Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 15, Issue 34, Pages 8548-8557Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200901379
Keywords
acetylation; dimethylaminopyridine; matrix isolation; nucleophilic catalysis; organocatalysis
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Funding
- Deutsche Forschungsgemeinschaft [SPP 1179]
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We have studied the formation of several N-acetyl-4-(dimethyl-amino)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 nonpoiar 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.
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