Thionation of Mesoionics with Isothiocyanates: Evidence Supporting a Four-Step Domino Process and Ruling Out a [2 + 2] Mechanism

Mesoionic heterocycles derived from 1,3-thiazolium-4-olates (thioisomünchnones) undergo thionation with aryl isothiocyanates to afford the corresponding 4-thiolate derivatives. Here, we document this transformation in detail, giving a crystallographic characterization of the solid-state structures. From the mechanistic viewpoint, the formal thionation process could be consistent with a [2 + 2] reaction of the exocyclic C−O bond of the thioisomünchnone with the CS double bond of the isothiocyanate moiety, which would be competing with a (3 + 2) process as usual in mesoionic rings. Theoretical computations at the [B3LYP/6-31G(d):PM3] level, in which only bond-forming and bond-breaking reactions and neighboring atoms are treated at the DFT level, do reproduce the experimental results and rule out the expected pathway. Calculations instead suggest the existence of a four-step domino pathway through several polar intermediates that agrees with the electronic nature of the substituents involved. The mechanistic hypothesis has further been corroborated by an experiment with isotopically 13C-labeled PhNCS that unambiguously shows the way in which the exchange reaction occurs.