Epoxides are prepared from alkenes by using peroxy acids. Epoxides are also prepared from a halohydrin with a strong base.
An epoxide is a three-atom ether ring with one oxygen and two carbon atoms. Epoxides are also referred to as oxiranes, and the simplest has the chemical formula of C2H4O. Epoxides are prepared from alkenes by using peroxy acids such as MMPP and MCPBA. A peroxy acid is an acid that contains the COOOH, or CO3H, group. Epoxidation is a chemical reaction that creates an oxirane ring (three-membered ether). In an epoxidation reaction, the carbon-carbon double bond, C=C, is converted to an oxirane. This is a concerted mechanism, and the stereoselectivity of this reaction is syn. The mechanism proceeds through an electrophilic addition.
Epoxides can also be prepared from a halohydrin with the addition of a strong base, such as sodium hydroxide (NaOH). A halohydrin is an organic molecule with a halogen and a hydroxyl group on adjacent carbons. Halohydrins are formed by using X2/H2O, such as Br2/H2O. An alkene is converted to a halohydrin with Br2/H2O via an electrophilic addition. Sodium hydroxide (NaOH) deprotonates the alcohol and, via a nucleophilic substitution, creates the epoxide.
Epoxide Formed with Halohydrin
An electrophilic addition of Br2/H2O to cyclohexene will form a halohydrin, 2-bromocyclohexanol, which is deprotonated using a strong base, forming an alkoxide. The alkoxide will undergo an intramolecular nucleophilic substitution reaction to form an epoxide.
Although epoxidation yields a stereoselective product (syn addition), it is still a racemic mixture, a 50:50 mixture of both enantiomers. In forming a cis epoxide, 50% of the product will be wedge/wedge, and 50% will be dash/dash. American chemist K. Barry Sharpless developed an enantioselective method of epoxidation that can yield not only a syn product but only one of the two possible syn isomers using diethyl tartrate (DET), and this method is commonly known as Sharpless epoxidation.
An enantioselective reaction refers to a chemical reaction in which one enantiomer of a chiral product is preferentially formed. An enantiomer is not superimposable on its mirror image; enantiomers and diastereomers are examples of chiral products. Sharpless epoxidation is used only with allylic alcohols; otherwise, the reaction will not function properly. t-butyl hydroperoxide (TBHP) is the oxidant, and titanium tetraisopropoxide (Ti[OiPr4] or Ti[OCH(CH3)2]4) is the catalyst. DET, is the chiral catalyst that provides the chiral environment that helps form the desired enantiomer.
Enantioselective Epoxidation
Sharpless epoxidation is enantioselective (forming preferentially one enantiomer), so one configuration of epoxide is preferred and will be the major product. When allyl alcohol undergoes Sharpless epoxidation, only one enantiomer will form instead of a racemic mixture.
