Computational Study on the Boundary Between the Concerted and Stepwise Mechanism of Bimolecular SNAr Reactions

The text-book mechanism of bimolecular nucleophilic aromatic substitutions (S N Ar) reactions is a stepwise process that proceeds via a so-called Meisenheimer intermediate. Only recently the alternative, concerted version of this mechanism has gained acceptance as more and more examples thereof have been reported. But so far only isolated examples of concerted S N Ar reactions have been described and a coherent picture of when a S N Ar reaction proceeds via a stepwise and when via a concerted mechanism has not yet been established. Here key factors are identified that influence the mechanistic choice of S N Ar reactions. Moreover, the electron affinity is used as a simple descriptor to make a prediction on whether a given aryl fluoride substrate favors a concerted or stepwise mechanism.