Activation Parameters for the Epoxidation of Substituted cis / trans Pairs of 1,2‐Dialkylalkenes by Dimethyldioxirane

The first activation parameter data for the reaction of dimethyldioxirane ( 1 ) with five cis/trans pairs of alkenes are reported. The epoxidation of cis ‐1,2‐dialkylalkenes ( 2 cis : R 1 = Me, R 2 = i Pr; 3 cis : R 1 = Me, R 2 = t Bu; 4 cis : R 1 = R 2 = Et; 5 cis : R 1 = Et, R 2 = i Pr; 6 cis : R 1 = Et, R 2 = t Bu) and trans ‐1,2‐dialkylalkenes ( 2 trans : R 1 = Me, R 2 = i Pr; 3 trans : R 1 = Me, R 2 = t Bu; 4 trans : R 1 = R 2 = Et; 5 trans : R 1 = Et, R 2 = i Pr; 6 trans : R 1 = Et, R 2 = t Bu) by 1 produced the corresponding epoxides, quantitatively and stereospecifically, as the sole observable products. Activation parameters of the epoxidation of the five pairs of alkenes, 2 cis – 6 cis and 2 trans – 6 trans , by 1 were determined using the Arrhenius method. Enhanced selectivity for cis ‐ vs. trans ‐alkene epoxidation was observed at lower temperatures. In general, the Δ G ‡ terms were larger and showed more variability for the reaction of 1 with trans ‐alkenes as compared to those for the corresponding cis isomers. The Δ H ‡ terms mirrored trends observed in Δ G ‡ because Δ S ‡ terms for all ten of the compounds were roughly identical. The ΔΔ G ‡ values, a comparison of the trans to the cis isomer data, yielded positive values of 1.2 to 1.8 kcal/mol for the five sets of data and appeared to be dependent on relative steric interactions. The experimental activation parameter data, consistent with predictions from ab initio calculations based on a spiro transition‐state model, showed that the lower reactivity of trans ‐alkenes is due to enthalpy factors. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)