Q3(b). MCPBA epoxidation of cis- and trans-but-2-ene

Introduction. m-Chloroperoxybenzoic acid (MCPBA) transfers its terminal peroxide oxygen to a C=C in one concerted step (the Bartlett “butterfly” transition state). Both C–O bonds form on the same face, so the geometry of the alkene is kept in the epoxide: the reaction is stereospecific syn addition.

Figure: Butterfly transition state (sketch): the peroxy O bridges both alkene carbons on one face.

Scheme: cis gives the meso epoxide; trans gives the racemic trans epoxide.

1. cis-But-2-ene gives only cis-2,3-dimethyloxirane: the two methyls stay on the same side. This epoxide has a mirror plane; it is (2R,3S), a meso compound. Attack on the top face or the bottom face gives the same molecule.
2. trans-But-2-ene gives only trans-2,3-dimethyloxirane. Attack on the top face gives (2R,3R) and on the bottom face (2S,3S); the two faces are equally accessible (enantiotopic), so the product is a racemate.
3. No cis/trans crossover is seen: there is no open-chain intermediate (carbocation or radical) in which rotation could occur. The reaction is therefore stereospecific (each alkene isomer gives its own stereoisomer), and in that sense 100 % stereoselective.
4. The alkene acts as the nucleophile (its π electrons attack the O–O σ* orbital), so alkyl groups on the C=C increase the rate; both butenes react faster than ethene.

Conclusion. MCPBA adds O syn in one step: cis-but-2-ene → meso cis-epoxide; trans-but-2-ene → racemic trans-epoxide. The alkene geometry is retained.

Application. Stereospecific epoxidation followed by anti ring opening (H₃O⁺) gives anti-1,2-diols; the method is used in making propylene oxide derivatives and epoxy resins.