3-Imidazolines, 1-hydroxy-, radicals

In electrochemical oxidation of l-hydroxy-3-imidazoline-3-oxides containing one to four H atoms at a-C, one observes in ESR-spectra not only triplet splitting of the nucleus 14N of the nitroxyl group (a v 15-16 G) but also splitting of the neighboring protons (a// 18-20 G), with multiplets corresponding to their number (from doublet to quintet) (101). Unlike spatially hindered hydroxylamines which show reversibility in electrochemical oxidation, hydroxylamines with H at a-C are oxidized irreversibly. Oxidation of hydroxylamines with nitroxyl radical proceeds easily and with quantitative yields (102). In the oxidation of asymmetric polylluorinated hydroxylamines with Mn02, isomeric polyfluorinated nitrones have been obtained (103). 

On studying electrochemical properties of l-hydroxy-3-imidazoline-3-oxides and their conversion into nitronyl nitroxyl radicals (NNR), the intermediate production of a nonaromatic radical cation (220) of AH -imidazole-1,3- dioxides (219) (Scheme 2.78) (101) was suggested. 

Oxidation gives stable radicals (442) which form cycloadducts with dipolarophiles (e.g. 443) (85TL4801). Radicals formed from 3-imidazoline 1-oxides or l-hydroxy-3-imidazolines can be formylated on nitrogen under Vilsmeier conditions and exocyclically brominated or chlorinated by NBS or NCS on a 4-methyl group. 

The formation of l-hydroxy-3-imidazolines proceeds under mild weakly alkaline or weakly acidic conditions, in contrast to the strongly acidic media usually employed for obtaining precursors of doxyl radicals (Keana, 1978). l-Hydroxy-3-imidazolines are easily oxidized to nitroxides by molecular oxygen, Pb02, Mn02, etc. 

Oxidation of 1-hydroxy derivatives (6a-g) with lead dioxide in nonpolar organic solvents leads smoothly to nitroxides, namely 2,2,5,5-tetra-alkyl-3-imidazoline-3-oxide-l-oxyls (7a-g) (Volodarsky and Kutikova, 1971). In the crystalline state radicals 7 can be stored for a long time without decomposition. Both radical precursors 6 and the radicals themselves enter into various substitution, condensation, and other reactions. 

For generating 3-imidazoline derivatives containing no N-oxide oxygen, 1,2-hydroxylaminoketones were employed, which are formed in the acidic hydrolysis of 1,2-hydroxylaminooximes (Volodarsky and Sevastyanova, 1971), The interaction of 1,2-hydroxylaminoketones with ketones and ammonia under mild conditions (in some cases at 20°C) leads to the formation of l-hydroxy-4-aryl- and l-hydroxy-4-alkyl-2,2,5,5-tetraalkyl-3-imidazolines (12) (Sevastyanova and Volodarsky, 1972 Volodarsky and Sevastyanova, 1973). Compounds 12 are colorless crystalline substances that have lower melting points and are more readily soluble in organic solvents than 3-imidazoline-3-oxides 6. When stored or recrystallized in air, they become partially oxidized to radicals. Oxi-... 

Interaction of 3-imidazoline-3-oxide nitroxides with Grignard reagent proceeds with the involvement of the radical center and leads to reduction products 6a,c and methoxy derivatives 35a,c. Meanwhile, in the interaction of methylmagnesium iodide with the methoxy derivatives 1,3-addition to the nitrone group occurs, and 1-methoxy-3-hydroxy-4-alkyl (aryl)-... 

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