Influence of ortho substituents on 17O NMR chemical shifts in phenyl esters of substituted benzoic acids

Abstract

Abstract17O NMR spectra for 29 phenyl esters of ortho‐, para,‐ and meta‐substituted benzoic acids, X‐C6H4CO2C6H5, at natural abundance in acetonitrile were recorded. The δ(17O) values of carbonyl and the single‐bonded oxygens for para derivatives gave good correlation with the σ+ constants. The δ(17O) values for meta derivatives correlated well with the σm constants. The influence of ortho substituents on the δ(17O) values of carbonyl oxygen and the single‐bonded oxygens was analyzed using the Charton equation containing the inductive, σI, resonance, σ+R, and steric, E, 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 17O signal and the electron‐withdrawing groups caused deshielding. In phenyl ortho‐substituted benzoates, the substituent‐induced positive inductive (ρI > 0), resonance (ρR > 0), and steric (δorthoE > 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(17O) 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.