Reactive Iron Porphyrin Derivatives Related to the Catalytic Cycles of Cytochrome P-450 and Peroxidase. Studies of the Mechanism of Oxygen Activation

Article abstract of DOI:10.1021/ja00233a021

The mechanism of oxidation of tetramesityliron(III) porphyrins IIITMP(X)> with peroxyacids has been examined.The reaction of Fe^IIITMP(Cl) (1) with peroxyacids in methylene chloride at -46 deg C afforded the corresponding oxoiron(IV) porphyrin cation radical IVTMP^.+(O)> (3).The kinetics of this process were complicated by an induction period that depended on the acidity of the peroxyacid used.By contrast, similar oxidations of Fe^IIITMP(OH) gave evidence for rapid ligand metathesis to afford an acylperoxoiron(III) complex Fe^IIITMP(OOC(O)Ar) (2).The decomposition of 2 to form 3 was found to be first order in 2 and catalyzed by acid.Electron-withdrawing substituents on the aryl portion of the peroxyacid facilitated this reaction (ρ = +0.5).The temperature dependence between -32 and -48 deg C indicated E_a = 4 +/- 0.4 kcal/mol, ΔH* = 3.6 +/- 0.4 kcal/mol, and ΔS* > -25 eu.The oxidation of 1-(m-chlorobenzoate) in toluene with peroxyacids afforded an iron(III) porphyrin N-oxide (5).The reaction required 2 equiv of peroxyacid and afforded 1 mol of a diacylperoxide.The presence of acid discouraged the formation of 5.Substituent effects in the peroxyacid were the opposite for the formation of 5 (ρ = -0.4) than the formation of 3.The results indicated that there are competing homolytic and heterolytic O-O bond cleavage reactions for 2 mediated by iron(III).