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

2009; Wiley; Volume: 15; Issue: 34 Linguagem: Inglês

10.1002/chem.200901379

1521-3765

V. Lutz, Jörg Glatthaar, Christian Würtele, Michael Serafin, Heike Hausmann, Peter R. Schreiner,

Chemical Reaction Mechanisms

We have studied the formation of several N-acetyl-4-(dimethylamino)pyridine (DMAP) salts (with Cl(-), CH(3)COO(-), and CF(3)COO(-) 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.