Reactions in strongly basic media. Part 10. Base-catalysed isomerisation of Z- to E-substituted chalcones

Abstract

The rate coefficients for the methoxide-catalysed isomerisation of substituted (Z)-chalcones have been measured in methanolic dimethyl sulphoxide (DMSO) at 30.0 °C, as well as for (Z)-chalcone at several temperatures. The reactions are first order in substrate and base. Further, log k₁ for the reaction of (Z)-chalcone and (Z)-4-nitrochalcone correlate with an acidity function, H_R^–, having slopes equal to 0.53 and 0.45, respectively. The effect of substitution has been assessed using the Hammett equation, with ρ equal to 1.99 for the β-phenyl group and 1.93 for the benzoyl group. For the (Z)-chalcone, the enthalpy and entropy of activation are 14.2 kcal mol^–1 and –14 cal mol^–1 K^–1, respectively, and the kinetic solvent isotope effect, k₂^MeOD/k₂^MeOH, is ca. 2.0. The rate coefficients for the amine-catalysed isomerisation of (Z)-chalcone and (Z)-4-nitrochalcone in DMSO have been measured at 30.0 °C. The amino bases are a series of secondary aliphatic amines and the Brønsted coefficients, β, are given as 0.39 and 0.38, respectively. The isomerisation is considered to proceed by rate-determining attack of the base at the β-carbon of the (Z)-chalcone to form a carbanion. The ‘cis’-conformer of the carbanion then rotates to form the ‘trans’-conformer, which eliminates the base to give the (E)-chalcone.