Evaluation of the polar–inductive and mesomeric effects exerted on contiguous functionalities by N-oxidopyridinium groups

Abstract

The polar–inductive and mesomeric effects (and mixtures thereof) of N-oxidopyridinium-2-, -3-, and 4-yl functionalities have been evaluated in terms of the σ_IB, σ_R^–, and σ scales previously defined. Values were generated from the phenyl para-¹³C shifts of PhCH₂C₅H₄NO and PhNHC₅H₄NO, respectively. The parameters obtained successfully account for the phenyl para-¹³C shifts of the sodium salts of anilinopyridine 1-oxides (VII), prepared in turn by deprotonation of the corresponding precursor nitrogen acids by sodium methylsulphinylmethanide. Results show that the N-oxidopyridinium ring exerts considerably more electron-withdrawing power, by both polar–inductive and mesomeric mechanisms, than in the absence of the N-oxide function.