Condensed oxidation

Anionic/oxidative reaction sequences have been developed in addition to the domino anionic/reductive processes. For example, with regard to the synthesis of novel diaryl heterocycles as COX-2 inhibitors [500], including rofecoxib (Vioxx) 2-972 [501] (which has recently been withdrawn from the market) or the pyrrolin-2-one derivative 2-973 [494], Pal and coworkers reported on a so-far unique domino aldol condensation/oxidation sequence (Scheme 2.218) [503]. 

Two schemes for particle combustion have been proposed which differ mainly in the consideration of the condensed oxide formed by the combustion reaction. 

In the first scheme the metal boiling point is less than the oxide boiling point and the model consists of a vaporising droplet of metal surrounded by a detached reaction zone where condensed oxides appear as fine droplets. The reaction rate is said to be controlled by the vapour phase diffusion of metal and atmospheric oxygen into the reaction zone as in Figure 5.6. 

Theoretical Studies of Liquid-Propellant Rocket Instability , Ibid, pp 1101-28 Pj) G.A. McD. Cummings A.R. Hall, "Perchloric Acid Flames Premixed Flames With Methane and Other Fuels , Ibid, 1365-72 P2) D.J. Carlson, Emittance of Condensed Oxides in Solid Propellants Combustion Products , Ibid, 1413-24 Qj) Ibid, "Perchloric Acid Flames Some Flame Temperatures and Burning Velocities , Ministry of Aviation,... 

Methoxypyridine-3-carboxaldehyde (148), prepared from 287 (88 JOCI367), serves as a key synthon for the construction of 8-azacoumarin (147) and the 1,9-diazaxanthone 392, via condensation, oxidation, and substitution product 391 (Scheme 120) [90JCS(P1)2409]. 

If the diffusion of the condensed oxide vapor into the particle is slow compared with the other two steps and if condensation is reversible, the outer surface layer of the particle reaches equilibrium, or pseudoequilibrium, with the oxide vapor, and further uptake is governed by... 

Diffusion of the condensed oxide vapor into particle... 

Not only must precursor fibers be self-supporting as extruded, they must also remain intact (e.g. not melt or creep) during pyrolytic transformation to ceramic fibers. Thus, precursor fibers (especially melt spun fibers) must retain some chemical reactivity so that the fibers can be rendered infusible before or during pyrolysis. Infusibility is commonly obtained through reactions that provide extensive crosslinking. These include free radical, condensation, oxidatively or thermally induced molecular rearrangements. 

Esters , lactones.4 In the presence of this catalyst, primary alcohols condense oxidatively to form esters. 

In many modern rocket propellant systems, condensable oxides are always present. Further the equilibrium constant of formation of carbon containing substances must contain graphite, since this is the form of carbon in its standard state. Thus, the condition presented by condensed phases must be considered. The equilibrium constant of formation of C02 follows from the simple reaction ... 

Of particular interest in the present chapter is the effect of test atmosphere on creep and creep damage mechanisms. While there are undoubtedly several factors that can promote creep cavitation and contribute to the observed changes in stress exponent and activation energy, the fact remains that the strain rates are substantially higher in air than in inert atmospheres, as shown in Fig. 8.12. This phenomenon is a direct consequence of the topotactic oxidation reaction of SiC whiskers exposed at the surface. As described by Porter and Chokshi,38 and subsequently by others,21,22 at high temperatures in air, a carbon-condensed oxidation displacement reaction occurs in which graphitic carbon and silica are formed at the whisker interface via... 

The advantages of PTC reactions are moderate reaction conditions, practically no formation of by-products, a simple work-up procedure (the organic product is exclusively found in the organic phase), and the use of inexpensive solvents without a need for anhydrous reaction conditions. PTC reactions have been widely adopted, including in industrial processes, for substitution, displacement, condensation, oxidation and reduction, as well as polymerization reactions. The application of chiral ammonium salts such as A-(9-anthracenylmethyl)cinchonium and -cinchonidinium salts as PT catalysts even allows enantioselective alkylation reactions with ee values up to 80-90% see reference [883] for a review. Crown ethers, cryptands, and polyethylene glycol (PEG) dialkyl ethers have also been used as PT catalysts, particularly for solid-liquid PTC reactions cf. Eqs. (5-127) to (5-130) in Section 5.5.4. 

