¹H and ¹³C-NMR Spectroscopic Study of Some 1H-4,5-Dihydroimidazolium Salts

Abstract

The ¹H y ¹³C-NMR spectra of some 1,3 and 1,2,3-trisubstituted 1H-4,5-dihydroimidazolium salts are analyzed.

Introduction

1H-4,5-Dihydroimidazolium salts are typical cyclic amidinium compounds where the cation is resonance stabilized and the positive charge can be delocalized either on the nitrogen atoms or on the C₂:

NMR spectra analysis and its comparison with the corresponding saturated compounds (imidazoli-dines 2), allows to reach conclusions about the contribution of such structures.

Experimental

¹H and ¹³C NMR spectra were recorded on a Bruker MSL-300 spectrometer using deuterochloro-form as the solvent.

Results and Discussion

The ¹H and ¹³C-NMR spectroscopic study of a series of 1,3-di and 1,2,3-trisubstituted 1H-4,5-dihydroimidazoliom salts 1 (Table) is presented.

Table 1.  

R1	R2	R3	X-
C6H5	H	C6H5	Cl-
p-CH3C6H4	H	p-CH3C6H4	Cl-
p-Cl-C6H4	H	CH2-C6H5	Cl-
C6H5	C6H5	CH3	I-
p-CH3C6H4	C6H5	CH3	I-
p-CH3OC6H4	C6H5	CH3	I-
p-NO2C6H4	C6H5	CH3	I-

Table 1.  

R1	R2	R3	X-
C6H5	H	C6H5	Cl-
p-CH3C6H4	H	p-CH3C6H4	Cl-
p-Cl-C6H4	H	CH2-C6H5	Cl-
C6H5	C6H5	CH3	I-
p-CH3C6H4	C6H5	CH3	I-
p-CH3OC6H4	C6H5	CH3	I-
p-NO2C6H4	C6H5	CH3	I-

In order to assign the heterocyclic hydrogens and carbons in the 1,2-diaryl-3-methyl substituted compounds, the spectroscopic study of the parent 1H-4,5-dihydroimidazoles 3 and their salts 4 had been carried out.

The unequivocal assignment of the hydrogen and carbon signals of the 1,2,3-trisubstituted salts has been done by the HMQC and HMBC spectra.

The important electronic deficit at the level of the heterocyclic ring in compounds 1 has been clearly demonstrated by comparison of the spectroscopic features of the salts 1 with the corresponding imidazolidines 2. The iminium structure contribution (A,C) was analyzed according to the chemical shifts and the heteronuclear ¹J¹³C-H coupling constants of the heterocyclic ring carbons and N-CH₃.