Is a Linear Relationship between the Free Energies of Activation and One-Electron Oxidation Potential Evidence for One-Electron Transfer Being Rate Determining? Intermediates in the Epoxidation of Alkenes by Cytochrome P-450 Models. 4. Epoxidation of a Series of Alkenes by Oxo(meso-te...

Article abstract of DOI:10.1021/ja00195a060

The mechanism of reaction of (porphyrin)Cr^V(O)(X) with alkenes has been investigated by combining the electrochemical determinations of redox potentials with the tools of kinetics and product identification.Studies include the following: (i) the dynamics of the reaction of oxo(meso-tetrakis(2,6-dibromophenyl)porphinato)chromium(V) ((Br8TPP)Cr^V(O)(X) with 16 alkenes (1,4-diphenyl-1,3-butadiene, 4-methoxystyrene, 1,1-diphenylethylene, 4-methylstyrene, 2,3-dimethyl-2-butene, cis-stilbene, styrene, 4-acetoxystyrene, cyclohexene, norbornene, cis-cyclooctene, 4-cyanostyrene, cis-2-pentene, cyclopentene, 1-hexene, 1-octene); (ii) the dynamics of the reaction of norbornene with five (porphyrin)Cr^V(O)(X) species (oxo(meso-tetraphenylporphinato)chromium(V), oxo(meso-tetrakis(2,4,6-trimethylphenyl)porphinato)chromium(V), oxo(meso-tetrakis(2,6-dichlorophenyl)porphinato)chromium(V), oxo(meso-tetrakis(2,6-difluorophenyl)porphinato)chromium(V), and oxo(meso-tetrakis(2,6-dibromophenyl)porphinato)chromium(V)).Alkenes were selected on the basis of their 1e oxidation potential so as to cover the widest range of E_1/2 possible (1.14-2.80 V (SCE)), and the 1e oxidation potentials for the (porphyrin)Cr^V(O)(X) varied from 0.790 to 0.975 V (SCE).Standard solutions of (porphyrin)Cr^V(O)(X) were obtained by controlled-potential bulk electrolysis of the corresponding (porphyrin)Cr^IV(O) and used in the kinetic (CH2Cl2 solvent, 30 deg C) and product studies.The time course for the change in concentration of (Br8TPP)Cr^V(O)(X), (Br8TPP)Cr^IV(O), and (Br8TPP)Cr^III(X) was simulated for the reaction of (Br8TPP)Cr^V(O)(X) with selected alkenes possessing "low, medium, and high" oxidation potentials.The time dependence of <(Br8TPP)Cr^V(O)(X)>, <(Br8TPP)Cr^IV(O)>, and <(Br8TPP)Cr^III(X)> as well as the could be accurately fit to a scheme involving the following: (i) bimolecular reaction of alkene with (Br8TPP)Cr^V(O)(X) to provide alkene oxidation product + (Br8TPP)Cr^III(X); (ii) reversible comproportionation of (Br8TPP)Cr^V(O)(X) + (Br8TPP)Cr^III(X) to provide (Br8TPP)Cr^IV(O) + (Br8TPP)Cr^IV(X)2; (iii) a spontaneous reversion (due to oxidation of solvent or solutes) of (Br8TPP)Cr^V(O)(X) to (Br8TPP)Cr^IV(O).The remainder of the alkene oxidations were followed by the more simplistic procedure of following disappearance of (Br8TPP)Cr^V(O)(X).The second-order rate constants (k₁) determined by either method were found to be in agreement.For about half the alkenes studied, the standard free energies for 1e oxidation by (Br8TPP)Cr^V(O)(X), to provide alkene-derived ? cation radicals (calculated from the potentials for 1e reduction of (Br8TPP)Cr^V(O)(X) and 1e oxidation of the alkenes to ? cation radicals), exceed the free energies of activation for epoxidation of alkenes by (Br8TPP)Cr^V(O)(X).A unified mechanism ...