3-Imidazoline-3-oxide

Diaryl- (85), diaroyl- (71), bis(4-substituted-l,2,5-oxadiazol-3-yl)furoxans (104) as well as exotic l,2,2,5,5-pentamethyl-4-(nitromethyl)-3-imidazoline 3-oxide-derived furoxan 22 (105) were obtained via corresponding nitrile oxides. 

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). 

In the IR-spectra of 4-haloalkyl-3-imidazoline-3-oxides there are two bands Vc=n due to the two conformers A and B (Fig. 2.12) (367, 389). Conformation B corresponds to the form with the lower frequency band Vc=n (Avc=n 30 cm-1), where the halogen atom is located in the plain of the C=N+-0-group in anti -position to the double bond. Conformation A corresponds to the... 

From a comparative analysis of H NMR spectra of structurally similar pairs of nitroxyl radicals of 3-imidazoline and 3-imidazoline-3-oxide, it was concluded that the nitrone group contributes to a more efficient long-range spin density delocalization in the conjugated n -system of functional groups bonded with atom C-4 (404). 

A strictly dehned region of chemical shifts of C2, C4, and C5 atoms in A-oxides of 4A-imidazoles allows to dehne clearly the position of the A-oxide oxygen atom (102). Chemical shifts of the a-C nitrone group in a-N-, O-, and S-substituted nitrones are located in the region of 137 to 150 ppm (388, 413). On the basis of 13C NMR analysis of 3-imidazoline-3-oxide derivatives, the position of tautomeric equilibria in amino-, hydroxy-, and mercapto- nitrones has been estimated. It is shown that tautomeric equilibria in OH- and SH-derivatives are shifted toward the oxo and thioxo forms (approximately 95%), while amino derivatives remain as amino nitrones (413). In the compounds with an intracyclic amino group, an aminonitrone (A) - A-hydroxyaminoimino (B) tautomeric equilibrium was observed (Scheme 2.76), depending on both, the nature of the solvent and the character of the substituent in position 2 of the heterocycle (414). 

The result of a study of 15 N NMR spectra of cyclic nitrones of 3-imidazoline-3-oxide and of the corresponding nitroxyl radicals has been reported (422). 

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. 

Unlike the 4H- imidazoles (219), (223), (224) electrochemical oxidation of the nitrone group in 4-R-3-imidazoline-3-oxides (228), (230-232), as in a-PBN and DMPO is of irreversible nature. Therefore, the formation of radical cations... 

The method of optically detected EPR (OD EPR) has been successfully employed to detect short-lived radical anions of cyclic nitrones DMPO and l,2,2,5,5-pentamethyl-3-imidazoline-3-oxide (PMIO) (229, R = H), generated by... 

Photoisomerization of nonconjugated nitrones, in particular derivatives of 3-imidazoline-3-oxide and pyrroline-N -oxide, appears to be irreversible (Scheme 2.83) (10, 452). 

A detailed analysis of photoisomerization of the nitrone group in the nitroxyl radical 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-3-oxide-l-oxyl, based on double electron-nuclear resonance methods, has shown that in the absence of oxygen the photochemical reaction occurs without affecting the radical center. The... 

Reduction of nitrone groups to hydroxylamines occurs readily with sodium borohydride (201). In particular, the action of NaBHj on l-hydroxy-3-imidazoline-3-oxides (268) leads to 1,3-dihydroxyimidazolidines (269), which on subsequent treatment with hydroxylamine hydrochloride afford 1,2-bishydroxyl-amines (270) (Scheme 2.99) (478). 

Owing to the existence of two centers for nucleophilic attack (at C2 and C5) in radical cations (220) obtained from the oxidation of 4-H -imidazole-1,3-dioxides (219), the formation of two products of methoxy group addition was observed, namely NNR (221) and NR of 3-imidazoline-3-oxide (222). The ratio of the products depends on the electronic nature of substitutes R1 and R2. Both, the donor character of R1 and acceptor character of R2 facilitate the formation of nitroxyl radicals (222) with the yield of (221) increasing with the inverted effect of the substituents. As was mentioned in Section 2.4, the results of preparative electrochemical oxidative methoxylation of 4H -imidazole-1,3-dioxides are similar to the results of chemical oxidation. 

