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.