Stereochemistry of phenyl alpha-nitronyl nitroxide radicals

An extensive investigation of the conformations adopted by the family of ph enyl alpha-nitronyl nitroxides has been carried out. A database containing 110 crystal structures was used in a statistical study of the solid-state g eometries and conformations of these radicals. This study revealed that the favoured conformations involve a twisted distortion in the imidazolyl ring s and a twist between the aromatic and heterocyctic rings in the molecules. As a consequence, these radicals show two types of preferred conformations in the solid state: the pseudo-anti enantiomeric pair and the pseudo-eclip sed pair, the latter type being the most statistically probable. A new chir al member of this group of radicals that bears a lactate moiety, (R)-1, and its corresponding racemic compound, (R,S)-1, have been prepared in order t o study the influence of chiral induction from the stereogenic centre on th e torsion angle between the aromatic and heterocyclic rings of the alpha-ni tronyl nitroxides. The X-ray crystal structures of the enantiopure and race -mic compounds, which both reveal chains of molecules sustained by strong O -H ... O hydrogen bonds between the carboxylic acid group and the ON group of the radical in the solid, as well as their magnetic properties have been determined. Remarkably, the molecules with a given stereogenic centre have a single helical sense between their component rings, even in the racemic crystal. Chiral induction from the stereogenic centre to the radical unit h as also been proved by CD spectroscopy in the solid state. The results of t hese experiments have been rationalised by ab initio calculations of the sp ectra.