Homoaromaticity example

Homoaromaticity is a term used to describe systems in which a stabilized cyclic conjugated system is formed by bypassing one saturated atom. The resulting stabilization would, in general, be expected to be reduced because of poorer overlap of the orbitals. The properties of several such cationic species, however, suggest that substantial stabilization does exist. The cyclooctatrienyl cation is an example ... 

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. °... 

Thus, homoaromaticity is a special example of energy-lowering homoconjugation in which delocalization through space leads to a closed cyclic array of (An + 2) electrons. 

As stated at the beginning of this Introduction, (homo)aromaticity refers to a special (thermodynamic) stability relative to some hypothetical reference state. It is therefore most attractive to use a thermochemical discriminator for the designation of homoaromaticity. However, such thermochemical methods suffer the same disadvantages when applied to homoaromaticity as they do in the case of aromaticity (see for example Garratt, 1986 Storer and Houk, 1992). There have been several recent studies using the heats of hydrogenation of potential homoaromatics in an attempt to classify these species (vide infra). Due, in the main, to the hypothetical nature of the localized model reference states there is some debate regarding these results (see Dewar and Holder, 1989 Storer and Houk, 1992). 

Various transition metal complexes of [5], [75] and their derivatives have been examined (for example, see Abel et al., 1958 Dunitz and Pauling, 1960 and Beddoes et al., 1970). Bleck et al. (1970) considered the available data on [78] were best explained in terms of a homoaromatic structure. 

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 ... 

Numerous derivatives of these colourfully named compounds have been synthesized and studied in the hope of lowering the energy of the homoaromatic transition state to the point where this symmetrical species becomes the ground state (for example, see Quast et al., 1986, and... 

Following the calculations (Williams and Kurtz, 1988) which led to the prediction that the bisannelated semibullvalene [108] would be homoaromatic, the Mullen group prepared the first (and, to date, only) example of a bisannelated semibullvalene ([109]) (Kohnz et al., 1989). The Cope rearrangement in [109] is extremely facile the estimated upper limit for the free energy of activation is 3.6 kcal mol-1. 

A range of variously substituted cis- and fram -cr-bishomobenzenes has been isolated or prepared as reactive intermediates (see, for example, Prinzbach and Stusche, 1970 Dalrymple and Taylor, 1971 Prinzbach et al., 1971 Whitlock and Schatz, 1971 Prinzbach and Schwesinger, 1972 Hashem and Weyerstahl, 1981) and none of these systems were proposed to be homoaromatics. Similarly, homoaromaticity is not considered to be of any significance in the cyclooctadienyne [110] (Meier et al., 1985). 

The primary piece of information obtained from most theoretical calculations is the molecular structure. If homoaromatic interactions are important in a molecule, they may cause the molecule to adopt an unusual geometry. In suitable radicals, ESR evidence has been taken to indicate systems of high symmetry which in turn has been interpreted in terms of homoaromatic interactions (Dai et al., 1990). A computational example of this effect is shown in the semiempirical calculations of Williams and Kurtz (1988) on the bisannelated semibullvalene [108]. Here simple configuration interaction... 

Another approach to evaluating homoaromaticity is to compute various reaction properties such as heats of reaction. A typical example of this approach is a recent paper by Storer and Houk (1992) using molecular mechanics calculations (MM2) of the heats of hydrogenation of triquinacene [118]. In this study they conclude that the anomalous heat of hydrogenation can be explained without invoking homoaromaticity. The use of this type of computational data suffers the same problems as experimentally measured values there is an ambiguity with regard to separation of structural and electronic effects and how to choose appropriate reference systems. 

NMR chemical shifts are also used to demonstrate the effects of homoaromatic interactions and they may be obtained from theoretical calculations. An example of this approach is the calculations of Svensson et al. (1991) on the 1,4-bishomotropylium ion, [45, R=H], They used the IGLO method of Schindler and Kutzelnigg (1982) to calculate the chemical shifts. 

As described, other nucleophilic reactions in the anthraquinone series also involve the production of anion-radicals. These reactions are as follows Hydroxylation of 9,10-anthraquinone-2-sulfonic acid (Fomin and Gurdzhiyan 1978) hydroxylation, alkoxylation, and cyanation in the homoaromatic ring of 9,10-anthraquinone condensed with 2,1,5-oxadiazole ring at positions 1 and 2 (Gorelik and Puchkova 1969). These studies suggest that one-electron reduction of quinone proceeds in parallel to the main nucleophilic reaction. The concentration of anthraquinone-2-sulfonate anion-radicals, for example, becomes independent of the duration time of the reaction with an alkali hydroxide, and the total yield of the anion-radicals does not exceed 10%. Inhibitors (oxygen, potassium ferricyanide) prevent formation of anion-radicals, and the yield of 2-hydroxyanthraquinone even increases somewhat. In this case, the anion-radical pathway is not the main one. The same conclusion is made in the case of oxadiazoloanthraquinone. 

The energetics consequence of homoaromaticity and homoantiaromaticity are much more readily documented in ions than in neutrals see, for example,... 

An example of the inherent complexities in assigning homoaromaticity in neutral hydrocarbons, even when accompanied by thermochemical, molecular mechanical and/or quantum chemical analysis, is shown by the competing studies of the energetics of triquinacene ... 

Following Section III, there will be a section (Section IV) in which the basic requirements for an ab initio investigation of homoconjugated molecules are sketched. As an illustrative example, the ab initio investigation of the homotropenylium cation will be described in detail where special emphasis is laid on an assessment of those molecular properties that are a direct reflection of the homoaromatic character of the molecule. The section will close by deriving detailed defini tions and requirements for homoaromaticity and homoantiaromaticity that are adjusted to the more recent developments in the field. 

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. 

The homotropenylium cation is the prima facie example of homoaromaticity, and therefore any useful definition of homoaromaticity has to cover this example. Haddon has calculated [at the HF/6-31G(d) level of theory using 5 d functions] the PES of the homotropenylium cation as a function of the 1,7 interaction distance by optimizing the geometry of the molecule for fixed values of R (1,7)M. The results of his POAV analysis are summarized in Figure 8. 

Neither 44 nor 45 possesses a homoaromatic covalent bond between the interacting C atoms and, accordingly, these cations represent examples of no-bond homoconjugation... 

This is a quantitative definition of homoaromaticity that is generally applicable and helps to specify exactly the point Rb in Figure 3, at which cyclopropyl homoconjugation starts. However, this definition is much more stringent than Winstein s definition because it excludes all those systems with 1,3-interactions that do not lead to a bond path (no-bond homoaromaticity). Hence, it describes homoaromaticity only for the case of cyclopropyl homoconjugation. For example, Kraka and Cremer have used this approach to describe cyclopropyl homoconjugation in norcaradiene (10)27 54. 

Bond homoaromaticity is literally identical with cyclopropyl homoaromaticity since no examples involving cyclobutyl or other rings are reported in the literature. Nevertheless, it is advisable to define bond homoaromaticity in a general way that leaves open the question whether there is any bond homoaromaticity beyond cyclopropyl homoaromaticity. 

Since the original development of the concepts of homoconjugation and homoaromaticity there has been a very large amount of work carried out to probe, test and find other examples of molecules or ions whose properties can be understood in this context. Several reviews of this work have appeared14-22. 

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