1.2- Diphenylimidazole, formation

Theoretical studies of the relative stabilities of tautomers 14a and 14b were carried out mostly at the semiempirical level. AMI and PM3 calculations [98JST(T)249] of the relative stabilities carried out for a series of 4(5)-substituted imidazoles 14 (R = H, R = H, CH3, OH, F, NO2, Ph) are mostly in accord with the conclusion based on the Charton s equation. From the comparison of the electronic spectra of 4(5)-phenylimidazole 14 (R2 = Ph, R = R3 = H) and 2,4(5)-diphenylimidazole 14 (R = R = Ph, R = H) in ethanol with those calculated by using ir-electron PPP method for each of the tautomeric forms, it follows that calculations for type 14a tautomers match the experimentally observed spectra better (86ZC378). The AMI calculations [92JCS(P1)2779] of enthalpies of formation of 4(5)-aminoimidazole 14 (R = NH2, R = R = H) and 4(5)-nitroimidazole 14 (R = NO2, R = R = H) point to tautomers 14a and 14b respectively as being energetically preferred in the gas phase. Both predictions are in disagreement with expectations based on Charton s equation and the data related to basicity measurements (Table III). These inconsistencies may be... 

Hydrazinium salts, 73 567-568 2-(4-Hydrazinocarbonylphenyl)-4,5-diphenylimidazole (HCPI), chemiluminescence reagent, 5 851 Hydrazone formation, microwaves in, 76 565... 

Involvement of AModo species in electrophilic C-iodinations needs to be considered since a number of imidazoles are known to form such compounds in basic medium. Charge-transfer complexes, too, are quite well known. They seem to be of the n -type through the unshared electron pair at N-3. Equilibrium constants for their formation are known to increase regularly in line with electron-donating powers of substituents (or vice versa). Some KCT values at 20°C (L M are imidazole (200), 1-methylimidazole (333), 1,2-dimethylimidazole (1165), 4-phenylimidazole (152), and 4,5-diphenylimidazole (141) (83BSB923). The charge-transfer complexes formed between iodine and imidazole-2-thiones appear to involve the sulfur atoms (88JA2586). 

Correlations between pK,d values and the equilibrium constants for the formation of iodine complexes with imidazoles suggest that the charge transfer complexes are of the /j-type involving donation of the unshared electron pair at N-3. For examples of K and pK3 values are imidazole, 202, 6.95 1-methylimidazole, 333, 7.33 4-phenylimidazole, 152, 6.10 4,5-diphenylimidazole, 141, 5.90 (83BSB923). 

The reaction of 4,5-diphenylimidazole-1,2-diamine 152 with substituted chalcones 153 in dimethylformamide for 1 h proceeds with the elimination of the hydrazine amino group, oxidation and formation of the appropriate 2,3,5,7-tetraaryl-5,6-dihydroimidazo[l,2-a]pyrimidin-6-ol 154 in low or moderate yields [126] (Scheme 4.47). The treatment of the same diamine with dibromo derivatives 155 produced imidazo[l,2-a]pyrimidines 156 [126]. 

Diphenyl-4-nitrosoimidazole is oxidized by amyl nitrate to the corresponding 4-nitro derivative [592], An unusual reaction is observed when 4,5-diphenylim-idazole and 2-bromo-4,5-diphenylimidazole are boiled with amyl nitrite. Instead of nitrosation the substrate undergoes nitration with the formation of 2-nitro-4,5-di-phenylimidazole [593] (Scheme 115). 

Although not particularly well known, nitrosoimidazoles appear to be quite stable compounds. The nitroso function can be reduced to amino, or oxidized to nitro. When 5-nitroso-2,4-diphenylimidazole is subjected to dropwise treatment with phenylhydrazine there is some reduction, but ring modification with the formation of the oxadiazole (224) accompanies this reaction (Scheme 123) (60G831). The nitroso function is able to take part in condensation reactions with compounds which possess active methylene groups, and related species (B-76MI40701). 

Reactions of Imidazoles. The action of sodamide on the chloro(ethynyl)-imidazole (413 R = Cl, = H) results in the formation of the isomer (413 R = H, R = Cl) similar transhalogenations have been observed for chloro(ethynyl)-pyrazoles and -1,2,4-triazoles. The imidazolium salt (414) undergoes base-induced ring-expansion to the pyrazine derivative (415). 2,4,5-Tribromo-l-methylimidazole reacts with xenon difluoride to give the imidazolidinetrione (416). Low-temperature H n.m.r. studies of the sensitized photo-oxygenation of imidazoles (417 R, R, R = H or Me) give direct evidence for the formation of unstable enr/o-peroxides (418) 2-methyl-4,5-diphenylimidazole yields the peroxy-derivative (419) in this reaction.Irradiation of the N-oxide (420) produces the diamine (422) by way of the oxaziridine (421)."" ... 

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