Although they really belong with other nucleophilic substitution reactions, we included the last few examples of epoxide-opening reactions here because they have many things in common with the reactions of bromonium ions. Now we are going to make the anal-ogy work the other way by looking at the stereochemistry of the reactions of bromonium ions, and hence at the stereoselectivity of electrophilic additions to alkenes. We shall first remind you of an epoxide reaction, where you saw this.

The epoxide ring opening is stereospecific: it is an SN2 reaction and it goes with inversion. The epoxide starts on the top face of the ring and the amino group therefore ends up on the bottom face. In other words, the two groups end up anti or trans across the ring. You now know how to make this epoxide—you would use cyclopentene and m-CPBA, and in two steps you could ‘add’ an OH group and a Me2N group anti across the double bond.

Now we can move on to look at the stereochemistry of electrophilic addition to alkenes.

مواضيع ذات صلة

The Mannich reaction

Base-promoted halogenation

Halogenation

Racemization

Thermodynamically stable enols: 1,3-dicarbonyl compounds

Kinetically stable enols

Summary of types of enol and enolate

The intermediate in the base-catalysed reaction is an enolate ion

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes