration, and the Sil-Si2 and Si2-Si3 bond distances of 2.240(2) and 2.244(2) A were intermediate between the Si=Si and Si-Si bond lengths of the precursor 19. This was explained by the delocalization of the positive charge over the Sil, Si2, and Si3 atoms, accompanied by the Sil-Si3 through-space orbital interaction, resulting in the overall homoaromaticity of 20. The hypothesis of homoaromaticity was further supported by the observation of an extremely low-field shifted signal of Si2, the central atom of the Sis homoaromatic system, at 315.7 ppm. 

The cyclobutenyl cation (92) and the homotropylium cation, CgHcf 93 are both prototypes of homoaromatic systems. 

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... 

In the light of the appreciable puckering found in the seven-membered ring of [16], Childs (1984) recalculated the expected chemical shifts for the exo and endo H(8) protons of [12]. He calculated the difference in chemical shift (A6) to be 6.9 ppm which is in good agreement with the observed AS - 5.86 ppm. However, his calculations revealed that both the exo and endo protons are shielded. This surprising result is opposed to the accepted intuitive view that in an aromatic/homoaromatic system protons with the H(8)(exo) orientation should be deshielded and those with the H(8)(cndo) orientation shielded. This result closely parallels the analogous calculated data for the homocyclopropenyl cation [2] (Schindler, 1987). 

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.)... 

Some authors have used the term antihomoaromaticity. We think that this term may be misleading since it implies either a system that, despite homoaromaticity, is destabilized or the anti form of a homoaromatic system, which may be considered as the aromatic form itself. 

Apart from this, it would be appropriate, although never done in practice, to define homoaromaticity with regard to a molecular property (such as energy, geometry, chemical shifts, etc.) in comparison to the reference(s) used. Various molecular properties reflect the special electronic features of homoaromatic systems with different sensitivity. Thus, for example, it is possible that NMR chemical shifts could suggest weak bond (electron) delocalization while an analysis of the molecular energy does not provide any indication of homoaromatic character. 

A better description of homoaromatic systems is provided by bond orders based on the virial partitioning analysis of the total electron density distribution, which we will discuss in the next section. 

In the seventies and eighties, ab initio calculations on potentially homoaromatic molecules were preferentially carried out with the Hartree-Fock (HF) method using minimal or double-zeta (DZ) basis sets. However, neither HF nor small basis sets are appropriate to describe a homoaromatic system. In the case of cyclopropyl homoconjugation, the use of a DZ + P basis set is mandatory since polarization (P) functions are needed to describe the bond arrangements of a three-membered ring. 

For a homoaromatic system, surface delocalization in the cyclopropyl ring is perpendicular to the bridging bond, thus forming a Hiickel aromatic electron ensemble which is delocalized in just one part of the bi(poly)cyclic system. 

For the magnetic susceptibility -y determined as a function of the interaction distance, a maximum value should be calculated for the homoaromatic system, i.e. the exaltation of the magnetic susceptibility should indicate homoaromatic electron delocalization. 

If the system in question possesses a CH2 group located above the ring in a similar way to the case of the homotropenylium cation, the shift difference between endo- and exo-orient ed proton should also adopt a maximum value for the homoaromatic system. 

The description of no-bond homoaromatic systems as frozen transition states is in line with the observation that their PES is rather flat in the direction of the interaction distance. This means that (a) homoaromatic stabilization energies are small (see Table 2 and the discussion presented above) and (b) relatively small energy increases lead to relatively large... 

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. 

In many respects the homotropenylium ion can be considered to be the archetype or benzene of homoaromatic systems. It is not only one of the earliest examples of a homoaromatic system to be described, but well more than forty substituted derivatives of the homotropenylium cation have now been reported69. These substituted ions have been examined by a broad range of experimental techniques and theoretical methods. It is interesting to note that unlike the trishomocyclopropenium cation, the initial homoaromatic system to be studied, characterization of the homotropenylium ion did not rely on a... 

In summary, the four major lines of approach to understanding the properties of the homotropenylium ion all lead to the same conclusion, namely that this ion is a cyclically delocalized, homoaromatic system. It should be stressed that this conclusion has been reached by using a combination of a battery of magnetic, spectroscopic, thermochemical, structural and theoretical techniques and these all give a consistent picture of the nature of the electron delocalization in the cation. 

It is clear from the consistent results of the various approaches used to probe the structure of 41 that this cation can be properly regarded as a homoaromatic system that meets the requirements set out in Section I.A above (see also Chapter 725). Substitution of the cation has also been demonstrated to lead to large changes in the structure and by no means can all the derivatives of 41 be classified as homoaromatic127. This sensitivity of homoaromatic delocalization to substitution parallels that demonstrated with the homotropenylium cations. 

The structures and electron delocalization in these boron-substituted derivatives of the cyclobutenyl/homocyclopropenium cations are fully consistent with their designation as homoaromatic systems. 

While considerable work has been reported on the bicyclo[3.1.0]hexenyl cation and its derivatives, the results of these studies have not been reviewed as extensively as those of the corresponding homoaromatic systems. The most detailed accounts of these systems are those of Koptyug132 and Barkhash133. Numerous reviews on the cyclohexadienyl cations have appeared132 134. 

In summary, the bicyclo[3.1.0]hexenyl cations clearly show that homoconjugation is an important factor in determining the chemistry and properties of these cationic systems. The properties of the bicyclo[3.1. Ojhexenyl cations are sharply different from those of the homotropenylium and cyclopropenium ions, reinforcing the designation of the latter two cations as being examples of homoaromatic systems. 

While 105 and 106 cannot be viewed as resonance structures of a common delocalized species, the question at issue is whether 105 and 106 can individually be regarded as homoaromatic systems In terms of thermochemical evidence, it has been recently pointed out that the available experimental data are among the most precise data available in the thermochemical literature234. The issue comes down to the interpretation of these data and the proper consideration of contributing factors such as strain, etc. 

We would stress that it is important in the consideration of a molecule or ion as a homoaromatic system to use as wide a range of the various criteria we have suggested as is... 

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