DMAP as an acylation catalyst
4-Dimethylaminopyridine (DMAP) is a pyridine with a donor group at C4: its resonance form with the negative charge on the ring nitrogen makes it more basic (pKaH = 9.6) and much more nucleophilic than pyridine. With acetic anhydride it forms an acylpyridinium ion, stabilized by the dimethylamino group and highly reactive.
The alcohol attacks the acylpyridinium ion and DMAP is released and re-enters the cycle. In catalytic amounts (1–10 mol%) and with an auxiliary base such as triethylamine, DMAP accelerates the acylation of hindered alcohols, such as tertiary ones, by several orders of magnitude compared with pyridine alone.