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|Title:||Homoaromaticity in homotropylium cations|
|Abstract:||<p>Investigations on the structure and the stability of homotropylium cations are presented in this thesis. These studies were necessitated by the lack of direct experimental evidence for homoaromatic delocalization and stabilization. The more commonly used NMR method has been shown to be inadequate. The structure of 2-hydroxyhomotropylium was determined. Inspite of the crystallographic disorder encountered, the cyclopropane bond lengths could be obtained. The internal cyclopropane bond (1.626(8) Å) was found to be longer than the other two cyclopropane bonds (1.488(7) Å). The structure of this cation was found to be in very good agreement with that calculated by Haddon for the parent homotropylium cation using MIND0/3. On the basis of the close similarity of the solid state and solution ¹³C NMR spectra of the 2-hydroxyhomotropylium cation, it was concluded that this cation has the same structure in both solution and the solid state. Structural studies on other model compounds fully confirm that the homotropylium cation is homoaromatic. A calorimetric approach was used to gauge the energetic importance of homoaromaticity. Heats of protonation of several alicyclic ketones were measured. The heat of protonation of 2,3-homotropone was found to be more exothermic than that of cyclohepta-2,4-dienone by 2.9 kcal/mole. It was concluded that homoaromatic delocalization results in a significant stabilization of the homotropylium cation. The chemistry of the homotropylium cations was examined. The acid catalyzed rearrangements of 8,8-dimethyl-2,3-homotropone and the circumambulatory rearrangements in homotropylium cations were studied in detail. Site preference for the methyl and hydroxy substituents in the seven-membered ring was probed by estimating the relative energies of the four possible positional isomers. It was found that these substituents prefer positions 2 and 4. The protonation of homotroponeirontricarbonyl complexes was carried out in different acid media to contrast the properties of the classical cations generated with those of the homotropylium cations. A new approach, based on the experimental determination of the structure and energy of the homoaromatic system under consideration, has been proposed in order to define homoaromaticity.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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