New 2,5‐dihydroimidazole‐1‐oxyls with functional side groups (N, O, S donors)

Abstract

A series of functionalized 2,s-dihydro-1-hydroxy-2-R1-2-RZ-sponding nitroxide radicals. The structures of 2',5'-dihydro-4-R3-imidazoles were prepared by cyclocondensation of a-l'-hydroxy-4',5',5'-trimethylspiro[4,5-diazafluoren-9,2'-imi0x0 hydroxylamines with ketones RIRZC=O in the presence dazole] (2) and of 2-(acetoxymethyl)-2,5-dihydro-2,4,5,5-teof ammonia (R' = R2 = 2-pyridy1, 4,5-diazafluoren-9-ylene, tramethylimidazole-1-oxyl (14) were determined by X-ray R3 = Me: R' = CH20COMe, CH20COPh, 2-thienyl, CH20H, diffraction. In the crystal 2 forms one-dimensional chains C02K, R2 = R3 = Me; R1 = Py, R2 = Me, R3 = Ph; R1 = R2 = through hydrogen bonding between N atoms of the CH20COPh, CHzOH, R3 = Me) and oxidized to the corre-hydroxylamine and imidazoline groups.Because of their stability nitroxide radicals have attracted much intere~t [~,~] and have found wide application as spin markers in biochemistry, medicine, and analytical chemist r~ [~,~] .Metal complexes of nitroxide~ [~] were studied, e.g., with the goal to obtain new molecular magnetic materiaW1.The group of Volodarsky developed the chemistry of nitroxides derived from 2,5-dihydroimida~ole[~~*~I. Our Munich group studied metal complexes of nitroxides already in 1967[1°1 following the history of organic nitroxides which was founded by Piloty and Graf Schwerinr"] and Wieland[121 in Munich. We were interested in the synthesis of 2,5-dihydroimidazole-l -oxyl radicals which contain additional donor atoms (N, 0, s) for coordination of metal ions.There are two pathways for the synthesis of donor-substituted 2,5-dihydroimidazole-1 -0xyls:-Cyclocondensation of a-0x0 hydroxylamines with functionalized ketones in the presence of ammonia to 1-hydroxy-2,5-dihydroimidazoles and subsequent oxidation to nitroxides[I31.-Derivatization of nitroxides. Thus, Reznikov et al.[I4] synthesized 4-substituted 2,5-dihydroimidazole-l-oxyls by deprotonation of the 4-methyl group of 4-methyl-2,5-dihydroimidazole nitroxides and addition of electrophiles. Results and DiscussionThe 2,5-dihydro-l-hydroxyimidazoles 1-7 were synthesized according to equation (1). NH40Ac was used as base and source for ammonia. It was observed that the rate of reaction and the yield increase in the case of ketones having electron-poor carbonyl groups. Due to problems of purification the yields of 4-6 are low. Compounds 8-10 were obtained by hydrolysis of the ester group of 4-6 and of ethyl 2,5-dihydro-1 -hydroxy-2,4,5,5-tetramethylimidazole-

New spirocyclic nitroxides of 2,5-dihydroimidazole series flanked by two mesogenic fragments

“…Thus, we decided to prepare the target radicals using 1,4-bifunctional derivative of cyclohexane as the starting compound. An approach to the synthesis of functionally substituted spirocyclic radicals of 2,5-dihydroimidazole series includes condensation of the corresponding 2-hydroxylamino ketones with cycloalkanones in the presence of ammonia followed by oxidation of intermediate 1-hydroxy-2,5-dihydroimidazoles [32][33][34][35] . Produced in such manner 3-imidazoline nitroxide radicals would be relatively inert toward many common chemical agents 36 and could be modified on available functional groups without destruction of radical center.…”

Synthesis, Structure, and Magnetic Properties of Transition Metal Complexes of the Nitroxide 2,5-Dihydro-4,5,5-trimethyl-2,2-bis(2-pyridyl)imidazole-1-oxyl

“…Starting Materials. The 2,5-dihydro-4,5,5-trimethyl-2,2-bis(2-pyridyl)imidazole-1-oxyl-radical L was prepared as published . All other reagents and solvents were purchased from commercial sources and used as supplied.…”

“…Recently we described the preparation of the radical 2,5-dihydro-4,5,5-trimethyl-2,2-bis(2-pyridyl)imidazole-1-oxyl ( L ),11a and a disilver complex of this ligand was structurally characterized 11b. We herein report the synthesis, the structures, and the magnetic properties a series of transition metal complexes with this nitroxide.…”

Spirocyclic Nitroxides as Versatile Tools in Modern Natural Sciences: From Synthesis to Applications. Part I. Old and New Synthetic Approaches to Spirocyclic Nitroxyl Radicals

“…The use of a cyclic ketone as a substrate in this procedure generates an SNR of the 3-imidazoline series. For instance, condensation of HAKs 246 with cyclopentanone, cyclohexanone, or heterocyclic ketones in the presence of excess NH 4 OAc in methanol yields 1-hydroxy derivatives 247a–g, whose subsequent oxidation with manganese dioxide (or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in the case of compound 247g ) leads to agents of radical polymerization and paramagnetic ligands, the corresponding SNRs 248a–d [ 155 ] , e–f [ 154 ] , and g [ 156 ] ( Scheme 43 ).…”