Transition metal oxidizing reagents

The oxidative cleavage of C=C bond is a common type of reaction encountered in organic synthesis and has played a historical role in the structural elucidation of organic compounds. There are two main conventional methods to oxidatively cleave a C=C bond (1) via ozonol-ysis and (2) via oxidation with high-valent transition-metal oxidizing reagents. A more recent method developed is via the osmium oxide catalyzed periodate oxidative cleavage of alkenes. All these methods can occur under aqueous conditions. 

The most widely employed transition metal oxidants for alcohols are based on Cr(VI). The specific reagents are generally prepared from chromic trioxide, Cr03, or a dichromate salt, [Cr207]2-. The form of Cr(VI) in aqueous solution depends upon concentration and pH the pKx and pK2 of H2Cr04 are 0.74 and 6.49, respectively. In dilute solution, the monomeric acid chromate ion [HCr03] is the main species present as concentration increases, the dichromate ion dominates. 

Water-gas shift reaction. The water-gas shift (WGS) reaction (reaction (2)) made by particles composed of a promoter element close to a supported cobalt particle leads to a change in the local CO/H2 ratio, which may affect the surface coverage of cobalt. As a result, both the activity and the selectivity of the catalyst can be altered. Some transition metal oxides are known to act as WGS reagents. 

Transition-metal oxides are useful oxidizing reagents for organic molecules and often participate in oxygen-atom transfer reactions [21]. A prototypical example is CH3Re03 (MTO), which serves as a versatile reagent for stoi-... 

In nonreactive molten salts, on the other hand, flux components are not incorporated into the product phase. Here, the molten salt acts more in the classical sense as a reagent to promote the reaction at a lower temperature than would be required by the ceramic, or direct, route (Section 5.2). This is accomplished by two attributes of molten salts an acid-base equilibrium that enables the general dissolution-recrystallization of metal oxides and a highly electropositive (oxidizing) environment that stabilizes the highest oxidation state of many transition metals (Gopalakrishnan, 1995), which can lead to mixed valency. A plethora of complex transition metal oxides have been synthesized in nonreactive molten alkali metal hydroxides, carbonates, and hypochlorites. Examples of such molten salt routes to mixed transition metal oxides include (Rao and Raveau, 1998) ... 

Transition metal oxidants such as permanganate, ruthenium tetroxide and diromium(VI) oxide are convenient and efficient reagents for routine cleavage reactions. The use of phase transfer catalysts (quaternary ammonium and phosphonium ions, primarily) has made it possible to solubilize transition metal oxides such as permanganate and chromatt in nonaqueous solvents, and to therdry increase the scope of these reactions substantially. ... 

Transition metal oxidants such as manganese and chromium oxidants have been widely used in the chemical industry over the years. They have a major disadvantage in that they produce large volumes of effluent containing the transition metals which are subject to more and more strenuous controls on discharge levels. Supported reagents or effluent recycle could be considered but neither is easy on an industrial scale and they are cures rather than prevention of the problem. 

Dehydrative amination is amination of the alcohol in the presence of dehydration catalysts. This method is similar to the previous process if we consider only the reagents (amine and alcohol) and the final products (alkylamine and water) but the chemical pathway involving acidic activation of the hydroxyl group of the alcohol is different. It is, nevertheless, sometimes difficult to definitively exclude one of the two pathways, especially when a transition metal oxide is used as a catalyst. Whatever the mechanism, the two methods lead only to water as a byproduct and meet the efficiency criteria. 

The catalytic oxidation of alcohols selectively to carbonyl compounds is one of the more important transformations in the synthetic organic chemistry. A large number of oxidants have been reported in the literature and most of them are based on transition metal oxides such as chromium and manganese [1-3]. A serious drawback to these reagents is the need to use them in large amounts, very... 

Sections 3.7.A and 3.7.B showed that alkenes are oxidatively cleaved hy transition metal oxidants. With certain reagents, 1,2-diols can also he oxidatively cleaved, usually to aldehydes or ketones. ... 

Diazoalkanes are fairly stable toward oxidation reagents. This is evident from one of the major reactions for their synthesis, the hydrazone dehydrogenation (see Subsect. 2.5.1). The diazoalkanes formed are stable against attack by the transition-metal oxides used as dehydrogenation reagents. 

This structural variation notwithstanding, only a few cationic transition-metal ions react efficiently with molecular oxygen under gas-phase conditions (see below). In contrast, many anionic metal complexes and clusters are readily oxidized by O2 to afford various metal-oxide anions [19]. From a conceptual point of view, however, anionic species appear to be inadequate reagents for the activation of hydrocarbons, because they generally require electrophilic attack. At present, only a few oxidations by transition-metal oxide anions have been reported to occur in the gas phase, and they are mostly limited to relatively polar substrates, such as the CH3OH CH2O conversion [20]. Because of the lower reactivity of hydrocarbons, their C-H bond activation by metal-oxide anions is likely to be limited to radical pathways driven purely thermodynamically, i.e., when Z)(0-H) exceeds Z)(C-H) of the substrate [21]. As radical-type pathways are prone to create selectivity problems, and over-oxidation is particularly difficult to control, the anionic route appears less attractive as far as partial oxidation of alkanes is concerned. 

Carbon-carbon double bonds, besides being susceptible to addition of oxygen or cleavage, can also react at allylic positions. Synthetic utility requires that there be good regioselectivity. Among the transition-metal oxidants, the CrOs-pyridine reagent in methylene chloride and a related complex in which 3,5-dimethylpyrazole is used in, place of pyridine are the most satisfactory for allylic oxidation. 

The metalloproteins consist of a metal complex imbedded in and bonded to a protein net of covalently bonded amino acids. The most commonly studied systems are the myoglobins and cytochromes, which contain Fe(II) or Fe(III) in a porphyrin complex, or the copper blue proteins, which have Cu(II) or Cu(I) compiexed most often by histidine nitrogens and cysteine and methionine sulfurs from the protein. Metalloproteins can be oxidized or reduced by standard transition-metal complex reagents, and the latter usually are chosen to ensure outer-sphere electron transfer. This area has been the subject of numerous reviews. 

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