Table 4. Calculated transition state properties for R=iPr (scheme 2)

Even for such a relatively small system, the role of dispersion interactions is shown to be crucial, along with the use of basis sets where the superposition errors are low.

Understanding the non covalent interactions (NCI) at play is highlighted as essential for the design of new synthetic routes and alternative reactants. An NCI analysis of the transition states enables the identification of non-covalent interactions that determine the reaction outcome, confirming the role of the electrostatic NCHᵟ+∙∙∙Oᵟ- interactions and highlighting new geometric schemes based on dispersion. Alternative mechanisms, such as proton-relays involving a water molecule or the Hajos-Parrish alternative, are shown to be higher in energy.