Homoaromatic compounds

Homoaromatic Compounds. When cyclooctatetraene is dissolved in concentrated H2SO4, a proton adds to one of the double bonds to form the homotropylium ion (107). In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The eighth carbon is an sp carbon and so cannot take part in the aromaticity. The NMR spectra show the presence of a diatropic ring current H/, is found at 5= - 0.3 at 5.1 6 Hj and H7 at... 

H2—He at 8.5 5. This ion is an example of a homoaromatic compound, which may be defined as a compound that contains one or more sp -hybridized carbon atoms in an otherwise conjugated cycle. °... 

In order for the orbitals to overlap most effectively so as to close a loop, the sp atoms are forced to lie almost vertically above the plane of the aromatic atoms. In 107, Hb is directly above the aromatic sextet and so is shifted far upfield in the NMR. All homoaromatic compounds so far discovered are ions, and it is questionable as to whether homoaromatic character can exist in uncharged systems. Homoaromatic ions of 2 and 10 electrons are also known. 

Two-/r-electron (n = 0) bis-homoaromatic compounds, such as bis(homotriborirane) anions 68, were... 

Homoaromaticity may still result if the delocalization in an aromatic compound is interrupted by more than one saturated linkage. In this case a bis-, tris-, or tetra-, etc., homoaromatic compound results. In the notation of Winstein (1967) the size of the saturated linkage (e.g. -CH2- and -CH2CH2-) is not considered in classifying the degree of homoaromaticity. Only the number of interruptions to delocalization is taken into account. Thus, if cycloheptatriene [5] were homoaromatic, it would be monohomobenzene. Similarly, all m-l,4,7-cyclononatriene [8] could be named trishomobenzene if homoaromatic. 

Homoaromatic compounds. When cyclooctatetraene is dissolved in concentrated H2S04, a proton adds to one of the double bonds to form the homotropylium ion 94.247 In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The... 

It is interesting to consider further the relationship between situations 3 and 4. Situation 3 will be reached if the transition state in 4 is sufficiently stabilized so that its relative energy drops below those of the valence tautomeric forms 42a and 42c. In other words, situation 3 corresponds to a frozen transition state 7. A no-bond homoaromatic compound is simply the realization of a frozen TS. 

Concepts such as the frozen transition state 47 or the frozen reaction path (Biirgi-Dunitz reaction path)48 were developed to obtain direct experimental information on transition states. So far, all attempts have failed to realize a frozen transition state experimentally, Cremer and coworkers49 58 have shown that a no-bond homoaromatic compound (PES situation 3), such as the homotropenylium cation, corresponds to a frozen transition state, and therefore its investigation provides ample information about the properties of transition states. 

If a molecule with no-bond homoaromaticity is investigated, the system in question possesses a non-classical structure with an interaction distance typical of a transition state rather than a closed-shell equilibrium structure. One can consider no-bond homoconjugative interactions as a result of extreme bond stretching and the formation of a singlet biradical, i.e. a low-spin open-shell system. Normally such a situation can only be handled by a multi-determinant description, but in the case of a homoaromatic compound the two single electrons interact with adjacent rc-electrons and form together a delocalized electron system, which can be described by a single determinant ab initio method provided sufficient dynamic electron correlation is covered by the method. 

With the available ab initio methods, one can also calculate infrared, Raman and ultraviolet spectra as well as many other molecular properties. However, none of these properties has been used extensively in investigations of homoaromatic compounds and therefore we refrain from discussing basic requirements in calculating them by ab initio methods. 

Schleyer was the first to fully realize the sensitivity of calculated NMR chemical shifts with regard to molecular geometry and use this for ab im n o/IGLO/NMR-based structural determinations in many cases"6. Cremer and coworkers realized the usefulness of this approach for the determination of geometries of potentially homoaromatic compounds not amenable to experiment49-58. 

Potential energy surface (PES) scans. Ab initio research on homoaromatic compounds always requires some exploration of the PES rather than just the investigation of equilibrium geometries. As discussed in Sections II.D and IV.B, it is essential to determine the shape of the PES as a function of homoconjugative interaction distances. The number and location of all stationary points have to be determined so that one can distinguish... 

Investigation of environmental effects. As has been stressed in this chapter, homoaromaticity is just a matter of a few kcal mol-1 stabilization energy in most cases, and therefore environmental effects may have a large impact on structure, stability and other properties of a homoaromatic compound. Future work in theory (as well as in experiment) has to clarify how environmental effects can influence electron delocalization, through-space interactions and bonding in homoaromatic molecules. The theoretical methods are now available to calculate solvent and counter ion effects (for homoaromatic ions in solution) or to study intermolecular and crystal packing forces in the solid state. 

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. 

Early MO descriptions of homoaromatic compounds were based on Huckel MO (HMO) theory. Through-space interactions between interacting C centres were modelled by assuming a value for the resonance integral For example, in the case of the homotropenylium cation, Winstein took P (C1,C7) = 0.5 and obtained a resonance energy comparable to that of the tropenylium cation. He concluded that, despite the insertion of the CH2 group into the Ti-system of the tropenylium cation, delocalization of TT-electrons is largely retained. 

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 ". ... 

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