Dioxygen manganese catalysts

Formation of free radicals in the presence of transition metal compounds may be the result of their interaction with hydrocarbon or dioxygen. Such catalysts as cobalt and manganese salts are usually introduced in the bivalent state. An inaease of chain initiation rate here may be connected with the activation of dioxygen by a metal complex via the following reaction [8] ... 

Fig. 8. The application of vesicles for photocatalytic water decomposition in sacrificial systems (a) — dihydrogen evolution in the vesicle cavity. Pt metal catalyst is anchored to the inner membrane // water interface (b) — dioxygen evolution in the bulk solution. Manganese oxide catalyst is anchored to the outer membrane // water interface of the vesicle...

The unique versatility of ruthenium as an oxidation catalyst continues to provide a stimulus for research on a variety of oxidative transformations. Its juxtaposition in the periodic table and close similarity to the biological redox elements, iron and manganese, coupled with the accessibility of various high-valent oxo species by reaction of lower-valent complexes with dioxygen make ruthenium an ideal candidate for suprabiotic catalysis. 

Although the decomposition of ozone to dioxygen is a thermodynamically favoured process,126 it is thermally stable up to 523 K and catalysts are needed to decompose it at ambient temperature in ventilation systems, in the presence of water vapour and at high space velocity. A limited number of catalysts have been evaluated and active components are mainly metals such as platinum, palladium and rhodium, and metal oxides including those of manganese, cobalt, copper, iron, nickel and silver. Supports that have been used include 7-alumina, silica, zirconia, titania and activated carbon.125,170... 

From the viewpoints of reaction mechanism and efficiency in organic synthesis, oxidation of phenols with dioxygen catalyzed by cobalt-, manganese- and related metal-amine complexes has been studied . In particular, much effort has been directed toward constructing new efficient catalysts by a combination of metals with... 

The results of catalytic epoxidation of various olefins, using f-BuOOH as the terminal oxidant and Mn(Me2EBC)Cl2 as the catalyst, are summarized in Table 3.4. The color of the reaction mixture turns to purple upon addition of f-BuOOH and the ultraviolet-visible spectrophotometry shows that the manganese is present predominantly in the tetravalent state, and no dioxygen evolution was observed. 

The oxidation of saturated hydrocarbons in the presence of iron- or manganese-containing catalysts can be achieved by using a variety of oxidants including alkyl hydroperoxides, peroxycarboxylic acids, iodosyl-benzene, dihydrogen peroxide, and dioxygen (9-11). It has been shown that chiral iron- and manganese-porphyrin complexes catalyze the asymmetric epoxidation of unfunctionalized alkenes (75). Except for a number of experiments in which up to 96 % enantiomeric excess (ee) has been reported (16,17), in most epoxidation reactions with chiral porphyrins only a low to moderate enantiomeric excess of the product is obtained (18,19). In association with these catalysts, alkyl hydroperoxides and iodosylbenzene are often used as primary oxidants (18,19). 

In nature, oxidation reactions are essential for aerobic life. Energy for cells is provided by the combustion of carbohydrates and fatty acids with dioxygen. Oxidation reactions are also involved in biosynthesis, metabolism reactions, and the detoxification of harmful compounds. In several of these reactions, iron or manganese enzymes are involved. These manganese and iron enzymes have frequently been used as a source of inspiration for the development of manganese- and iron-based oxidation catalysts. 

Electrocatalytic oxidations (mainly epox-idation) of alkenes by manganese porphyrins [77, 78] and a Schiff-base [79] and iron and cobalt porphyrins [78] have been achieved. Hydrogen peroxide or the superoxide ion (O ) was generated electrochem-ically by reduction of dioxygen in solvents containing an acid or acid anhydride, the metal compounds as catalysts, and olefins as substrates, in the presence or absence of an axial base. The reaction was believed to take place through the formation of a high valent metal 0x0 porphyrin, produced... 

Recently, Hulsken et al. have employed STM to probe the mechanism by which manganese porphyrin complexes achieve epoxidation of alkenes with dioxygen [69]. Importantly, this study demonstrates the positive role of solid-support immobilization in precluding the formation of inactive oxido-bridged catalyst dimers (a common deactivation pathway). 

Catalytic systems based on various metal oxides under MW-assisted, solvent-free, aerobic conditions were compared for benzyl alcohol oxidation (Table 18.6) [23]. The manganese oxides were prepared by solution-based or solid-state reaction procedures [24], while V2O5, CuO, Fe Og, C02O3, and NiO were obtained via sol-gel or precipitation methods [25]. The benzyl alcohol oxidation was performed in a glass reactor where the slurry of catalyst with alcohol was stirred under dioxygen pressure and MW irradiation (Scheme 18.10, Table 18.6). 

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