Epoxidation of Alkenes by Dimethyldioxirane: Kinetics, Activation Parameters and Solvent Studies

The reaction of dimethyldioxirane with a series of cis/trans-1,2-dialkylalkenes was carried out and produced the corresponding epoxides in high yield. As expected, the relative reactivity at 23 ºC of the cis-alkenes was at least 8-fold greater than that of the trans-counterparts with the magnitude of the relative reactivity increasing with increased steric bulk. Enhanced selectivity for cis- versus trans-alkene epoxidation was observed at lower temperatures. The reaction of dimethyldioxirane with selected alkenes was carried out in various solvent conditions (dried acetone:acetonitrile (1:9), dried acetone:methanol (1:9), dried acetone:carbon tetrachloride (1:9) and acetone:water (Xwater = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05)) and produced the corresponding epoxides in high yield. The reactivity of dioxirane with simple di- and trialkylalkenes was enhanced as the polarity and hydrogen bonding capability of the solvent system were increased. Little to no change in reactivity was observed in the non-polar solvent system. Epoxidation of trisubstituted alkenes by dioxirane showed a greater rate enhancement in polar protic solvents compared to that for the epoxidation of the disubstituted alkenes. The epoxidation of an allylic alcohol by dimethyldioxirane showed a large increase in the non-polar solvent system compared to that in acetone. The reaction of dimethyldioxirane with the allylic alcohol also exhibited less of a rate increase in polar protic systems than its alkyl counterpart. Activation parameters for the epoxidation of cis/trans-1,2-dialkylalkenes by dioxirane in dried acetone and the previously mentioned solvent systems were determined using the Arrhenius method. In general, the ∆G‡ and ∆H‡ terms were greater for the reaction of dimethyldioxirane with trans-alkenes as compared to those for the corresponding cis-isomers regardless of solvent or alkyl steric bulk. The calculated ∆S‡ terms appeared essentially independent of steric bulk or solvent composition and were roughly identical, within experimental error, for all of the five cis/trans pairs. The ∆∆G‡ values, a comparison of the trans- to the cis-isomer data, yielded values of 1.2 to 1.8 kcal/mol in dried acetone for the five pairs of alkenes and appeared to be dependent on relative steric interactions. The ∆∆G‡ values for the epoxidation of cis/trans-alkenes carried out in solvents other than acetone showed no change from the value obtained in acetone. The experimental activation parameter data in dried acetone were consistent with predictions from ab initio calculations based on a spiro transition state model