Nitroxides as reaction intermediates

Abstract

Vinyl nitroxides 4 are obtained by oxidation of the nitrones 3, as was shown by esr studies and by identification of the reaction products. The formation of 4d–f is even observed in oxidation of the hydroxylamines 1d–f, nitroxides 2d–f and nitrones 3d–f being the intermediates. The high reactivity of the vinyl nitroxides 4 at their β-position is illustrated by the reactions of 4a with various compounds affording the nitroxides 7–10, respectively. Compound 4c reacts with its precursor 3c to give 11, 12, or 13, depending on the reaction conditions. From oxidation of 3a, c, and e the dimerization products 5a, c, and e, respectively, could be isolated. Whereas further oxidation of 5d yields 6d, the acyl nitroxides 14a and c are formed in the oxidation of 5a and c, respectively.The formation of quinone 23 in the reaction of 2-methyl-2-nitrosopropane with potassium tert-butoxide in isopropyl alcohol in the presence of oxygen is discussed. The nitroxide 20 has been detected in the reaction mixture. Imines 24 react with nitrosobenzene giving nitroxides 26. These are further oxidized by nitrosobenzene to afford nitrones 27. Whereas 27a and b could be isolated, 27c and d undergo further reaction yielding the diimines 30c and d along with dinitrone 29.The formation and reactions of imino nitroxides 31 and of the nitronyl nitroxide 41 are discussed. Electron spin resonance studies revealed the high reactivity of the imidazolyl-1,3-dioxides 46 and the imidazolyl-1-oxides 50, which easily form radicals 47–49 and 51, respectively, which are derived from secondary reactions.

Oxidation of quinolones with peracids (an in situ EPR study)

“…[24,25] However, no contribution of any further nuclei to the hyperfine structure is evident. At the higher oxidant concentrations (CPBA : Q x up to 5), further spectra arise with a well-defined hyperfine structure as described later.…”

C−H Reactivity of the α‐Position in Nitrones and Nitronates

“…Despite the addition to nitrones, N ‐vinylnitroxyl radicals (such as 74a ) react with dienes, nitroso compounds and nitrile oxides, leading to the products 94 – 96 of formal addition to the C=C bond (Scheme ), as demonstrated by ESR spectroscopic studies …”

“…Due to the presence of spin density on the carbon atom, vinylnitroxyl radicals are much more reactive than classic non‐conjugated nitroxyls . Radicals 5 have short lifetimes; however, their lifetimes can still be long enough to observe these species by ESR spectroscopy.…”

“…Upon oxidation of nitrones with lead dioxide or nickel peroxide in dilute solutions, the corresponding N ‐vinyl nitroxyl radicals 74 are generated (Scheme ) . The lifetime of radicals 74a , b , e is long enough to observe them by ESR spectroscopy.…”

“…Radicals 74a , c and 74e easily dimerize through their C‐ends to afford bis‐nitrones 75a , c , e , respectively, in good yields . Adduct 75d could not be isolated due to its fast oxidation to ene‐nitrone 76d (Scheme ), while for the sterically hindered radical 74b , the formation of the corresponding dimer was not observed.…”