Mechanism, Chemoselectivity, and Stereoselectivity of NHC‐Catalyzed Asymmetric Desymmetrization of Enal‐Tethered Cyclohexadienones

Density functional theory calculations were performed to study the mechanism, chemoselectivity, and stereoselectivity for the N‐heterocyclic carbene (NHC)‐catalyzed asymmetric desymmetrization of enal‐tethered cyclohexadienones. The results showed that the whole reaction includes nucleophilic addition, homoenolate intermediate generation, Michael addition, protonation, esterification, and catalyst elimination. Protonation and esterification were demonstrated to be mediated by AcOH/AcO–, in stark contrast to the experimental speculation that MeOH provided the proton. The Michael addition step determined the chemoselectivity and stereoselectivity. The observed β‐chemoselectivity was attributed to the hydrogen‐bond and favorable π–π interaction in the β‐addition transition states. Moreover, the β‐addition transition state with (S,R,S)‐configuration was most stable, in agreement with the experimental results that the product derived from this structure was preferentially generated. Further noncovalent interaction analysis showed that stronger π–π interaction between cyclohexadienones and the NHC catalyst, as well as C–H···π interactions, made the (S,R,S)‐configuration transition state the most energetically favored.