11.9:
Preparation of Epoxides
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Epoxides result from alkene oxidation, which can be achieved by a) air, b) peroxy acids, c) hypochlorous acids, and d) halohydrin cyclization.
Epoxidation with Peroxy Acids
Epoxidation of alkenes via oxidation with peroxy acids involves the conversion of a carbon–carbon double bond to an epoxide using the oxidizing agent meta-chloroperoxybenzoic acid, commonly known as MCPBA. Since the O–O bond of peroxy acids is very weak, the addition of electrophilic oxygen of peroxy acids to alkenes occurs with ease, thereby following syn addition. Hence, the epoxides are produced with the retention of the alkene configuration.
Epoxidation via Air Oxidation
Although peroxy-mediated epoxidation is the most common method for alkene oxidation, ethylene oxide is synthesized at the industrial scale via air oxidation by treating a mixture of ethylene and air in the presence of a silver catalyst.
Cyclization of Halohydrins
Cyclization of halohydrins of alkenes in the presence of a base also yields epoxides, and the reaction follows the SN2 substitution mechanism. Hence, the nucleophile—the oxygen anion—and the leaving group—the chloride ion—must orient anti to each other in the transition state to make the halohydrins cyclization feasible.
In noncyclic halohydrins, this anti-relationship is achieved by an internal rotation. For instance, in 1-chloro-2-methyl-2-propanol, shown in Figure 1, the hydroxyl and the chloro group are not oriented anti to each other. To achieve the anti-relationship, the carbon-bearing chloro group undergoes an internal rotation, thereby making the nucleophile attack—from the backside of the C–X bond—and the ejection of the leaving group feasible. Thus, the epoxides formed via halohydrin cyclization also retain the alkene configuration.
Similarly, the cyclic halohydrins must undergo conformational changes to achieve the anti-relationship. For example, the halohydrin of cyclohexane, shown in Figure 2, undergoes a conformational change from diequatorial to diaxial to successfully form an epoxide.
