Illustrated Glossary of Organic Chemistry

Anchimeric assistance (neighboring group participation): The interaction of an electron pair (either lone pair or covalent bond pair) with an adjacent reaction center (site of bond changes) during the course of a reaction mechanism.

In this S_N2 reaction the sulfur lone pair displaces iodide ion (the leaving group) in an intramolecular mechanism step, to give a cyclic sulfonium ion. This mechanism step results in inversion of configuration at the carbon that was bonded to the leaving group. In a second, intermolecular mechanism step, iodide ion attacks the backside of the carbon-sulfur bond, resulting in the second inversion of configuration at this carbon atom. (The curved arrows for the anchimeric assistance step are shown in red.) Without anchimeric assistance (mechanism pathway B) the stereochemical configuration of the reaction product would be different.