2.4.5- Triarylimidazoles, oxidation

Oxidation of azole anions can give neutral azole radicals which could, in principle, be tt (139) or a- (140) in nature. ESR spectra indicate structure (141 hyperfine splittings in G) for imidazolyl radicals, but both tt- and cr-character have been observed for pyrazolyl radicals. Tetrazolyl radicals (142 4 143) are also well known (79AHC(25)205). Oxidation of 2,4,5-triarylimidazole anions with bromine gives l,l -diimidazolyls (144) which are in equilibrium with the dissociated free radical (145) (70AHQ 12)103). 

A peculiar case of fast and reversible (i.e., tautomeric) isomerizations initiated thermally or photochemically is represented by triarylimidazole dimers 349a formed by oxidation of triaryUmidazoles (66JA3825 90MI2) and diarylhetarylimidazoles (Ar = Het) (Scheme 140) [97MI553]. 

The anodic oxidation of 2,4,5-triarylimidazole was studied in aprotic solvents.291-295 The 2,4,5-triarylimidazole anions undergo a one-electron oxidation, forming dimeric bis-(2,4,5-triarylimidazolyls).294 The isomeric bis imidazolyls consist of imidazole and isoimidazole systems. The dimerization is a result of a nucleophilic attack of 2,4,5-triarylimidazole anions on the electrochemically generated 2,4,5-triarylimidazolium cations. 

D. M. White and J. Sonnenberg, Oxidation of triarylimidazoles. Structures of the photochromic and piezochromic dimers of triarylimidazyl radicals. J. Am. Chem. Soc. 1966, 88, 3825. 

The relationship of 149 and 150 to their various dimers and the photo-chromic, thermochromic, and piezochromic properties of these materials have attracted much interest. The nature of the dimers formed on oxidation of triarylimidazoles was elucidated by White and Sonnenberg494 their properties were studied by groups in America495-499 and Japan.500-511 The chromism of whatever kind arises from the color of the triarylimidazolyl radicals formed on dissociation of the dimers. Similar conclusions have been reached for dimers of tetraarylpyrrole and similar compounds.488,512-516... 

Reactions of the sodium salts of 2,4,5-triarylimidazoles with bromine and oxidations of A -hydroxybenzimidazoles to cyclic nitroxyls have been discussed elsewhere (Section 4.02.1.4.11). 

Electron-donor substituents facilitate the polarographic oxidation of triarylimidazoles. At 280°C iron(II) oxalate reduces l-(o-nitrophenyl) imidazole mainly to the amino compound (140) with a trace of azo compound, but when there is a methyl group at C-2 or C-4 some of the condensed tricyclic product 141 is isolated. ... 

Chemical basis. The oxidation products of triarylimidazoles gives rise to several Isomeric dimers, which can thermally or photochemically dissociate. The dimers (2 scheme below, and 3 scheme below) can be interconverted via the colored triaryli-mldazolyl free radical (1 scheme below). 

Scouting at DuPont continued for a brief period, during which Dr. Lawrence A. Cescon saw a reference to Japanese work that described photochromic behavior of certain oxidation products of triarylimidazoles (lophines). Soon other laboratories in Japan, Germany, England and the United States embarked on studies of these materials. 

Oxidation of Triarylimidazoles. Structures of the Photochromic and Piezo-Chromic Dimers of IViarylimidazyl Radicals. D.M. White and J. Sonnenberg (Gen. Elec. Res. Develop. Center Schenectady, N.Y.). J. Am. Chem. Soc. 88 (16), 3825-9 (1966) (Eng.). Two dimers of the 2,4,5-triphenylimidazyl radical have been prepd. and characterized. One compd., an isoimidazyliso imidazole (I), is photochromic and thermochromic. The isomeric compd., a bis(isoimidazole) (II), is a piezochromic solid which rapidly dissociates to tiiphenylimidazyl radicals in soln. Interconversions of I and II have been carried out and indicate that I is more stable than II both in soln. and in the solid state. Various substituted triarylimidazoles have been converted to dimers with analogous photo- and piezochromic properties. 15 references. 

These were the first to show the broad range of chemistry that could be initiated photo-chemically with hexaarylbiimidazoles. The first of these reported in detail some of the properties of HABIs, and the second addressed the reactivity of different HABIs. This work involved the synthesis of over a dozen different triarylimidazoles and their oxidation to the corresponding dimers. It was thus possible to gain a better understanding of substituent effects. 

Colored triarylimidazolyl free radicals 1 are formed from the thermal or photolytic dissociation of hexaarylbiimidazoles, the oxidation products of triarylimidazoles 4 [2-10]. Two hexaarylbiimidazole isomers have been reported [11], In solution, the more stable 2 is produced at room temperature and 3 is formed below —20°. Interconversion of 2 and 3 involves 1. Structures were inferred from infrared data [11]. 

Further exploration of triarylimidazolyl radical chemistry was undertaken. Two types of oxidations by 2 were studied abstraction of electrons and of hydrogen atoms. Both reactions normally yield triarylimidazole 4 as the reduction product, the proton being obtained from the solvent if necessary. Results are summarized in Scheme 7.2. Examples of representative reactions are the subject of mechanistic studies reported in subsequent papers [15,17,22-24],... 

Table 7.4. Cyclic voltammetric oxidation-reduction of triarylimidazoles in acetonitrile solutions...

Table 7.5. Oxidation potentials of some triarylimidazoles in acetonitrile solvent ...

TM. Sheets at Towanda sought to improve the economics of the coated product, and achieved success by co-oxidizing the triarylimidazoles from which TCTM-HABI and o-Cl-HABl are manufactured. Surprisingly, the mixture showed improved photofixing capability. 

TCDM-HABl is formed by co-oxidation of the above triarylimidazoles. 

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