Homoaromaticity Winstein

In view of the vague knowledge of bonding in the sixties, we consider it more appropriate to stress Winstein s general understanding of homoaromaticity, which covered both bond and no-bond homoaromaticity. Winstein s requirements for homoaromaticity can be listed as follows ... 

The term homoconjugation (a through-space interaction of orbitals) was introduced by Winstein and Adams (1948) in connection with the cholesteryl cation [1]. Applequist and Roberts (1956) were the first to progress to the concept of homoaromaticity in considering the homocyclopropenium cation [2], Soon after this Doering et al. (1956) invoked pseudoaromatic ... 

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. 

Grutzner and Winstein (1968) proposed one of the more interesting homoaromatic anionic systems — the bicycloaromatic system [147]. Their evidence was equivalences in the lH NMR spectrum that could not be separated over a temperature range of -35°C to 100°C. If this molecule is undergoing a rapid conversion, then the barrier is less than 11.8 kcal mol-1. 

It is interesting to apply this definition, which is a clear improvement over Winstein s original definition of homoaromaticity, to a particular case, namely the homotropenylium cation. 

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. 

The trans isomer 76 was found to be more stable than the cis form 75 by 6 kcalmol"1. Since the experimental 13C NMR shift values exclude the possibility of a closed form, it would appear that Warner and Winstein prepared 76 rather than 75. Cremer and coworkers also pointed out that agreement between the calculated and experimental 13C NMR spectrum can only be obtained by re-assigning the shifts assigned to C(l) and C(2)183. When this is done, the chemical shifts resemble those of 77 rather than the homoaromatic models 11 or 71. This conclusion is supported by the calculated geometry of 76 which suggests the existence of an almost isolated double bond that does not interact with the pentadienyl system (Scheme 32)l83. 

The concept of homoaromaticity29 1001-1006 was advanced by Winstein in 1960. It represented a challenge to experimental and theoretical chemists alike. - The question of homoaromatic overlap has been mainly studied in six-Jt-electron Hiickeloid systems,29 1001 1006 although several two-jt-electron homoaromatic systems have been discovered subsequently.230 1006 1010-1013... 

The concept of homoaromaticity was introduced by Winstein to account for the relative stability of molecules in which the cyclic conjugation of n orbitals is... 

The term homoaromatidty has been coined by Winstein [43, 44]. The rupture of a cydic conjugation due to the insertion of a saturated fragment such as CH2 partly preserves the aromatic stabilization of the original aromatic molecule or ion. Winstein suggested that homoaromaticity, a type of aromaticity, is found for cations that have neither the u-electron... 

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. 

Cremer and coworkers have shown that the analysis of p(r) provides a basis for a rigorous definition of homoaromaticityUtilizing the definitions of covalent bonding, bond order, Ti-character and Ti-delocalization (Section III.E), they translated Winstein s definition of homoaromaticity " (Section III.A) into density language " ... 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

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