Mechanistic Studies by Electron Paramagnetic Resonance Spectroscopy on the Formation of 2-(N-Chloroimino)-5,5-dimethylpyrrolidine-1-oxyl Radical from 5,5-Dimethyl-1-pyrroline 1-Oxide and Hypochlorite-Treated Ammonia

Article abstract of DOI:10.1021/jo00086a013

Mixtures of ammonium ion (NH4(1+)) or ammonia (NH3), hypochlorous acid (HOCl), and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) gave the radical 2-(N-chloroimino)-5,5-dimethylpyrrolidine-1-oxyl (1).The first step in the formation of 1 was the reaction of HOCl and NH4(1+) to provide ammonia chloramines: monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3).Chloramine composition and the formation of 1 were dependent on pH, the ratio of NH4(1+) to HOCl, and, at acidic pH, on the concentration of chloride in the medium.Conditions were chosen to isolate the individual chloramines in solution for further study.NCl3 and DMPO gave 1; however, NHCl2 and NH2Cl required additional oxidants such as HOCl or PbO2 to produce 1.Studies with 15N-labeled chloramines confirmed that NHCl2 and NH2Cl reacted with DMPO to form N-chloro intermediates that yielded 1 upon subsequent oxidation.Light had no effect on the formation of 1, and UV irradiation did not enhance the EPR signal intensity but caused rapid decay, indicating that radical intermediates of ammonia chloramines were not involved.The mechanism of formation of 1 appeared to involve temporary attachment of chloramine Cl to the nitroxide oxygen of DMPO which activated its β-carbon for nucleophilic addition of the chloramine N.Subsequent N-chlorination and/or dehydrochlorination, depending on the reactive chloramine, would then provide 1.However, nucleophilic addition of H2O to the activated β-carbon of DMPO was competitive because 5,5-dimethyl-2-hydroxypyrrolidine-1-oxyl (DMPO-OH) or 5,5-dimethyl-2-pyrrolidone-1-oxyl (DMPOX) radicals were sometimes observed as minor products along with 1.Analogous chloroimine radicals were not obtained from the reaction of ammonia chloramines with 3,3,4,4-tetramethyl-1-pyrroline 1-oxide (M4PO) and N-tert-butyl-α-phenylnitrone (PBN), although their 2-oxo nitroxyl derivatives and hydroxyl adducts were formed as radical products suggesting that nucleophilic addition of H2O was dominant with these nitroxides.