Epoxides conversion

A symmetry boundary condition was imposed perpendicular to the base of the mold. Since the part is symmetric, only half of the part cross-section needed to be simulated. The initial conditions were such that resin was at room temperature and zero epoxide conversion. The physical properties were computed as the weight average of the resin and the glass fibers. 

Part cures were characterized by exothermic reaction wave propagation. Figures 6a-9b show the development of the reaction waves. The waves propagate from the walls of the part towards the center. A comparison of the temperature and epoxide conversion profiles revealed that the highest temperature corresponded to the highest conversion. As the part initially heats the resin/glass matrix nearest the walls heats fastest however, as the part exotherms the temperatures in the interior of the part exceeded the wall temperatures. The center temperature does not become the hottest temperature until the waves intersect. It must be noted that the hottest temperature does not always occur at the center of the part. The wave velocities are proportional to the wall temperatures. In Figures 6a to 9b the mold temperature was 90 C and the press temperature was elevated to 115 C. Since the press does not heat the part until after it is wound, the press temperature was elevated to accelerate the reaction wave from the press so that the waves would intersect in the center of the part. 

Alkene conversion to epoxide (%) Epoxide selectivity (%) Epoxide conversion (%) Cyclic carbonate selectivity (%)... 

Many different pathways, mechanisms, and enzymes are associated with activation. These include dehalogenation, AT-nitrosation of secondary amines, epoxidation, conversion of phosphothionates to phosphate, metabolism of phen-oxyalkanoic acids, oxidation of thioethers, hydrolysis of esters and peroxides. The following is a summary. 

Kenson and Lapkin [173] measured the rate of isomerization of the epoxide to acetaldehyde on a supported silver catalyst and found the rate of epoxide conversion REO 3.9 X 10 4 PE0 at 200°C with an activation energy of 9.8 kcal mol-1. Acetaldehyde may be an intermediate in the combustion of ethylene oxide, but, as such, it is unimportant because it is rapidly oxidized. 

Kasang G. and Schneider D. (1974) Biosynthesis of the sex pheromone disparlure by olefin-epoxide conversion. Naturwissenschaften 61, 130-131. 

The kinetic and deactivation models were fitted by non-linear regression analysis against the experimental data using the Modest software, especially designed for the various tasks -simulations, parameter estimation, sensitivity analysis, optimal design of experiments, performance optimization - encountered in mathematical modelling [6], The main interest was to describe the epoxide conversion. The kinetic model could explain the data as can be seen in Fig. 1 and 2, which represent the data sets obtained at 70 °C and 75°C, respectively. The model could also explain the data for hydrogenated alkyltetrahydroanthraquinone. 

The first attempt to relate changing dielectric properties to kinetic rate equations was by Kagan et al.S9), working with a series of anyhydride-cured epoxies. Building on Warfield s assumed correlation between d log (g)/dt and da/dt, where a is the extent of epoxide conversion, they assumed a proportionality between a and log (q), and modeled the reaction kinetics using the equation... 

On account of high activity and selectivity of this silica catalyst Dll-10, the influence of temperature was studied in the range of 150 °C to 300 °C (Fig. 15.4) (23). It is advantageous to run the reaction at about 250 °C to 300 °C. In this range, complete epoxide conversion is achieved within 6 h TOS in combination with the highest aldehyde selectivities of 45%. The formation of 19 and 21 are increased by increasing the temperature. Compound 20 passes a maximum at around 200 °C. 

Because it is often possible to control the stereochemical orientation of substituents on a cyclic array, Baeyer-Villiger cleavages of substituted cyclic ketones have been used extensively in the stereocon-trolled syntheses of substituted carbon chains. An asymmetric synthesis of L-daunosamine intermediate (30) from a noncarbohydrate precursor employed the cyclopentenol (28), prepared in optically pure form (95% ee) from 2-methylcyclopentadiene using asymmetric hydroboration (Scheme 8). Stereoselective epoxidation, conversion to Ae ketone and regioselective Baeyer-Villiger oxidation afforded lactone (29). 

