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.