Oxidation of hydrocarbons. 17. Solvent and substituent effects on the oxidation of styrene derivatives by quaternary ammonium permanganates

Abstract

When alkenes are oxidized by quaternary ammonium or phosphonium 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 β-bromo and β-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.