Synthesis and tautomerism of 2-phenacyl-1H-benzimidazoles and their hydrogen bromide salts

Dzvinchuk, I. B.; Nesterenko, A. M.; Polovinko, V. V.; Ryabitskii, Aleksey B.; Lozinskii, M. O.

doi:10.1007/s10593-011-0860-7

Cited by 5 publications

(10 citation statements)

References 15 publications

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“…Since treatment of 2-methylbenzheteroazoles with benzoyl chloride and triethylamine may afford a variety of products [16,18,19], the nature of the intermediate products can be doubtful. When reaction takes place at 100 °C, the 2-methyl group in 2-methylbenzoxazole undergoes double benzoylation, i.e.…”

Section: Resultsmentioning

confidence: 99%

Synthesis and Structural Characterization of Substituted 2-Phenacylbenzoxazoles

Skotnicka

Kolehmainen

Czeleń

et al. 2013

IJMS

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1H and 13C NMR spectra of eleven 2-phenacylbenzoxazoles (ketimine form) show that their CDCl3-solutions contains also (Z)-2-(benzo[d]oxazol-2-yl)-1-phenylethenols (enolimine form). Intramolecular hydrogen bonding in the latter tautomer was found to be significantly weaker than that one in respective (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines. Integrals of the 1H NMR signals were used to evaluate the molar ratio of the tautomers. Strong electron-donating substituents were found to stabilize the ketimine tautomer. pKT (negative logarithm of the equilibrium constant, KT = [ketimine]/[enolimine]) was found to be linearly dependent on the Hammett substituent constant σ. The results of the MP2 ab initio calculations reveal enolimine including an intramolecular OH···N hydrogen bond to be the most stable form both with electron-donor and electron-acceptor substituents. The stability of ketimines is an intermediate of those found for enolimines and enaminones i.e., (E)-2-(benzo[d]oxazol-2(3H)-ylidene)-1-phenylethanones. 13C CPMAS NMR spectral data reveal that in the crystalline state the ketimine tautomer is predominant in p-NMe2 substituted congener. On the other hand, enolimine forms were detected there when the substituent has less electron-donating character or when it is an electron-acceptor by character.

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Section: Resultsmentioning

confidence: 99%

Synthesis and Structural Characterization of Substituted 2-Phenacylbenzoxazoles

Skotnicka

Kolehmainen

Czeleń

et al. 2013

IJMS

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“…For the substituents studied, presence of the enolimine O in DMSO-d 6 solution has never been reported. 1 H and 13 C as well as 1 H, 13 C HMBC NMR spectra suggest that K and E forms are present in such solution [4]. There is no doubt about the presence of the former tautomer, but unequivocal distinguishing between enolimines and enaminones cannot be based only on the simple NMR spectra [1][2][3].…”

Section: Resultsmentioning

confidence: 99%

“…There is no doubt about the presence of the former tautomer, but unequivocal distinguishing between enolimines and enaminones cannot be based only on the simple NMR spectra [1][2][3]. It is why the 1 H, 13 C HMBC technique has been used earlier to prove which of K and E is present in solution [4]. Using of the IR spectra to identify the form present in the crystalline state E [4] also seems unreliable.…”

Section: Resultsmentioning

confidence: 99%

“…It is why the 1 H, 13 C HMBC technique has been used earlier to prove which of K and E is present in solution [4]. Using of the IR spectra to identify the form present in the crystalline state E [4] also seems unreliable.…”

Section: Resultsmentioning

confidence: 99%

“…Tautomerism of 2-phenacylbenzimidazoles has been already studied by the NMR methods: 2-phenacylbenzimidazoles (ketimines K) in DMSO-d 6 solution were found to equilibrate with 2-(1,3-dihydro-2H-benzo[d]imidazol-2ylidene)-1-phenylethan-1-one (enaminones) [4]. Since an unequivocal distinguishing between enolimines O and enaminones E (any of these two forms can be present in solution), based only on the simple NMR spectra, may be difficult [1][2][3], ab initio calculations were performed in the present paper in order to support or deny the earlier conclusions [4]. The substituent effect on tautomerism of such derivatives is always significant [1][2][3], so the series of substituted 2-phenacylbenzimidazoles is worth further study.…”

Section: Introductionmentioning

confidence: 99%

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Tautomeric Equilibria in Solutions of 2-Phenacylbenzimidazoles

Skotnicka

Czeleń

Gawinecki

2019

Heteroatom Chemistry

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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.

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