2.4.5- Triarylimidazole anions

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). 

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. 

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). 

When the dihydro-1,2,4-triazinone 76 is heated at 180°C with red phosphorus and hydroiodic acid, there is rupture of the 1,2-bond and subsequent ring closure to give 4,5-diphenylimidazolin-2-one (77) (see Scheme 17) and reductive ring contractions to imidazoles (81) have also been reported " in the 1,3,5-triazine series. A further interesting ring contraction is that which occurs when 4H-l,3,5-thiadiazines (78) are treated with aliphatic amines at room temperature. The triarylimidazoles (81) may be formed by initial formation of the 1,3,5-thiadiazine anion (79) which probably loses sulfur... 

---

See also in sourсe #XX -- [ ]

---