General Mechanism - Electrophilic Process Catalysis

Three salient features emerge from consideration of the mechanism depicted:

1. The first step's rate is more rapid than for pyridine because DMAP is a stronger base due to the electron donating effect of the p-dialkylamino group.

2. The intermediate salt(s) formed by the first step are more stable that their pyridine analogues (due to resonance stabilization) and they can sometimes be isolated. Thus, their high concentration facilitates the reaction in spite of the poorer-than-pyridine leaving group characteristics of DMAP relative to pyridine.

3. The second step is formulated as proceeding by general base catalysis. This is consistent with the finding that acid anhydrides are beter acylating agents than acid chlorides. This step is promoted by the presence of strong bases (e.g., TEA) that are known to release DMAP from its salts.

In summary, DMAP derives its remarkable ability to catalyze not just from its strong base characteristic but from its unique structure.

1. M. Balasubramanian, R. Murugan, E. F. V. Scriven, Specialty Chemicals, June 2001