Structure and Reactivity of the 1,3-Dioxolan-2-ylium Ion System

Abstract

<p>The 1,3-dioxolan-2-ylium ion is an important intermediate occurring in many carbohydrate transformations. The system has been widely studied, yet conflicting views on the effect of substituents on the ground state structure of 1,3-dioxolan-2-ylium ions have been presented. This thesis embodies the results of a series of investigations, utilizing a number of complementary techniques, to examine the effect of substitution on the structure and reactivity of a homologous series of 1,3-dioxolan-2-ylium ions.</p> <p>The possible use of the 1,3-dioxolan-2-ylium system as a model for the transition state structures of nucleophilic displacement reactions has been explored. The intramolecular nucleophilic attack of an acetate group on a 1,3-dioxolan-2-ylium ion has been investigated for a series of C(4)-aryl substituted cations. A Hammett study revealed that this isomerization reaction proceeds via a step-wise mechanism involving a carbenium ion intermediate. In contrast, when the C(4) substituent is hydrogen or methyl, a concerted isomerization mechanism is operative, as revealed by semi-empirical calculations. It is suggested that when the C(4) substituent is sufficiently electron donating, the system may adopt a trigonal bipyramidal geometry and hence may serve as a model for the Sn2 transition state.</p> <p>Dual substituent parameter (DSP) correlations were used to establish the dependence of the ¹³C chemical shifts of the 1,3-dioxolan-2-ylium ion system on the electron donating power of the C(4)-aryl substituent. These correlations support the inclusion of an ionic resonance contributor to the ground state description of the system. The weight given to the ionic resonance contributor increases with better electron donating C(4)-substituents.</p> <p>X-ray crystallography has been used to determine the solid state structures of two 1,3-dioxolan-2-ylium ion salts with different C(4) substituents. The changes in bond lengths observed in going from a hydrogen to a phenyl substituent at C(4) are partly attributed to the increased importance of the ionic resonance structure in the aryl system.</p> <p>Semi-empirical calculations at the AM1 level revealed the effects of differential C(4)-substitution. Better electron donating substituents lowered the isomerization barrier and increased the relative importance of the ionic resonance contributor in the ground state, in accord with the experimentally determined results. The ability of a CF₃ substituent at C(2) to achieve similar changes in structure and reactivity was also established.</p>