Diels-Alder cycloaddition versus ring-opening esterification: A computational study of the mechanism of formation of oxa-norbonene lactones from the reaction of furfuryl alcohol and itaconic anhydride

摘要

The reactions of furfuryl alcohol with itaconic anhydride can proceed via four plausible pathways - two pathways involving initial Diels-Alder cycloaddition followed by lactonization and two pathways involving initial esterification followed by intramolecular Diels-Alder cycloaddition - to afford two distinct norbornene products bearing either a five- or six-membered butyrolactone ring respectively. DFT calculations reported herein show that the first-step Diels-Alder cycloaddition reactions have barriers of 8.5 and 11.8 kcal/mol which are far lower than the barriers of 29.6 and 36.9 kcal/mol for the esterification pathways. The calculated energies for the experimentally observed five- and six membered oxa-norbornene lactone isomers are exergonic with reaction energies of -17.1 and -12.8 kcal/mol respectively. Thermodynamically the five-membered isomer is more stable adduct than the six-membered adduct. Thus the most preferred pathway is the one involving initial [4 + 2] cycloaddition followed by lactonization to form the five-membered butylrolactone ring. Substitution of methyl groups on the carbon bearing the hydroxyl group shows that the substituents do not affects the reaction pathways markedly.