An ELF analysis of the C–C bond formation step in the N-heterocyclic carbene-catalyzed hydroacylation of unactivated C–C double bonds

The changes in electron-density along the C–C bond-formation step in the N-heterocyclic carbene-catalyzed hydroacylation of unactivated double bonds has been studied by an electron localization function (ELF) analysis at the B3LYP/6-31G** level in order to characterize the reaction mechanism. Analysis of DFT reactivity indices and the natural bond orbital and ELF analysis at the most relevant points of the intrinsic reaction coordinate indicate that the reaction path takes place through a two-stage one-step mechanism with non-polar character. In the first stage a hydrogen atom is transferred from the hydroxyl group of Breslow intermediate 12 to the terminal olefinic carbon atom, to yield a pseudodiradical species. The barrierless C–C bond formation in this species constitutes the second stage of the reaction allowing the formation of the corresponding alcohoxy intermediate 13. The present theoretical study establishes that this mechanism is completely different to that involved in the intramolecular Stetter reaction, which is initialized by a polar Michael-type addition.