Imine formation and enamine formation appear different because one leads to a product with a C=N double bond and the other leads to a product with a C=C double bond. Actually, though, the reactions are quite similar. Both are typical examples of nucleophilic addition reactions in which water is eliminated from the initially formed tetrahedral intermediate and a new C=Nu double bond is formed.
Imines are formed in a reversible, acid-catalyzed process that begins with nucleophilic addition of the primary amine to the carbonyl group, followed by transfer of a proton from nitrogen to oxygen to yield a neutral amino alcohol, or carbinolamine. Protonation of the carbinolamine oxygen by an acid catalyst then converts the –OH into a better leaving group (-OH2+), and El-like loss of water produces an iminium ion. Loss of a proton from nitrogen gives the final product and regenerates the FIGURE 19.8
Mechanism of imine formation by reaction of ketone or aldehyde with a primary amine. The key step is nucleophilic addition to yield a carbinolamine intermediate, which then loses water to give the imine.