NMR and semiempirical conformational analysis of the 2-aryl-1,3-dihydroxy-4,4,5,5-tetramethylimidazolidines

Article abstract of DOI:10.1016/S0040-4020(97)10145-4

2-Aryl-1,3-dihydroxy-4,4,5,5-tetramethylimidazolidines (1) were synthesized and their ¹H and ¹³C NMR spectra recorded. Quantum mechanical semiempirical calculations were also performed for a better understanding of the signals recorded in the NMR spectra of imidazolidines. The conformation of the imidazolidine ring was initially studied for the 2-methyl-1,3-dihydroxy-4,4,5,5-tetramethylimidazolidine (2), used as a model molecule. The results of the calculations obtained for structure 2 showed that the methyl groups are located in axial and equatorial positions. At these positions, the methyl groups are affected by the magnetic anisotropic effects of carbon-nitrogen and carbon-carbon bonds of the imidazolinyl ring with different intensities. Semiempirical calculations for structure 2, suggested that the effect of the γ-oxygen on the carbon atoms of methyl groups (γ-effect) might lead to an alteration of the electronic charge density and consequently to a change in the diamagnetic shielding on these groups. These data were used for the analysis of the NMR spectra of compound 1. The diamagnetic shielding effects, estimated from the calculated electronic charge densities on the carbon atoms of methyl groups, are in agreement with the signals observed in the NMR spectra of compound 1. By combining the contribution of the anisotropic and γ-effects, it appears that the axial methyl groups are located relatively closer to the γ-oxygens in compound 1.