Tautomeric Equilibria in Solutions of 2-Phenacylbenzimidazoles

Agnieszka Skotnicka,Przemysław Czeleń,Ryszard Gawinecki

doi:10.1155/2019/4364207

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.

Highlights

It was found recently that in chloroform solution 2-phenacylbenzoxazoles (K in Scheme 1, X = O) are in equilibrium with (Z)-2-(benzo[d]oxazol-2-yl)-1-phenylethenols [1]
The MP2 ab initio calculations supported the energetic preference of the O form. 13C CPMAS NMR spectra proved that in the crystalline state the ketimine tautomer K is predominant only in the p-NMe2 substituted congener
We have previously shown that 1-methyl-2-phenacylbenzimidazoles are similar in chemical properties to 2-phenacylbenzoxazoles [2]

Summary

Introduction

Since the –N=C(–)CH2CO– moiety is present in 2-phenacylbenzimidazoles (Scheme 1, X = NH), these compounds and 2-phenacylbenzoxazoles and 1-methyl-2phenacylbenzimidazoles are expected to behave In drawing such conclusions one has to be very careful: minor modifications in the molecule may affect the tautomeric equilibria significantly [1,2,3]. 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

K ketimines

Results and Discussion

H H H p-NO2 p-NO2 p-NO2

Syntheses

Conclusions