OXYGEN TRANSFER FROM PERCARBOXYLIC ACIDS AND ALKYL HYDROPEROXIDES TO (MESO-TETRAPHENYLPORPHINATO)IRON(III) AND -CHROMIUM(III)

Article abstract of DOI:10.1021/ja00221a029

The second order rate constants (k_YOOH) for the reaction of (meso-tetraphenylporphinato)iron(III) chloride ((T-PP)Fe^III(Cl)) with a series of percarboxylic acids and alkyl hydroperoxides (species YOOH) were determined in methanol by the trapping of the hypervalent iron-oxo porphyrin species with 2,4,6-tri-tert-butylphenol.The oxygen donors, TOOH, were chosen so that the leaving species, YOH, possessed pK_a values spaced between 3.8 and 16.7.Values of k_YOOH for oxidation of (TPP)Cr^III(Cl) to (TPP)Cr^V(O) have been determined in dichloromethane by following the formation of ((TPP)Cr^IV)2O.The latter arises by non-rate-determining trapping of (TPP)Cr^V(O) by unreacted (TPP)Cr^III(Cl).A plot of log k_YOOH for (TPP)Fe^III(Cl) versus the log of k_YOOH for (TPP)Cr^III(Cl) exhibits two linear portions.A slope of about unity is seen when YOOH represents percarboxylic acids.This reflects the similarity in mechanism for the reactions of percarboxylic acids with (TPP)Fe^III(Cl) and (TPP)Cr^III(Cl).Since phenylacetic acid is obtained as the product in the reaction of both (TPP)Fe^III(Cl) and (TPP)Cr^III(Cl) with phenylperacetic acid, the common mechanism is one of heterolytic scission of the percarboxylic acid O-O bond.With (TPP)Cr^III(Cl), a linear free-energy plot of log k_YOOH versus the pK_a of the leaving YOH is shared by both percarboxylic acids and hydroperoxides.This finding is to be expected if oxygen transfer from hydroperoxides to (TPP)Cr^III(Cl) shares with the percarboxylic acids the common mechanism of O-O bond heterolysis.A change in mechanism of oxygen transfer in the reactions of (TPP)Fe^III(Cl) with percarboxylic acids and alkyl hydroperoxides is shown by the breaks in plots of the log k_YOOH versus either the pK_a of YOH or log k_YOOH for (TPP)Cr^III(Cl).The rate-limiting step on reaction of the less acidic alkyl hydroperoxides with (TPP)Fe^III(Cl) is suggested to involve homolytic O-O bond scission to provide an intimate <(TPP)Fe^IV(OH)(X)-^.O-alkyl> pair which may then dissociate as such or after H^. transfer dissociate to (^+.TPP)Fe^IV(O)(X) + HO-alkyl.Differentiation of mechanism on the basis of the products of YOH oxidation cannot be made.Details are discussed in the paper.The various linear free-energy plots exhibit a surprising independence of steric effects in the hydroperoxides.