THE MECHANISM OF RUO4-MEDIATED OXIDATIONS OF SATURATED-HYDROCARBONS - SOLVENT EFFECTS AND SUBSTITUENT EFFECTS

Adamantane and cis-decalin have been oxidised by RuO4 in organic solvents of different polarity. No large variations in the rate of reaction were observed. cis-Decalin was oxidised in a series of acetone-water mixtures. From these experiments a Grunwald-Winstein m-value of 0.31 was obtained, indicating a nonpolar transition state (TS) for the reaction. A series of 1-substituted adamantanes were oxidised with RuO4. A correlation of the rates of reaction with the Taft sigma gave rho* of -1.68 (-2.08 excluding hydrogen as a substituent), indicating a polar TS. This contradicting evidence could be explained by two mechanisms: either by a hydride-abstraction reaction with the formation of an intimate ion pair or by a reaction with a pre-equilibrium formed from the hydrocarbon and RuO4 followed by a concerted rate-determining step: RH + RuO4 half arrow right over half arrow left RH.O4Ru] --> R-O-RuO3H.