Imidazolidine nitroxides

Nitrone cycloaddition reactions with alkynes have been widely used for the synthesis of imidazolidine nitroxides (736) and (737), containing chelating enam-ino ketone groups (821). Different heterocyclic systems were obtained, such as 3-(2-oxygenated alkyl)piperazin-2-ones (738) (822), also compounds containing the isoxazolo[3,2-i]indole ring system (739) (823) and a new class of ene-hydroxylamino ketones- (l )-2-( 1-hydroxy-4,4,5,5-tetraalkylimidazolidin-2-ylidene)ethanones (740) (824) (Fig. 2.46). 

Various a-fluorinated imidazolidine nitroxides have been obtained126 127 from 3-imida-zoline-3-oxide, and other five-membered N-oxide derivatives by reactions with xenon difluoride in CH2C12 (Scheme 53). 

Loss of the EPR signal during Fe(CN)g (hcf) oxidation of imidazolidine nitroxides having a hydrogen at position 4 of the heterocycle (HIMD) supports the assumption that the HIMD fragmentation is facilitated by proton abstraction from the position 4 of the oxoammonium cation formed as a result of the initial one-electron HIMD oxidation. ... 

Recently Keana and co-workers (1978) suggested a new method of synthesizing imidazolidine nitroxides. According to this method, condensation of 2,3-diamino-2,3-dimethylbutane with a ketone in the presence p-toluenesulfonic acid leads to imidazolidine 16. Oxidation of the latter with m-chloroperbenzoic acid gives the corresponding nitroxide 17, with a... 

Hence, polarization of the C=N bond of imidazolinium nitroxyl salts provides easy and unique addition of nucleophilic agents (i.e., cyanide ion, Grignard reagent, and hydride ion) with the formation of imidazolidine nitroxides, whereas the radical center participates when, in reactions of this type, as in the case of 3-imidazoline-3-oxides, the functional group becomes transformed and the heterocycle is opened. 

Morat and Rassat (1972), using N-labeled nitroxides, showed that in the IR spectra of piperidine nitroxides the band of stretching vibrations of the nitroxide grouping (v. q) lies in the region of 1373 7 cm The Vn- o band in di-tert-butyl nitroxide is manifest at 1345 cm" (Hoffmann and Henderson, 1961). Other nitroxides show absorption in the region of 1310-1370 cm (Forrester et al, 1968). In contrast to these data, in imidazoline and imidazolidine nitroxides the Vn- q is found at 1440 4 cm and in fN-labeled nitroxides it is shifted 26 cm down-... 

Recently, it was shown that the chemical straaure of the initial alkoxyamine had a strong effect on the outcome of the polymerization. Using imidazolidin-N-oxyl nitroxide-based alkoxyamines, it was observed that the 1-phenylethyl alkyl moiety led to a successful living and controlled polymerization of styrene, whereas the p-nitrophenyloxycarbonyl-2-prop-2-yl... 

It should be noted that self-condensation of aminonitriles to 4-oxo-imidazolidines is accompanied by the formation of sterically hindered piperazine derivatives, the starting compounds for nitroxides of the piperazine series (Yoshioka et al, l972b). Stable 4-oxoimidazolidine nitroxides, as well as piperidine derivatives, have been patented as heat and light stabilizers, inhibitors, etc. (see Murayama et al, 1976). 

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