Iron(II)/reductant(DH2)-induced activation of dioxygen for the hydroxylation and ketonization of hydrocarbons; mimics for the cytochrome P-450 hydroxylase/reductase system

Several metal complexes [(FeII(DPAH)2 (DPAH2 = 2,6-dicarboxyl pyridine), FeII(PA)2 (PAH = picolinic acid), FeII(bpy)2(2+), FeII(OPPh3)4(2+), (Cl8TPP)FeIIIX (X = Cl, OH, SCH2Ph) [Cl8TPP = tetrakis (2,6-dichlorophenyl)porphyrin], (TPP) FeIIICl (TPP = tetraphenylporphyrin), and CuI(tpy)2+ (typ = 2,2'-6,2"-terpyridine)] in combination with one of several reductants [DH2; PhNHNHPh (mimic of dihydroflavin), PhNHNH2, ascorbic acid (H2asc), and PhCH2SH (model ligand for cysteine residue)] catalytically activate O2 (1 atm) for the hydroxylation of saturated hydrocarbons (e.g. c-C6H12-->c-C6H11OH). This chemistry closely parallels that of cytochrome P-450 proteins, and both appear to involve a Fenton-like reactive intermediate), [LxFeOOH(DH)]. With cyclohexane as the substrate the dominant product is its ketone (as well as significant amounts of its hydroperoxide). 1,4-Cyclohexadiene (with two double-allylic carbon centers) undergoes dehydrogenation to give benzene, but also yields substantial amounts of phenol via ketonization of an allylic carbon. The 1:1 FeII(bpy)2(2+)/(PhNHNH2 or H2asc), FeII(PA)2/H2asc, and (Cl8TPP)FeIIICl/PhNHNH2 combinations initiate the autoxidation of 1,4-cyclohexadiene with turnover numbers (moles of product per mole of reductant) from 71 to 26, respectively (alpha-tocophenol reduces the turnover numbers by 20-80%). With respect to aerobic biology, the present results indicate that dysfunctional transition metals (degradation products of metalloproteins) in combination with biological reductants activate O2 for reaction with organic substrates. The level of activation is similar to that for Fenton reagents and cytochrome P-450 hydroxylases. Hence, dysfunctional transition metals, reductants, and O2 are a hazardous combination within a biological matrix.