1H NMR SPECTRAL CHARACTERISTICS OF PYRIDO[1,2-A]BENZIMIDAZOLE AND ITS DERIVATIVES

Abstract

A systematic study of proton NMR spectra of pyrido[1,2-a]benzimidazole and 65 of its mono-, di- and poly-substituted derivatives with substituents in the pyridine and/or benzene fragment was carried out. The choice of certain structures is related to the great practical significance, as well as the complexity of interpreting their 1H NMR spectra. As a result of the research, a number of general patterns of arrangement (characteristic ranges) on the scale of chemical shifts of signals of hydrogen atoms associated with the nucleus of the studied heterocycles have been established. In the 1H NMR spectrum of pyrido[1,2-a]benzimidazole and most of its derivatives, the H1 signal of the pyridine cycle associated with a carbon atom was released in the weakest field, which experienced a strong electron acceptor effect of the nodal endocyclic nitrogen atom. The signal of another heteroaromatic proton H2 was usually detected in the strongest field region of the spectrum. The H8 signal had the smallest chemical shift among the aromatic protons. As the analysis of 1D 1H NOE spectra of a series of pyrido[1,2-a]benzimidazoles showed, this position of the heterocycle was the reaction center for the introduction of an electrophilic particle under SEAr reaction conditions. The H9 signal was detected in a weaker field relative to other aromatic protons. The influence of the electronic nature of the substituent and its position in the condensed heterocycle on the distribution of the electron density in the molecule has been established. It is concluded that the transmission of electronic effects of substituents in pyrido[1,2-a]benzimidazoles differs from benzoid systems. A number of controversial proton signals were attributed by means of 1H-1H NOESY and 1H-15N HMBC spectroscopy. The conducted studies will allow to use the data of 1H NMR spectroscopy to determine potential reaction centers in pyrido[1,2-a]benzimidazoles and other similar condensed pyridine derivatives with a nodal nitrogen atom. For citation: Begunov R.S., Fakhrutdinov A.N., Sokolov A.A., Savina L.I., Bashkov N.E. 1H NMR spectral characteristics of pyrido[1,2-a]benzimidazole and its derivativese. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 5. P. 43-53. DOI: 10.6060/ivkkt.20246705.6971.