Kinetics and Mechanism of Reductive Dioxygen Activation Catalyzed by the P-450 Model System. Iron Picket Fence as a Catalytic Center

Article abstract of DOI:10.1021/ja00301a016

Picket-fence porphyrin (TpivPP)-iron-N-methylimidazole-O2 complex is used as an artifical P-450, and the decomposition rates are investigated in detail in the presence of HCl and H2-colloidal platinum supported on poly(vinylpyrrolidone) with or without addition of benzoic anhydride.From the decay rates of the oxy complex followed by electronic spectrum under a variety of conditions, pseudo-first-order rate (with the complex) constants are obtained.The pseudo-first-order constants are proportional to first order with the colloidal platinum and first order with dihydrogen.Analysis of the dependence of the rate constants on the acidity strongly suggests the simultaneous participation of the protonated and unprotonated oxy complexes in the transition states.Cyclohexene used as a guest does not affect the rate at all, demonstrating that the product-forming step comes later than the rate-determining step.It is also ascertained that H2 favorably competes with cyclohexene in the product-forming step under the conditions of the rate measurements.However, competitive oxidation of the present artifical P-450 porphirin is satisfactorily slow, and solvent oxidation is not appreciable.Products of the present acid-catalyzed reductive decomposition of the oxy complex are corresponding ferric (deoxy) complex, trans-cyclohexan-1,2-diol ethyl ether drived from cyclohexene oxide and ethanol.Slow regeneration of the ferrous oxy complex from the ferric complex leads to the effective recycling (turnover) of the artifical P-450 system.