Please use this identifier to cite or link to this item:
|Title:||Thermochemical and mechanistic investigation of carbocation isomerizations|
|Authors:||Despard, Alexander Nicholas|
|Abstract:||<p>This thesis describes experiments focussed on the isomerizations of carbocations, in particular, hydroxy-substituted carbocations produced by the protonation of ketones or phenols. Three classes of isomerization were investigated with the goal of determining the effect of substituents on the energetics of isomerization (carbocation stability, activation energy) and to probe the mechanisms of these reactions. It was proposed that differential scanning calorimetry (DSC) could be used to measure heats of isomerization to determine the effect of substituents on the energies of carbocations. Results presented in chapters 2, 3 and 4 demonstrate that DSC is a reliable technique for the determination of accurate and reproducible heats of isomerization of carbocations in superacid solution. As described in chapter 2, methyl migrations in cyclohexadienyl, cyclohexenyl and cyclopentenyl cations were investigated and the effect of moving methyl groups on to charge bearing positions of the carbocations was quantified. In addition, the energetic advantage of having a hydroxy group on the central 3-position rather than the terminal 1-position of the pentadienyl cation π-system was determined. Methyl substitution has pronounced effects upon the energetics of the isomerization of protonated bicyclo[3.1.0]hexenones to protonated phenols or cyclohexadienones and these results are described in chapter 3. These effects are consistent with the known charge distributions in the product and reactant cations. In addition, it was shown that the improved charge stabilization in the cyclohexadienyl cation product is a driving force for the isomerization, in contrast to the conclusions of an earlier study. A series of protonated cyclopropyl ketones in triflic acid were found to undergo ring expansion to oxolanylium ions except for two bicyclic ketones where steric constraints prevented ring expansion and ring opening to protonated enones occurred instead. The course of each reaction was determined and the reactant and product cations were characterized by 1 H and 13 C NMR spectroscopy. For use in later mechanistic studies, the stereochemistry of a set of dimethyl substituted cyclopropyl and oxolanylium compounds were established. The heats of isomerization of protonated cyclopropyl ketones were measured and found to fall in a narrow range which indicated that substituent effects were similar in the reactant and product cations. The substituents on the cyclopropyl ring and on the carbonyl carbon affected the rates of ring expansion of protonated cyclopropyl ketones which, provided information about the charge distribution in the reactant and the transition state, and the charge stabilizing abilities of the substituents. The list of possible mechanistic pathways was narrowed with a combination of information (product studies, substituent effects, entropies of activation). Finally, evidence is provided for the occurrence of a concerted 1,3-sigmatropic migration during the ring expansion of protonated cyclopropyl ketones in triflic acid. However, other reaction pathways can be made competitive with minor changes in reactant structure or reaction medium.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
Items in MacSphere are protected by copyright, with all rights reserved, unless otherwise indicated.