Kinetics of C–H bond and alkene oxidation by trans-dioxoruthenium(VI) porphyrins

Abstract

A. Series of [Ru^VILO₂] complexes (H₂L =Para-substituted tetraphenylporphyrins) have been synthesised and characterized, and the kinetics and mechanism of oxidation of the C–H bond and alkenes investigated. The complexes were selective towards tertiary C–H bonds in saturated alkanes but were almost inactive towards secondary C–H bonds. However, they were reactive towards aromatic hydrocarbons and the second-order rate constants (k₂) for the oxidation of ethylbenzene and cumene by [Ru(tpp)O₂](tpp = 5, 10, 15, 20-tetraphenylporphyrinate) were 2.21 × 10^–4 and 3.16 × 10^–4dm³ mol^–1 s^–1 respectively. A kinetic isotope effect (K_H/K_D) of 11.7 was found for the allylic oxidation of cyclohexene by [Ru(tpp)O₂]. The major organic products of the oxidation of alkenes in CH₂Cl₂–MeOH mixtures were epoxides and [Ru(tpp)O₂] gave a monomeric product formulated as [Ru^IV(tpp)O]·EtOH. or [Ru^IV(tpp)(OH)₂·]EtOH. Similar reactions with [Ru^VI(oep)O₂](oep = 2,3,7,8,12,13,17,18-octaethylporphyrinate) gave [{Ru^VI(oep)(OH)}₂O] in non-co-ordinating solvents. The observed rate law for alkene oxidation was rate =K₂[Ru^VI][alkene]. There exists an almost linerar free-energy relationship between log k₂ and E_½(one-electron oxidation potentials of alkenes) with slope =–1.1 V^–1 for the [Ru(tpp)O₂] system. Activation parameters have been determined for the oxidation of styrene, norbornene and cyclooctene by [Ru^VILO₂]. Non-linear and U-shaped Hammett plots were observed for the oxidation of substituted styrenes. The mechanism of alkene oxidation is proposed to involve a continum of transition states, the structures of which may change and be stabilized by defferent substituents.