Oxygen transition-metal catalysis

Dialkyl peroxides, like diacyl peroxides and peroxyesters, are characterized by homolysis of the 0-0 bond, which is promoted thermally, photochemically or by transition metal catalysis. The combination of steric factors, and the poor leaving group ability makes simple dialkyl peroxides (ROOR) almost unreactive in heterolytic oxygen atom transfer.128 Consequently, no further mention of these species will be made. 

The crossover product, propionaldehyde-l,3-d-3- C 12, clearly demonstrated that the isomerization occurred via intermolecular 1,3-hydrogen shift. These results are consistent with a modified metal hydride addition-elimination mechanism which involves exclusive 1,3-hydrogen shift through oxygen-directed Markovnikov addition of the metal hydride to the carbon-carbon double bond (Scheme 12.2). The directing effect of functional groups on the selectivity of transition metal catalysis is well presented [9], and an analogous process appears to be operative in the isomerization of allylamines to enamines [10]. 

In spite of the many modern techniques available to the chemist, the known chemistry of polynuclear cobalt (III) complexes is essentially that deduced by Werner 60 years ago. Since his work, no new polynuclear cobalt complexes have been prepared and characterized and no new reactions uncovered. Modem work in this area is being aimed at attaining a better understanding of the electronic structures inherent in polynuclear ions, which would be of value in a variety of active fields. The chemistry of polynuclear complexes is important in such new areas as synthetic oxygen carriers, electron transfer reactions, and transition metal catalysis. The fact that these new investigations are solidly based on Werner s pioneer investigations testifies to the genius with which he opened up a new area of coordination chemistry, with only the simple chemical techniques available to him. His work in the area of polynuclear cobalt(III) ammine complexes should continue to serve as a model of solid research for some time to come. 

If the metal cation is too electrophilic, CO coordination will be too strong, possibly by coordination via its oxygen atom, and CO will act as a poison rather than participating in the polymerization [40], The moderate electrophilicity of Pd" catalysts makes them tolerant also to a variety of heteroatom functionalities in the olefin substrate. In this respect, polyketone catalysis can have a wider applicability than early transition metal catalysis of polyolefins, which is highly intolerant of functional groups. 

IA Weinstock, KE Hammel, MA Moen, LL Landucci, S Ralph, CE Sullivan, RS Reiner. Selective transition-metal catalysis of oxygen delignification using water-soluble salts of polyoxometalate (POM) anions. Part It. Reactions of a-[SiVWii04o] with phenolic lignin-model compounds. Holzforschung 52 311-318, 1998. 

The epoxidation of olefins was discovered by a Russian chemist, N. Prileschajew, in 1909 using peracids RCO3H, and this reaction has been used for a long time. Per-acids often are dangerous (explosive), however, and must therefore imperatively be avoided. Transition-metal catalysis is now used together with hydroperoxides ROOH (most often R = t-Bu, in short TBHP) whose first example was discovered by Hawkins in 1950 with [V2OS] as catalyst. The catalysts most often are d° complexes with an oxophilic Lewis acid able to bind an oxygen atom of hydroperoxide. 

The transition metal catalysis of this tandem (cascade) reaction involving [3,3]-sigmatropic rearrangement (since it would be the carbonyl oxygen of the acetate forming a new bond to the p-acetylenic atom) followed by the formal Myers-Saito cyclization of allene 3.524 results in the formation of aromatic ketones 3.525 in up to 94% yield (Scheme 3.29) [265]. 

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

Binary systems of ruthenium sulfide or selenide nanoparticles (RujcSy, RujcSey) are considered as the state-of-the-art ORR electrocatalysts in the class of non-Chevrel amorphous transition metal chalcogenides. Notably, in contrast to pyrite-type MS2 varieties (typically RUS2) utilized in industrial catalysis as effective cathodes for the molecular oxygen reduction in acid medium, these Ru-based cluster materials exhibit a fairly robust activity even in high methanol content environments of fuel cells. 

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