17O vs 13C NMR chemical shifts as effective indicators of π-electron-density distribution in hindered methyl benzoates

17O NMR Substituent Chemical Shift (SCS) values on the carbonyl and methoxy oxygens in methyl 2,6-dimethyl-4-X-benzoates 2’ have been measured in acetonitrile-d and compared with those of the 2,6-unsubstituted methyl 4-X-benzoates 2, the former exhibiting very little sensitivity to the effect of 4-substituents and not appearing to be related to π-electron density changes in such sterically hindered systems. Conversely 13C NMR SCS values of the methoxy carbon of the ester function in either 2 and 2’, although modest in size, exhibit a very good dual substituent parameter (DSP) correlation with the polar and resonance effect of 4-substituents and are therefore effective indicators of local π electron densities. Moreover, 13C NMR SCS values of the methoxy carbon of 2’ exhibit an excellent linear correlation with those of 2,6-unsubstituted benzoates 2. Once more this agrees with our previous conclusion that the resonance interaction between the aromatic ring and the ester group is essentially related to a π-polarization effect, not involving charge-transfer between the two moieties and therefore not sensitive to steric hindrance to their coplanarity.