The nucleophilic substitution, amination, aldol-type condensation, oxidation, and hydrolysis of the l//-pyrazino[2,3-c][l,2,6]thiadiazine 2,2-dioxide system, structurally related to pteridine, were studied in detail <03HCA139>. Chlorinated pyrazines were directly oxidized to their corresponding iV-oxides using dimethyldioxirane in a completely regioselective fashion <03HEC221 >. 1,6-Dibenzoyl-5//, 10//-diimidazo[ 1,5-a 1, 5 -[Pg.374]

One cannot help but notice that the above reactions lead to related products characterized by the presence of two oxygens in a 1,3-relationship as either a jS-hydroxycarbonyl (if both components are aldehyde or ketones) or a )S-dicarbonyl system (in the case of esters). Both of these functionalities are useful in subsequent conversions and we see that the synthetic utility of the reactions used to prepare these adducts is broadened further. Typical transformations are shown in Scheme 2.26 for the product 70 of an acetone aldol condensation. Oxidation of 70 leads to the formation of the corresponding dicarbonyl compounds 71, while the 1,3-diol 72 is formed as a result of reduction of 70 and the a,j8-unsaturated carbonyl compound 73, formed via dehydration of 70. 

Apart from esterifications, dehydrations, condensations, oxidations, Diels-Alder alkylation, and hydrogenations also have been coupled to PV. In the production of alkyl coating resins at temperatures as high as 150-300° C, the use of microporous silica PV-membranes has been successfully investigated. A mixture of acids, acid anhydrides, and alcohols reacts to form a resin and water. In a kilogram-scale unit, energy savings turned out to be 40% and reactor efficiency increased by 30%. 

Previous sections have shown that catalysis by solid acids has received much attention due to its importance in petroleum refining and petrochemical processes. Conversely, relatively few studies have focused on catalysis by bases, even if acid and base are paired concepts. Base catalysts, however, play a decisive role in several reactions essential for fine-chemical syntheses [248-251]. Solid-base catalysts have many advantages over liquid bases. Examples of successfijl reactions include isomerization, aldol condensation, Knoevenagel condensation, Michael condensation, oxidation and Si—C bond formation. Various reviews have discussed catalysis by solid bases [248-255]. 

In order to optimize the processes and materials used in these applications, the vaporization of condensed oxide phases, their thermochemistry as well as that of the gaseous oxide species has to be known. These demands initiated investigations of oxide vapors. 

Gaseous hydrogen, containing a suspended second phase was found (as expected) to generate static (promote charge separation). This added to understanding of the need to avoid static inside purification systems where condensed oxidant phases could be in contact with H2. 

Examples of phase transfer catalysis include hydrolysis, condensation, oxidation, and polymerization ". There are many industrial applications. 

Volatilization and adsorption of lube oil by the catalysts is followed by catalytic oxidation to CO2 at higher temperatures as is shown in Figure 8. In the Pyran -GCMS experiments, CO2 is the only non-condensable oxidation product measured from the combustion of lube oil in the "lean" gas stream by either Catalyst "C" or the gamma-alumina. The onset of CO2 evolution with Catalyst "C" occurs at approximately 180°C, which is just above the temperature range for the lube oil volatilization curve which is also shown. Lube conversion to CO2 is virtually complete with Catalyst "C" as determined by CO2 mass balance (95-100%) and the absence of detectable condensable species. 

Table 6-3 summarizes a scheme for formation of oxidation dyes and provides some examples of the types of dyes that have been isolated from these reactions. This scheme shows that a dye precursor (e.g., p-phenylenediamine) is oxidized to its corresponding diiminium ion (IX), and this active intermediate condenses with an electron-rich dye coupler, forming a dinuclear product that is oxidized to an indo dye. This reaction may stop at the dinuclear dye stage, or additional condensation-oxidation... 

Synthesis of five-membered heterocycles via a TOP sequence from activated alcohols affords 4,5-benzo-fused-2-substituted imidazoles, oxazoles and thiazoles in synthetically useful yields over the presumed oxidation-condensation-oxidation sequence (eq lOS). ... 

Oxidizing Agent Grinding Agent Condensation Oxidation Yieidb (%)... 

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