3.1 Reaction Mechanism
The catalytic transformation of (E )-2-((4-methylbenzylidene)amino)phenol (denoted as R ) was selected as the computational model because it gives 4-tolylbenzoxazole with excellent yield of 99%. We present the entire catalytic cycle in Scheme 3 with more details considered, especially for the tautomerization from zwitterion intermediate Int1 , which is generated by absorption of the NHC catalyst (denoted as Cat ) to the aldimine R , to the imidoyl azolium Int4 . There are three possible pathways calculated and more discussions will be given below. The following processes are successively deprotonation from the phenolic hydroxy group under assistance of[DQH](4’-hydroxy-[1,1’-biphenyl]-4-olate), ring-closure, and finally desorption of the product P from Cat . In the following part of this section, we will give detailed discussions about this catalytic cycle step by step. The Gibbs free energy profiles of the whole reaction along with some key geometry parameters of all transition states are presented in Figure 1. Unless otherwise stated, the energies of Cat + R were set as the reference of 0.0 kcal/mol.
In the first process, the nucleophilic attack of the C1 atom inCat to the C2 atom in R may occur from either theRe- or Si- face. The resulting transition stateRe-TS1 was predicted to locate 3.1 kcal/mol higher thanSi-TS1 , and the corresponding absorption productSi-Int1 was found to be 8.3 kcal/mol more stable than that Re-Int1 . Thus we abandoned all the following calculations related with Re-Int1 . Besides, the barrier via Si-TS1 was calculated to be 20.3 kcal/mol, indicating that the absorption is able to occur smoothly under the experimental conditions (30 °C).
Given the zwitterion intermediate Si-Int1 , three possible pathways for its oxidation to generate the imidoyl azoliumInt4 were calculated, in particular the direct oxidation byDQ via transition state TS2” (DOx), the direct [1,2]-proton transfer of the H6 atom via TS2’ to give aza-Breslow intermediate Int3 and then followed by oxidation byDQ to form Int4 (DPTOx), and the successive transformations of [1,4]-proton transfer of the H5 atom viaTS2 /spontaneous [1,5]-proton transfer of the H6 atom/oxidation by DQ via TS3 to form Int4(SPTOx).
In the DOx pathway, the hydride H6 was abstracted from the C2 atom inSi-Int1 to the O7 atom in DQ . As shown by the optimized geometry parameters in Figure S1, the length of the C2–H6 bond is elongated from 1.11 Å in Si-Int1 to 1.23 Å inTS2” , while distance between the H6 and O7 atoms is shortened from 1.27 Å in TS2” to 0.96 Å in [DQH], which clearly indicates formation of the new bond. The barrier of this elementary step was predicted to be 41.0 kcal/mol (Figure 1), which is too high to be overcome under the experimental conditions. Consequently, the reaction goes through the DOx strategy was excluded.