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.