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 wi th chiral centers, (b) which structures provide multiple structures and (c) under what conditions (temperature and solvent) are they obtained. It is s hown that nitroxides with one chiral center produce only one EPR/ENDOR spec trum in toluene or ethanol (293-193 K); the nitroxyl function is not a chir al center under these conditions. Nitroxides with two chiral centers (alpha - and beta-position) give two EPR/ENDOR spectra consistent with a pair of d iastereomeric configurations (see series II), but this observation was not apparent in every case (see series III). This anomaly was attributed to a s tereoselective production of certain nitroxides which apparently do not pro duce diastereomeric mixtures in alcohols as solvent. Further examples of mu ltiple EPR/ENDOR spectra were found when the position of chirality is in th e alpha- and beta-position to a remote spin center such as the nitroxide fu nction (see series TV and V). Finally, EPR/ENDOR spectra were obtained from nitroxide derivatives wherein the absolute configuration at the alpha-carb on atom was known. Distinctly different EPR/ENDOR spectra were indeed obtai ned for every diastereomer, thus establishing this point unequivocally. Cop yright (C) 1999 John Wiley & Sons, Ltd.