Kinetics and Mechanism of Oxygen Transfer in the Reaction of p-Cyano-N,N-dimethylaniline N-Oxide with Metalloporphyrin Salts. 5. The Influence of Imidazole Ligation of (meso-Tetrakis(2,6-dimethylphenyl)porphinato)manganese(III) Chloride on the Rates of Oxygen Transfer from N-Oxide to...

Article abstract of DOI:10.1021/ja00245a039

Equilibrium constants for mono- and bisligation of imidazole (ImH) with (meso-tetrakis(2,6-dimethylphenyl)porphinato)manganese(III) chloride ((Me8TPP)Mn^IIICl) have been determined so that the concentrations (dry CH2Cl2) of the three species (Me8TPP)Mn^IIICl, <(Me8TPP)Mn^III(ImH)>Cl, and <(Me8TPP)Mn^III(ImH)2>Cl may be calculated at different ImH concentrations.The equilibrium constants for ligation of the one and two imidazoles are K₁ = 245 M^-1 and β₂ = 1.80 x 10⁵ M^-2.The reaction of p-cyano-N,N-dimethylaniline N-oxide (NO) with the manganese(III) porphyrin (under the pseudo-first-order conditions of _i >> <(Me8TPP)Mn^IIICl>_i and in the presence and absence of ImH) is first order in both NO and manganese(III) porphyrin, and the rate-controlling step involves oxygen transfer with formation of higher valent manganese-oxo porphyrin species plus p-cyano-N,N-dimethylaniline (DA).From the dependence of the pseudo-first-order rate constants (k_obsd) upon _i, and a knowledge of the equilibrium constants for imidazole ligation there has been calculated the second-order-rate constants for the kinetic terms k₁<(Me8TPP)Mn^IIICl>, k₂<<(Me8TPP)Mn^III(ImH)>Cl>, and k₃<<(Me8TPP)Mn^III(ImH)2>Cl>.Comparison of the second-order rate constants (k₁ = 3.4 X 10^-2 M^-1 s^-1, k₂ = 5.53 M^-1 S^-1, and k₃ = 7.32 X 10^-2 M^-1 s^-1) establishes that ligation by one imidazole increases the rate of reaction of the manganese(III)porphyrin with NO by ca. 166-fold.Bis-imidazole ligated species are blocked to reaction with NO.The higher valent manganese-oxo porphyrin species formed from the reaction of NO with <(Me8TPP)Mn^III(ImH)>Cl has been shown to be the principal epoxidizing agent from the dependence of the percentage yield of epoxide upon the concentration of ImH in reactions with cis-cyclooctene using constant initial concentrations of (Me8TPP)Mn^IIICl and NO.Epoxidation reactions are not rate controlling, and epoxide is formed in competitive processes that involve the reaction of higher valent manganese-oxo porphyrin species with DA (and its oxidation products) and alkene.With exception of the sterically hindered trans-β-methylstyrene, the percentage yield of epoxide at 1.0 M alkene is essentially independent of the type of alkene.Increase in the concentration of alkene and the 1e^- oxidizable 2,4,6-tri-tert-butylphenol fails to trap all higher valent manganese-oxo porphyrin species.This result is interpreted as being due to the initial formation of an intimate pair of oxo species and DA with competition between dissociation of DA and oxo species and oxidation of DA within the intimate pair.Epoxidation of alkene by the oxo species occurs after the latter dissociates from the intimate pair.