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

摘要

~(17)O NMR spectra for 29 phenyl esters of ortho-, para,- and meta-substituted benzoic acids, X-C_6H_4CO_2C _6H_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 γ~+ constants. The δ(~(17)O) values for meta derivatives correlated well with the γ_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, γ_I, resonance, γ~+R, and steric, EsB, 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 (p_I>0), resonance (p_R>0), and steric (δ_(ortho)EsB>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.