In addition to the oxidative alkoxylation of 4H -imidazole and oxazolidine derivatives, the reaction was also used with other cyclic aldo-nitrones such as DMPO, derivatives of 3-imidazoline-3-oxide (228-232) (506), and derivatives... 

Oxidative Fluorination of Nitrones to a-Fluorosubstituted Nitroxyl Radicals Formation of nitroxyl radicals by the radical cation route was observed in reactions of various nitrones with xenon difluoride in dry methylene chloride (520, 523). In this reaction, more than 40 nitrones, including 4H -imidazole N,N -dioxides (219), 4H -imidazole TV-oxides (223) and (224), 2H -imidazole N -oxides (225), 2H -imidazole TV,TV-dioxides (226), 3,3,5,5-tetramethylpyrroline N -oxide (TMPO), derivatives of 3-imidazoline-3-oxides (231) and (232), have been examined. ESR spectra of nitroxyl radicals containing one or two fluorine atoms at a-C have been registered (Scheme 2.108) (523). In the case of... 

It was found (616) that the course of the heterogeneous reaction of 1-hydroxy-5,5- dimethyl-2,4- diphenyl-3-imidazoline-3-oxide with PhLi depends on the crystalline phase of the starting compound, which can be obtained, predominantly, in cyclic or open chain tautomeric forms. 

The formation of derivatives of 2,3,6,8-tetraazabicyclo-[3.2.1]3-octene (425) arises from an intramolecular nucleophilic addition to the nitrone group of hydra-zone (424). Compound (424) was prepared by reaction of 2-acyl-3-imidazoline-3-oxides (423) with hydrazine. From the cis- and frans-derivatives (424), exo- and enr/o-isomers (425) were obtained (Scheme 2.197). The reaction of intramolecular cyclization does not occur in cases with monosubstituted hydrazones (316). 

Mitasov MM. Structural Inverstigations of 3-imidazoline-3-oxides, 3-imidazolines and Corresponding Nitroxides by the Methods of Vibration Spectroscopy, PhD Dissertation, Novosibirsk Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences, Novosibirsk, 151, 1984. 

When l-hydroxy-3-imidazoline 3-oxides (263) are treated with HCl they dehydrate forming imidazole 3-oxides (Scheme 153) (73CHE1175). Reduction of 4//-imidazole N-oxides with borohydride leads to either 1-hydroxy-imidazolines or -imidazolidines, depending on the position of the oxide function. Under the same conditions 4//-imidazole 1,3-dioxides give 1,3-dihydroxyimidazolidines (76CHE1280). The 4iT-imidazole AT-oxides are prepared by heating 5,5-disubstituted l-acyloxy-3-imidazoline 3-oxides in vacuo. 

Cyclohexanedione- and cycloheptanedione-l,2-dioximes react with ketones in acid medium to give 2f/-imidazole 1,3-dioxides a-aminooximes give 3-imidazoline 3-oxides under similar conditions (Scheme 50) (80CHE628). 

Lithiation of l,2,2,5,5-pentamethyl-3-imidazoline 3-oxide 92 leads to the formation of the dipole-stabilized organolithium 93 (Scheme 19). Compared with 92, both C-4 and C-5 resonate at higher frequency, whereas N-3 resonates at lower frequency (Table 7). The Li NMR resonance appears significantly lower in frequency relative to that of j -BuLi (0.34 ppm), indicating more covalent character of the C-Li bond in 93 <2001MRC681>. 

H(14)1313, 90JOC1772). Substituents also affect the equilibrium between l-hydroxy-3-imidazoline 3-oxides and the open-chain hydroxyiminonitrones electron-withdrawing groups in the... 

Ethanolic HCl aromatizes 4-aryl-3-imidazoline 3-oxides (128), and heating in vacuo or with 10% sodium hydroxide may also be effective (Equation (46)) <86S704>. 

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