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ortho ‐Substituent effects in N ‐arylacetamides. NMR and molecular mechanics investigation
Author(s) -
Kolehmainen Erkki,
Laihia Katri,
Kauppinen Reijo,
Rasała Danuta,
Puchała Agnigeszka
Publication year - 1995
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610080903
Subject(s) - substituent , chemistry , acetanilide , chemical shift , moiety , steric effects , ring (chemistry) , stereochemistry , nmr spectra database , carbon 13 nmr , proton nmr , nuclear magnetic resonance spectroscopy , medicinal chemistry , computational chemistry , crystallography , organic chemistry , spectral line , physics , astronomy
1 H, 13 C, 15 N and 17 O NMR spectra of N ‐phenylacetamide (acetanilide) and 21 ortho ‐substituted acetanilides were measured and assigned. The observed NMR parameters are related to the Hammett substituent parameters and conformational characteristics of the acetamido moiety estimated by molecular mechanics calculations. Significant relationships were found for the 13 C NMR chemical shifts of C‐5 ( para to substituent) and the direct spin–spin coupling constant, 1 J (C, H), of C‐3 ( ortho to substituent) with Hammett substituent parameters. For 15 N NMR chemical shifts of the amido nitrogen, no general correlation with the Hammett substituent parameters was found. The interactions between functionalities contiguous to the carbonyl group and the amino nitrogen in acetanilides are effectively hampered owing to the increased twist angle between the planes containing the phenyl ring and the H–N–C fragment in derivatives bearing bulky ortho substituents. Especially in ortho ‐halogen‐substituted derivatives the 15 N NMR chemical shift of the amino nitrogen is very clearly related to the twist angle between the phenyl ring and H–N–C fragment. For 17 O NMR chemical shifts of the acetamido carbonyl, the observed variations can be related to the steric inhibition of resonance between the benzene ring and the acetamido group.