A New Synthesis of α-Asymmetric Nitroxide Radicals

Article abstract of DOI:10.1021/jo00126a035

A new methodology for the synthesis of α-asymmetric bicyclic nitroxides is described.Reduction of certain homoallylic nitroenone with 3.0 equiv of SmI2 in THF or THF-Et2O at -50 deg C gave a stable nitroxide radical-enolate intermediate, a kind of radical-anion species.The subsequent addition of electrophiles such as acyl chlorides at -50 deg C resulted in the O-acylation of the enolate moiety to form the corresponding neutral nitroxide radical.The reduction process was quite sensitive to the substrate structure, amount of SmI2, solvent composition, and reaction temperature.The optimum reaction conditions for this two-step reaction were determined.Analytically pure stable nitroxide radicals were isolated in good to fair yields when benzoyl chlorides substituted by electron-withdrawing groups were used as the electrophile for O-acylation.Although the use of organosilyl chlorides and alkylating reagents as the electrophile seemed to provide the corresponding radical products in the reaction mixture, pure radical products were not obtained.The mechanism of formation of the nitroxide radical-enolate intermediate was investigated by EPR spectroscopy and the product analysis.