Study of the Effect of N-Protonation and N-Methylation on the 1H and 13C Chemical Shifts of the Six-Membered Ring in Benzazoles and 2-Substituted N,N-Dimethylamino Derivatives

Article abstract of DOI:10.1002/mrc.1260240505

The 1H and 13C NMR spectral data of benzimidazole, benzoxazole, benzothiazole and N-methylbenzimidazole and of their 2-N,N-dimethylamino derivatives are reported.The spectra were recorded in acetone-water solution.The chemical shifts of these molecules are compared with those of the corresponding cations obtained by N-protonation and N-methylation of the neutral molecules.The carbon atoms C-4, C-5, C-6 and C-4a show a qualitatively identical behaviour, moving to higher field when the cations are formed from the benzazoles examined, but a different behaviour is found for C-2, C-7 and C-7a, which depends on the heterocyclic ring, on the presence of the substituent in position 2 and on the type of cation formed (N-protonation and N-methylation cause different effects).The conversion of benzazoles into the corresponding cations causes all 1H chemical shifts to move to lower field.KEY WORDS: Heterocycles, Benzazoles, Amino Derivatives, Cation formation, 1H and 13C NMR