Electronic effects on keto–enol tautomerism of p-substituted aryl-1,3-diketone malonates

• Published in: Journal of molecular structure 2015-12, Vol.1101, p.162-169
• Main Authors: Jiménez-Cruz, Federico, Ríos-Olivares, Hector, García-Gutiérrez, José Luis, Mar, Lubanski Fragoza
• Format: Article
• Language: English
• Subjects: 1,3-diketones-malonates; Hammett LFER; NMR; Substituent effect; Tautomeric equilibrium constants; Tautomerism
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/j.molstruc.2015.08.022

Electronic effects on keto–enol tautomerism of 1-(p-substituted aryl)-1,3-diketone malonates 1–10 were determined and established by NMR-Hammett linear free energy relationships. In addition, tautomeric equilibrium constants were determined for the title compounds by 1H NMR in DMSO-d6, showing an increasing to diketo tautomer with the increasing in the temperature. In this context, the enol form becomes less stable with increasing temperature suggesting that the intramolecular hydrogen bonding of the cis enol-keto form are increasingly broken at higher temperatures. In general, the enolic form (rather than the keto) is the most stable conformation due to its six member cyclic structure fulfilled by an internal O–H … O hydrogen bond. •Electronic effect of p-substituent in aryl-1,3-diketone malonates were established by NMR-Hammett LFER.•Tautomer equilibrium constants were determined by 1H NMR in DMSO-d6.•Keto–keto tautomer was better evidenced with the increasing of the temperature.•The enolic form is more stable than the keto form.