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

The oxidations of a series of 21 alkylaromatic compounds by <i>trans</i>-[Ru<sup>VI</sup>(L)(O)<sub>2</sub>]<sup>2+</sup> (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in CH<sub>3</sub>CN. 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[Ru<sup>VI</sup>]/d<i>t</i> = <i>k</i><sub>2</sub>[Ru<sup>VI</sup>][ArCH<sub>3</sub>]. The kinetic isotope effects for the oxidation of toluene/<i>d</i><sub>8</sub>−toluene and fluorene/<i>d</i><sub>10</sub>−fluorene are 15 and 10.5, respectively. A plot of Δ<i>H</i><sup>⧧</sup> versus Δ<i>S</i><sup>⧧</sup> is linear, suggesting a common mechanism for all the substrates. In the oxidation of para-substituted toluenes, a linear correlation between log <i>k</i><sub>2</sub> and σ<sup>0</sup> values is observed, consistent with a benzyl radical intermediate. A linear correlation between Δ<i>G</i><sup>⧧</sup> and Δ<i>H</i><sup>0</sup> (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 <i>trans</i>-[Ru<sup>VI</sup>(L)(O)<sub>2</sub>]<sup>2+</sup>. The slope of (0.61 ± 0.06) is in reasonable agreement with the theoretical slope of 0.5 predicted by Marcus theory.