Theoretical study on the mechanism of the reaction of alkylmethylimidazolium cation with benzaldehyde involved in the base-catalyzed Baylis–Hillman reaction

Alkylmethylimidazolium ionic liquids were reported to be non-innocent solvents in Baylis–Hillman reactions under mildly basic conditions. Here we present a theoretical explanation on why imidazolium-based ionic liquids are reactive solvents in such a kind of reactions. By performing density functional theory calculations, we have investigated the reactions of a representative imidazolium cation, 1-butyl-3-methylimidazolium cation ([bmim]+) with benzaldehyde (one of the reactants in Baylis–Hillman reactions) in the absence and presence of base, 1,4-diazabicyclo[2.2.2]octane (DABCO), to understand the side-reaction mechanism involved in Baylis–Hillman reactions. In the absence of DABCO, the barrier of the reaction is calculated to be as high as 54.53 kcal mol−1, implying the high stability of [bmim]+. However, with the assistance of DABCO, it is found that C2 atom of [bmim]+ is very easily deprotonated, which significantly reduces the positive charge on C2 and hence remarkably enhances the nucleophilic ability of C2 atom. The overall energy barrier for the base-catalyzed reaction is remarkably reduced to 24.81 kcal mol−1, indicating the increasing reactivity of [bmim]+ towards benzaldehyde under mildly basic conditions.