Triarylamines oxidation

Activated thioethers Triarylamines Oxidative cleavage of the C—S bond [117, 118]... 

Fewer examples are reported for organic electrode reactions some alkyl halides were catalytically reduced at electrodes coated with tetrakis-p-aminophenylporphy-rin carboxylate ions are oxidized at a triarylamine polymer and Os(bipy)3 in a Nafion film catalytically oxidizes ascorbic acid Frequently, modified electrodes fail to give catalytic currents for catalyst substrate combinations that do work in the homogeneous case even when good permeability of the film is proven... 

The reaction with ammonia or amines, which undoubtedly proceeds by the SnAt mechanism, is catalyzed by copper and nickel salts, though these are normally used only with rather unreactive halides. This reaction, with phase-transfer catalysis, has been used to synthesize triarylamines. Copper ion catalysts (especially cuprous oxide or iodide) also permit the Gabriel synthesis (10-61) to be... 

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). 

Bipolar Molecular Glasses. Recently, bipolar molecular glasses have been described that allow both injection of holes and electrons (Fig. 3.30). 2- 4-[bis(4-methylphenyl)amino]phenyl -5-(dimesitylboryl)thiophene (PhAMB-lT, 68) and 2- 4-[bis(9,9-dimethylfluorenyl)amino]phenyl -5-(dimesitylboryl)thiophene (F1AMB-1T, 69) show oxidation potentials of 0.62 and 0.58 V, and reduction potentials of —2.13 and —2.01 V vs. Ag/0.01 Ag+, respectively [145]. Oxidation as well as reduction leads to stable radical ions. With the conversion rules given above, the HOMO and LUMO levels can be estimated to be approximately at —5.3 and —2.8 eV. In comparison, for the bipolar compound 70, consisting of triarylamine and oxadiazole moieties, the values are —5.5 and — 2.7eV [129]. However, in this case no data on the stability of the radical ions are available. 

The use of a chemical mediator can alter the chemoselectivity of an electrochemical reaction. In the reaction illustrated in Scheme 2, -methylstyrene was oxidized using both direct electrolysis and mediated conditions [10]. The current density, amount of charge passed, temperature, and other variables were all kept constant. The only difference was the addition of 6.4 mole percent of tris(4-bomophenyl)amine to the mediated reaction. The direct electrolysis tended to afford the product of a four-electron oxidation (7). When 3.5 F mole of electricity was passed, a 55% yield of (7) was obtained along with a 45% yield of (6). With additional current (6.4 F mole ), a 75% yield of (7) was obtained. The mediated process led to a preponderance of the product from the two-electron oxidation. When 3.5 F mole of electricity was passed in the experiment using the triarylamine mediator, a 93% yield of (6) was obtained along with only 6% of the four-electron oxidation product. 

Preannihilative electrochemical oxidation of the phenanthrene anion has given a green emission13,64 spectrally nearly identical to the previously reported room-temperature phenanthrene phosphorescence which is a single broad peak.71 Chemical oxidation of the chrysene anion with Wurster s blue perchlorate produced an emission containing three bands at 19,800, 18,600, and 17,400 cm"1 which seem to correspond to the known phosphorescence bands of chrysene (19,500,18,500, and 16,600 cm-1). Chemical oxidation of the radical anion of N-methylcarbazole has possibly led to phosphorescent emission from this triarylamine.7... 

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. 

Indirect Electrochemical Oxidations Using Triarylamines as Redox Catalysts... 

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... 

Table 11. Indirect electrochemical oxidations using triarylamines as mediators... 

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)). 

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. 

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] ... 

Triarylamine is a purely organic molecule which is interesting as a chro-mophore in e.g. display technology. The molecule can be switched between a reduced colourless, and an oxidized blue state. The sensitization to nanos-tructured TiC>2 electrodes provides the substantial surface area required for a strong coloration. It is, however, believed that the electron transfer involved in the redox reaction takes place mainly within the sensitizer layer, and does not involve the substrate. Instead, there is an eventual electrical contact between the back-contact and the sensitizer layer [98]. For a quantum chemical modelling of the system, the inclusion of the substrate is in this case not likely to be essential. For a molecule of this size, it is possible to apply standard quantum... 

