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Carbon–13 NMR studies of tautomerism in some 2‐substituted imidazoles and benzimidazoles
Author(s) -
Papadopoulos E. Paul,
Hollstein Ulrich
Publication year - 1982
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270190406
Subject(s) - chemistry , substituent , steric effects , imidazole , medicinal chemistry , tautomer , intermolecular force , proton nmr , proton , nmr spectra database , nitrogen , stereochemistry , organic chemistry , spectral line , molecule , physics , quantum mechanics , astronomy
The 13 C NMR spectra of several 2‐substituted imidazoles and benzimidazoles have been measured. The substituent was CH 3 , COOH and CONHR, where R = H, n ‐Bu, p ‐tolyl or m ‐chlorophenyl. Carbons 4 and 5 in the imidazoles and the carbon pairs 8/9, 4/7 and 5/6 become equivalent by proton transfer from N‐1 to N‐3, possibly through intermolecular association. The rate of this proton exchange increases with concentration and temperature. It decreases with extension of the 2‐substituent (rate CH 3 ≫CONH‐phenyl > CONH‐ p ‐tolyl ≃ CONH‐ m ‐chlorophenyl ≫ CONH‐ n ‐butyl) due to steric hindrance at the site of the (benz)imidazole nitrogen.