title: Activation of the SN2 Reaction by Adjacent π Systems: The Critical Role of Electrostatic Interactions and of Dissociative Character.
abstract: The activation of the SN2 reaction by π systems is well documented in textbooks. It has been shown previously that this is not primarily due to classical (hyper)conjugative effects. Instead, π-conjugated substituents enhance favorable substrate-nucleophile electrostatic interactions, with electron-withdrawing groups (EWG) on the sp(2) system leading to even stronger activation. Herein we report computational and experimental results which show that this activation by sp(2) EWG-substitution only occurs in a fairly limited number of cases, when the nucleophile involves strong electrostatic interactions (usually strongly basic negatively charged nucleophiles). In other cases, where bond breaking is more advanced than bond making at the transition state, electrophile-nucleophile electrostatic interactions are less important. In such cases, (hyper)conjugative electronic effects determine the reactivity, and EWG-substitution leads to decreased reactivity. The basicity of the nucleophile as well as solvent effects can help to determine which of these two regimes occurs for a given electrophile.
authors.affiliation: Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain , Place Louis Pasteur 1 box L4.01.02, B-1348 Louvain-la-Neuve, Belgium.
authors.forename: Raphaël
authors.initials: R
authors.lastname: Robiette
authors.affiliation: Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain , Place Louis Pasteur 1 box L4.01.02, B-1348 Louvain-la-Neuve, Belgium.
authors.forename: Tran
authors.initials: T
authors.lastname: Trieu-Van
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol BS8 1TS, United Kingdom.
authors.forename: Varinder K
authors.initials: VK
authors.lastname: Aggarwal
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol BS8 1TS, United Kingdom.|Department of Chemistry, Katholieke Universiteit Leuven , Celestijnenlaan 200F, B-3001 Heverlee, Belgium.
authors.forename: Jeremy N
authors.initials: JN
authors.lastname: Harvey