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Influence of ortho substituents on 17 O NMR chemical shifts in phenyl esters of substituted benzoic acids
Author(s) -
Nummert Vilve,
Mäemets Vahur,
Piirsalu Mare,
Koppel Ilmar A.
Publication year - 2011
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.1784
Subject(s) - chemistry , steric effects , substituent , medicinal chemistry , inductive effect , chemical shift , resonance (particle physics) , acetonitrile , benzoic acid , stereochemistry , proton nmr , polar effect , nmr spectra database , organic chemistry , spectral line , astronomy , physics , particle physics
17 O NMR spectra for 29 phenyl esters of ortho ‐, para ,‐ and meta ‐substituted benzoic acids, X‐C 6 H 4 CO 2 C 6 H 5 , at natural abundance in acetonitrile were recorded. The δ ( 17 O) values of carbonyl and the single‐bonded oxygens for para derivatives gave good correlation with the σ + constants. The δ ( 17 O) values for meta derivatives correlated well with the σ m constants. The influence of ortho substituents on the δ ( 17 O) values of carbonyl oxygen and the single‐bonded oxygens was analyzed using the Charton equation containing the inductive, σ I , resonance, σ + R , and steric, E s B , substituent constants. For ortho derivatives, excellent correlations with the Charton equation were obtained when the data treatment was performed separately for derivatives containing electron‐donating + R and electron‐attracting − R substituents. The electron‐donating substituents in ortho ‐, meta ‐, and para ‐substituted esters resulted in shielding of the 17 O signal and the electron‐withdrawing groups caused deshielding. In phenyl ortho ‐substituted benzoates, the substituent‐induced positive inductive ( ρ I > 0), resonance ( ρ R > 0), and steric ( δ ortho E s B > 0) effects were found. The steric interaction of ortho substituents with ester group was found to produce a deshielding effect on the carbonyl and single‐bonded oxygens. For ortho derivatives with − R substituents, the resonance term was insignificant and the steric term was ca . twice weaker as compared to that for derivatives with + R substituents. The δ ( 17 O) values for ortho ‐substituted nitrobenzenes, acetophenones, and benzoyl chlorides showed a good correlation with the Charton equation as well. In ortho ‐substituted nitrobenzenes the inductive, resonance and steric effect were found to be ca . 1.7 times stronger as compared to that for phenyl ortho ‐substituted benzoates. Copyright © 2010 John Wiley & Sons, Ltd.