The Willgerodt-Kindler Reaction: mechanistic reality check 2.

Continuing an exploration of the mechanism of this reaction, an alternative new mechanism was suggested in 1989 (having been first submitted to the journal ten years earlier!).[1] Here the key intermediate proposed is a thiirenium cation (labelled 8 in the article) and labelled Int3 below.

The model chosen is the same as before (B3LYP+GD3+BJ/Def2-TZVPP/Solvent=water) but now includes a specific base (ammonia) to help remove and add protons. Species 8 (Int3) sits in the middle of the rearrangement mechanism and can account for isomerisation in which (above) the Ph and H substituents of the starting ketone end up transposed. It also has the apparent merit that cations such as 8 are known as crystal structures[2],[3]+ DOI: 10.5517/cc112bct,[3]+DOI: 10.5517/cc112bfw. As you can see from the relative free energies (FAIR data at DOI: 10.14469/hpc/7336) that of Int3 is 50 kcal/mol higher than the reactant, and the  transition state leading to it is even higher. So whereas species such as 8 (Int3) can exist (albeit substituted with sterically hindering groups), they probably play no actual role in the mechanism of this reaction.

The hunt continues for a mechanism for which the computed energies along the reaction path are ≤ 31 kcal/mol at 403K, which would correspond approximately to a half life of ~60 minutes.  


  1. M. Carmack, "The willgerodt-kindler reactions. 7. The mechanisms", Journal of Heterocyclic Chemistry, vol. 26, pp. 1319-1323, 1989.
  2. R. Destro, V. Lucchini, G. Modena, and L. Pasquato, "X-ray Structures and Anionotropic Rearrangements of Di-tert-butyl-Substituted Thiiranium and Thiirenium Ions. A Structure−Reactivity Relationship", The Journal of Organic Chemistry, vol. 65, pp. 3367-3370, 2000.
  3. H. Poleschner, and K. Seppelt, "XeF2/Fluoride Acceptors as Versatile One-Electron Oxidants", Angewandte Chemie International Edition, vol. 52, pp. 12838-12842, 2013.

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