KINETICS OF C-H BOND AND ALKENE OXIDATION BY TRANS-DIOXORUTHENIUM(VI) PORPHYRINS

A series of [Ru(VI)LO2] 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 x 10(-4) and 3.16 x 10(-4) dm3 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(IV)(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 linear free-energy relationship between log k2 and E1/2 (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(VI)LO2]. 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 continuum of transition states, the structures of which may change and be stabilized by different substituents.