Kinetics of ionisation of 3-phenylcoumaran-2-one and reprotonation of the resulting carbanion

Abstract

The kinetics of the reversible ionisation of 3-phenylcoumaran-2-one (pK_a = 8.9), to form a carbanion, are reported in 50% (v/v) water–dioxane mixtures at 25 °C. Proton abstraction is catalysed by hydroxide ion, water and general bases which generate a Brønsted β_B of 0.52 indicative of a fairly symmetrical transition state. Rate limiting proton abstraction by hydroxide ion is confirmed by a primary kinetic isotope effect, k_H/k_D, of 3.81. Protonation of the enolate carbanion occurs by the hydronium ion and general acids but not by water. In acidic solution, below pH 5, O-protonation occurs initially to generate the neutral enol and ketonisation occurs by C-protonation of the minor enolate anion species in an overall pH independent step. The pK_a of the enol is 6.0 and the pK_E is calculated to be 2.9. The intrinsic rate constant, log (k_o/dm³ mol^–1 s^–1), of 2.60 is similar to those of other carbon acids of pK_aca. 9, suggesting that a relatively small amount of molecular and solvent reorganisation is required in order to stabilise the generated charge in the transition state. A rate–equilibrium correlation for proton transfer to hydroxide ion from carbon acids activated by a mono carbonyl group has been extended in the thermodynamically favourable direction to cover pK_a units with no signs of significant curvature in the direction predicted by Marcus theory.