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

The mechanism of oxidation of tetramesityliron(III) porphyrins [Fe^IIITMP(X)] with peroxyacids has been examined. The reaction of Fe^mTMP(Cl) (1) with peroxyacids in methylene chloride at -46 °C afforded the corresponding oxoiron(IV) porphyrin cation radical [Fe^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 oxidation 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 °C indicated E_a= 4 ± 0.4 kcal/mol, ΔH* = 3.6 ± 0.4 kcal/mol, and ΔS* > -25 eu. The oxidation of l-(m-chiorobenzoate) 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 for 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).