Enantiopure and Racemic Chiral Nitronyl Nitroxide Free Radicals: Synthesis and Characterization

A synthetic route to a series of homochiral (and achiral) nitronyl nitroxides derived from rac-(and meso)-3,4-dimethyl-3,4-dinitrohexane is described. The two forms of this precursor, meso and rac, were identified unambiguously and reduced to the corresponding meso- and rac-diamines. The rac-diamine was condensed with an enantiopure aldehyde specifically designed to give a diastereomeric mixture of imidazolidines easily separated by flash chromatography. Both enantiopure diamines were then obtained by acidic hydrolysis of these imidazolidines. The absolute configurations of the diamino precursors were determined by X-ray crystallography of single crystals of a manganese(II) complex [(R,R,R)-5E·Mn(hfac)2] of a nitroxide containing a third chiral center of known configuration. A series of homochiral nitronyl nitroxides was then prepared from the enantiopure diamino precursors by condensation with aldehydes, followed by oxidation. Their structural properties were compared to those of their achiral meso counterparts and it was found that the puckering of the five-membered ring is dependent on the chirality of the imidazolyl unit. The radicals show marked optical activity, as explored by polarimetry and circular dichroism spectroscopy. These paramagnetic building blocks of known absolute configuration include chiral centers adjacent to the oxyl groups, and since they exhibit C2 symmetry they are well suited for coordination chemistry studies because stereochemical complexity is minimized. They have been specifically designed for further developments of the metal-radical approach to molecular magnetic materials. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)