Figure 1. Gibbs free energy profiles of the whole reaction and some key geometry parameters of all transition states involved. All bond lengths are given in angstrom (Å).

The DPTOx pathway assumes a typical [1,2]-proton transfer of the H6 atom from the C2 atom to the N3 atom so as to provide the aza-Breslow intermediate Int3 , which has been definitely verified to be difficult under mild conditions due to the highly strained three-membered ring involved in the transition state (C2–N3–H6 inTS2’ ). Not surprisingly, the Gibbs free energy barrier viaTS2’ was predicted to be 44.1 kcal/mol, which is obviously an unreasonable obstacle for the mild experimental conditions.

In the SPTOx pathway, two successive proton transfers were proposed to give Int3 , in particular the proton H5 shifted from the O4 atom to the negative N3 atom via transition state TS2 , followed by the migration of the H6 atom from the C2 atom back to the O4 atom to generate intermediate Int3 . The energy barrier via TS2was calculated to be 9.6 kcal/mol, and the tautomerization fromInt2 to Int3 was demonstrated to be spontaneous via the flexible scanning of the O4–H6 bond length (Please see Figure S2 for more details). The subsequent elementary step is to oxidateInt3 to Int4 via hydride transfer of the H5 atom from the N3 atom to DQ . The gradual elongation of the N3–H5 bond length (from 1.02 Å in Int3 to 1.33 Å in TS3 , Figure S1) and the continued shortening of the O7–H5 bond length (from 1.14 Å in TS3 to 0.96 Å in [DQH]^– , Figure S1) clearly indicated the abstraction of the hydride H5 byDQ . As shown in Figure 1, transition state TS3 was predicted to locate 24.2 kcal/mol higher than the intermediateInt3 , which is much lower than that goes through the DOx or DPTOx pathway. Taking all these discussions into consideration, the SPTOx pathway was concluded to be most energetically favorable for oxidation from Si-Int1 to the imidoyl azoliumInt4 .

The third process is to abstract the H6 atom from the phenolic hydroxy group by the [DQH]^– to afford the zwitterion intermediate Int5 . It was confirmed to be also barrierless by the flexible scanning of the O4–H6 bond length (Please see Figure S3 for more details). The fourth process is the ring closure undergoing through nucleophilic attack of the O4 atom to the C2 atom to form intermediate Int6 via transition state TS4 . The geometry optimizations indicates that the distance between the C2 and O4 atoms decreases from 2.39 Å in Int5 to 1.92 Å in TS4and finally become 1.44 Å in Int6 , which clearly demonstrates formation of the C2-O4 bond. The energy barrier of this step was predicted to be 21.3 kcal/mol, indicating that this intramolecular cyclization process can be accomplished under the experimental conditions. Finally, formation of the C2=N3 double bond promoted release of the final product P from the catalyst. The very low energy barrier via transition state TS5 (2.2 kcal/mol, shown in Figure 1) indicates that it is easy for the catalyst to be recycled, which corroborates the potential for the NHC as a good leaving group.