Carbazole cation-radicals

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. 

Carbazole, like most aromatic amines, oxidizes readily via electron transfer. We recognized early that electron transfer may be an important initiation process for polymerizing the N-vinyl derivative. Some years ago we showed (29) that cycloheptatrienyl cation could act as an efficient one-electron transfer reagent, producing the appropriate cation radicals from reactive amines such as phenothiazine and tetramethyl-p-phenylene-diamine. It was also suggested that the product of the reaction between cycloheptatrientyl cation and carbazole itself was the carbazole cation radical. However, our recent work (21) has demonstrated that one-electron oxidation of carbazole leads directly to the 3,3-dicarbazoyl cation radical (VII). 

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. 

Despite the reactivity of carbazole cation-radicals, that of 9-ethylcarbazole has been trapped by nucleophiles, e.g., I", NO2,475 Thus, iodo and nitro derivatives of 9-ethylcarbazole result, along with 9,9 -diethyl-3,3 -bicarbazole, when 9-ethylcarbazole is oxidized by iodine in the presence of iodide and nitrite.473 Oxidation in the absence of nucleophiles, e.g., by nitrosonium tetrafluoroborate, permits the synthesis of the cation-radical fluoroborate salts of the 3,3 -bicarbazoles476 The reactivity of the cation-radical of 9-vinylcarbazole has been reviewed recently.11,477... 

Intramolecular coupling or nucleophilic substitutions sometimes occur upon oxidation of the cation radical the new product frequently then undergoes an additional anodic electron-transfer reaction to yield a new cation radical. For example, tri-p-substituted triphenylamines form stable cation radicals upon oxidation at their first anodic wave. Reynolds et alt (1974) have shown, however, that further oxidation of the cation radicals leads to the appearance of a new reversible redox couple at potentials slightly positive with respect to the first anodic wave. This new couple was shown to occur at potentials where the corresponding carbazole is oxidized, so that the overall process involves conversion of the amine to the carbazole cation radical by reaction scheme (85). 

Side reactions of the dication (e.g. hydrolysis), and perhaps further oxidation of the carbazole at potentials where the dication forms, limits the yield of carbazole cation radical in some cases. A similar reaction sequence reported by Ronlan and Parker (1970) involves oxidation of . [7a] which produces a cyclized product, the 9,10-dihydrophenanthrene [7b]. On the CV time scale [7b] forms a stable cation radical, but on coulometric oxidation it shows an napp-value of two and produces the phenanthrene [7c]. A similar intramolecular coupling occurs upon oxidation of... 

The direct electrochemistry of redox proteins such as ferredoxin and blue copper protein was studied. The additional effect of poly(L-lysine) on the redox behaviour of horse heart cyt c at functional electrodes has been reported Electropolymerized films such as PAn undergo redox reactions producing a colour change. This is described in Sect. 5.2. The anodic oxidation of poly(iV-vinylcarbazole) films was shown to involve initially the cross-linking of the polymer chains by oxidation of the carbazole moieties and dimerization of the resulting pendant carbazole cation radicals The resulting dimeric carbazole unit is more easily oxidized than the monomer and undergoes a further (reversible) two-electron oxidation. 

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. ... 

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. ... 

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)]. 

Nitrosonium terafluoroborate initiates the cation radical-mediated imino-Diels-Alder reaction of IV-arylimines with IV-vinylpyrrolidinones to give cw-4-(2-oxopyr- (g) rolidin-l-yl)tetrahydroquinolines.175 Also, 2,4,6-triphenylpyrylium tetrafluoroborate catalyses the Diels-Alder addition of IV-arylimines with IV-vinylpyrrolidinone and IV-vinylcarbazole to yield the corresponding 2-oxopyrrolidin-l-yl- and carbazole-9-yl-tetrahydroquinolines.176 The tricyclic core (151) of the batzelladine alkaloids has been prepared by a diastereoselective 4 + 2-annelation of the vinylcarbodiimide (150)... 

