Epoxidation of alkenes

Alkenes can be oxidized to epoxides using a ‘peroxyacid‘ such as m-chloroperoxybenzoic acid (MCPBA). Notice the presence of a third oxygen in the peroxyacid functional group.

 The π electrons in the alkene double bond attacking the ‘outer’ oxygen of the peroxyacid and cleaving the reactive O-O peroxide bond.

Uncatalyzed epoxidation of an asymmetric alkene generally results in two diastereomeric epoxide products, with the epoxide adding either from above or below the plane of the alkene.

Oxacyclopropane (ethylene oxide) is used for many purposes, but probably the most important reaction is ring opening with water to give 1,2-ethanediol (ethylene glycol, bp 197^o). This diol, mixed with water, is employed widely in automotive cooling systems to provide both a higher boiling and lower freezing coolant than water alone:

Exercises

1. Predict the product of the reaction of cis-2-hexene with MCPBA (meta-chloroperoxybenzoic acid)

a) in acetone solvent.

b) in an aqueous medium with acid or base catalyst present.

2. Predict the product of the reaction of trans-2-pentene with magnesium monoperoxyphthalate (MMPP) in a chloroform solvent.

3. Predict the product of the reaction of trans-3-hexene with MCPBA in ether solvent.

4. Predict the reaction of propene with MCPBA.

a) in acetone solvent

b) after aqueous work-up.

5. Predict the reaction of cis-2-butene in chloroform solvent.

Answers

1.     a) Cis-2-methyl-3-propyloxacyclopropane

b) Racemic (2R,3R)-2,3-hexanediol and (2S,3S)-2,3-hexanediol

2.     Trans-3-ethyl-2-methyloxacyclopropane.

3.     Trans-3,4-diethyloxacyclopropane.

4.    a) 1-ethyl-oxacyclopropane

b) Racemic (2S)-1,2-propandiol and (2R)-1,2-propanediol

5. Cis-2,3-dimethyloxacyclopropane