Kinetics and Mechanism of the Oxidation of Alkylaromatic Compounds by a trans-Dioxoruthenium(VI) Complex

The oxidations of a series of 21 alkylaromatic compounds by trans-[Ru ^VI(L)(O)₂]²⁺ (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in CH₃CN. Toluene is oxidized to benzaldehyde and a small amount of benzyl alcohol. 9,10-Dihydroanthracene is oxidized to anthracene and anthraquinone. Other substrates give oxygenated products. The kinetics of the reactions were monitored by UV-vis spectrophotometry, and the rate law is: -d[R^VI]/dt = k₂[Ru^VI][ArCH₃]. The kinetic isotope effects for the oxidation of toluene/d₈-toluene and fluorene/d₁₀-fluorene are 15 and 10.5, respectively. A plot of ΔH‡ versus ΔS‡ is linear, suggesting a common mechanism for all the substrates. In the oxidation of para-substituted toluenes, a linear correlation between log k₂ and σ⁰ values is observed, consistent with a benzyl radical intermediate. A linear correlation between ΔG‡ and ΔH⁰ (the difference between the strength of the bond being broken and that being formed in a H-atom transfer step) is also found, which strongly supports a hydrogen atom transfer mechanism for the oxidation of these substrates by trans-[Ru ^VI(L)(O)₂]²⁺. The slope of (0.61 ± 0.06) is in reasonable agreement with the theoretical slope of 0.5 predicted by Marcus theory.