Of epoxidation reactions

The most striking chemical property of epoxides is their far greater reactivity toward nude ophilic reagents compared with that of simple ethers Epoxides react rapidly with nude ophiles under conditions in which other ethers are inert This enhanced reactivity results from the angle strain of epoxides Reactions that open the nng relieve this strain... 

The primary type of epoxide reaction remains the nucleophilic ring-opening reaction. Research on the development of novel catalysts or catalytic systems for epoxide opening continues to be a highly active area of study. 

This strong directing effect can exert stereochemical control even when opposed by steric effects. Several examples of epoxidation reactions are given in Scheme 12.11. Entries 4 and 5 illustrate the hydroxyl directing effect. Other substituents capable of hydrogen bonding, in particular amides, also can exert a yyu-directing effect.67... 

Synthesis and Characterization of Lignin-like Model Epoxide. A series of epoxidation reactions were performed with hydroxypropyl guaiacol (HPG) as model compound and ECH. Molar ratios of ECH HPG varied from 1 to 10. Pelletized KOH and a quarternary ammonium salt (QAS) served as catalyst and reagent. Toluene was the solvent. Catalyst concentration and method of addition were varied. Experimental details are given elsewhere (11). 

Several significant reviews have appeared in recent years which contributed greatly to current knowledge of epoxide reaction mechanisms. Among them may be cited excellent discussions by Winetein and Henderson,am Kliel, 0 and Parker and Isaacs. Older articles of an encyclopedic nature include those of Bodforss,1 Meerwein,11 and Tiffeneau.1117 The well-known review by StreifcwieeerlMa may be consulted for a broader treatment of nucleophilic displacement reactions in general. 

Condensations of hydroxylic nucleophiles with ethylene oxides, when considered as a whole, probably constitute the largest single body of epoxide reactions. Included among these nucleophiles are water itself, the aliphatic Alcohols, and the aromatic alcohols or phenols. The present section will be devoted to the reactions of theee substances. 

The mechanisms and the regio- and stereo-selectivity of epoxide reactions have been reviewed.39 The review also covers the role of epoxides in biologically important reactions. The achiral and chiral catalysts used in these reactions are discussed. A second review40 discusses the ring-opening reactions of oxiranes with carbon nucleophiles. 

The relative rates and stereochemistry of epoxidation reactions of 5-substituted-adamantan-2-ones with two sulfur ylids (methylenedimethylsulfurane and its oxy-sulfurane analogue) have been studied in DMSO and in benzene.318... 

High-valent oxo-complexes, isolated or in situ-generated, interact most often with electron-rich n -systems 1 or suitable C-H bonds with low bond dissociation energy (BDE) in substrates 3 (Fig. 2). These reactions may occur concerted via transition states 1A or 3A leading to epoxides 2 or alcohols 4. On the other hand, a number of epoxidation reactions, such as the Jacobsen-Katsuki epoxidation, is known to proceed by a stepwise pathway via transition state IB to radical intermediate 1C [39]. Similarly, hydrocarbon oxidation to 4 can proceed by a hydrogen abstraction/S ... 

In epoxidation reactions allyl alcohol can act as a prochiral alkene. Enantio-merically pure glycidol isomers (see Table 1.1) may be used to make S-propanolol 9.61, a drug for heart disease and hypertension. The mechanistic details of the epoxidation reaction with V5+ and Mo6+ complexes as catalysts were discussed in Section 8.6. The basic mechanism of epoxidation reaction, the transfer of an oxygen atom from f-butyl hydroperoxide to the alkene functionality, remains the same. 

Table 5.3 Summary of epoxidation reactions in ionic liquids... 

Deviations from the above pattern of epoxide reactions have been explained in terms of two-step processes in which a relatively stable solvated benzyhc [173] or allylic [184] carboninm ion intervenes. The products arise by solvent attack on the intermediate carboninm ion, as a second step governed either stereochemicaUy by the accessibility of the two faces of the carboninm ion, or by the stability of the product. 

The most detailed studies of epoxide reactions in the absence of external nucleophiles have employed boron tri-... 

In concluding our discussion of the reactions of epoxides, the meth-anolysis of 3,4-epoxy-l-butene might be mentioned. This reaction has been studied in detail, and it illustrates very nicely some of the principles of epoxide reactions which we have considered. In the presence of sulfuric acid the only product isolated was 2-methoxy-3-buten-l-ol (III).27,28 When sodium methoxide was employed as the catalyst, the major product of the reaction was l-methoxy-3-buten-2-ol (IV),28 but some 2-methoxy-3-buten-2-ol was also obtained. 

Desulfurization. Desulfurization of olefin episulfldes (thiiranes) to olefins proceeds much more readily than the deoxygenation of epoxides. Reaction of... 

Figure 3.5. Comparison of heat flow as measured by the DSC and calculated heat from the extent of epoxide reaction (measured by NIR Spectroscopy) for TGDDM with 27% DDS at 177 °C for the times indicated. The difference arises from the change in heat capacity change during cure which requires a baseline correction to the DSC trace. Adapted from de Bakker et al. (1993b).

The arylbutylselenide 53 readily prepared from the corresponding aryl bromide 54 and lithium butylselenide is an excellent catalyst for the performance of epoxidation reactions in a fluorous biphase system. With 5 mol % of the catalyst 53, various polysubstituted and functionalized olefins 55 are epoxidized in a biphasic system of bromoperfluorooctane and benzene using hydrogen peroxide (60% in water, 1.5-2.0 equiv) leading to epoxides 56 in good to excellent yields Eq. (25). 

The successful examples of epoxidation reactions using MMPP have so far been carried out using a hydroxylic solvent to dissolve the reagent. This has usually been a low molecular weight alcohol or, where the substrate to be epoxidised is... 

Before leaving the subject of epoxidation reactions we should make brief mention of the control of stereochemistry involving the use of neighbouring functional... 

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