Collect. Czech. Chem. Commun. 2011, 76, 1737-1763
Published online 2012-01-13 10:45:23

17O NMR study of ortho and alkyl substituent effects in substituted phenyl and alkyl esters of benzoic acids

Vilve Nummert, Vahur Mäemets, Mare Piirsalu, Signe Vahur and Ilmar A. Koppel*

Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia


17O NMR spectra for 44 ortho-, meta- and para-substituted phenyl and alkyl benzoates (C6H5CO2C6H4-X, C6H5CO2R) at natural abundance in acetonitrile were recorded. Substituent effects on the 17O NMR chemical shifts, δ(17O), of the carbonyl oxygen and the single-bonded phenoxy (OPh) and alkoxy (OR) oxygens have been studied. The δ(17O) values of the carbonyl oxygen for para derivatives showed a good correlation with the σ° constants. The δ(17O) values of carbonyl oxygen for ortho derivatives were found to be described well with the Charton equation containing the inductive, σI, resonance, σ°R, and steric, EsB, substituent constants in case the data treatment was performed separately for electron-donating +R and electron-attracting –R substituents. The electron-donating +R ortho and para substituents in substituted benzoates caused shielding and the electron-withdrawing –R substituents produced deshielding of the O signal. The steric interaction of ortho substituents with the ester group decreased the electron density at the carbonyl oxygen. In alkyl benzoates the δ(17O) values were found to be described well with the inductive, σI, and the steric, EsB, substituent constants.

Keywords: NMR spectroscopy; Substituent effects; Structure–activity relationships; 17O NMR spectra; ortho Effect; Phenyl benzoates; Alkyl benzoates.

References: 89 live references.