Epoxide hydroxyl reactions

Using measured conversion rates and the heat evolution data, it is possible to estimate the relative heats of reaction of the epoxide-primary amine and the epoxide-hydroxyl reactions, the major reactions in this system. This calculation requires reliable estimates of the rate of appearance of hydroxyl groups and the rate of disappearance of the epoxide groups at early stages of the cure, as measured by FT-IR spectroscopy. These measurements can be corroborated by N-15 nmr monitoring of the rate of formation of secondary amines. 

Although much more selective than the uncatalyzed reaction, the base-catalyzed reaction has some dependence on stoichiometry. At a ratio of epoxide to acid of 1 1, essentially all the product is the hydroxy ester. However, when an excess of epoxide groups is present. Reaction 7 proceeds until all the acid is consumed, after which the epoxide-hydroxyl reaction (Reaction 9) starts. This is illustrated in Figure 4. 

We emphasize that the above results have been observed only in the oxidation of sulfides and phenols, reactions known to follow radical mechanisms. A thorough investigation of the catalytic potential of the materials in other oxidation reactions (epoxidation, hydroxylations, etc.) is warranted. 

In situ EPR experiments in the presence of different substrates (allyl alcohol, benzene, phenol, or toluene) reveal that type A species is involved in epoxidation reactions. Species B is more active than A in ring hydroxylation reactions. A comparison of the toluene results with those of phenol/benzene suggests that while species B is involved in ring hydroxylations, the A-type species are possibly involved in... 

At odds with other similar peroxo metal complexes, V0(02)pic(H20)2, 36, performs non-selective epoxidation reactions. On this occasion Mimoun proposed a mechanism where a homolytic rupture of one metal-peroxo oxygen bond produces the active oxidant (Scheme 14). When aromatic substrates are allowed to react with 36, hydroxylation reaction takes place by way of the same active species as indicated in Scheme 15. 

The remaining chapters deal with a variety of catalysts for effecting oxidation reactions. Chapter 5 describes three simple protocols for the controlled oxidation of primary or secondary alcohols. The importance of stereocontrolled epoxidation and hydroxylation reactions is reflected by the fact that Chapter 6, directed at this field, is one of the most extensive sections of the book. An interesting example of an enantioselective Baeyer-Villiger reaction is featured in Chapter 7, together with an industrially important ketone to enone conversion. Oxidative carbon-carbon... 

The homopolymerization reactions of impure TGDDM (MY720) in the presence and absence of a BF3 NH2C2H5 catalyst and, also, pure TGDDM were monitored by FTIR as a function of cure temperature from 177 to 300 °C. The intensities of the epoxide, hydroxyl, ether and carbonyl bands at 906, 3500, 1120 and 1720 cm-1 respectively were determined from spectral differences and are plotted as a function in cure conditions in Figs. 10,11,12 and 13 respectively. The 906,1120 and 1720 cm-1 band intensities were normalized to the 805 cm-1 band and the 3500 cm-1 to the 1615 cm 1 band. The 805 and 1615 cm-1 bands are associated with the phenyl group which is assumed to chemically unmodified during the homopolymerization reactions. 

Hence, the most plausible explanation of our FTIR observations of the simultaneous appearance of hydroxyl, carbonyl and ether groups upon TGDDM epoxide consumption is epoxide isomerization and/or oxidation followed by epoxide-hydroxyl chain extension reactions. 

Fig. 22a and b. (i) Epoxide-hydroxyl and (ii) secondary amine-epoxide reactions that form (a) inter-molecular crosslinks and (b) intramolecular rings... 

A kinetic model which includes both amine-epoxide and hydroxyl-epoxide addition reactions, with hydroxyl autocatalysis has been proposed by Zukas 103,104). The starting point was an expression for the rate of consumption of epoxide by reaction with primary or secondary amine and hydroxyl groups... 

In practice the epoxide-amine cure is often accelerated by the addition of catalysts such as boron trifluoride complexes, and the boron trifluoride-ethylamine adduct (BFE) is widely used for this purpose. In addition to catalysing the epoxide-amine reactions, BFE can initiate homopolymerisation of epoxide. The accelerating effect of BFE is illustrated by DSC scans for the TGDDM/DDS/BFE system in Figure 12. The multiple-peaked exotherm associated with the BFE-catalysed TGDDM/DDS cure indicates that the kinetics of this system are more complex than those for the cure with amine alone. For this system the overall heat of reaction was found to decrease with increasing BFE concentration 89). For DDS alone Q0 was about 110 kJ per mole epoxide while the value for BFE alone was 75 kJ/mole, and the DDS/BFE values were between these limits. It appears that the proportion of epoxide homo-polymerisation relative to amine or hydroxyl addition increases with increasing BFE concentration. 

May, S. W., and Abbott, B. J. 1973. Enzymatic Epoxidation. 2. Comparison between Epoxidation and Hydroxylation Reactions Catalyzed by Omega-Hydroxylation System of Pseudomonas oleovorans. J. Biol. Chem., 248,1725-1730. 

Figure 10.1. Examples of epoxidation and aromatic hydroxylation reactions.

The major by-product in the double hydroxylation reaction is the a-hydroxy ketone F which forms presumably by protiodesilylation of the transient, intermediate epoxide B. In order to exclude tree m-chlorobenzoic acid that might cause this side reaction, MCPBA is purified and added very slowly to the substrate in the presence of excess, finely powdered potassium bicarbonate. In the case of the example presented above, the mechanism presumably is as follows ... 

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