Homoaromatic molecules

Aromatic compounds exhibit a cyclic conjugated array of it electrons. If the array of adjacent orbitals is intermpted by one or several saturated units (e.g., a CHj group), some degree of aromaticity can still be retained if conjugation goes through a bond or through space. Winstein coined the term homoaromaticity to 

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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 ... 

Photoelectron and electron transmission spectroscopy indicate that there is appreciable interaction between the acetylene units of [129] (Houk et al., 1985). Both homoconjugation and hyperconjugation are proposed. Dewar and Holloway (1984) suggest that the through-bond interactions dominate. Similar thermochemical studies to those performed with the triquinacene series were carried out on [129] and some acyclic homoconjugated acetylenes (Scott et al., 1988). From these data it was concluded that decamethyl[5]pericyclyne should be classed as a homoaromatic molecule. As already discussed for the triquinacene series, the species used as non-homoaromatic models (and the calculated compensations for strain energies) may be inappropriate and thus this conclusion should be treated with some caution. Using our probes for homoaromaticity we were not able to obtain any evidence in support of the homoaromaticity of [129] (Williams and Kurtz, unpublished results). 

FIGURE 6. One-dimensional representations of the potential energy surface (PES) of molecule 42 shown as a function of the interaction distance R. Situation 1 corresponds to the bicyclic molecule 42a, situation 2 to the open monocyclic molecule 42c, situation 3 to the no-bond homoaromatic molecule 42b with non-classical structure and situation 4 to a valence tautomeric equilibrium between 42a and 42c with the homoaromatic form 42b being the transition state. See text... 

Setting out the requirements for homoaromaticity in this manner, it should be easy to distinguish homoaromatic from non-homoaromatic molecules. Clearly, an appropriate geometry or structure of the species in question is required. This pertains not only to the appropriate placement of the AOs at the homoconjugative centres but also to the structural changes associated with the cyclic delocalization of (4q + 2) 7t-electrons. This cyclic delocalization should also be reflected by the stability of the system and its spectroscopic properties, including in particular its NMR spectrum. 

It has also been shown that surface delocalization can adopt a preferential direction if the cyclopropyl group interacts with a -conjugated system. There are basically two directions of surface delocalization as indicated in Scheme 9 for vinylcyclopropane (20) and divinylcyclopropane 47. In homoaromatic molecules, surface delocalization is perpendicular to the 1,3-bond while in homoantiaromatic molecules it is parallel to the 1,3-bond27-85. 

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. 

In the following, we will discuss ab initio descriptions of the prototype of homoaromatic molecules, namely the homotropenylium cation (45). Theoretical work on this cation reflects all the problems involved in an ab initio investigation of homoaromatic compounds44 49 53 56 69 73 79 93117-119... 

The resonance energy of a homoaromatic molecule is < -2 kcalmol 1 and that of a homoantiaromatic molecule > 2 kcalmoT1. Typical values are between 2 and 10 kcal-moT1 indicating that homoconjugative stabilization or destabilization is normally just a matter of a few kcalmol1. 

For the magnetic properties of a no-bond homoaromatic molecule, one can expect typical values. 

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 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. 

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. 

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