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

The kinetics of the reversible ionisation of 3-phenylcoumaran-2-one (pKa = 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 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, kH/kD, 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 pKa of the enol is 6.0 and the pKE is calculated to be 2.9. The intrinsic rate constant, log (ko/dm3 mol–1 s–1), of 2.60 is similar to those of other carbon acids of pKaca. 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 pKa units with no signs of significant curvature in the direction predicted by Marcus theory.