Just as direct addition to C=O becomes substitution at C=O when there is a leaving group at the carbonyl carbon, so conjugate addition becomes conjugate substitution if there is a leaving group, such as Cl, at the β carbon atom. Here is an example: substitution replaces Cl with OMe, just as it would have done in a reaction with an acyl chloride.
As with substitution at C=O, this apparently simple reaction does not involve a direct dis placement of the leaving group in a single step. The mechanism starts in exactly the same way as for conjugate addition, giving an enol intermediate.
Now the leaving group can be expelled by the enol: the double bond moves back into its original position in an elimination reaction—the sequence is often called an addition–elimination
reaction. The ‘new’ double bond has the more stable E configuration. In the next example, two consecutive conjugate substitution reactions give a 1,1-diamine.
At first sight, the product looks rather unstable—sensitive to water, or traces of acid perhaps. But, in fact, it is remarkably resistant to reaction with both. The reason is conjugation: this isn’t really an amine (or a diamine) at all because the lone pairs of the nitrogen atoms are delocalized through into the carbonyl group, very much as they are in an amide. This makes them less basic, and makes the carbonyl group less electrophilic.
