Structural Analyses of N‐Acetylated 4‐(Dimethylamino)pyridine (DMAP) Salts

We have studied the formation of several N‐acetyl‐4‐(dimethylamino)pyridine (DMAP) salts (with Cl−, CH3COO−, and CF3COO− counterions), which are considered to be the catalytically active species in DMAP‐catalyzed acetylation reactions of alcohols. Combined crystal structure analyses, variable temperature matrix IR and NMR spectroscopy as well as computational techniques at the UAHF‐PCM‐B3LYP/6‐311+G(d,p)//B3LYP/6‐31G(d) level were utilized to examine the structures and dynamics of salt formation. We found clear evidence for the formation of tight ion pairs that are stabilized by dynamic hydrogen‐bonding interactions. In nonpolar solvents, the nucleophilicity of acetate in its N‐acetyl‐DMAP salt only allows a steady‐state concentration smaller 1 % at room temperature. Thus, we propose additional hydrogen‐bonding interactions with alcohols to be the key stabilization factor in subsequent acetylations. Detailed insights into N‐acetyl‐ and N‐trifluoroacetyl‐4‐(dimethylamino)pyridine salts by crystal structure analyses, NMR techniques, and novel diffusion‐controlled matrix IR experiments reveal that the nucleophilicities as well as the hydrogen‐bonding capabilities of the counterion with the pyridinium cation are crucial for the formation of the tight ion‐pair salts.