RESEARCH ARTICLE

 

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

 

 

Marrigje M. Conradie; Alfred J. Muller; Jeanet Conradie^*

Department of Chemistry, University of the Free State, Bloemfontein, 9300 South Africa

 

 

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ABSTRACT

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 from a 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.

Keywords: β -Diketone, thienyl, crystal structure, keto-enol tautomerism, resonance

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Acknowledgements

Financial assistance by the South African National Research Foundation under grant number 2067416 and the Central Research Fund of the University of the Free State are gratefully acknowledged.

 

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Received 23 July 2007
Revised 23 November 2007
Accepted 23 November 2007

 

 

* To whom correspondence should be addressed. E-mail: conradj.sci@ufs.ac.za