Conclusions
The mechanism of the oxidative nonpolar inversion reaction catalyzed by NHC to achieve benzoxazoles and roles of the NHC catalysis in facilitating the reaction were investigated in very details. It was revealed that the reaction goes through generally five processes, i.e. absorption of the catalyst, oxidation of the zwitterion intermediate to imidoyl azolium, deprotonation followed by ring closure, and finally desorption of the catalyst. As for the oxidation process, the successive [1,4]-/[1,5]-proton transfers followed by oxidation byDQ was demonstrated to be much more energetically favorable than the direct oxidation or the direct proton transfer followed by oxidation pathway. Oxidation of the C2 atom was indicated to be the RDS, and the barrier was predicted to be 24.2 kcal/mol, a reasonable value for experimental conditions (30 °C). Mechanism of the non-catalyzed reaction was also calculated, and the excessive exothermic property of the initial step was concluded to be the main reason for its extremely high barrier. In the NHC-catalyzed reaction, this unfavorable transformation was subtly prevented due to the specific sequence and amount of reagents addition. We conclude that the intrinsic requirement of NHC catalyst formation in situ and the exactly equal amount of the Pre-NHC and t -BuOK are the two core factors that enable the reaction to occur under mild conditions.