Characterization of the Crystalline Nature of the Racemates of Novel Chiral Five-Membered Cyclic Nitroxides

ABSTRACT

Racemic samples of chiral five-membered cyclic nitroxides, 2,2,5,5-tetrasubstituted pyrrolidine-1-oxyls bearing one or two 4-hydroxyphenyl groups on the stereogenic centers [C(2) or C(2) & C(5)] adjacent to the NO radical moiety, exist as a racemic conglomerate in the crystalline state, whereas those having one or two 3-hydroxyphenyl groups at the same positions belong to a racemic compound. The key intermolecular interactions in controlling whether these racemates crystallize in a homochiral way or a heterochiral manner have been investigated by X-ray crystallographic analysis and magnetic susceptibility measurements.

Keywords:

• chiral nitroxide radical
• electron paramagnetic resonance spectroscopy
• magnetic susceptibility
• racemic compound
• racemic conglomerate
• X-ray crystallographic analysis

Notes

^a Measured by DSC. All samples were decomposed at the indicated temperature.

^b Determined by HPLC analysis using a chiral stationary phase column (Daicel Chiralcel OD-H, 0.4 × 25 cm) and a mixture of hexane and 2-propanol (9:1) as the mobile phase.

^c Measured in THF. c 1.00.

^d Measured in THF at 25°C.

^e Curie constants.

^f Weiss temperature.

^g Data quoted from Ref. 1.