Diels-Alder cycloaddition versus ring-opening esterification: Acomputational study of the mechanism of formation of oxa-norbonenelactones from the reaction of furfuryl alcohol and itaconic anhydride
The reactions of furfuryl alcohol with itaconic anhydride can proceedviafour plausible pathways–two path-ways involving initial Diels-Alder cycloaddition followed by lactonization and two pathways involving initialesterification followed by intramolecular Diels-Alder cycloaddition–to afford two distinct norbornene productsbearing either afive- or six-membered butyrolactone ring respectively. DFT calculations reported herein showthat thefirst-step Diels-Alder cycloaddition reactions have barriers of 8.5 and 11.8 kcal/mol which are far lowerthan the barriers of 29.6 and 36.9 kcal/mol for the esterification pathways. The calculated energies for theexperimentally observedfive- and six membered oxa-norbornene lactone isomers are exergonic with reactionenergies of−17.1 and−12.8 kcal/mol respectively. Thermodynamically thefive-membered isomer is morestable adduct than the six-membered adduct. Thus the most preferred pathway is the one involving initial[4 + 2] cycloaddition followed by lactonization to form thefive-membered butylrolactone ring. Substitution ofmethyl groups on the carbon bearing the hydroxyl group shows that the substituents do not affects the reactionpathways markedly.
An article published by Elsevier B.V. and also available at https://doi.org/10.1016/j.comptc.2018.05.019
E. Fosu et al. Diels-Alder cycloaddition versus ring-opening esterification: Acomputational study of the mechanism of formation of oxa-norbonenelactones from the reaction of furfuryl alcohol and itaconic anhydride, https://doi.org/10.1016/j.comptc.2018.05.019