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 en olate anion at high pH. The pK(a) of 3-phenylcoumaran-2-one is 8.39 in water at 25 degrees C and the 3-phenyl substituent increases the carb on acidity by 10(4). However, despite this ready carbanion formation, the conventional addition-elimination mechanism for hydrolysis of 3-ph enylcoumaran-2-ones is confirmed by a solvent kinetic isotope effect o f 0.63 and a Bronsted beta(1g) of -0.6, This is compatible with rate l imiting formation of a tetrahedral intermediate.