Carbazoles oxidative dimerization

Similarly, 1-hydroxy-3-methylcarbazole (23) and 2-hydroxy-3-methylcarbazole (52) could lead to bis(0-demethylmurrayafoline A) (204) and bis-2-hydroxy-3-methylcarbazole [l,l-bis(2-hydroxy-3-methylcarbazole)] (213), respectively, as shown in Scheme 3.9. Although bis(0-demethylmurrayafoline A) (204) is a non-natural carbazole derivative, the isolation of 2-hydroxy-3-methylcarbazole (52) and bis-2-hydroxy-3-methylcarbazole [l,l-bis(2-hydroxy-3-methylcarbazole)] (213) from M. komigii supports the oxidative dimerization of 2-hydroxy-3-methylcarbazole (52) (Scheme 3.9). 

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). 

The temperature dependence of the electrical resistance values of the polymer in the range of 30-125°C is such that it can be used as a thermal sensor." When solid carbazole crystals are immobilized on an electrode surface, oxidative dimerization and polymerization can also be achieved in solid state." By means of electrochemical nanolithography, conducting nanopatterns due to the selective oxidative crosslinking of PVK can be produced. 

Bringmann and Lin used tert-butylperoxide (DTBP) in the oxidative dimerization of carbazole (62) to synthesize 63, which is further oxidized with pyr-idinium cholorochromate (PCC) to give bismurrayaquinone A (Scheme 8.22). ... 

Scheme 8.22 Synthesis of bismurrayaquinone A using DTBP-induced oxidative dimerization of carbazole.

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. 

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. ... 

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... 

Hydrazones of thiochroman-4-ones are converted into thiochromans under Wolff-Kishner-Huang conditions65 and into the azo dimers by silver oxide.66 Fisher indolization of the phenylhydrazones (33) gives 6,ll-dihydrobenz[6]indolo[2,3-d]thiopyrans (34), which by hydride loss form the thiopyrylium salts (35), or on dehydrogenation produce the pseudoazulenes (36) (heterocyclic analogs of the carcinogen, benz[a]-carbazole), as shown in Scheme 6.87-73... 

Oxidative coupling has been observed for benzene (52), methyl substituted benzenes (53), triphenylethylene (54), triphenyl-amines (55-59), anilines (57), carbazoles (60,61), iminobibenzyls (62), and heterocyclic phenols (71,72). Intramolecular anodic coupling reactions are used for synthesizing specific ring structures (63-68). Both dimer and octamer of dibenzothiophene have been detected (69,70)... 

The formation of carbazole cation-radicals upon anodic oxidation of di-and triarylamines has been reported.474 The main requirement is that the amine used should form a persistent cation-radical (and hence not dimerize) so that it may be oxidized to the dication. It is this species which cyclizes to the carbazole. The carbazole formed is then oxidized to its cation-radical. This is the observed product if the reactive positions in the carbazole are blocked. In general, those substituents which confer persistence on the amine cation-radical are also those which confer it on the carbazole cation-radical. 

Change of solvent to benzene also yields these dimers which are accompanied by the adducts (78) and the furan (79). The adducts (78) are formed via a hydrogen abstraction pathway. The photochemical isomerization of the spirocyclopropane derivatives (80) affords the carbazole derivatives (81). This reaction involves ring-opening of a cyclopropyl bond followed by cyclization of the resultant biradical. An oxidative step must also be operative. 

Carbazoles are oxidized at controlled potential at a platinum anode in acetonitrile to cation radicals that are stable when the 3-, 6-, and 9-positions are blocked [223]. The radical cation from carbazole dimerizes predominantly at the 3-position to 3,3-dicarbazyl, which is further oxidizable to the quinoidal dication at the potential used. In the presence of pyridine, which may cause a rapid deprotonation of the N-H proton 9,9 -dicarbazyl is the isolated product. 

Further oxidation of the formyl group using Corey s conditions by treatment with manganese dioxide and potassium cyanide in methanol afforded methyl 6-methoxycarbazole-3-carboxylate (133), which is a naturally occurring carbazole alkaloid. Alternatively, compound 129 can be regioselectively bromi-nated at the 5-position. Subsequent cleavage of the methyl ether and prenylation at C-5 with the dimeric 7i-prenylnickel bromide complex afforded micromeline... 

Electrochromic polymer films have been made where the carbazole unit is not part of the main chain of the polymer, e.g., poly-iM-vinylcarbazole (29) [107—109]. When electrochemically polymerized, it has been shown that the cation radical of the carbazole unit easily dimerizes to form a 3,3 -bicarbazolyl that is more easily oxidized than the parent monomer. These films also show a colorless to green transition upon oxidation. Similarly, films of poly[3-(3-bromocarbazol-9-yl)propyl]methylsiloxane (30) have been prepared where the carbazole dimerizes creating a cross-linked film that switches between colorless and green as with the other polymers [110]. 

The oxidation of alkyl-substituted carbazoles generally produces dimers rather than polymers. An exception is A -vinylcarbazole, which can be oxidized in dichlor-omethane to produce a polymer (o- = 10" S cm ) [189]. This polymer as well as poly-3,6-dibromo-N-vinylcarbazole can be electrochemically doped to conductivities up to lO " S cm" [190]. 

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