For H-acid catalysis stereospecifity is lowered and appears to be the same as in the reactions initiated with trifluroacetic acid instead of the ammoniumyl salt. For the cation-radical mechanism the sterically hindered base 2,6-bis(tert-butyl)pyridine does not inhibit the cyclization triarylamine retards this reaction photosensibilized one-electron oxidation of a diene leads to the same products, which are formed in the presence of the ammoniumyl salt. As shown, in the majority of cases only the cation radical chain mechanism of the diene-diene cyclization is feasible (Bauld, Bellville, Harirchian 1987). Meanwhile, cyclodimerizations of 2,4-dimethylpenta-1,3-diene (Gassman Singleton 1984) and 1,4-dimethylcyclohexa-1,3- or -1,4-diene (Davies et al. 1985) proceed through both mechanisms. 

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. 

Cyclic voltammetry revealed that the N-atoms of 86a, 88a and 91a are oxidized at lower potentials than the trivalent P atoms. Comparison of these data with those observed with model compounds shows a very weak electronic delocalization via the P centers for copolymers 86a and 91a. In contrast, the low first oxidation potential observed for 88a (Table 4.5) is assumed to result from an electronic communication between the N moieties through the connecting P centers [59b]. The equivalence of the oxidation potentials for the oxidized polymers (Table 4.5) suggests the presence of electronically isolated triarylamine fragments in these derivatives. Note that the involvement of the P lone pair in jt-delocalization... 

Annelations via Friedel-Crafts acylation were applied to racemic syntheses of [njhelicenes with n up to 6. The 12-step synthesis of [6]helicene (1) by Newman and Lednicer employed stepwise diannelation, followed by oxidative aromatiza-tion and then resolution with the complex of a chiral n-acceptor, a-2,4,5,7-tetrani-tro-9-fhiorohdeneaminooxypropionic acid (TAPA) and 1 [25]. The one-step diannelation was ubiquitously applied to syntheses of triarylamine [4]helicenes,... 

Figure 6.34 Variation of the diffusion coefficient, Dapp, following a potential step from 0.2 to 1.0 V for the oxidation of a triarylamine in a mixed monolayer with a redox-inactive compound as a function of the mole fraction x1. The electrolyte is l-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Reprinted with permission from P. Bonhote, E. Gogniat, S. Tingry, C. Barb , N. Vlachopoulos, F. Lenzmann, P. Comte and M. Gratzel, /. Phys. Chem., B, 102, 1498 (1998). Copyright (1998) American Chemical Society...

Arylamines are commonplace, as components of molecules with medicinally or electronically important, catalytic active, or structurally interesting properties. An aryl-nitrogen linkage is included in nitrogen heterocycles such as indoles [4,5] and benzopyrazoles, conjugated polymers such as polyanilines [6-12], and readily oxidized triarylamines used in electronic applications such as 4,4 -bis(3-methylphe-... 

Arylamines display electronic properties that are favorable for materials science. In particular, arylamines are readily oxidized to the aminium form, and this leads to conductivity in polyanilines, hole-transport properties in triarylamines, stable polyradicals with low energy or ground-state, high-spin structures, and the potential to conduct electrochemical sensing. The high yields of the palladium-catalyzed formation of di- and triarylamines has allowed for ready access to these materials as both small molecules and discrete oligomeric or polymeric macromolecules. 

Aminations of five-membered heterocyclic halides, such as furans and thiophenes, are limited. These substrates are particularly electron-rich. As a result, oxidative addition of the heteroaryl halide and reductive elimination of the amine are slower than for simple aryl halides (see Sections 4.7.1 and 4.7.3). In addition, the amine products can be air-sensitive and require special conditions for their isolation. Nevertheless, Watanabe has reported examples of successful couplings between diarylamines and bromothiophenes [126]. Triaryl-amines are important for materials applications because of their redox properties, and these particular triarylamines should be especially susceptible to electrochemical oxidation. Chart 1 shows the products formed from the amination of bromothiophenes and the associated yields. As can be seen, 3-bromothiophene reacted in higher yields than 2-bromothiophene, but the yields were more variable with substituted bromothiophenes. In some cases, acceptable yields for double additions to dibromothiophenes were achieved. These reactions all employed a third-generation catalyst (vide infra), containing a combination of Pd(OAc)2 and P(tBu)3. The yields for reactions of these substrates were much higher in the presence of this catalyst than they were in the presence of arylphosphine ligands. 

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. 

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