A Computational Examination of the Uncatalyzed Meinwald Rearrangement of Monoterpene Epoxides

Abstract

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 αpinene oxide (two different mechanisms leading to trans-pinocamphone and α-campholenal), cis-limonene oxide (leading to trans-dihydrocarvone), translimonene 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-4hexene (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.

Regiodivergent Reductive Opening of Epoxides by Catalytic Hydrogenation Promoted by a (Cyclopentadienone)iron Complex

“…Such large difference is in agreement with the observed reactivity trend, with the aromatic epoxides reacting smoothly (Scheme ) and the aliphatic ones requiring a Lewis acid cocatalyst to give reasonable conversions (see Table S1 in the Supporting Information and Scheme ). To investigate the role of Lewis acids, we computed the BF 3 -cocatalyzed ring opening of aliphatic epoxide 2l′ (Scheme ). It was found that BF 3 -mediated reactions have lower barriers than those without BF 3 (Scheme ) by 15.3 and 11.3 kcal/mol for the formation of primary and secondary alcohols, respectively.…”