Prediction on the Origin of Selectivities in Base-controlled Switchable NHC-catalyzed Transformations.

The base-controlled mechanisms for N-heterocyclic carbene (NHC)-catalyzed divergent [3+3] and [3+2] annulation reactions were examined by using the DFT method. The reaction initiates with the complexation of NHC and enal to give the Breslow intermediate, which diverges afterward. Then, the azomethine imine can either react with the Breslow intermediate to give the six-membered ring product or the β-carbon protonation occurs for forming the enolate intermediate controlled by different bases. The formed enolate intermediate reacts with azomethine imine to afford the five-membered ring product. The calculated results show that only the base K2 CO3 can facilitate the structural transformation between homoenolate and enolate to switch the chemoselectivity; therefore, the [3+3] annulation happens preferentially in the presence of base DBU while the other situation occurs with K2 CO3 as base. The NCI analysis results reveal that the stereoselectivity is predominately determined by the π⋅⋅⋅π, C-H⋅⋅⋅O, and C-H⋅⋅⋅N interactions. The obtained mechanistic insights should provide valuable clues for the rational design of these kinds of divergent reactions.