Dynamics of reaction of [meso-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato]-iron(III) hydrate with various alkyl hydroperoxides in aqueous solution. 4. Comparison of kinetic parameters and D2O solvent isotope effects

Article abstract of DOI:10.1021/ja00016a025

The second-order rate constants (k_ly) for the reactions of the alkyl hydroperoxides (Ph)₂(MeOCO)COOH and (Ph)(Me)₂COOH with [meso-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato]iron(III) hydrate [(1)Fe^III(X)₂ (X = H₂O or OH_-)] have been determined in water over the pH range 1-13 by using the one-electron oxidizable trapping agent 2,2′-azinobis(3-ethylbenzthiazolinesulfonate) (ABTS). The complex pH dependencies of k_ly reflect the pH dependency of the structure of reactants and intermediates as shown in Scheme II. Plots of log k_ly vs pH were fitted by a steady-state kinetic expression (eq 1) shown previously to describe the pH dependence of the reaction of (CH₃)₃COOH with (1)Fe^III(X)₂. The dependencies of the rate constants for O-O bond cleavage (K₂ and k₄, Scheme II) of the ligated hydroperoxides YOOH to the pK_a of the leaving group YOH (δ(log k₂)/δpκ_aYOH and (δ(log k₄)/δpκ_aYOH can be isolated from the pH dependencies of the k_ly values and are determined to be -0.22 and -0.13, respectively. Thus, the sensitivity of rate constants for the cleavage of the O-O bond of ligated hydroperoxide to its polarity is small. Kinetic deuterium solvent isotope effects for reaction of (Ph)₂(MeOCO)COOH with (1)Fe^IIIX₂ have been found to be in accord with those determined previously for (CH₃)₃COOH.