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Theoretical Study of the Reaction Mechanism of Limonene Epoxide Isomerization
  • Julián Sánchez-Velandia,
  • Sol Mejia,
  • Aida Villa
Julián Sánchez-Velandia
Universidad de Antioquia

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Sol Mejia
Pontificia Universidad Javeriana
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Aida Villa
Universidad de Antioquia
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Abstract

Reaction mechanisms of limonene epoxide isomerization for obtaining dihydrocarvone and carvenone as main products were proposed using three computational methodologies. It was deduced that carvenone can be obtained with a low energetic barrier with respect to dihydrocarvone that is its intermediate; these findings are in accordance with the experimental low selectivity of dihydrocarvone. Apparently, the change in the functional (from B3LYP to PBE, with the same Pople basis set, 6-31++g(d,p)) produces a change in the energetics of the reactions for the synthesis of dihydrocarvone, while in the second part of the reaction mechanism (for obtaining carvenone) did not change after modification of the level of theory (from B3LYP to PBE vs MP2). Based on these results, it is proposed that the reaction mechanism for obtaining both dihydrocarvone and carvenone occurs towards the rearrangement of the carbocation pathway. Reactivity of the carbocations was also analyzed towards the GAPHOMO-LUMO and the charge analysis showed that in some cases, electron withdrawing can be established. The course of the reaction showed in almost all cases that bonds are pure covalent instead of closed-shell interactions. Furthermore, thermodynamic analysis of their stereochemical configuration was also determined; it was concluded that the transformation of cis-limonene epoxide into carvenone is the most spontaneous and exothermic reaction. On the other hand, based on the Gibbs energy it is concluded that cis-limonene epoxide isomerization into trans-dihydrocarvone is the most probable thermodynamic pathway with respect to the other obtained configurations. Finally, kinetic analysis suggests that the determining step corresponds to the elimination of hydrogen to obtain an endo C=C of the carvenone.