Mechanistic elucidation of Diels-Alder cycloaddition reactions between quinoflavonoid and substituted butadiene using LOL, ELF, QTAIM, and DFT studies

Computations of the Diels-Alder reactions of quinoflavonoid with diene at the density functional theory (DFT) with the hybrid functional B3LYP using the basis set 6-311(d,p) level reveal concerted mechanisms via asynchronous transition states. We have studied in these work six reactions in order to highlight the reaction mechanism. The low activation energies for these reactions mean that are not energetic. In particular, the activation energy of the reaction of isoprene with quinoflavonoid is 16.29 kcal/mol. The effect of the solvent is clear from the founded results; it lowers the activation energy for the reaction between the quinoflavonoid and isoprene from 16.29 to 14.39 kcal/mol. The reaction between dienophile and diene methylsorbate is experimentally unfeasible according to previous studies; however we have shown that this reaction is feasible theoretically. The value negatives of nucleus-independent chemical shifts (NICS) confirm the aromaticite of transition state for all the reactions. In addition, intrinsic reaction coordinate (IRC) confirms the transition state for these reactions. The quantum theory of atoms in molecules (QTAIM) watches the high value of electron density and the value negatives of Laplacian that confirms the stability of the transition state structure. The electron localization function (ELF) and the localized orbital locator (LOL) confirm more this stability.

Automated summary

This study utilizes density functional theory (DFT) with the hybrid functional B3LYP to compute the Diels-Alder reactions of quinoflavonoid with diene. It reveals concerted mechanisms via asynchronous transition states. The results suggest low activation energies for these reactions, with the solvent affecting the activation energy. The study also indicates theoretical feasibility for a reaction previously considered experimentally unfeasible.