Mechanism of RuO4-mediated oxidations of saturated hydrocarbons, isotope effects, solvent effects and substituent effects

Article abstract of DOI:10.1002/(SICI)1099-1395(199607)9:7<507::AID-POC811>3.0.CO;2-L

Adamantane and 1,3,5,7-tetradeuterioadamantane were oxidized by RuO₄ in two solvent systems, CCl₄-CH₃CN-H₂O and acetone-water, yielding two kinetic deuterium isotope effects (KIEs), 4-8 ± 0-2 and 7.8 ± 0.l, respectively, very similar to those obtained in analogous reactions with cu-decalin and perdeuteriocis-decalin, 4-8 and 6-8. These results were interpreted as primary KIEs and small or negligible secondary KIEs. From this, sp²-hybridized intermediates were not involved in the reaction path. The kinetic effect of the solvent was investigated by performing the reaction in aqueous acetone and acetonitrile. The rates were correlated with Grunwald-Winstein Y values and with Reichardt E_T(30) values. Both correlations showed the reaction to be only moderately dependent on the solvent polarity. 1-Substituted adamantanes were oxidized in CCl₄-CH₃CN giving a Taft ρ* value of -2.5 ± 0.1. These results were regarded as support for a reaction consisting of a pre-equilibrium with formation of a substrate-RuO₄ complex followed by a rate-determining concerted reaction. Chemical Equation Presented. The results did not support a reaction mechanism with a carbocation or radical intermediate, or a scheme with two competing reactions, one with a carbocation intermediate and the other with a concerted mechanism.