The reaction mechanism
of the isomerization of α and β-pinene
epoxides with Fe species as catalysts was studied with density functional
theory (DFT) calculations and an experimental methodology. β-pinene
epoxide can be isomerized into myrtanal and myrtenol in four steps,
while in the case of perillyl alcohol, two additional steps are necessary.
On the other hand, high selectivity to myrtanal obtained experimentally
can be explained by the number of steps and the kind of the hydrogen
transference that is easier for this compound in comparison with myrtenol
and perillyl alcohol. A thermodynamic analysis showed that transformation
into myrtenol, myrtanal, and perillyl alcohol is spontaneous but transformation
into myrtanal is the most favorable. In the case of α-pinene
epoxide rearrangement, a mechanistic study was carried out toward
the optimization of the possible intermediates. Synthesis of campholenic
aldehyde and carveol from α-pinene epoxide was carried out through
three steps after the coordination of oxygen to iron, showing that
in contrast to carveol formation, campholenic aldehyde synthesis is
spontaneous. Analysis of ∇2ρ, the total energy
density (H = V + G), and the |V|/G ratio evaluated
at the bond critical point of the Fe–O bond showed for both
epoxides that such interaction is closed shell instead of covalent.
Apparently, α-pinene epoxide can be isomerized faster that β-pinene
epoxide with Fe3+, which is in agreement with previous
experimental results. This is the first report where a reaction mechanism
of isomerization of monoterpenes epoxides is studied based on very
detailed experimental and computational methodologies.
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