Meisenheimer Complexes in S_NAr Reactions: Intermediates or Transition States?

Meisenheimer's missing! Anionic σ-complexes, best known as Meisenheimer complexes, have long been assumed to be intermediates in S_NAr 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 ¹²C/¹³C kinetic isotopes effects, determined using a new method based on ¹⁹F NMR, and DFT calculations.