4 -Phenylimidazole, tautomerism

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... 

Recently the synthesis of NHC complexes by a base catalyzed tautomeric rearrangement has been introduced. Af-Phenylimidazole coordinates via the unsubstituted nitrogen atom to Mn to give complex 82 (Fig. 26a) [198]. Subsequent attempts to deprotonate the C2 carbon atom in complex 82 led to the intermediate... 

Phenylimidazole was synthesized by treatment with Raney Ni of the corresponding thiohydantoin derivative. This desulfurization was considered to take place via the tautomeric dimercaptoimidazole, as shown in eq. 13.67.143... 

Matters become more complex still in neutral conditions because now tautomeric effects can be superimposed upon steric and electronic effects. In an 5e2 process 2-methyl-4-phenylimidazole is methylated by dimethyl sulfate to give a mixture of the 4-phenyl and... 

The increasing interest in C-NMR spectroscopy has resulted in a number of such studies in the imidazole field. Carbon-13 chemical shifts in a variety of solvents have been reported for imidazoles, 4-methyl-imidazole and its cation, phenylimidazoles, 1-acetylimidazole, and Fe(III) porphyrin-imidazole complexes.The influence of Mn(II) and Cu(II) on nuclear relaxation rates in imidazole has been studied C-2 appears at lowest field (136.2), while C-4 and C-5 are at 122.3 Hz. While C-NMR studies have been of some value in conformational studies,the shifts are of limited value only in ascertaining positions of tautomeric equilibrium for rapidly interconverting tautomers. ... 

Many workers in recent years have been interested in the tautomeric equilibria pertaining to oxygenated imidazoles such as imidazolones, " " N-hydroxyimidazoles, and imidazole iV-oxides. The earlier results have been critically summarized. In the solid state l,2-dimethyl-5-phenylimidazol-4-one exists in the OH form, but insolubility prevented studies in solution. Theoretically l-methylimidazol-5-ones can exist in the forms 93-96 shown in Eq. (22). French workers ... 

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