Leaving groups epoxidation reactions

Although an epoxide and an ether have the same leaving group, epoxides are much more reactive than ethers in nucleophilic substitution reactions because the strain in their three-membered ring is relieved when the ring opens (Figure 11.2). Epoxides, therefore, undergo nucleophilic substitution reactions with a wide variety of nucleophiles. 

Overall the stereospecificity of this method is the same as that observed m per oxy acid oxidation of alkenes Substituents that are cis to each other m the alkene remain CIS m the epoxide This is because formation of the bromohydrm involves anti addition and the ensuing intramolecular nucleophilic substitution reaction takes place with mver Sion of configuration at the carbon that bears the halide leaving group... 

Unlike other ethers, epoxide rings con be cleaved by base as well as by acid. Although an ether oxygen is normally a poor leaving group in an reaction (Section 11.3), the strain of the three-membered ring causes epoxides to react with hydroxide ion at elevated temperatures. 

Other leaving groups are sometimes used. Sulfates, sulfonates, and epoxides give the expected products. Acetals can behave as substrates, one OR group being replaced by ZCHZ in a reaction similar to 10-101. Ortho esters behave similarly, but the product loses R OH to give an enol ether. ... 

Whereas phosphonium ylides normally react with carbonyl compounds to give alkenes, dimethylsulfonium methylide and dimethylsulfoxonium methylide yield epoxides. Instead of a four-center elimination, the adducts from the sulfur ylides undergo intramolecular displacement of the sulfur substituent by oxygen. In this reaction, the sulfur substituent serves both to promote anion formation and as the leaving group. 

A simple example of a ring-opening substitution reaction is the acid-catalysed hydrolysis of epoxides. In the example shown, protonation of the epoxide oxygen improves the leaving group, and an Sn2 reaction may then proceed using water as the nucleophile. Three-membered rings must of necessity... 

The proposed mechanism of this reaction is based on the nucleophilic attack of the alkyllithium compound at the carbenoid carbon atom or at the a-lithiooxy carbene. The dilithium compound 102 gives the alkene 103 by the loss of lithium oxide (equation 56). When an alkoxy residue, which is a better leaving group than U2O, is offered in the a-position of the corresponding dilithium compound, the elimination of lithium alkoxide takes place instead of lithium oxide. This is illustrated by the reaction of epoxide 104 that delivers the allylic alcohol 105 upon treatment with n-butyllithium (equation The... 

The fact that an epoxide gives a reaction analogous to a halide indicates that the oxygen and halogen are leaving groups in a nucleophilic substitution step. 

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