Figure 2. (a) The proposed mechanism for the reaction without NHC catalysis, and (b) the predicted Gibbs free energy profiles along with some key parameters in each transition states (bond length in Å).
The computational results reveal that the initial step of the reaction is barrierless (See Figure S4 for more details) and remarkably thermodynamically favorable, indicating strong preference of the proton transfer from phenolic hydroxy group to the tert-butyl alcohol anion (t -BuO). The energy barrier for the conformational transformation via TS1nc was predicted to be 4.5 kcal/mol, which is very easy to be overcome under mild conditions. However, the following conversion fromInt2nc to the benzoxazole productPnc was demonstrated to be very difficult via neither of the two proposed pathways because of their extremely high barrier (44.1 or 53.8 kcal/mol, respectively).