Scheme 4. The main change in the initial step of the non-catalyzed reaction.
Taking all above discussions into consideration, for either the NHC-catalyzed or non-catalyzed reaction, the oxidation by DQwas disclosed to be the rate-determining step (RDS), but the barrier of the latter strategy (44.1 kcal/mol) is about 20 kcal/mol higher than the former one (24.2 kcal/mol). Actually, due to participation of NHC, the reaction mechanism has been fundamentally altered, leading to the whole barrier changes from the accumulate obstacles including the conformational isomerism, the ring closure and the oxidation processes to the barrier of the elementary oxidation step. In addition, the energies of the highest point in the two energy profiles (TS3and TS3nc , respectively) are almost equal (23.6 vs 23.5 kcal/mol), indicating that it should be the excessive exothermic change in the initial deprotonation step that leads to the unreasonable barrier of the non-catalyzed reaction. As illustrated in Scheme 4, it is no surprise that the latter state locates more than 20 kcal/mol lower than the initial state because this is typically a displacement reaction of strong acid with strong base to give relatively weak acid and weak base. While with participation of NHC, this transformation is subtly prevented by controlling the reaction procedure. In particular, exactly equal amount (20 mol%) of triazolium salt Pre-NHC and the baset -BuOK were firstly stirred in the DMSO solvent for 15 min to yield the NHC catalyst in situ . Afterwards, there was littlet -BuOK left to react with the substrate which was later added to the system along with the oxidant. In a word, by properly controlling the order and amount of reagents added, the reaction mechanism is completely changed by use of NHC catalyst. This strategy effectively prevents formation of the excessive stable intermediate, and thus resulting in substantial reduction in energy barrier of the whole reaction.