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.