Triarylimidazole

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). 

C. Tautomerism and Photochromism by Homolytic Cleavage IN Triarylimidazole Dimers... 

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]. 

This transformation can also be carried out under solvent-free conditions in a domestic oven using acidic alumina and ammoniiun acetate, with or without a primary amine, to give 2,4,5-trisubstituted or 1,2,4,5-tetrasubstituted imidazoles, respectively (Scheme 15A) [69]. The automated microwave-assisted synthesis of a library of 2,4,5-triarylimidazoles from the corresponding keto-oxime has been carried out by irradiation at 200 ° C in acetic acid in the presence of ammonium acetate (Scheme 15B) [70]. Under these conditions, thermally induced in situ N - O reduction occurs upon microwave irradiation, to give a diverse set of trisubstituted imidazoles in moderate yield. Parallel synthesis of a 24-membered library of substituted 4(5)-sulfanyl-lff-imidazoles 40 has been achieved by adding an alkyl bromide and base to the reaction of a 2-oxo-thioacetamide, aldehyde and ammonium acetate (Scheme 15C) [71]. Under microwave-assisted conditions, library generation time was dramatically re-... 

A direct synthesis of triarylimidazoles and triarylimidazolines has been accomplished by the dicobalt octacarbonyl-catalyzed reaction of benzyla-mine derivatives with carbon tetrachloride. When the reaction temperature is 150°C a complex product is formed and yields of heterocyclic products are poor. By lowering the reaction temperature to 120°C or reducing the reaction time, or by using [Mo(CO)6] and [f)5-C5H5Mo(CO)3]2 as the... 

The mechanism of photooxygenation has been studied for a range of substituted imidazoles 45 (Scheme 14). It proceeds via the endoperoxide 46, which has been detected by H-NMR spectroscopy in certain instances, giving 47 for triarylimidazoles, 48 reversibly at low temperatures for R = alkyl, R = R = and a variety of different products for other... 

B. S. Tanaseichuk, Triarylimidazole Radicals and their Dimers, Cliein. Heterocycl. Compd. 8, 1173-1180 (1972). 

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. 

Keywords benzil, aromatic aldehyde, NH4OAC, zeolite HY, microwave irradiation, silica gel, triarylimidazole... 

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. 

Acylamino ketones on heating with ammonium acetate are converted into imidazoles. 2,4,5-Triarylimidazoles 235 were prepared in this way from the amides 234 (Scheme 113) <1973CB2415>, and numerous variations are possible for this reaction. 

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... 

SheUce, K., Kakade, G., Shingate, B., and Shingare, M. 2008. Microwave-induced one-pot synthesis of 2,4,5-triarylimidazoles using glyoxylic acid as a catalyst under solvent-free conditions. RASAYAN Journal of Chemistry, 1 489-94. 

UV spectroscopy has been applied to studies of 2,4,5-triarylimidazole radicals, imi-dazolones and various thioimidazoles. In addition, the rates of hydrolysis, alcoholysis and aminolysis of imidazolides such as 1-acetylimidazole can be determined readily since such azolides show characteristic intense absorptions at wavelengths longer than those of the imidazole products. In the C-acyl- and C-aroyl-imidazoles, those which have the carbonyl group attached at C-2 exhibit a bathochromic shift of about 20 nm, while in m- and p-substituted 2-aroylimidazoles intense absorptions (log,o e 3.5-4.3) appear in the 295-309 nm region, with slightly less intense peaks at about 233-266 nm (74CRV279). 

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). 

In the presence of phosphorus trichloride, phenacylbenzamides react with primary aryl-amines with the formation of 1,2,5-triarylimidazoles (62), albeit in less than 10% yields... 

Treatment of 4//-l,3,5-thiadiazines (198) with aliphatic amines at room temperature gives 2,4,5-triarylimidazoles. These may be formed through the Stt 1,3,5-thiadiazine anion (199), which could lose sulfur via a thia-a-homoimidazole (200 Scheme 114). 

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