Addition Reactions Involving Epoxides

Schechter 55) proposed that the catalytic effect of hydroxyl groups on the epoxide-amine addition reaction involved a termolecular activated complex formed in the concerted reaction of amine, epoxide and hydroxyl. Smith 57) suggested a modified mechanism in which the same activated complex is formed in a bimolecular reaction between an adduct formed from epoxide (E) and the proton donor (HX), and the amine ... 

Most of the reported DSC studies involving base-catalysed cure are concerned with dicyanodiamide (DICY) which is a crystalline material, melting at 208 °C. At ambient temperature the solubility in epoxy resins is very low but above about 100 °C it is sufficiently soluble to initiate cure. DICY has free amino-hydrogen groups which can partake in addition reactions with epoxides ... 

An intermolecular iron-catalyzed ring expansion reaction involving epoxides and alkenes provided tetrahydrofurans via radical processes <07CEJ4312>. Cp2TiCl is able to promote cyclization of 2,3-epoxy alcohols containing a p-(alkoxy)acrylate moiety to form tetrahydrofurans <07TL6389>. As shown in the following example, an intramolecular addition of carbon radicals to aldehydes was reported to afford tetrahydrofuran-3-ols... 

Epoxide groups may homopolymerise or react with active hydrogen atoms in other molecules, usually termed hardeners or co-reactants, to produce copolymers. Cure of an epoxy resin may involve either or both of these reactions, and can be very complex, since reaction of an epoxide with one functional group can produce new functional groups for additional reaction with epoxides, and so on. 

Further dechlorination may occur with the formation of substituted diphenyhnethanes. If enough aluminum metal is present, the Friedel-Crafts reactions involved may generate considerable heat and smoke and substantial amounts of hydrogen chloride, which reacts with more aluminum metal, rapidly forming AlCl. The addition of an epoxide inhibits the initiation of this reaction by consuming HCl. Alkali, alkaline-earth, magnesium, and zinc metals also present a potential reactivity hazard with chlorinated solvents such as methylene chloride. 

The most common method of epoxidation is the reaction of olefins with per-acids. For over twenty years, perbenzoic acid and monoperphthalic acid have been the most frequently used reagents. Recently, m-chloroperbenzoic acid has proved to be an equally efficient reagent which is commercially available (Aldrich Chemicals). The general electrophilic addition mechanism of the peracid-olefin reaction is currently believed to involve either an intra-molecularly bonded spiro species (1) or a 1,3-dipolar adduct of a carbonyl oxide, cf. (2). The electrophilic addition reaction is sensitive to steric effects. 

Epoxides bearing electron-withdrawing groups have been most commonly synthesized by the Darzens reaction. The Darzens reaction involves the initial addition of an ct-halo enolate 40 to the carbonyl compound 41, followed by ring-closure of the alkoxide 42 (Scheme 1.17). Several approaches for inducing asymmetry into this reaction - the use of chiral auxiliaries, reagents, or catalysts - have emerged. 

Z-vinyl iodide was obtained by hydroboration and protonolysis of an iodoalkyne. The two major fragments were coupled by a Suzuki reaction at Steps H-l and H-2 between a vinylborane and vinyl iodide to form the C(ll)-C(12) bond. The macrocyclization was done by an aldol addition reaction at Step H-4. The enolate of the C(2) acetate adds to the C(3) aldehyde, creating the C(2)-C(3) bond and also establishing the configuration at C(3). The final steps involve selective deprotonation and oxidation at C(5), deprotection at C(3) and C(7), and epoxidation. 

This section deals with additives used to initiate and accelerate reactions of isocyanates and epoxides. Polyurethanes are very versatile materials, capable of being formulated into a wide variety of products (B-64MI11501, B-78MI11504, B-71MI11500). Many important applications involve the production of foams catalysts play a key role in balancing the rates of the various reactions involved. 

At present, we can say that copolymerization initiated by various salts proceeds by an anionic mechanism, after dissociation of the initiators in the reaction medium. The primary step is the addition of the initiator anion to the epoxide. In the initiation by Lewis bases, i.e. by tertiary amines, initiation involves formation of a primary active centre of an anionic character. This active centre is probably generated by interaction of the tertiary amine with the anhydride and an allyl alcohol. The allyl alcohol can be formed by a base-catalyzed isomerization of the epoxide. In the presence of a proton donor, the formation of active centres is possible through interaction of tertiary amine, anhydride and proton donor without epoxide isomerization. Another way of initiation consists in a direct reaction of epoxide with tertiary amine yielding an anionic primary active centre. We believe that in both kinds of initiation in the strict absence of proton donors, the growing chain end has the character of a living polymer. The presence of proton donors, however, gives rise to transfer reactions. 

Because of their high refractive index, these materials possess desirable optical and thermal properties, in addition to being impervious to a variety of chemicals [5]. Analogous reactions involving carbon disulfide and epoxides have provided extremely interesting examples of atom-exchange polymerization processes. 

The addition criterion tends to be confusing when applied to a molecule like ethylene where addition occurs at both ends of the double bond. The reader is advised, in such cases, either to use the symmetry criterion or to choose epoxidation as the test reaction for the addition criterion. For additional examples involving the heterotopic faces of not only olefins and carbonyl compounds... 

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