Tautomeric Equilibria in Solutions of 2-Phenacylbenzimidazoles

Abstract

Detailed NMR spectral analysis of DMSO-d6 solutions of the series of substituted 2-phenacylbenzimidazoles (ketimine form, K) reveals two from three tautomeric forms. Integrals of the 1H NMR signals are used in establishing the molar ratio of tautomers. The experimental analyses are supported by quantum-chemical calculations, which satisfactorily reproduced the experimental trends. Although the reported semiempirical quantum-chemical calculations show that enaminone E, i.e., 2-(1,3-dihydro-2H-benzo[d]imidazol-2-ylidene)-1-phenylethan-1-one, was thermodynamically most stable, the results of MP2 ab initio calculations reveal the following order of stability: ketimine > enolimine > enaminone (substituents do not affect this sequence). 13C CPMAS NMR spectral data reveal that in the crystalline state the enolimine tautomer O is predominant in the p-CH3 and p-NO2 substituted congeners.

Tautomerism and Self-Association in the Solution of New Pinene-Bipyridine and Pinene-Phenanthroline Derivatives

“…It seemed that the more polar keto tautomer was better stabilized by the less polar (compared to CD 3 CN) CD 2 Cl 2 and CDCl 3 . The chlorinated solvents were in fact strong proton donor solvents; thus, the keto tautomer 3k could be more favored through intermolecular H-bonding involving the free carbonyl group, as reported in other tautomeric systems [23,24].…”

Substituent and Solvent Polarity on the Spectroscopic Properties in Azo Derivatives of 2-Hydroxynaphthalene and Their Difluoroboranes Complexes

“…It is known that systematic change of substituent and benzoannulation have a fundamental impact (qualitative and quantitative) on the properties of compounds exhibiting tautomerism [ 29 , 30 , 31 , 32 ]. Presumably, the properties of BF 2 -carrying molecules may also be tuned in this way.…”