Kinetics of C-H Bond and Alkene Oxidation by trans-Dioxoruthenium(VI) Porphyrins

Article abstract of DOI:10.1039/DT9910002933

A series of 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 (tpp = 5,10,15,20-tetraphenylporphyrinate) were 2.21*10^-4 and 3.16*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 .The major organic products of the oxidation of alkenes in CH2Cl2-MeOH mixtures were epoxides and gave a monomeric product formulated as IV(tpp)O>*EtOH or IV(tpp)(OH)2>*EtOH.Similar reactions with VI(oep)O2> (oep = 2,3,7,8,12,13,17,18-octaethylporphyrinate) gave <IV(oep)(OH)>2O> in non-co-ordinating solvents.The observed rate law for alkene oxidation was rate = k₂VI>.There exists an almost linear free-energy relationship between log k₂ and E_1/2 (one-electron oxidation potentials of alkenes) with slope = -1.1 V^-1 for the system.Activation parameters have been determined for the oxidation of styrene, norbornene and cyclooctene by 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 continuum of transition states, the structures of which may change and be stabilized by different substituents.

Full text of DOI:10.1039/DT9910002933

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