An Ab Initio Study of SN2 Reactivity at C6 in Hexopyranose Derivatives. II. Role of Populations, Barriers, and Reaction Path Curvature

This paper continues our investigation into a simple dipole-dipole interaction model proposed to explain the dramatically reduced SN2. reactivity at the primary C6 position of galacto-configured pyranose systems relative to their gluco isomers. We present ab initio calculations (B3LYP/6-31+G(d,p)) on six model structures that show that this effect is not a major influence. Reactant rotameric equilibria as well as free-energy reaction barriers and reaction path curvature were evaluated. Results point to a number of other factors that could account for the observed reactivity differences. Our results cast doubt on the general relevance of transition structure dipole-dipole repulsions to SN2 reactivity

An Ab Initio Study of SN2 Reactivity at C6 in Hexopyranose Derivatives. I. Influence of Dipole-dipole Interactions in the Transition Structure

It is widely accepted that dipole-dipole interactions in the SN2 transition structure can play a dominant role in determining reaction rates. a model of this type was proposed some years ago to explain the remarkably low reactivity of galactopyranose-6-O-sulfonates toward SN2 displacement, and similar arguments have recently been restated in the context of gas-phase reactions. in this paper, we present ab initio calculations (B3LYP/6-31+G(d,p)) on model structures and an analysis of charge densities using the theory of atoms in molecules. We find that the maximum possible impact of local dipole-dipole interactions is insufficient to account for the observed reactivity differences