Cu -phenoxyl radical

Compound 1 (Fig. 18.18) reversibly forms an analogous ferric-superoxo/Cu adduct at 60 °C, as demonstrated by resonance Raman spectroscopy. However, warming the sample to 40 °C results in a rapid four-electron reduction of the bound O2 ligand, generating a ferryl/Cu /phenoxyl radical derivative (Fig. 18.18) [Collman et al., 2007a]. 

Table 2 Selected spectroscopic data for Cu -phenoxyl radical complexes. ...

In an early report of a catalytic alcohol oxidation system, (27) was treated with KOH under O2 in neat primary alcohol substrate (EtOH, Pr OH, or hydroxy acetone). As many as 12 turnovers to aldehyde product were observed (10 h). " Similar turnovers of LiOCH2Ph to benzaldehyde were obtained with the Cu -phenoxyl radical species (19) (R = SPh). Complex (20) also converted primary alcohols to aldehydes with turnover numbers of 32. Higher turnover numbers (cf. 300 per day) were found using [(22 )Cu]+ (R = OMe, R = N02). Of particular relevance to the GAO mechanism, exposure of a mixture of (22)H2 and CuCl to O2 resulted in the generation of H2O2. 

Surprisingly, it was found that [Cun(L"BuMet )]2+ possesses an. S t = ground state where the phenoxyl radicals are ferromagnetically coupled to the Cu(II) ion (145). This makes mechanism (b) quite appealing for the monocations. 

Figure 29. Relative orientations of magnetic orbitals of the Cu(II) ion (d c2 2) in the x,y plane relative to that of the half-occupied Jt orbital of the phenoxyl radical. Here a is the Cu-O-C bond angle and P the dihedral angle between the x,y plane and the plane of the phenyl ring of the coordinated phenoxyl and St is the expected electronic ground state (204).

In contrast to the active site of galactose oxidase, to pre-catalyst 13, and to the system reported by Stack et al., the proposed catalytic species 15 does not imdergo reduction to Cu intermediates, as the oxidation equivalents needed for the catalysis are provided for solely by the phenoxyl radical Hgands. Since the conversion of alcohols into aldehydes is a two-electron oxidation process, only a dinuclear Cu species with two phenoxyl ligands is thought to be active. Furthermore, concentrated H2O2 is formed as byproduct in the reaction instead of H2O, as in the system described by Marko et al. [159]. 

Moreover, under certain conditions these phenolic compounds could also act as pro-oxidants. In the presence of redox-active metal ions such as Cu or Fe, phenolic compounds react with O2 to generate phenoxyl radicals. Under normal growth conditions phenoxyl radicals can be rapidly deactivated by polymerization or enzymatic reduction. However, if the phenoxyl radical concentrations are too high and/or the lifetime is increased, they could initiate DNA damage or lipid peroxidation and exhibit cytotoxicities. Curcumin, demethoxycurcumin, and bisdemethoxycurcumin have been reported to induce... 

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. 

Fig. 21. Proposed catalytic mechanism for substrate oxidation by galactose oxidase. (A) Substrate binding displaces Tyr-495 phenolate which serves as a general base for abstracting the hydroxylic proton. (B) Stererospecihc pro- hydrogen abstraction by the Tyr-Cys phenoxyl radical. (C) Inner sphere electron transfer reducing Cu(II) to Cu(I). (D) Dissociation of the aldehyde product.

Wang, Y., DuBois, J. L., Hedman, B., Hodgson, K. O., and Stack, T. D. P., 1998, Catalytic Galactose Oxidase Models Biomimetic Cu(II)-Phenoxyl-Radical Reactivity, Science 279 5379540. 

The metrical details of a coordinated phenoxyl radical have been determined through an X-ray crystal structure of [L(° 9Ci-](Q04)-3MeCN the only other metal-phenoxyl radical complex to be structurally defined by X-ray diffraction is a Cu species, which is discussed in Section... 

Figure 9 Relative orientations of the magnetic orbitals on Cu (dx2-y2) and a coordinated phenoxyl radical (half-occupied tt) that result in the expected overall spin state (5tot)- The angles a and / are defined as follows a — Cu—O—C bond angle, / = dihedral angle between the x, y plane and the phenoxyl ring plane (reproduced by permission of Wiley-Interscience from Muller et al. ).

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