The Activation of Molecular Oxygen

These compounds, unlike their near-neighbors MeReO(dithiolate) 2 and MeRe(dithiolate)L, catalyze the oxidation of selected substrates by 

The first indication came with the finding that 25 is air-sensitive, unlike the others. The preparation of 25 is represented by the chemical reaction  

The findings will be illustrated for a typical substrate, tri(para-tolyl)-phosphine. Typical conditions are 1-30 mM PT0I3, 0.1-9.1 mM 02 and 5-500 pM 25. The major product was T0I3PO ( 90%), accompanied by a small amount of Tol3PS, evidently from side reactions between PT0I3 and a dithiolate ligand of 25. 

The kinetic pattern follows the Michaelis-Menten pattern, the rate law for which is, 

The value of kx proved to be so small that it could be omitted from the denominator. The initial rate increased with PT0I3 towards saturation limit in accord with Eqn. 41. The series of reagents P(C6H4R)3 were studied, but the rate constant pattern is not uniform because the active catalyst, 25-PR3, as well as the substrate is changing (56). 

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Figure 28. Possible mechanistic model for the activation of molecular oxygen over Au/Ti-Si02 catalysts [92].

The activation of molecular oxygen by copper plays a central role in synthetically useful stoichiometric and catalytic oxidative conversions of organic molecules and in biological systems.4"26... 

A dinuclear Fe(II) centre is bridged by two carboxylate and one hydroxide group. It is thought that the activation of molecular oxygen is a consequence of the ability of the active site to shuttle the FeuFen/ FeIIFeIII/FeIIIFeIn redox changes. 

One pecuhar oxygen buffer reaction is the well-known thermobarometric equation of Buddington and Lindsley (1964), which furnishes information on both T and the activity of molecular oxygen. The reaction uses the equilibrium among titanomagnetite (or ulvospinel ), magnetite, hematite, and ilmenite components in the hemo-ilmenite and spinel phases ... 

The biochemical importance of flavin coenzymes ap-pears to be their versatility in mediating a variety of redox processes, including electron transfer and the activation of molecular oxygen for oxygenation reactions. An especially important manifestation of their redox versatility is their ability to serve as the switch point from the two-electron processes, which predominate in cytosolic carbon metabo-lism, to the one-electron transfer processes, which predomi-nate in membrane-associated terminal electron-transfer pathways. In mammalian cells, for example, the end products of the aerobic metabolism of glucose are C02 and NADH (see chapter 13). The terminal electron-transfer pathway is a membrane-bound system of cytochromes, nonheme iron proteins, and copper-heme proteins—all one-electron acceptors that transfer electrons ultimately to 02 to produce H20 and NAD+ with the concomitant production of ATP from ADP and P . The interaction of NADH with this pathway is mediated by NADH dehydrogenase, a flavoprotein that couples the two-electron oxidation of NADH with the one-electron reductive processes of the membrane. 

As shown by Table 3, most of the Group VIII metal-peroxo complexes are obtained from the direct interaction of dioxygen with the corresponding reduced forms. A considerable effort has been devoted to this subject in the last decade with the hope that selective oxidations of hydrocarbons could be achieved by the activation of molecular oxygen under mild conditions12,56 133,184 and several such examples have actually been shown to occur. 

D. J. Kosman and J. J. Driscoll, Solvent exchangeable protons and the activation of molecular oxygen the galactose oxidase reaction, Prog. Clin. Biol. Res., 274 (1988) 251-267. 

Although dioxygen is the terminal acceptor of both electrons and protons (and H+) in the molybdenum and tungsten oxidases, 02 is not directly involved in substrate oxidation. The activation of molecular oxygen is effected by (non-Mo- and non-W-containing) prosthetic groups such as flavin-adenine dinucleo-... 

The activation of molecular oxygen. (G. Henrici-Olive and S. Olive, Angew. Chem. Internat. Edn., 1974, 13, 29). 

Any mechanism for monooxygenases must include the activation of molecular oxygen to form some type of electrophilic oxygen species. One approach to... 

Reoxidation of Fe(II) Porphyrins. Model systems for the activation of molecular oxygen via coordination to an Fe(II) porphyrin have not been reported because of the rapid irreversible autooxidation of the Fe(II) to the Fe(III) oxo-bridged dimer (C) (Reaction 16). Since the... 

An analogous mechanism is proposed for the activation of molecular oxygen and the cooperative binding of two oxygen molecules on the anionic silver clusters in which the first adsorbed O2 serves as an activator [351]. Since anionic silver clusters have generally lower VDE values than gold clusters, weaker electron acceptors such as O2 can already induce electron transfer and activate them, which is not possible in the case of Au . The first oxygen... 

Scheme 1 Products of the activation of molecular oxygen on metals. ...

Taken together, the various reactions and interconversions of these manganese porphyrin complexes have allowed the examination of each step in the activation of molecular oxygen by the mechanism suggested for P-450. Detailed mechanistic studies of the 0-0 bond cleavage event in 29 by kinetics, substituent effects, and product analysis showed that the reaction proceeds via heterolysis to produce 27 when acid is present, whereas homolysis is predominant in the absence of acid but in the presence of hydroxide ion (95). Under basic conditions, homo-lytic cleavage of the 0-0 bond of 29 forms Mn (=0)TMP (28) and an acyl-oxyl radical. Thus, when an alkyl peroxy acid is employed, decarboxylation competes with electron transfer, as shown in Scheme IX, to afford a mixture of 27 and 28. Yuan and Bruice have proposed a similar heterolysis mechanism based on the kinetic analysis for the reaction of mCPBA with catalytic amounts ofMn TPP(/(W). 

As already mentioned, the activation of molecular oxygen can often be circumvented if peroxides are used as activated oxygen donors. Efforts to identify the reactive oxygen species in these peroxide-supported reactions have been pursued for many years . The species that has been spectroscopically detected in these reactions has the spectroscopic signature of a ferryl intermediate , but evidence is lacking that this intermediate is the same as that produced by the activation of molecular oxygen. To the contrary, the reactions with peroxides have been shown to produce EPR signals tentatively attributed to tyrosine radicals but no such radicals have been observed under normal turnover conditions. Furthermore, as noted earlier, the peroxide-mediated reactions do not always faithfully reproduce the normal reactions. 

THE UNIQUE ROLE OF IRON CATIONS IN THE ACTIVATION OF MOLECULAR OXYGEN IN THE GAS PHASE... 

The activation of molecular oxygen by transition metal complexes is of fundamental interest for chemistry, biology, and medicine. As it is well known from biochemistry, iron is crucial in the transportation, storage, and activation of oxygen, and with respect to these processes only copper is of comparable relevance. 

In comparison, is the only cation of the row transition metal cations, which mediates both, the activation of molecular oxygen as well as that of the olefin. Thus the experimental findings indicate that the extraordinary role of iron with respect to the activation of molecular oxygen is - at least partially - due to an intrinsic property of this element. 

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