Tuning the Catalyst Reactivity of Imidazolylidene Catalysts through Substituent Effects on the N-Aryl Groups.

A series of imidazolium salts with various N-aryl groups were synthesized, and their catalytic activities were evaluated to investigate the contribution of the N-aryl groups to the catalytic activity in the synthesis of γ-butyrolactone through an a3→d3-umpolung addition. Imidazolylidenes with 2,6-diethylphenyl groups were effective catalysts, and several mechanistic studies, including a deuterium kinetic isotope effect study, revealed that both steric and kinetic effects were responsible for the enhanced catalytic activity.