Influence of strain on chemical reactivity. Relative reactivity of torsionally distorted double bonds in MCPBA epoxidations

Article abstract of DOI:10.1021/ja00034a042

The second-order reaction rates were measured for the MCPBA epoxidation in CH₂Cl₂ for a series of cyclic olefins including bridgehead olefms and trans-cycloalkenes. As expected, strained bridgehead alkenes and trans-cycloalkenes showed faster reaction rates than nonstrained cis-cycloalkenes. The MM-2 steric energies of alkenes, alkanes, and their corresponding epoxides were calculated to evaluate the strain energy released in each reaction (ΔSE). Plots of log k_rel vs olefin strain did not show a good correlation. However, the plot of log k_rel vs ΔSE (which is defined as the steric energy difference between olefin and the corresponding epoxide) showed a good correlation for each set of di- and trisubstituted olefins. This result suggests that ΔSE directly reflects strain energy relief in the transition state. From the slope for the plot log k_rel vs ΔSE, it was thought that approximately 42% of strain (ΔSE) was released in the transition state for the MCPBA epoxidation. Also, trialkyl-subtituted alkenes were found to be about 50 times more reactive than dialkyl-substituted alkenes in cases where the strain energy relief (ΔSE) is the same. The reaction rate is also plotted versus ionization potential of the olefin, assuming that the major orbital interaction lies between the LUMO of the peracid and the HOMO of the olefin. Although, in some cases, a rough correlation of the reaction rate with the ionization potential of the olefin exists, the frontier orbital interaction is not viewed as the dominant factor since conjugated alkenes, which have higher HOMO energies than simple olefins, are not more reactive in MCPBA epoxidation.