Mechanistic Studies on N-Heterocyclic Carbene-Catalyzed Umpolung of β,γ-Unsaturated α-Diketones

The mechanism of N-heterocyclic carbene-catalyzed umpolung of β,γ-unsaturated α-diketone was studied using both density functional theory (DFT) calculations and experimental methods. In contrast to the originally proposed mechanism, the calculations revealed a more complicated process involving both nucleophilic O-acylated homoenolate and electrophilic α,β-unsaturated acyl azolium intermediates. The experimental studies confirmed the existence of the aforementioned two key intermediates. A revised mechanism has been proposed to demonstrate the detailed mechanistic insights into the product formation.