Metal-catalyzed oxidations

Storage in amber glass afforded some protection, whereas storage in foil-wrapped glass showed the slowest rate of oxidation. Metal-catalyzed photooxidation of hGH was found to specifically oxidize His21, one of three residues involved in a cation-binding site. " Other potential sites of oxidation, including three Met and two additional His, were found to be unaffected by photooxidation of hGH. 

Oxidations Metal-catalyzed aerobic oxidation of organic compounds has been reviewed. Aerial oxidation of primary and secondary alcohols is mediated by TEMPO in the presence of HCl and NaN02- Secondary benzylic alcohols undergo aerial oxidation (or with r-BuOOH) based on catalysis by AuCl - neocuproine," but another report describes the oxidation of both primary and secondary alcohols (to acids and ketones, respectively) using nanoclusters of gold that are stabilized by poly(A-vinyl-2-pyrtolidone). ... 

PROBABLE FATE photolysis photooxidation definitely occurs, photooxidation half-life in water 3.2-160 days photooxidation half-life in air 1.19-11.9 hrs oxidation metal-catalyzed oxidation occurs in aerated surface waters, oxidation by peroxy radicals is important, photochemically produced hydroxyl radicals degrades compound in daylight hours, half-life 8 hrs hydrolysis not an important process volatilization not an important process sorption slight potential for adsorption onto organic materials, adsorption to sediment will be moderate biological processes biodegradation can occur other reactions/interactions chlorine present in water could chlorinate the eompound can be washed out by rain... 

PROBABLE FATE photolysis, photooxidation of volatized phenol and photoylis of phenolic anion may both take place at moderate rates, atmospheric and aqueous photolytic half-life 1.9-7.2 days oxidation metal-catalyzed oxidation may take place in highly aerated waters, photooxidation half-life in water 3.2-160 days, photooxidation half-life in air 2.28-22.8 hrs hydrolysis not important volatilization very little, if any, volatilization of phenol occurs sorption not important biological processes no bioaccumulation, but very extensive biodegradation in natural waters other reactions/interactions chlorination of water may produce chloro-phenols... 

An exception is, of course, metal-catalyzed oxidation with molecular oxygen. 

Transition-metal-catalyzed oxidations may or may not proceed via peroxocomplexes. Twelve important industrial organic oxidation processes catalyzed by transition metals, many of which probably involve peroxo intermediates, have been tabulated (88). Even when peroxo intermediates can be isolated from such systems, it does not necessarily foUow that these are tme intermediates in the main reaction. 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

Citric acid also inhibits color and flavor deterioration in frozen fmit. Here again the function is to inhibit enzymatic and trace metal-catalyzed oxidation. 

Isomerization of ethylene oxide to acetaldehyde occurs at elevated temperatures ia the presence of catalysts such as activated alumina, phosphoric acid, and metallic phosphates (75). Iron oxides also catalyze this reaction. Acetaldehyde may be found as a trace impurity ia ethylene oxide. 

Joining two heteroatoms to a ring by radical combination is not presently a common route to heterocycles. It might become more important if the art of metal-catalyzed redox reactions keeps advancing at the present pace. Current examples are the conversion of 1,5-dithiols to 1,2-dithiepanes by oxidants such as FeCla, and the oxidation of 1,3-propane-bis-hydrazines to 1,2,3,4-tetrazepines (Sections 5.18.4.1 and 5.18.10.1). 

There are some aspects in the raw dry NR grades for adhesive manufacturing to be considered. NR tends to suffer oxidative degradation catalyzed by metals (mainly copper). The susceptibility of NR to oxidation can be measured using the plasticity retention index. The better grades of rubber have the higher plasticity retention index. 

Polyhaloporphyrins as unusual ligands for metals and metal-catalyzed oxidations 97ACR251. 

For the reactions of other 1,3-dipoles, the catalyst-induced control of the enantio-selectivity is achieved by other principles. Both for the metal-catalyzed reactions of azomethine ylides, carbonyl ylides and nitrile oxides the catalyst is crucial for the in situ formation of the 1,3-dipole from a precursor. After formation the 1,3-di-pole is coordinated to the catalyst because of a favored chelation and/or stabiliza-... 

The first, and so far only, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reaction of nitrile oxides with alkenes was reported by Ukaji et al. [76, 77]. Upon treatment of allyl alcohol 45 with diethylzinc and (l ,J )-diisopropyltartrate, followed by the addition of diethylzinc and substituted hydroximoyl chlorides 46, the isoxazolidines 47 are formed with impressive enantioselectivities of up to 96% ee (Scheme 6.33) [76]. 

Zinc-tartrate complexes were applied for reactions of both nitrones and nitrile oxides with allyl alcohol and for both reaction types selectivities of more than 90% ee were obtained. Whereas the reactions of nitrones required a stoichiometric amount of the catalyst the nitrile oxide reactions could be performed in the presence of 20 mol% of the catalyst. This is the only example on a metal-catalyzed asymmetric 1,3-dipolar cycloaddition of nitrile oxides. It should however be no-... 

Modern variants are the enzyme-catalyzed and the transition-metal-catalyzed Baeyer-Villiger reaction, allowing for an oxidation under mild conditions in good yields, with one stereoisomer being formed predominantly in the enzymatic reaction ... 

Compared with the related reactions of tutrones, there have only appeared a few pnbhcacions of metal-assisted or metal-catalyzed 1,3-dipolar cycle additions of nltnle oxides This Is due to... 

Metal deactivators (MD) act, primarily, by retarding metal-catalyzed oxidation of polymers they are, therefore, important under conditions where polymers are in contact with metals, e.g., wires and power cables. Metal deactivators are normally polyfunctional metal chelating compounds (e.g.. Table la, AO 19-22) that can chelate with metals and decrease their catalytic activity [21]. 

Besides the oxidative and transition-metal-catalyzed condensation reactions discussed above, several other syntheses were developed to generate PPP and PPP derivatives. 

Oxidants Available for Selective Transition Metal-catalyzed Epoxidation... 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

Table 6.1 Oxidants used in transition metal-catalyzed epoxida-tions, and their active oxygen content.

Table 6.12 Transition metal-catalyzed epoxidation of olefins with H202 as terminal oxidant.

Nital. A soln of l-5ml of nitric acid (d 1.42 g/cc) in 100ml of 95% ale, used for etching metals. A nital soln contg about 15% by vol of Grasselli reagent, nitric acid and ethanol, used for etching Bi, decomposed vigorously. Explns were also reported when mtal mixts came in contact with other metals. It is believed that the decompn was caused by the reduction of the nitric acid to oxides, which catalyzed the decompn (Ref 1). 

In low density reactant compacts, the reaction is believed to involve gas phase oxygen diffusion whereas under conditions of improved contact, in high-density material, the mobile species is identified as Fe2+. The metal catalyzes decomposition of the oxidant (KMn04), an effect that is inhibited by small quantities of certain additives (e.g. NaF). There is a large and specialist literature devoted to self-heating reactions. 

R. A. Sheldon and J. K. Kochi in Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981, p. 392. 

In contrast to the asymmetric procedures discussed above, the metal-catalyzed oxidation of alkyl aryl sulphides by t-butylhydroperoxide carried out in a chiral alcohol gives rise to chiral sulphoxides of low optical purity290 (e.e. 0.6 9.8%). Similarly, a very low asymmetric induction was noted when prochiral sulphides were oxidized by sodium metaperiodate in chiral alcohols as solvents291. 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

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