Triarylamine radical cations

The same approach allows preparation of various pyrylium carboranes from the corresponding 4/f-pyran carboranes 174a,b and 175b by the action of acetyl perchlorate,245 perchloric acid,244 and triarylamine radical cation salts,244,245 as well as electrochemically.243 The oxidation of condensed 4H-pyran 345 with trityl perchlorate, 2,3,5,6-tetra-substituted 4f/-pyrans 431 and 153 with tropylium tetrafluoroborate or 153 with heterocyclic salt 393 led to useful preparations of pyrylium salts 394,330 395a,359 and 395b,360 respectively. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Triarylamine radical cations may also be used in an indirect electrochemical oxidative cleavage of thiol esters [142] ... 

Thus, deprotection of benzylic ethers, oxidation of activated thioethers [117], and deprotection of gem-dithioacetals [117,118] could be completed by means of such electrogenerated triarylamine radical cations. Even a homogeneous Kolbe reaction (indirect oxidation of carboxylate ions) was shown to be feasible [19] (Table 5). 

Deprotection of Derivatives of Carboxylic Acids.—Methyl esters are demethylated with anhydrous trifluoroacetic acid, in yields ranging from quantitative to 2%. Triarylamine radical cations react with benzyl esters in an oxidative cleavage reaction to give carboxylic acids yields are best with p-methoxybenzyl esters. ... 

Light emission occurs during the reaction of numerous radical anions of aromatic hydrocarbons with radical cations such as Wurster s red 103, Wurster s blue 104 or radical cations derived from triarylamines of the type 105, 106. 

For oxidations, the cation radicals of aromatic compounds like 9,10-diphenyl-antracene, thiantrene, phenoxathiine, or dibenzodioxine are likely candidates. Their reactivity towards nucleophiles, however, limits their application to media of low nucleophilicity. Sometimes the stability of such cation radicals can be enhanced through blocking the reactive positions by substituents. For example, para-substituted triarylamines deliver cation radicals with often excellent stability even in methanol. The stability is further increased by incorporation of urzAu-substituents. Other mediators which have been applied in indirect electrosyntheses are those which are able to abstract hydrogen atoms or hydride atoms. 

As for halogens as oxidizing reagents, bromine has proved more useful than its homologs chlorine and iodine. It was employed as early as 1879 on di- and tetra-methyl-p-phenylenediamine [27-29] and early in this century, Wieland used it to generate the aminium salts of triarylamines and tetraarylhydrazines [30, 31]. Since bromine adds readily to unsaturated as well as to some strained ring compounds, it is not expected to be very useful in the context of the radical cations discussed here. 

The current organic photoreceptors are triarylamines, triarylmethanes, hy-drazones, oxadiazoles, pyrazolines, oxazoles, and more recently, stilbene derivatives. The polymer matrix, on the other hand, is constituted by polyesters and polycarbonates (Fig. 5). The common presence of aromatic amines as substituents in all these materials contributes to efficient hole transport [44]. The nonbonding electron pair on the nitrogen atom, in fact, confers on these molecules a low oxidation potential, and consequently, the production of a chemically stable radical cation with the possibility of an effective overlap of nonbonding molecular orbitals between neighboring molecules. 

Cyclic voltammetry in THF of the triarylamine-based dendrimer 68 indicated the presence of at least four unresolved waves, giving rise to delocalized radical cations [138]. 

It should be noted here for triarylamine networks and dendrimers [104], respectively, the radical cations have interesting properties like the formation of high-spin polyradicals with ferromagnetic coupling [105] or conducting polymers [106]. Very often, triarylamines have been used as the hole-transport layer in electroluminescent devices [107]. 

R SR Gem-dithioacetals C / R SR 9,10-Diphenylanthracene and triarylamines Rapid decomposition of radical cation by cleavage (R = aromatic) or nucleophilic attack (R = aliphalic) (oxidative deprotection of carbonyl compounds) [117]... 

A few promised chapters on the acidity of anilines, on polyanilines and on radical cations of triarylamine and phenylenediamine were not delivered. We hope to include these chapters in a future supplementary volume. 

