Radicals carbazole

Chakrabarti and Chakraborty suggested that the carbazole radical leaks from the solvent cage and abstracts hydrogen from other molecules to form (1). Ghosh et al (49) on the other hand have shown that N-benzoyl carbazole (175) undergoes photo-Fries rearrangement only in the presence of iodine in methanolic solution through a separate mechanism. 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

The oxygen transfer to the -acetylenic carbon results in the very intense benzoyl cation, whereas the transfer to the a-carbon, via a series of fragments corresponding to the loss of OH , CO and CO2, respectively, leads to annelated heterocycles such as the radical cation of carbazole, as a result of elimination of CO81. The generation of the benzoyl cation was rationalized as shown in Scheme 1281. 

A4 (251), B2 (252), B3 (253), B4 (254), and B5 (255) from Streptomyces cyaneus 2007-SVi. The antiostatins represent the first carbazole derivatives with an acetamido group or a substituted urea chain, and exhibited strong inhibitory activity against free radical-induced lipid peroxidation (228) (Scheme 2.62). 

Intramolecular cyclization of diphenylamines to carbazoles is one of the most versatile and practical methods. This has been achieved photochemically, thermally in the presence of elemental iodine at 350°C, or with platinum at 450-540°C, via free radicals with benzoyl peroxide in chloroform, or by using activated metals such as Raney nickel or palladium on charcoal. Most of these methods suffer from low to moderate yields, and, in some cases, harsh reaction conditions (8,480). 

Aza[60]fulleronium ion 28 has been isolated and characterized by X-ray crystallography via the oxidation of 2 with the radical cation hexabromo(phenyl)carbazole (HBPC ) in dry ODCB [27]. The counter-ion is the silver(I) complex carborane ion Ag(CBjjHgCl5)2]. The salt was crystallized as dark green crystals by diffusion of hexane vapor. The solid is reasonably air stable. has almost the same structure... 

The formation of bicarbazoles can also be achieved electrolytically in acetonitrile solution once again, at the anode, the carbazole cation radical is produced and dimerizes. The further oxidation of 3,3 -bicar-bazole under these conditions with loss of two electrons and formation of a dication-diradical was demonstrated. Anodic oxidative coupling occurs at nitrogen best in the presence of collidine, presumably via deprotonation. 

A species believed to be the monomer cation radical of 9-ethylcarbazole as a green solution in acetonitrile formed by oxidation of 9-ethylcarbazole with iodine-silver(I) perchlorate, was detected by ESR spectroscopy, although the perchlorate of the cation radical could not be isolated subsequent treatment with potassium iodide gave 9,9 -diethyl-3,3 -bicarbazole. The borofluoride salts generated as crystalline materials by oxidation of carbazole or 9-methylcarbazole with tropylium borofluoride in acetonitrile followed by precipitation with methanol are not salts of the monomer cation-radicaP as originally believed. Russian workers have suggested that nitration of carbazole proceeds via a cation radical. ... 

Carbazole oxidized by nickel peroxide in the absence of light and in the presence of 2-methyl-2-nitrosopropane gave the radical 71, an observation taken as additional evidence for the intermediacy of radical cations, trapped in this case by the nitrosoalkane, in oxidative dimerization of carbazoles (see Section II,A,2). 

Substitution at carbazole nitrogen and the formation of 74, presumably via a radical cation, resulted from reaction with the cation radical salt 75 in acetonitrile. ... 

Nitrosocarbazole condenses with 9-aminocarbazole to give the azo compound 122, heating of which produces dimers in a ratio similar to that observed by oxidation of carbazole (see Section II,A,2) it was therefore believed to involve the carbazol-9-yl radical. 3-Nitro-9-nitrosocarbazole was shown to serve as a nitrosating agent for A-methylaniline it converted aziridine to ethene and nitrous oxide. ... 

Triethyltin hydride promotes the radical decomposition of 128 giving carbazole. The dimer 129 (R = H) decomposed in dry benzene at room temperature giving carbazole (10%) and 9-anilinocarbazole (20%) and other unidentified products. However 129 (R = Me) gave 130, which on warm-... 

The conversion of substituted diphenylamines and triphenylamines to carbazoles at platinum anodes in CH3CN-Et4NC104 takes place if the intermediate cation-radical is fairly stable. Thus the anodic oxidation of (V-ethylbis(p-fert-butylphenyl)amine (87) gave 3,6-di-ferf-butyl-Af-ethyl-carbazole (88) in 15% yield152 [Eq. (72)]. 

Electrochemical and spectroscopic techniques have been used to study the oxidation of carbazole and 71 of its derivatives.207,208 Positions 3, 6, and 9 of the carbazole nucleus are the most reactive sites, as expected from Hiickel theory. The products isolated are symmetric carbon-carbon (3,3 ) and nitrogen-nitrogen (9,9 ) dimers. Substitution of carbazoles in the 3-, 6-, and 9-positions prevents anodic dimerization at these positions the electrochemical formation of a stable radical-cation is possible.209 The electrochemical oxidation of iminobibenzyl and several related compounds have been investigated in CH3CN-Bu4NC104 and their electrochemistry was compared with that of related carbazoles.210... 

---