The formation of cation-radicals by carbazoles is well documented. In 1968, it was observed that oxidation of carbazoles with lead tetraacetate in acid conditions led to the formation of the cation-radical of the corresponding 3,3 -dicarbazole. The radicals are persistent and uninfluenced by air or water.466 This work was subsequently pursued further, and similar oxidations were effected by other typically single-electron oxidants.467 Indications from electrochemical work that the simple cation-radicals are the reactive intermediates appeared about the same time as the initial observation.468 This work, too, has been followed-up in depth. Electrochemical... 

Tail-to-tail coupling of radicals obtained in the anodic oxidation of triphenyl-amines results in the formation of tetraphenylbenzidines. Oxidation of triarylamines to the di-cation results in the formation of the carbazoles, as observed for Ai -alkyl-p,p -disubstituted diphenylamines [1-3, 78]. The cation radicals of triarylamines with substituents in the para position of the aryl groups, which can protect them against nucleophilic attack, are very stable and can be used as organic redox catalysts for indirect electrochemical oxidation reactions. Depending on the substitution pattern on the phenyl group the oxidation potentials of the triarylamines can be tuned over a wide range [Eoy. = 0.7-2.0 V) and many of these have been used as redox catalysts in numerous indirect electrochemical reactions [1-3, 79-83]. 

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. 

Direct photolysis of 50-53 in 02-saturated acetonitrile solution also leads to the corresponding carbazoles with a quantum yield of ca 0.64 in all cases (equation 13)161. Apparently, substituents have only a little effect on the chemical yield of carbazole produced by steady-state irradiation in aerated acetonitrile. However, an attempt to carry out such a photocyclization reaction by using photoinduced electron-transfer sensitization has failed, presumably due to fast back electron transfer that quenches the net reaction. It is also interesting to note that chemical oxidation and electrochemical oxidation of 50-53 does not result in carbazoles. Instead, benzidine products are formed. These results are consistent with the AMI calculations, which suggest that the cyclization reaction is both kinetically and thermodynamically more favorable from the triplet state than from the cation radical or dication. 

A particularly interesting series of reactions has been characterized for the cation radical of the olefin N-vinyl carbazole (NVC [50]). 

Solvations of the charged species accelerate the transfer of electrons and the ionizations are enhanced by polar solvents.Charge transfer reaction studies with tetracyanoethylene, an acceptor, and A -vinyl carbazole, a donor, in benzene solution demonstrated that both cation radicals and anion radicals form. This can be used in a subsequent cationic polymerizations ... 

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 radical cation presumably dimerizes to the di(AT-vinyl carbazole) cation. 

A similar mechanism has been found in the case of other polyheterocycles. For example, the deposition of polycarbazole has been shown to involve the initial formation of cation radicals. The electrooxidation of carbazole in the nonaqueous medium was thoroughly studied by Ambrose and Nelson [10], who established that a dicarbazyl radical resulting from the coupling of carbazole cations at the 3-position is formed. These authors did not report polymerization of carbazole. However, others who have investigated the oxidation of carbazole [11] reported polymerization. An iodinated, chemically polymerized material was also reported [12]. The mechanism for the formation of polycarbazole in dimethyl formamide (DMF) or aqueous medium is shown in Scheme 2 [11]. 

Cation radicals of carbazolyl chromophore can be formed by quenching its excited state by the electron acceptor, DMTP, in DMF. The measured absorption spectra were composed of the carbazole cation and the counter acceptor anion. The former is observed in the wavelength region above 600 nm, while the latter is seen below 600 nm. The molecular structures of PVCz, Poly(N-carbazolylethyl vinyl ether) and polyurethanes studied and the absorption spectra of their cation radicals are shown in Fig. 8. The spectra of the polyvinyl-ethers and polyurethanes are very broad and show a peak at 780 nm (25, 28-30). PVCz has a maximum at 770 nm and no shoulder is detected. It is worth noting that this spectral shape is common to all PVCz polymer cations with different n-values from 4 to 1100 and is observed even for PVCz-4. 

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. 

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... 

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