Determination of the enol form of asymmetric 1,3-dicarbonyl compounds: 2D HMBC NMR data and DFT calculations

Abstract

In this study, a series of asymmetric aryl 1,3-dicarbonyl compounds were synthesized and their enol forms were observed via experimental data and theoretical calculations. According to the 1 Hand 13 C-NMR results, all the investigated compounds were found as a single enol form in CDCl 3 solution. Moreover, their HMBC spectra were applied to identify the observed enol forms and correlations between certain protons and carbon atoms were considered. The dihedral angles of the asymmetric compounds that have aryl units on both sides were calculated by DFT to understand the reason for the observed enol forms. Small dihedral angles caused longer conjugation, resulting in more stable compounds and it was found that the observed enol forms were based on small dihedral angles, namely, resonance is the driving force. Furthermore, the compounds possessing both aryl and alkyl moieties prefer the enol form towards the aromatic ring side due to longer conjugation.

A Golden Synthetic Approach to 2-(1H-Pyrrol-1-yl)anilines and Pyrrolo[1,2-a]quinoxalines through a Gold Carbene Intermediate

“…14a b When the NMR data for the resulting ynone derivatives with a 1,3-β-keto ester structure were analyzed, it was discovered that tautomerization occurred in a ratio of 1:1 for 1a and 1b and 1:3 for 1e – g , in agreement with the literature. 14c d However, the NMR spectra of ynone derivatives with only one carbonyl group and symmetric 1,3-diketo structures revealed no tautomerization.…”