Keto-enol/enolate equilibria in the isochroman-4-one system.: Effect of a β-oxygen substituent

The enol of 1-tetralone was generated flash photolytically, and rates of its ketonization were measured in aqueous HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4-, (CH2OH)(3)CNH3+, and NH4+ buffers. The enol of isochroman-4-one was also generated, by hydrolysis of its potassium salt and trimethylsilyl ether, and rates of its ketonization were measured in aqueous HClO4 and NaOH. Rates of enolization of the two ketones were measured as well. Combination of the enolization and ketonization data for isochroman-4-one gave the keto-enol equilibrium constant pK(E) = 5.26, the acidity constant of the enol ionizing as an oxygen acid pQ(a)(E) = 10.14, and the acidity constant of the ketone ionizing as a carbon acid pQ(a)(K) = 15.40. Comparison of these results with those for 1-tetralone shows that the beta -oxygen substituent in isochroman-4-one raises all three of these constants: K-E by 2 orders of magnitude, Q(a)(E) by not quite 1 order of magnitude, and Q(a)(K) by nearly 3 orders of magnitude. The beta -oxygen substituent also retards the rate of hydronium-ion-catalyzed ketonization by more than 3 orders of magnitude. The origins of these substituent effects are discussed.