Reactions with Molecular Oxygen

The presence of oxygen can modify the course of a fiee-radical chain reaction if a radical intermediate is diverted by reaction with molecular oxygen. The oxygen molecule, with its two unpaired electrons, is extremely reactive toward most free-radical intermediates. The product which is formed is a reactive peroxyl radical, which can propagate a chain reaction leading to oxygen-containing products. 

Ketones and carboxylic esters can be a hydroxylated by treatment of their enolate forms (prepared by adding the ketone or ester to LDA) with a molybdenum peroxide reagent (MoOs-pyridine-HMPA) in THF-hexane at -70°C. The enolate forms of amides and estersand the enamine derivatives of ketones can similarly be converted to their a hydroxy derivatives by reaction with molecular oxygen. The M0O5 method can also be applied to certain nitriles. Ketones have also been Qc hydroxylated by treating the corresponding silyl enol ethers with /n-chloroperoxy-... 

As an example that uses structural formulas and Equation, consider the energy change that takes place during the combustion reaction of propane (C3 Hg). Recall from Chapter 3 that combustion is a reaction with molecular oxygen. The products of propane combustion are carbon dioxide and water ... 

Addition of a radical to the C6-position of thymine residues in DNA generates the C5-thymine radical 74 (Scheme 8.27). Reaction with molecular oxygen, followed by reduction, yields the hydroperoxide 75. Decomposition of the hydroperoxide ultimately yields the hydantoin nucleobase 78 via the ring-opened derivatives 76 and 77 ti,2 for the decomposition of 75 in aqueous solution is slow, with a 9 h for the tra i-isomer and 20 h for the c -isomer of the nucleoside)... 

In the presence of oxygen, the situation is more complicated, as more active species can be formed, which can contribute to NO decomposition or formation in the plasma. It was found [46] that the excited states of N2 do not contribute significantly to NO evolution, but mainly influence the dissociation or quenching of 02. As the concentration of 02 increases, their contribution to 02 dissociation also increases. Only nitrogen atoms in the ground state (N(2D)) or excited state (N(4S)) may contribute directly to NO formation by reactions with molecular oxygen ... 

The family of poly(pyrazol-l-yl)borates has been widely used as supporting ligands in nickel coordination chemistry.556,557 Complex (191) is an example, where unusual cysteine coordination is achieved at a tris(pyrazolylborate)nickel(II) template.601 (191) undergoes rapid reaction with molecular oxygen to presumably form a sulfinate. 

However, ubihydroquinone, a two-electron reduced form of ubiquinone, can produce superoxide on reaction with molecular oxygen ... 

The methylated carbenes lb-d are easily generated in argon matrices by photolysis of the corresponding diazo compounds.24 The IR and UV/vis spectra are very similar to that of la. In addition, the carbenes were chemically characterized by their reaction with molecular oxygen to give the corresponding quinone O-oxides 7b-d. 

The carboxylic acid derivatives li-lm can only be matrix-isolated if the corresponding quinone diazides 2i-2m are irradiated with monochromatic blue light (k = 436 nm).81 91 92 UV or broad-band visible irradiation rapidly results in the decarboxylation of the carbenes. As expected, the IR and UV/vis spectra of the carbenes are very similar to that of la. Oxygen trapping results in the formation of the photolabile carbonyl oxides 7. Thus, the carbenes li-lm were identified both spectroscopically and by their characteristic reaction with molecular oxygen. 

Redox Reactions with Molecular Oxygen and Water... 

FIGURE 8.22 Reaction of a free radical with an unsaturated lipid and the subsequent rearrangement of double bonds and reaction with molecular oxygen. 

Oxidation. Oxidative studies are designed to reveal the primary oxidative degradation products due to oxidation. Oxidative degradants in drug substances and drug products normally are the result of reaction with molecular oxygen. As in the case of acid/base hydrolysis, appropriate controls are necessary. 

This section will cover aspects of monohydride terminal surface reactions that were carried out under free-radical conditions. The description will be circumscribed to the reactions with molecular oxygen and monounsaturated compounds. Mechanistic information for these reactions is scarce mainly due to the complexity of the system, and mechanistic schemes are often proposed in analogy with radical chemistry of organosilane molecules. H—Si(lll) has a band gap of about 1.1 eV while the HOMO LUMO gap in (Me3Si)3SiH is within 8-11 eV and, therefore, has very important consequences for the reactions with nucleophilic and electrophilic species where frontier orbital inter-... 

Conditions that permit oxidation of organoboranes to alcohols using molecular oxygen,139 sodium peroxycarbonate,140 or amine oxides141 as oxidants have also been developed. The reaction with molecular oxygen is particularly effective in perfluoroalkane solvents.142... 

For carbon nucleophiles sequential addition of 2-potassio-2-nitropropane and oxygen to 4-arylidene-2-phenyl-5(47/)-oxazolones 623 has been reported (Scheme 7.200). The process involves a Michael reaction of the 2-nitropropane anion followed by reaction with molecular oxygen and elimination of nitrous acid to yield 2-aryl butenoic acid imides 626. 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

Background and Possible Intermediates. Accepting the premise of carbanion formation in the basic media, the mode of reaction with molecular oxygen can now be considered. Sprinzak (8) reported that the autoxidation of fluorene in basic media proceeds by direct reaction of the fluorenyl carbanion with oxygen to form initially the hydroperoxide, which decomposes to yield 9-fluorenone, as depicted below. 

Another aspect of the gas-phase homogeneous oxidation reaction at high temperatures is that the major reaction of the alkyl radicals (ethyl and propyl) is their reaction with molecular oxygen to form alkene and 02H radical ... 

F.F. Fenter, RD. Lightfoot, J.T. Niiranen, and D. Gutman. Kinetics of the CCI3 Association Reaction with Molecular-Oxygen at 298 K and 333 K and from 1 to 760 Torr of Total Pressure. J. Phys. Chem., 97 5313-5320,1993. 

Lithiation of dibenzofuran with butyllithium and mercuration both occur at the 4-position. Thallation occurs at the 2-position, however (57IZV1391). The mercury and thallium derivatives serve as a source of the iodo compounds by reaction with iodine. Bromodibenzofurans undergo bromine/lithium exchange with butyllithium and the derived lithio compounds may be converted into phenols by reaction with molecular oxygen in the presence of a Grignard reagent, into amines by reaction with O-methylhydroxylamine, into sulfinic acids by reaction with sulfur dioxide, into carboxylic acids by reaction with carbon dioxide and into methyl derivatives by reaction with methyl sulfate (Scheme 100). This last reaction... 

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