Direct observation of enol ether type radical cations such as are expected to be important in the fragmentation of nucleotide C4 radicals is not possible by the time-resolved laser flash photolysis technique owing to the lack of a suitable chro-mophore. However, it has recently been demonstrated that if such an LFP experiment is conducted in the presence of a triarylamine then any diffusively free enol ether radical cations oxidize the amine to the corresponding highly colored ami-nium radical cation. In this manner the overall rate constant for fragmentation and cage escape may be determined (Scheme 4) [1 Ij. 

Radical cations of triaiylamines are known to be single electron transfer oxidizing agents. Very recently, these substances have been used as efficient mediators for gcm-difluorodesulfurization as shown in Scheme 56. ° Furthermore, triarylamines have recently been shown to be highly effective mediators for monofluorodesulfurization of P-lactams. Severe passivation of the anode takes place during anodic fluorination of P-phenylthio- P-lactams, which is quite different from the case of a-phenylthio-P-lactams 60. However, selective anodic fluorodesulfurization proceeds efficiently without passivation when tris(2,4-dibromophenyl)amine is employed as a mediator as shown in Scheme 57. ... 

Due to the utility of the methods and the significance of the materials, the use of Pd-catalyzed fV-arylation reactions in oligoaniline and polyaniline synthesis has remained an active area of research [344, 345]. For example, in recent work Kulszewicz-Bajer and coworkers have employed Pd-catalyzed amination reactions for the synthesis of AT-aryl poly(w,-p-aniline) derivatives that can be oxidized to high-spin radical cations [346, 347]. The preparation of these materials was accomplished by coupling 1,3-dibromobenzene with A TV -diarylphenylenediamine derivatives (Eq. 65). Palladium-catalyzed A -arylation reactions have also been employed for the preparation of partially annulated poly(w-anilines) [348] and other triarylamine-derived polymers [349, 350]. 

The strongly oxidizing SbCl5 is an effective oxidant for the preparation of cation-radical salts of hexachloroantimonate (SbCl ) from a variety of organic donors, such as para-substituted triarylamines, fully-substituted hydroquinone ethers, tetraarylethylenes, etc.176 For example, the treatment of the hydroquinone ether EA (2 mmol) with SbCl5 (3 mmol) in anhydrous dichloromethane at — 78°C immediately results in an orange-red solution from which the crystalline cation radical salt readily precipitates in quantitative yield upon the slow addition of anhydrous diethyl ether (or hexane)173 (equation 36). 

This process is more efficient when being redox-catalyzed [110, 111]. Thus the use of electrogenerated cation radicals of some triarylamines (Ar = Tol, p-Br-C6H4 [112]) as mediators provides even higher yields of the corresponding carbonyl compounds. 

Cation-Radicals of Triarylamines in Optical-Recording Media... 

The second claim by lida et al. (2007) refers to a composite of an organic electroluminescence device comprising a luminescent material and triarylamine cation-radicals that open a possibility to use a lowered working voltage and to enhance the device durability. Scheme 8.7 represents one of the examples from this patent. 

Triarylamine cation radicals seem to be able to react in two different ways 1 — as simple one-electron transfer agents (mechanism A, Sect. 2.3) 2 — as chemical catalysts (mechanism B, Sect. 2.3) for oxidations in which the redox step is... 

The indirect anodic cleavage of carbon-hydrogen bonds in the benzyl position using triarylamine mediators was also used for mild and selective deblocking of hydroxy, carboxyl, and amino groups. The primarily formed cation radical of the protective group is readily deprotonated in the benzyl position by an added base (Eq. (107)). This benzylic radical is easily further oxidized to the benzyl cation which subsequently is cleaved by attack of a nucleophile, such as water (Eq. (108)). 

Typical examples for type 1 are the anodic cleavages of two carbon-sulfur bonds in 1,3-dithianes [46] or dithiolanes [47]. This reaction is especially effective if performed under the conditions of indirect electrolysis using triarylamine cation radicals as regenerable oxidative mediators [47] ... 

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