Iron(II)/hydroperoxide (Fenton reagent)-induced activation of dioxygen for (A) the direct ketonization of methylenic carbon and (B) the dioxygenation of cis-stilbene

Aluminum derivative peroxides in the (t-BuO)3Al–2t-BuOOH catalytic system as a source of electron-excited dioxygen: a quantum chemical study on a model

The t-BuOOH catalytic decomposition leads to reactive oxygen species including chemically activated and chemically excited (O₂(b¹Σ+g) and O₂(¹Δ_g)) dioxygen.

The iron(II)/reductant (DH2)-induced activation of dioxygen for the demethylation of N-methylanilines: reaction mimic for the cytochrome P-450/reductase system

Several iron(II) complexes [FeIIL+; FeII(DPAH)2 (DPAH2 = 2,6-dicarboxyl pyridine), FeII(PA)2 (PAH = picolinic acid) and FeII(bpy)2(2+)(bpy = 2,2'-bipyridine)] in combination with a reductant [DH2; PhNHNHPh (mimic of dihydroflavin)] catalytically activate O2 (1 atm) for the demethylation of N-methylanilines [PhN(CH3)2, PhNH(CH3)]. This chemistry, which appears to involve a Fenton-like intermediate [LxFeIVOOH(DH) (1c)], mimics that of the cytochrome P-450 monooxygenase/reductase proteins.

Aromatic hydroxylation by Fenton reagents (reactive intermediate [Lx+FeIIOOH(BH+)], not free hydroxyl radical (HO.))

Several iron complexes [FeII(bpy)2(2+), FeII(OPPh3)4(2+), and FeII(PA)2] in combination with hydrogen peroxide (HOOH) catalytically hydroxylate aromatic substrates (ArH). The base-induced nucleophilic addition of HOOH to the electrophilic iron center yields the reactive intermediate of Fenton reagents [FeIILx2+ + HOOH<-->Lx+FeIIOOH(BH+)(1)]. The latter includes a 'stabilized' hydroxyl radical that is able to replace an aromatic hydrogen (H) with a hydroxyl group (HO) via an initial addition reaction. With PhCH3 and PhCH2CH3 as substrates free HO. (from the radiolysis of H2O) reacts via aryl addition (97 and 85%, respectively) to give Ar-Ar as the predominant product, whereas 1 favors H-atom abstraction from the alkyl group (50 and 80%, respectively) and the only detectable products from aryl addition are the respective substituted phenols (o:p-ArOH). Other substituted benzenes (PhX) undergo addition by free HO at the ortho and para aryl carbons (o:p ratio, 2), followed by dimerization and elimination of two H2O molecules to yield substituted biphenyls. In contrast, 1 reacts with PhX to yield substituted phenol (ArOH; o:p ratio, 0.5-1.1). With phenol (PhOH) as the substrate, reaction with 1 yields mainly catechol (o-Ar(OH)2; o:p ratio, 20). In a solvent matrix of MeCN:H2O (3:1 mol:mol ratio) the reaction efficiencies with FeII(bpy)2(2+) and FeII(OPPh3)4(2+) for the hydroxylation of benzene to phenol are 36 and 42%, respectively (product per HOOH).

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.