Knoevenagel condensation versus Michael addition reaction in ionic-liquid-catalyzed synthesis of hexahydroquinoline: a SMD–DFT study

Abstract

Knoevenagel condensation and Michael addition reaction are two different mechanisms for C–C bond formation. In this work, M06-2X method is used to compare activation free energies of Knoevenagel condensation and Michael addition reaction in the gas and ionic liquid phases. The role of 2-ethyl imidazolium hydrogen sulfate in the synthesis of hexahydroquinolines is investigated. To model ionic liquid phase, SMD continuum universal solvation model is used. Data show that Knoevenagel condensation and Michael addition have activation free energies of 45 and 13 kcal mol−1 in the gas phase and + 24 and + 12 kcal mol−1 in the ionic solution phase, respectively. The final cyclization process has zero barrier energy. Analyzing TS structures by the quantum theory of atoms in molecules (QTAIM) show that the TS structure of Knoevenagel step is similar to the product, while the TS structure of Michael addition step is similar to the reactants.