Mechanism of hydrolysis of coumaran-2-ones

The hydrolysis of coumaran-2-one and 5-substituted 3-phenylcoumaran-2-ones is preceded by a pre-equilibrium involving the formation of an enolate anion at high pH. The pKa of 3-phenylcoumaran-2-one is 8.39 in water at 25 °C and the 3-phenyl substituent increases the carbon acidity by 104. However, despite this ready carbanion formation, the conventional addition–elimination mechanism for hydrolysis of 3-phenylcoumaran-2-ones is confirmed by a solvent kinetic isotope effect of 0.63 and a Brønsted 1g of –0.6. This is compatible with rate limiting formation of a tetrahedral intermediate.