Cantillo David, Avalos Martín, Babiano Reyes, Cintas Pedro, Jiménez José L, Light Mark E, Palacios Juan C
Departamento de Química Orgánica e Inorgánica, QUOREX Research Group, Facultad de Ciencias, Universidad de Extremadura, E-06071 Badajoz, Spain.
J Org Chem. 2009 May 15;74(10):3698-705. doi: 10.1021/jo900028c.
Mesoionic heterocycles derived from 1,3-thiazolium-4-olates (thioisomunchnones) 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 thioisomunchnone 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 (13)C-labeled PhNCS that unambiguously shows the way in which the exchange reaction occurs.
