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!).[cite]10.1002/jhet.5570260518[/cite] 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[cite]10.1021/jo991731o[/cite],[cite]10.1002/anie.201307161[/cite]+ DOI: 10.5517/cc112bct,[cite]10.1002/anie.201307161[/cite]+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.
