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

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journal contribution
posted on 01.12.2003, 00:00 by William W. Y. Lam, Shek-Man Yiu, Douglas T. Y. Yiu, Tai-Chu Lau, Wing-Ping Yip, Chi-Ming Che
The oxidations of a series of 21 alkylaromatic compounds by trans-[RuVI(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[RuVI]/dt = k2[RuVI][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-[RuVI(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.