Premium
15 N‐NMR study of activated enamines . Structural dependence of δ ( 15 N) and n J (N,H) in primary, secondary and tertiary enamino‐ketones, esters and amides
Author(s) -
Kozerski Lech,
KamienskaTrela Krystyna,
Kania Lidia,
Von Philipsborn Wolfgang
Publication year - 1983
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19830660722
Subject(s) - chemistry , steric effects , geminal , chemical shift , lone pair , delocalized electron , crystallography , conjugated system , double bond , stereochemistry , nmr spectra database , kinetic isotope effect , proton , nuclear magnetic resonance spectroscopy , molecule , spectral line , deuterium , organic chemistry , polymer , physics , quantum mechanics , astronomy
The pulse sequence INEPT was used to obtain proton‐coupled 15 N‐NMR spectra in natural isotope abundance for enamines substituted in 2‐position with electron‐with‐drawing groups. The chemical shifts and coupling constants are discussed in terms of their relationship to structural features such as multiple N ‐alkyl substitution, double‐bond configuration, H‐bonding, N‐lone‐pair delocalization within the conjugated system, and steric effects. It is concluded that 15 N chemical shifts are a sensitive probe for local structural modifications at the N‐atom and conformational changes in a remote part of a conjugated molecule, while one‐bond N,H‐coupling essentially reflects N‐hybridization and subtle local geometric distortions. Stereospecific three‐bond N,H spin coupling to olefinic protons (4.0 ± 0.2 Hz) has been found a characteristic feature of ( Z )‐isomers in all investigated compounds, whereas two‐bond coupling to olefinic protons ( 2 J (N,H) = 0.5 to 5 Hz) is observed in ( E )‐isomers. The sensitivity to solvents and steric properties of remote substituents renders geminal coupling a useful probe for studying electronic effects in the CN bond.