Theoretical characterization of the mechanism of Alder-ene reactions

Abstract

[eng] The mechanism of Alder-ene reactions between n-hexene and heptanal and n‑tetradecene and heptanal was characterized using reaction force, reaction force constant and reaction electronic flux concepts. Density-functional theory studies were performed at the B3LYP/6-311G(d,p) level. Structural analysis shows the changes in bond angles and distances throughout the reaction path. Thermodynamic analyses suggest these reactions are exothermic with activation energies between 21.1 and 34.2 kcal/mol. The proposed mechanisms follow three basic reaction steps in the transformation from reagents to products. First, the hydrogen shift from the ene to the enophile. Then, there is an electronic rearrangement produced by π and σ electrons. Finally, σ-bond formation and structural relaxation occur, leading to the final product. The mechanism found, based on the quantum descriptors, was confirmed to be the right one.