Use of Oxygen as Oxidant

The oxidative carboxylation of olefins appears to be a very interesting synthetic methodology for synthesizing CCs, starting from cheap and easily available reagents such as C02 and 02 (Equation 7.16). 

The direct oxidative carboxylation of olefins has great potential, and many advantages. Notably, it does not require the C02 to be free of dioxygen this is an especially attractive feature, as the cost to purify C02 is extremely high, and may discourage its use. Moreover, the direct oxidative carboxylation of olefins can couple two processes-the epoxidation of olefins, and the carbonation of epoxides. Hence, the process makes direct use of those olefins that are available commercially at low price, and which represent an abundant feedstock. Such an approach also avoids having to isolate the epoxide. 

Very few examples have been reported of the direct carbonation of olefins examples include the direct functionalization of propene [174, 175] and styrene [176, 177]. 

When using RhClP3 as a catalyst, under homogeneous conditions, Aresta et al. demonstrated [176, 178, 179] the formation of two classes of compounds due to two alternative modes of oxygen transfer to the olefin  

The reaction mechanism has been shown [180] to consist of (i) interaction of the Rh-catalyst with 02 to afford a dioxygen species (ii) conversion of the Rh-02 complex into a peroxocarbonate (Equation 7.17a) by reaction with C02 and (iii) a one-oxygen transfer to the olefin with formation of the Rh-carbonate which, in principle, should no longer be active as a catalyst (Equation 7.17b). 

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Particularly challenging is the use of oxygen as oxidant, since it has been proved to be extremely difficult to achieve selectivity with this reagent. In the present case, it was found that GO is a general catalyst to perform the aerobic oxidation of benzylic hydrocarbons [27]. Similarly, benzylic alcohols can also be oxidized to the corresponding carbonylic compound by molecular... 

Respiration, or biological oxidation, is the use of oxygen as an electron receptor in the cataboHc degradation of an organic and can occur either aerobically or anaerobically. Aerobic respiration uses free oxygen as an electron receptor whereas anaerobic respiration uses inorganic oxygen. In both cases, however, water and carbon dioxide are the principal end products. 

Secondary alcohol oxidases catalyze the oxidation of secondary alcohols to ketones using molecular oxygen as oxidant. A secondary alcohol oxidase from polyvinyl alcohol-degrading bacterium Pseudomonas vesicularis var. povalolyticus PH exhibited activity toward several... 

The low specificity of electron-donating substrates is remarkable for laccases. These enzymes have high redox potential, making them able to oxidize a broad range of aromatic compounds (e.g. phenols, polyphenols, methoxy-substituted phenols, aromatic amines, benzenethiols) through the use of oxygen as electron acceptor. Other enzymatic reactions they catalyze include decarboxylations and demethylations [66]. 

A structured ruthenium catalyst (metal monolith supported) was investigated by Rabe et al. [70] in the ATR of methane using pure oxygen as oxidant. The catalytic activity tests were carried out at low temperature (<800 ° C) and high steam-to-carbon ratios (between 1.3 and 4). It was found that the lower operating temperature reduced the overall methane conversion and thus the reforming efficiency. However, the catalyst was stable during time on-stream tests without apparent carbon formation. 

Platinum supported on carbon (Pt/C) was tested as solid catalysts in the oxidation of sucrose using molecular oxygen as oxidant (Scheme 10). The reaction was carried out in water and under atmospheric pressure. The support strongly influences the reaction and Pt/C was found more efficient than Pt/Alumina at 353 K. Over Pt/C, at a pH of 9, mono-, di-, and tricarboxylate derivatives were mainly obtained with a tricarboxylate yield of 35% [103]. 

Table 9.1 Oxidation of sulfides ArSR to sulfones catalyzed by LDH-OSO4 using molecular oxygen as oxidant.

Stereospecific Epoxidation of 2-hexene on Photoirradiated Ti02 Powders Using Molecular Oxygen as Oxidant... 

Copper-modified Ca-phosphate catalysts were also used to convert benzene into phenol in the presence of oxygen and ammonia at 450 °C, where N20 was formed under the reaction conditions [148]. The addition of ammonia to the 02-H20 feed promoted the phenol yield, but the yield was small (<1.3%). The use of N20 as oxidant was better than the 02-NH3 feed. 

Presented examples show that under the use of oxygen as the most cheap and ecologically pure oxidizer and simple catalytic system [Cu2+... substrate... base] we are succeeded in developing of principally novel processes of oxidative modification of vegetable raw materials with obtaining of valuable from practical point of view products. Invaluable contribution into developing of such processes was made by Professor N.M. Emanuel who stood at the beginning of works. 

A stereocontrolled synthesis of the /ra j-tetrahydrofuran units in Annonaceae acetogenins that relies on the Sharpless asymmetric dihydroxylation protocol is outlined in Scheme 60 <1999TA2551>. In the first step, the disubstituted double bond of the starting material is dihydroxylated followed by monoprotection as a methoxymethyl ether. Einally, a cobalt-catalyzed oxidation using molecular oxygen as oxidant furnishes the /ra j-tetrahydrofuran. 

For small-scale applications, the cost may be more important than the efficiency of a given plant. In most cases, the partial oxidation reactors give a highly expensive plant layout because of the inherent, high cost of air separation (unless a low-cost source of oxygen is available). The use of air as oxidant makes the final separation very difficult, especially if carbon monoxide is needed either in a pure form or with hydrogen. 

Oxidation of unsaturated compounds. Methyl ketones are produced from 1-aIkenes using molecular oxygen as oxidant [catalyst Pd(OAc)2-pyridine]. On the other hand, methyl 3,3-dimethoxypropanoate is formed when eth l acrylate is oxidized on activated carbon-supported molybdovanadophosphate and Pd(OAc>2 in acidic ethanol. Treatment of enones with LiAlH4 under dry oxygen gives 1,3-diols. ... 

A third topic, not specific to the use of dioxygen as oxidizing agent, concerns selectivity and, in particular, the closely related aspect of over-oxidation. This problem is most severe in the partial oxidation of saturated hydrocarbons, because activation of alkanes is usually more difficult than that of any of the oxygenated products (e.g., alcohols and aldehydes), which are easily oxidized further. The reason is simply that C-H bonds of functionalized alkanes are generally weaker than those of the parent hydrocarbons. Moreover, as far as metal catalysis is concerned, the polar oxidation products can... 

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