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).