The Effect of the Leaving Group in N-Heterocyclic Carbene-Catalyzed Nucleophilic Aromatic Substitution Reactions

We report here that the reactivity order of the leaving group is F > Cl ≥ Br > I in N-heterocyclic carbene-catalyzed CSNAr reactions of aryl halides bearing an α,β-unsaturated amide. Based on a qualitative Marcus analysis, the nature of the transition state in this catalytic CSNAr is primarily determined by the potential energy of the Meisenheimer complex, even though it is not involved as a discrete intermediate in the reaction pathway.