EPR and ENDOR study of chiral nitroxides

Publisher Website
Google Scholar
Abstract
Racemic and achiral alkyl aryl nitroxides were examined at various temperatures. The restricted rotation of the aminoxyl substituents was indicated both by the β‐proton coupling constants and by the inequivalence of the aromatic ortho protons. These results are interpreted by the presence of two different rotamers which may lead to two splitting constants for the β‐proton. The capability for inter‐ and intra‐molecular hydrogen bond formation in the nitroxides investigated was demonstrated. However, complexes consisting of chiral radicals and chiral auxiliaries do not show hyperfine structures which unambiguously indicate diastereomeric species. Therefore, hydrogen bonding between chiral molecules is not sufficient for chiral recognition if an alteration of the hyperconjugation angle does not occur. Simple steric interactions can result in the doubling of the β‐proton coupling constant, independently of diastereomeric complex formation.