A Kinetic Study on the Reactivity of a cis-Dioxoruthenium(VI) Complex. Oxidation of Alcohols, Aromatic Hydrocarbons, and Alkenes by cis-[RuVI(Tet-Me6)O2]2+

The kinetics of oxidation of alcohols, aromatic hydrocarbons, and alkenes by cis-[RuVI(Tet-Me6)-O2]2+ (Tet-Me6 = N,N,N′,N′-tetramethyl-3,6-diazaoctane-1,8-diamine) have been studied. In general, the oxidation chemistry of cis-[RuVI(Tet-Me6)O2]2+ is similar to those of trans-dioxoruthenium(VI) compounds such as trans-[RuVI(N2O2)O2]2+ (N2O2 = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) and trans-[RuVI(pytn)O2]2+ (pytn = N,N′-dimethylbis(2-pyridylmethyl)-propylenediamine). The oxidation is first-order with respect to cis-[RuVI(Tet-Me6)O2]2+ and organic substrates and is accompanied by large and negative ΔS‡. Large kH/kD values were found for the oxidation of alcohols. A linear Hammett plot for the oxidation of para-substituted benzyl alcohols with a slope of -1.0 was obtained. For alcohol oxidation, a charge-transfer mechanism involving hydride abstraction has been proposed. Oxidation of toluene, ethylbenzene, and cumene by cis-[RuVI(Tet-Me6)O2]2+ gave benzaldehyde, acetophenone/sec-phenylisopropyl alcohol, and 2-phenyl-isopropyl alcohol, respectively. A kH/kD value of 11 has been found for the oxidation of ethylbenzene. In the oxidation of these aromatic hydrocarbons, the key step likely involves C-H bond activation in the transition state. Allylic oxidation is the major reaction pathway in the cyclohexene oxidation. In the oxidation of alkenes, both C=C bond cleavage product(s) and epoxides were found. The Hammett plot for the oxidation of para-substituted styrenes is linear with a slope of -2.0. The oxidation likely proceeds through a charge-transfer mechanism.