Ligation and Reduction of Iron(III) Porphyrins by Amines. A Model for Cytochrome P-450 Monoamine Oxidase

Article abstract of DOI:10.1021/ja00274a055

The scope and mechanism of the ligation and reduction of iron(III) porphyrins by amines are presented.The reaction is general and proceeds with the overall stoichiometry 2PFe(III)Cl + 5RNH2 -> 2PFe(II)(H2NR)2 + R'CH=NH + 2RNH3(1+)Cl(1-).Imines are the only amine-derived products.The ability of the amine to coordinate iron and its possession of a >CHNH moiety are essential for reduction.The reaction path antails two successive reversible ligation steps followed by two one-electron reductions of the bis(amine)-ligated low-spin iron(III) adduct.In benzene the kinetics are biphasic.The second ligation is rate limiting 1(ligation > k(reduction) > k₂(ligation)>.In dimethylformamide the first reduction step is rate limiting.The influence of nonreducing amines, porphyrin substituent, axial ligands, and deuteration of the substrate upon the kinetics leads to the formulation of the first reduction as a reversible outer-sphere electron transfer to the porphyrin periphery.This is followed by a rapid conversion of the generated aminium cation radical to an α-aminocarbinyl radical.The latter completes the reduction in an irreversible second step.The relevance of this chemistry to related transformation catalyzed by cytochrome P-450 is discussed.