NMR Conformational Analysis and Theoretical Calculations for 2‐Aryl‐ 1,3‐dihydroxy‐4,4,5,5‐tetramethylimidazolidines

Abstract

Conformational studies of 1,3-dihydroxy-4,4,5,5-tetramethyl-2-(pyridin-1-yl)imidazolidine (1a) and 1,3dihydroxy-4,4,5,5-tetramethyl-2-(pyridin-3-yl)imidazolidine (1b), carried out by using 1D 1 H-and 13 C-NMR and 2D HMQC, HMBC, and NOESY experiments and with the aid of theoretical calculations, indicate that the OH groups are trans to the pyridinyl substituent. Because the two 1 H-NMR signals of the Me groups are distinguishable and do not change between 290 and 380 K, it is proposed that 1a and 1b have each only one conformation in this temperature range. This behavior was not found with 1,3-dihydroxy-4,4,5,5-tetramethyl-2-(pyridin-2-yl)imidazolidine (1c) because its Me 1 H-NMR signals cross over at 300 K. Hence, more than one conformation must be present, beyond those produced by simple inversions. Theoretical calculations including temperature and solvent effects were performed to provide further information on the conformational analysis and to help to assign the NMR data. The combination of NMR measurements and quantum-chemical calculations is shown to be a very promising strategy for conformational analysis studies in solution.

Conformational Analysis of 5,4′-Dihydroxy-7,5′,3′-trimethoxyisoflavone in Solution Using 1H NMR: A Density Functional Theory Approach

“…This approach certainly seems theoretically more justified than the frozen ϕ 1 torsion angle procedure, as long as the Gibbs fee energy barrier for structural interconversion in solution is not high at room temperature. It is opportune to mention that a somehow similar situation concerning imidazolidine compounds was reported some years ago where two methyl groups in axial and equatorial positions become indistinguishable at 300 K, producing a single NMR signal, implying a rapid axial–equatorial inversion of the Me groups, and a relatively low energy barrier. This experimental finding in DMSO- d 6 solution is closely related to the results describe here in chloroform solution for compound 9 .…”

Experimental and Theoretical Investigation of Epoxide Quebrachitol Derivatives Through Spectroscopic Analysis

“…The synthesis of azide derivatives from l -quebrachitol 1 can be cited as an example, where a combination of theoretical and experimental data led to the determination of the reaction mechanism . In conformational analysis, another important area, a combination of experimental spectroscopic data and theoretical ab initio results can lead to the determination of the conformational population in equilibrium at a given temperature, such as for example reported for tetramethylimidazolidine compounds . In many organic chemistry applications, substituted six-membered ring species are present, usually in condensed media, with spectroscopic methods being used routinely for experimental structural determination.…”