Thienyl-containing β-diketones: Synthesis, characterization, crystal structure and keto-enol kinetics

1-phenyl-3-(2-thenoyl)-1,3-propanedione, Hbth, pK_a′ = 9.006(8) and 1,3-di(2-thenoyl)-1,3-propanedione, Hdtm, pK_a′ = 8.893(3) were prepared by the Claisen condensation of ethyl 2- thiophenecarboxylate with an appropriate ketone under the influence of lithium diisopropylamide (LDA). The group electronegativity of the thienyl group is 2.10 (Gordy scale) as calculated froma linear group electronegativity vs. methyl ester IR carbonyl stretching wavenumber relationship. A crystal structure determination of Hbth (orthorhombic, Pbca, Z = 8, R = 0.0290) shows asymmetrical enolization on the side of the phenyl group. The preferred enol isomer of β-diketones containing more than one aromatic moiety that crystallizes in the solid state is determined by the resonance driving force stabilization of the thienyl or any other aromatic group, rather than the stabilization by resonance due to the phenyl group. The slow conversion of the enol isomers to the keto-enol equilibrium position was followed in CDCl₃ solution by NMR spectroscopy.