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Hydrogen Bonding and Deuterium Isotope Effects in 13C NMR Spectra of Phenylene Enaminones Derived from Dehydroacetic Acid

Enaminones have been recently a subject of comprehensive studies due to their wide applications in organic synthesis [1] and coordination chemistry, as well as their potential pharmacological importance. Studies have shown that enaminone derivatives possess anti-inflammatory, anticonvulsant, antimalarial and cardiovascular activities. Hydrogen bonds are an important structure and reactivity factor, and also a bioactivity modulator. The aim of this work is to investigate the intra- and intermolecular hydrogen bonding structure in solution of enaminones derived from dehydracetic acid by employing NMR and DFT methods. X-ray structural analysis has confirmed the existence of the keto-amine form (1a) in the solid state, stabilized by formation of intramolecular H-bonds [2]. However, the fundamental question is whether the nature of the intramolecular H-bond N― H∙ ∙ ∙ O and the proton transfer (Fig. 1) will be affected by solvents of different proton donor and acceptor abilities and the substitution pattern (ortho, meta, para) of enaminone isomers. In order to get further insight into these interactions secondary deuterium isotope effects on 13C chemical shifts have been measured and analyzed.

enaminones; hydrogen bonding; deuterium isotope effects; NMR

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