Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Addition Reactions Involving Carbonyls and Nitriles
9.9 Mechanism for Addition Reactions Involving Imines

Imines are very important functional group in organic chemistry; most are used as ligands for the extraction of metals. The polarity of the imine functionality is very similar to that of the carbonyl functionality. The knowledge gained and used for the addition reactions of the previous section can be applied to predict the outcome of just about any addition reaction with imines. As you can imagine, the addition of an electrophile to the imine is regiospecific and will add to the nitrogen since it is more electronegative than carbon as shown in Reaction (9-69).

(9-69)

The next step of the addition reaction mechanism involves the nucleophile adding to the electrophilic carbon as shown in Reaction (9-70).

(9-70)

Thus, the overall hypothetical reaction is shown in Reaction (9-71), note again, it a regiospecific reaction with the electrophile adding to the nitrogen and the nucleophile adding to the electrophilic carbon.

(9-71)

Of course, it is possible for the sequence of the addition steps to take place in a different sequence in which the first step involves the nucleophile adding to the electrophilic carbon of the imine, as shown in Reaction (9-72).

(9-72)

In the next step, the electrophile will add to the iminium intermediate that was created by the addition of the nucleophile, as shown in Reaction (9-73) to give a final neutral product.

(9-73)

The overall hypothetical addition reaction is the same as given in Reaction (9-71). The next section will cover the addition of water to imines.

9.9.1 Addition of Water to Imines

In this section, we will discuss the addition of water to imines, which is accomplished in the presence of an acid catalyst. The addition of water to imine functionalities is a reversible reaction and an example is shown in Reaction (9-74).

(9-74)

The mechanism for the addition of water to the imine functionality is shown in Reaction (9-75).

(9-75)

You will notice that the product of the addition reaction of water results in an intermediate that was formed by the addition of amines to carbonyl compound (Reaction 9-63). As a result, it is possible to convert an imine to a carbonyl compound as shown in Reaction (9-76).

(9-76)

As we will see in later chapters, these reactions are very important for the synthesis of molecules that have these functionalities.