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

Abstract

A series of [RuVILO2] 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 (KH/KD) 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 [RuIV(tpp)O]·EtOH. or [RuIV(tpp)(OH)2·]EtOH. Similar reactions with [RuVI(oep)O2] (oep = 2,3,7,8,12,13,17,18-octaethylporphyrinate) gave [{RuVI(oep)(OH)}2O] in non-co-ordinating solvents. The observed rate law for alkene oxidation was rate = K2[RuVI][alkene]. There exists an almost linerar 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 [RuVILO2]. 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.