Scheme 2. Experimental details and the proposed tentative mechanism.
More details about the experimental results by Biju and co-workers60 were shown in Scheme 2. The aldimine 1 was treated with the carbene that is generated in situ from triazolium salt Pre-NHC using KOt -Bu as the base and DQ as the oxidant. The optimal solvent was demonstrated to be DMSO, and the temperature was set at 30 °C. Various 2-arylbenzoxazole and 2-arylbenzothiazole were obtained with yield up to 99%. The tentative mechanism indicates five steps for the whole catalytic cycle, namely absorption of NHC to the substrate to give zwitterion A, followed by [1,2]-proton transfer to generate the aza-Breslow intermediate B, oxidation by DQ, ring closure under assistance of the Lewis base, and finally desorption of the catalyst. This cycle provides inspired perspectives to get better understanding to the general process of the catalytic mechanism, but its rationality and feasibility still need to be verified. For example: since the [1,2]-proton transfer usually difficult,61 is the one-step transformation from intermediate A to aza-Breslow intermediate B feasible or not? Since the triazolium salt Pre-NHC and the base KOt -Bu were added in exactly equal equivalent, how it can be used in proton abstraction from phenolic hydroxy group in compound D? What kind of roles do the NHC catalyst play in enabling the reaction to occur? In order to pursue deep insights into all above questions, we conducted detailed density functional theory (DFT)62 studies to the entire catalytic cycle.