Superoxo-complexes, mechanism formation

It is now generally understood that the mechanism of hydroxylation by cytochrome P-450 proceeds by two successive one-electron reduction steps of the heme center, transforming dioxygen into a peroxide species bonded to iron (Scheme l).73,78 Well-defined steps (25) - (31) involve (a) the formation of a high-spin Fein-enzyme-substrate complex (26) (b) one-electron reduction of (26) to a high-spin ferrocytochrome (27) (c) addition of dioxygen to form a superoxo-Fe111 complex (28) and (d) one-electron reduction of the superoxo complex to a peroxide complex (29). 

These copper-mediated reactions very often involve dinuclear intermediates, but detailed mechanistic studies on stoichiometric systems are relatively few. The key features are the formation of p-peroxo or p-superoxo complexes by electron transfer from cop-per(i) to dioxygen. The co-ordinated oxygen may then act as an electrophile to the aromatic ring. A possible mechanism for the ortho-hydroxylation of phenol by dioxygen in the presence of copper catalysts is shown in Fig. 9-29. 

Figure 9-29. A possible mechanism for the oxidation of phenol to 1,2-benzoquinone by dioxygen in the presence of copper(i) salts. The key steps involve the formation of a peroxo or superoxo complex, followed by electrophilic attack upon the benzene ring.

The mechanism of metal phthalocyanine catalysed oxidation by molecular oxygen -isobutyraldehyde system is not established at this stage. The iron[14], manganese[15] and cobalt tetrasulphonato-[16] phthalocyanines are known to form superoxo complexes with dioxygen and are known to catalyse autoxidation reactions[13]- The acyl radical formation thus can be initiated by interaction of metal phthalocyanine-dioxygen superoxo complex with isobutyraldehyde. The acyl radical in presence of oxygen can yield acylperoxy radical or peracid as the oxidising speceis[17]. 

Superoxo 1 1 (Co O2) complexes are usually unstable relative to formation of bridged /i-peroxo complexes. In fact the mechanism of formation of most /x-peroxo complexes involvesintermediacy of the 1 1 superoxo complex [45], Wilkins, ef a/. [46-48] have presented kinetic evidence for such a mechanism for the oxygenation of cobalt(II) amine complexes in water, equations (9), (10). 

The kinetics and mechanism of the O2 oxidation of2-aminophenol (ap) to 2-aminophenoxazin-3-one (apx) under ambient conditions, catalyzed by the recently synthesized tetrakis(3,5-di-teAt-butyM-hydroxyphenyl)-dodeca-chlorophthalocyaninatocobalt(ll), (R4PCC0), (Figure 41) have been studied by spectrophotometry . The rate of ap formation is first-oider in [R4PCC0] and obeys Michaelis-Menten t) e kinetics with respect to [ ]. The suggested mechanism involves rate-determining inner-sphere election transfer from coordinated ap to coordinated O2 in the superoxo complex. 

In light of the accepted mechanism for cytochrome P-450 (97-100), a superoxo-Cu(II) intermediate is further reduced, leading to dioxygen activation. Accordingly, a monomeric peroxo or hydroperoxo copper(II) complex serves as a synthetic model for these intermediates of copper-containing monooxygenases. However, no well-characterized complexes of these types are available to date. Formation of a monomeric hydroperoxo or acylperoxo complex was reported to occur when a trans-/u-l,2-peroxo complex, [(Cu(TPA))2(02)]z+, was treated with H+ or RC(O)+, but no details of the structures and properties of the complexes were provided (101). A related complex, a monomeric acylperoxo cop-per(II) complex, was synthesized (102). Low-temperature reaction of a dimeric copper(II) hydroxide complex, [Cu(HB(3,5-iPr2pz)3)]2(OH)2, with 2 equivalents of m-CPBA (3-chloroperoxybenzoic acid) yielded a monomeric acylperoxo complex whose structure was characterized by... 

Time-resolved spectroscopy (stopped-flow ultraviolet-visible (UV-vis) spectroscopy at -90° C, proprionitrile or acetonitrile, [O2] S> [complex]) has been used to characterize intermediates and evaluate the mechanism of the peroxo complex formation (see Fig. 16) (196). Based on the similarity of the spectral features with known superoxo copper(lI) and peroxo-dicop-per(ll) complexes (262, 268, 281) the mechanism shown in Scheme 17 was proposed, and the spectra of the superoxo copper(II) and peroxo-dicop-per(II) complexes were determined (see Table XI). For steric reasons and in... 

Other DFT calculations have shown that co-adsorption of H2O and O2 on Aug clusters, free or supported on MgO(lOO), leads to the formation of an 02- -H20 complex involving partial proton sharing or proton transfer, and leading to a hydroperoxy-like complex (HO2) [178]. This favors the activation of the 0—0 bond, i.e. the bond extension to values characteristic of a peroxo- or superoxo-Uke state. Consequently, the reaction with CO can occur with a small activation barrier of -0.5 eV, either through an Eley-Rideal mechanism if O2 is adsorbed on the top face of Aug clusters or through a Langmuir-Hinshelwood mechanism if O2 is adsorbed on the periphery of the cluster. 

Both mechanisms (40) and (42), applied to either aquacopper(I) or copper(I) complexed with either unidentate amines or chelates, imply the transient formation of copper(II) superoxo or a p-peroxo species. The evidence supporting this view had been primarily kinetic, i.e. no direct detection of such intermediates could be achieved [340]. The situation has changed with the introduction of Karlin s ligands [201,383], which afford copper(I) complexes that form relatively stable... 

The proposed mechanism involves H-atom abstraction from the phenolic OH group by the superoxo adduct of Co(NMe-salpr) [202], followed by rearrangement and peroxy complex formation, and decomposition via a dioxetan intermediate ... 

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