Conformational Studies by Dynamic NMR. 84. Structure, Conformation, and Stereodynamics of the Atropisomers of N-Aryl-tetrahydropyrimidines

The existence of stereolabile atropisomers for a number of N-aryl-tetrahydropyrimidines in solution has been deduced from the observation of the anisochronous NMR signals of prochiral methylene groups. The interconversion barriers for these atropisomers have been measured by line shape analysis of dynamic NMR spectra at various temperatures: a Molecular Mechanics modeling resulted in good agreement with these values. In an appropriate case, distinct NMR signals for the two enantiomeric forms could be observed at ambient temperature in a chiral environment. Evidence was also obtained for an exchange process occurring between two conformers experiencing a very biased equilibrium. Single-crystal X-ray diffraction of one such compound yielded a molecular structure in good agreement with the results obtained by ab initio calculations.