A study of the reaction of methyl (£)-cinnamate with quaternary ammonium permanganates in methylene chloride solutions has been completed. The rate of reaction is fastest for those ions which permit the interionic distance in the quaternary ammonium ion pair to be minimized. The Hammett p value for the reaction is 0.95 at 20 °C, and an inverse secondary deuterium isotope effect is observed at the ß-position but not at the «-position. The sign of the p value for this reaction is in direct contrast to the reported value (-0.6) for the oxidation of alkyl vinyl ethers in aqueous dioxane solutions. On the other hand, the fact that the activation parameters for the two reactions are almost identical suggests that the reaction mechanisms cannot be widely divergent. An attempt has been made to visualize a mechanism in which the reactions can proceed through similar intermediates but via either an electron-rich or an electron-deficient transition state depending on the demands of the substituents. The apparent change in mechanism is considered to be simply a reflection of nature's ability to find the lowest energy surface between two points.

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When alkenes are oxidized by quaternary ammonium or phosphoniurn permanganates in polar organic solvents, such as acetone, the structures of the cations have little or no effect on the rates of reaction. In less polar solvents, such as methylene chloride or toluene, the rates of reaction are, however, dependent on' the identity of the quaternary ammonium or phosphonium ions. It thus appears as if the reacting species may exist as solvent-separated ion pairs in polar solvents and as intimate ion pairs in nonpolar solvents. The rates of reaction are also very sensitive to substituent effects, a nonlinear (concave upward) Hammett plot being obtained for the oxidation of substituted P-bromo and P-methoxystyrenes. The mechanism is best visualized as proceeding by way of a continuum of transition states that can vary from electron rich to electron poor depending on the capacity of the substrate structures to accommodate either negative or positive charges. Lorsque des alctnes sont oxydCs par des permanganates d'ammonium ou de phosphonium quaternaires, dans des solvants organiques polaires c o m e l'acttone, les structures des cations n'ont pas ou peu d'effet sur les vitesses des rkactions. Dans des solvants moins polaires, c o m e le chlomre de mkthylbne ou le tolubne, les vitesses dkpendent toutefois de I'identitC des ions quaternaires. I1 semble donc que les entitCs qui rCagissent peuvent exister sous la forme de paires d'ions solvatCsdans les solvants polaires et de paires d'ions intimes dans les solvants non-polaires. Les vitesses de rkaction sont aussi trbs sensibles aux effets de substituants: on obtient une courbe de Hammett qui n'est pas 1inCaire (concave vers le haut) lors de l'oxydation de P-bromo et de P-mCthoxystyri?nes substituks. La meilleure reprksentation du mkcanisme de la rCaction implique des Ctats de transition continus qui varient suivant leur nature riche ou pauvre en Clectrons, qui dCpend elle-mkme de la capacitk des structures des substrats d'accommoder soit des charges positives ou nCgatives.[Traduit par la revue]