Sulfonylation of Five-Membered Aromatic Heterocycles Compounds through Nucleophilic Aromatic Substitution: Concerted or Stepwise Mechanism?

Heterocyclic sulfones are relevant in medicinal chemistry, drug design and organic synthesis, and the mechanism of sulfonylation of five-membered heterocycles is not clear. In this work, it is presented a computational study on the mechanism of sulfonylation of five-membered heterocycles compounds. Concerted and stepwise mechanisms for the nucleophilic aromatic substitution (S_NAr) involved in the reaction were examined. Effect of the neighbouring group (NG), heteroatom in the ring (HT), nucleophile (NU), and leaving group (LG) in fourteen reactions were examined. Analysis of the intrinsic reaction coordinate, reaction force, natural bond orbital (NBO) charges and bond indexes give a reasonable insight about the mechanism for the reactions studied. Results revealed activation free energies ranging from 20.8 kcal/mol up to 38.6 kcal/mol. Reaction force analysis showed that structural changes in the reactant are the most important component associated with the energy of activation with more than 60 % of the contribution. Most sulfonylation reactions in this study proceed through a concerted mechanism; only two occur though a stepwise-Meisenheimer-intermediate mechanism.