N-Heterocyclic Carbene (NHC)-Catalyzed sp(3) beta-C-H Activation of Saturated Carbonyl Compounds: Mechanism, Role of NHC, and Origin of Stereoselectivity

Activation of inert sp(3) beta-C-H bonds has attracted widespread attention and been developed with significant progress in recent years, but understanding the mechanism of this kind of reaction continues to be one of the most challenging topics in organic chemistry. In this paper, the possible reaction mechanisms and origin of stereoselectivity in the reaction between saturated carbonyl compounds with enones generating cyclopentenes catalyzed by N-heterocyclic carbene (NHC) have been investigated using density functional theory. The computational results show that the additive DBU plays an important role in NHC-catalyzed C-H activation. Analyses of the natural bond orbital charge and global reaction index indicate that NHC can lower the energy barrier of the entire reaction by activating the alpha/beta-C-H bond rather than by strengthening the nucleophilicity of the reactant as a Lewis base. This is remarkably at variance from previous reports. In addition, the pi-pi stacking between the phenyl of the enone and the conjugated system of the NHC-bounded enolate intermediate has been found by the analyses of distortion/interaction and atom-in-molecule to be responsible for the stereoselectivity. These results shed light on the detailed reaction mechanism and the significant role of the NHC organocatalyst and offer valuable insights into the rational design of potential catalysts for this kind of highly stereoselective reaction.