Epoxy rings

Epoxy/Carboxy Cure Sites. Epoxy/carboxy cure sites probably represent the most important alternative to labile chlorine containing monomers. There has been increasing interest in them due to the discovery of the highly efficient quaternary ammonium salt-based accelerators (29—34). The reaction between the epoxy ring and carboxyUc acid can happen in the following three ways ... 

The three-membered epoxy ring is highly strained and is reactive to many substances, particularly by with proton donors, so that reactions of the following schematic form can occur ... 

It has been shown in the course of this work that the reactivity of the epoxy ring is enhanced by the presence of the ether linkage separated from it by a methylene link. 

The epoxy ring may then be readily attacked not only by active hydrogen and available ions but even by tertiary amines. For example, with the latter it is believed that the reaction mechanism is as follows ... 

They will be formed as epoxy rings are opened during cure. 

This product will contain new hydroxyl groups that can react with other epoxy rings, generating further active hydroxyl groups, e.g. 

In addition to the catalytic reactions the resins may be cross-linked by agents which link across the epoxy molecules. These reactions may be via the epoxy ring or through the hydroxyl groups. Two examples of the former are ... 

Melt reaction mechanisms of tertiary aliphatic amine catalyzed phenolic-epoxy reactions were proposed to begin with a trialkylamine abstracting a phenolic hydroxyl proton to form an ion pair (Fig. 7.36). The ion pair was suggested to complex with an epoxy ring, which then dissociated to form a /1-hydroxycther and a regenerated trialkylamine.87... 

Biernath et al. concluded that phenolic novolac and epoxidized cresol novolac cure reactions using triphenylphosphine as the catalyst had a short initiation period wherein the concentration of phenolate ion increased, followed by a (steady-state) propagation regime where the number of reactive phenolate species was constant.85 The epoxy ring opening was reportedly first order in the steady-state regime. 

The ion thus produced may itself react with another epoxy-group in a process which forms the first crosslink (Reaction 4.16). This reaction may occur at both ends of the molecule of the diglycidyl ether, so that a crosslinked stmcture can easily be built up from these substances. Reaction becomes complicated by the fact that the epoxy-group may also react with the hydroxy-groups that form as the epoxy ring opens up during cure. Thus the finished resin may contain a complicated array of structures within the three dimensional network. 

A comprehensive theoretical investigation of PECD in a series of substituted oxiranes has been presented by Stener et al. [53] using the B-spline method. Variations in the predicted dichroism were found as both a function of initial orbital, and of the chemical substitution about the epoxy ring. These substitutions do not induce very significant geometry changes in the optimized ring structure. 

It is obvious that the unstable character of an ether comes from the epoxy ring. On the other hand, linear ethers are perfectly stable. Obviously, ethers with structures similar to the examples above are those that are mainly responsible for the accidents bonded to the decomposition of ethers. 

The epoxy ring opens easily when nucleophilic reagents are present and releases a large quantity of energy due to the cyclic tension. This leads to reactions that can be hardly controlled. 

The epoxy ring does not open more easily when certain acids are present. When the molecule is unstable, risks are higher as shown in the accident described below ... 

Z = 4 D, = 1.52 R = 0.049 for 1,606 intensities. An error in the atomic coordinates for C-6 prevents display of this molecule. The atomic coordinates for C-l 1 were omitted, and those for H-61 and H-62 appear to be in error. The linkage, orientational angles between the hexenopyranosyl and the pyranoside ring are 0-5 -C-l -0-4-C-4 = +83°, C-l -0-4-C-4-C-5 = —139°. The epoxy ring is equilateral, with C-C = C-0 = 146 pm, and the ring-angles equal to 60 0.3°. The results of this analysis were correlated with those from the n.m.r. spectrum of the compound in solution. 

This group of eight representatives (85-92) is characterized by the 60,70-epoxy ring. 

Polymer linkages that do not release condensates on being formed include polyurethanes (diisocyanate/polyalcohol) and epoxy (ring opening of the epoxy... 

Mori reported an improved synthesis of (3S,4P,6 ,10Z)-faranal (37), the trail pheromone of the Pharaoh s ant (Monomorium pharaonis) [84]. As summarized in Scheme 55, the key-reaction was the coupling of iododiene A with iodide E. The geometrically pure A was prepared by the zirconocene-mediated carbo-alumination reaction, and E was prepared from B by the asymmetric cleavage of its epoxy ring to give C (77% ee), which could be purified via its crystalline 3,5-dinitrobenzoate D. 

Table 3 Stereochemistry of natural pheromones containing an epoxy ring (main components) and field attractancy of the synthetic racemate... 

Species Main pheromonal component Stereochemistry of the epoxy ring (main component) ... 

The stereochemistry of each enantiomer separated by the chiral HPLC has been studied after methanolysis of the epoxy ring. Examining the H NMR data of esters of the produced methoxyalcohols with (S)- and (R)-a-methoxy-a-(tri-fluoromethyl) phenylacetic acid by a modified Mosher s method [181], it has been indicated that the earlier eluting parent epoxides are (3S,4R)-, (6S,7R)-, and (9R,10S)-isomers (Table 7) [75, 76, 179]. The above three chiral HPLC columns show different resolution abilities but a different elution order is not observed. The resolution profile by the reversed-phase OJ-R column has been generalized with molecular shapes of the epoxy compounds considering the... 

Examination of the H-NMR spectrum in comparison with that of 246 indicated the presence of two ethyl chains (Me—CH2 signals at 0.82 and 1.08 ppm with 7 = 7 Hz) and also confirmed the presence of an epoxy ring (H-15 at 2.92... 

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