Thionation of mesoionics with isothiocyanates: evidence supporting a four-step domino process and ruling out a [2+2] mechanism

Cantillo, David, Avalos, Martin, Babiano, Reyes, Cintas, Pedro, Jimenez, Jose L., Light, Mark Edward and Palacios, Juan C. (2009) Thionation of mesoionics with isothiocyanates: evidence supporting a four-step domino process and ruling out a [2+2] mechanism. Journal of Organic Chemistry, 74 (10), 3698-3705. (doi:10.1021/jo900028c).

Abstract

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 C-13-labeled PhNCS that unambiguously shows the way in which the exchange reaction occurs.

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Contributors

Author: Mark Edward Light

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