The conversion of alkenes into epoxides is important not only because it is one of the most reliable routes leading from oxidation level 1 to level 2, but also because reactions of non-symmetrical epoxides with nucleophiles invariably proceed as an attack at the less substituted carbon with inversion of configuration. Thus, hydride reduction of epoxides represents an additional option for the preparation of alcohols (Scheme 2.62), especially valuable for the synthesis of optically pure isomers from epoxides obtained by the Sharpless oxidation. It is also of merit that as a result of alkene-epoxide conversion, a nucleophilic moiety (double bond) is transformed into an electrophilic epoxy ring. The latter... 

We observed that in each case the highest enantiomeric excess is obtained at the lowest temperature [entries 1-5 and 10-14 for catalyst (31), and 6-9 and 15-18 for catalyst (24)]. The increase in ee is coupled with a decrease in epoxide conversion at lower temperatures. A comparison of the two catalysts shows that the biphenyl catalyst (31) is much more reactive (as it is under the Oxone conditions) at lower temperatures and far more selective, regardless of solvent. 

These olefins were not as reactive towards catalyst (31) as the previously tested substrate 1-phenylcyclohexene, therefore the majority of reactions were carried out at —40 °C. Again, the dichloro methane/acetonitrile conditions produce the better results in terms of both enantiomeric excess and epoxide conversion, over the dichloromethane conditions. Triphenylethylene was extremely unreactive when compared with all the other alkenes tested (entries 3, 4, 10 and 11). The best ee obtained was for l-phenyl-3,4-dihydronaphthalene at —40 °C in dichloromethane/acetonitrile, which in 3h gave 100% conversion and 65% ee. This is even more remarkable when one considers that l-phenyl-3,4-dihydro-naphthalene also gave the poorest result in dichloromethane (7% ee). 

The most widely used method for the preparation of epoxides involves oxidation of an aUcene by a peracid, °° via a direct one-step transfer of an oxygen atom. More highly (alkyl) substituted alkenes react fastest showing that electronic effects are more important than steric effects in this reaction. Steric effects do, however, control the facial selectivity of epoxidation conversely hydrogen-bonding groups, such as OH and NH, can direct the reaction to the syn face. 

As discussed in a previous section, a number of studies have been conducted to increase the rate of cationic polymerization of epoxides. In curing applications, polymerization should be rapid enough for high output of production. In a recent work, the effect of addition of tetraethylene glycol (TEG) or polyEPB on the rate of photoinitiated cationic polymerization of CY179, limonene dioxide (LDO), and 1,2,7,8-diepoxyoctane (DEO) has been investigated [150]. These hydroxyl containing additives were shown to obviously accelerate the polymerization, increase the total epoxide conversion and decrease the induction period. 

Entry 5 is an interesting example that entails both enzymatic and hydrolytic epoxide conversion. In the first step, an enzymatic hydrolysis proceeds with retention of the configuration at the tertiary center. This reaction is selective for the 5-epoxide. The remaining 7 -epoxide is then subjected to acid-catalyzed hydrolysis, which proceeds with inversion at the center of chirality (see p. 186). The combined reactions give an overall product yield of 94%, having 94% e.e. ... 

Epoxidation. Conversion of alken as catalyst proceeds at low temperature 1 e... 

Oil Iodine Valued Iodine Value Acid Value S % Ox. Epoxy Equiv. Wt.(approx.) Av. functionality, moles epoxy mole of oil % Epoxidation % Conversion of double bond... 

O2. Quantitative conversion of cw—cyclooctene into the epoxide (conversion >95%) was obtained with a much higher substrate/catalyst ratio (1000/1) than previously... 

Epoxidation Conversion of an alkene to an epoxide, usually by treatment with a peroxy acid. 

Figure 1 shows the principal metabolic pathways of B tP (a representative PAH) and the formation of some conjugates. In the interest of simplicity, further metabolic transformations, such as the P450 catalyzed oxidation of phenols to phenol-epoxides, conversion of phenols to phenol-dihydrodiols, all the enzymatic steps, and the stereochemistry of the metabolites are omitted. 

Fig. 13. Comparison of the epoxide conversion in stoichiometric DGEBA— poly(oxypropylene)triamine system at 100°C by DSC ( ), SEC(D), and NIR (A). From Ref. 64.

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