Recognition of chirality in nitroxides using EPR and ENDOR spectroscopy

The question regarding chirality in nitroxides was rigorously investigated, namely, (a) are multiple EPR and ENDOR spectra expected from nitroxides with chiral centers, (b) which structures provide multiple structures and (c) under what conditions (temperature and solvent) are they obtained. It is shown that nitroxides with one chiral center produce only one EPR/ENDOR spectrum in toluene or ethanol (293–193 K); the nitroxyl function is not a chiral center under these conditions. Nitroxides with two chiral centers (α- and β-position) give two EPR/ENDOR spectra consistent with a pair of diastereomeric configurations (see series II), but this observation was not apparent in every case (see series III). This anomaly was attributed to a stereoselective production of certain nitroxides which apparently do not produce diastereomeric mixtures in alcohols as solvent. Further examples of multiple EPR/ENDOR spectra were found when the position of chirality is in the α- and β-position to a remote spin center such as the nitroxide function (see series IV and V). Finally, EPR/ENDOR spectra were obtained from nitroxide derivatives wherein the absolute configuration at the α-carbon atom was known. Distinctly different EPR/ENDOR spectra were indeed obtained for every diastereomer, thus establishing this point unequivocally.