Meisenheimer Complexes in SN Ar Reactions: Intermediates or Transition States?

Meisenheimer's missing! Anionic σ-complexes, best known as Meisenheimer complexes, have long been assumed to be intermediates in SN Ar reactions. New evidence now suggests that these intermediates may only be formed in select cases and that a concerted pathway is more common. These claims are supported by 12 C/13 C kinetic isotopes effects, determined using a new method based on 19 F NMR, and DFT calculations.