A Computational Examination of the Uncatalyzed Meinwald Rearrangement of Monoterpene Epoxides

Epoxides are relatively reactive compounds and may undergo decomposition or rearrangement reactions at elevated temperatures, and gas chromatographic analysis of essential oils may cause thermal decomposition or rearrangement of epoxide components at gas chromatographic temperatures. Density functional theory (DFT) calculations were carried out using the B3LYP functional at the 6-311++G**//6-31G* level of theory on the Meinwald rearrangements of α-pinene oxide (two different mechanisms leading to trans-pinocamphone and α-campholenal), cis-limonene oxide (leading to trans-dihydrocarvone), trans-limonene oxide (leading to cis-dihydrocarvone), 6,7-epoxymyrcene (leading to 2-methyl-6-methylene-7-octen-3-one), and 1,2-epoxy-2,5-dimethyl-3-vinyl-4-hexene (two mechanisms leading to 5-methyl-3-vinyl-4-hexenal and 8-methyl-5,7-nonadien-2-one). Free energies of activation for the uncatalyzed Meinwald rearrangement of common monoterpene epoxides are relatively large, being of the order of 70 kcal/mol, and are, therefore, not predicted to be important reactions at gas chromatographic temperatures.