Copper phenoxyl radical

In studies of the reactivity of copper-phenoxyl radical species, particular emphasis has been placed on performing alcohol oxidations, either stoichiometrically or catalytically, and by using O2 as the final oxidant. One goal has been to obtain mechanistic insights into GAO function. In addition, the GAO reaction sequence has been used as inspiration for the development of new chemical catalysts for alcohol oxidations and H2O2 generation (a bioinorganic success story ). ... 

Jazdzewski BA, Tolma WB (2000) Understanding the copper-phenoxyl radical array in galactose oxidase contributions from synthetic modeling studies. Coord Chem Rev 200-202 633-685... 

Flavonoids have the ability to act as antioxidants by a free radical scavenging mechanism with the formation of less reactive flavonoid phenoxyl radicals [Eq. (1) and (2)]. On the other hand, through then-known chelating ability these compounds may inactivate transition metals ions (iron, copper), thereby suppressing the superoxide-driven Fenton Reaction, Eqs. (3) and (4), which is currently believed to be the most important route to activate oxygen species [51]. 

In a later article, complexes of Ni(II), Cu(II), Pd(II), and V02+ ions with the same tetra-substituted porphyrin were reported. Stepwise oxidation of these complexes gave products for which the authors proposed quinonoid, monoradical, and diradical structures. The most prolonged oxidations yielded the diradical products, which were isolated as dark purple crystals, relatively stable in air (40). The monoradical vanadyl complex was observed to be diamagnetic, suggesting antiferromagnetic coupling between the phenoxyl radical and unpaired electron on vanadium, whereas in the copper complex no such coupling was observed. More detailed studies of these systems seem warranted. 

The catalytic mechanism of GOase has been extensively studied (Fig. 8) 63,64). The primary alcohol first coordinates to the active species A, leading to the formation of the metal-phenoxyl radical complex B. This species undergoes proton abstraction from the substrate by the axial tyrosinate (Tyr495), followed by a rapid intramolecular electron transfer from the intermediate ketyl radical anion with reduction of Cu to Cul The copper(I) species C reacts with dioxygen to form the hydroperoxo copper(II) complex D with the liberation of the aldehyde. Finally, dihydrogen peroxide is released to give back the active form of the enzyme. 

Ethanol, benzyl alcohol or 2-butanol coordinate to the proposed dinuclear copper complex (Fig. 16) to produce the active species I (Fig. 17, A). After H abstraction by one of the two phenoxyl groups, a radical is created which is oxidized to the corresponding carbonyl derivative by the second phenoxyl radical. In the case of 2-propanol and diphenylmethanol, two alcohol molecules coordinate to the dinuclear moiety resulting in species II (Fig. 17, B). The H abstractions by the two phenoxyl ligands generate two... 

Figure 7.1 (a) Resonance structures for the phenoxyl radical (b) phenol carbon atoms that undergo C-C or C-0 coupling reactions and (c) reaction scheme for the copper-catalyzed oxidative coupling of DMP (R = CH3). 

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