Aromaticity 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... 

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. 

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... 

After a brief survey of the history of valence-bond isomers of aromatic compounds, new syntheses and the reactions of these isomers reported in the last decade are reviewed. In the second chapter, the valence-bond isomers of homoaromatic compounds, especially benzene derivatives, are described and in the third chapter those of heterocyclic compounds. Photoreactions of perfluoroalkylated aromatic compounds afford valence-bond isomers in high yields. These isomers are very stable and useful for the synthesis of highly strained compounds. Therefore, the emphasis is put on the chemistry of trifluoromethylated benzvalenes, Dewar thiophenes, and Dewar pyrroles. 

Later on Winstein extended the concept of homoconjugation for a nonclassical MO joining the atoms taking part in its formation into a cycUc system. A particular systems is that of homoaromatic compounds 233,341-344) aromaticity and... 

Since the publication of Winstein s paper, much research has focused on understanding the bonding mode in these molecules, broadening definition of aromaticity as weU as reaction chemistry of homoaromatic molecules. Homoaromatic compounds include cationic, anionic, and neutml homoaromatic molecules. Cationic homoaromatic compounds are relatively more common. The homotropylium cation (CgHg" ) is among the most studied example of a cationic homoaromatic compound. 

There is no doubt that the term aromaticity is one of the most widely used terms in chemistry. At the same time, aromaticity may well be one of the most widely misused terms in chemistry, not by being attributed to compounds that do not qualify as aromatic, but by becoming so unspecified that it is applied to too many compounds that show widely different physico-chemical properties. Labels that are so broad that they apply to a multitude of compounds are bound to cause confusion, just as confusion may arise when there are too many Smiths, Browns, and Jones in the same locality. To reduce the latter confusion, people have been given two and even three names hence, it is understandable that similar attempts were made to discriminate among several types of aromaticity, like pseudo-aromaticity, homoaromaticity, quasi-aromaticity, etc. (Table 2). 

One molecule that might be expected to be homoaromatic, if the phenomenon can exist in neutral species, is triquinacene (Fig. 9.7) the three double bonds are held rigidly in an orientation which appears favorable for continuous overlap with concomitant cyclic delocalization of six n electrons. Indeed, its potential aromaticity was one of the reasons cited for the synthesis of this compound [58], A measurement of the heat of hydrogenation of triquinacene found a value 18.8 kJ mol 1 lower than that for each of the next two steps (leading to hexahydrotriqui-nacene) [59]. This was taken as proof of homoaromaticity in the triene, i.e. that the compound was 18.8 kJ mol-1 (4.5 kcal mol 1) stabler than expected for an... 

The term in-plane aromaticity has been used for molecules such as the didehy-drophenyl cation (see Section II). However, we stress that the compounds in question are homoaromatic rather than aromatic molecules, which can be directly related to a-aromaticity Therefore, the appropriate notation should be homo-o--aromaticity rather than in-plane aromaticity. The concept of c-aromaticity is very controversial. One can completely avoid this term by referring to the mode of electron delocalization as was done by Cremer While 7r-aromaticity and homo-7r-aromaticity are connected with ribbon delocalization of electrons along a conjugative cycle, molecules that have been considered to be either a-aromatic or homo-o--aromatic (in-plane aromatic) seem to prefer delocalization of electrons over a surface defined by the participating atoms (see the discussion in... 

The problem of the reference compound is inherent to most chemical concepts. By definition a suitable reference compound is a compound that possesses the same properties as the target compound with the exception of the electronic and structural features to be investigated. In most cases, such a compound cannot be found since changes in the (electronic) structure automatically lead to changes in all properties and hinder meaningful comparison. This is the reason why many chemical concepts are discussed at a qualitative rather than a quantitative level. In fact, as has been forcefully described by Binsch, attempts to quantify a concept very often lead to the collapse of the whole concept. This potential collapse-by-quantification problem exists for the concepts of homoconjugation and homoaromaticity just as it does for the concept of aromaticity. 

---