Flash Photolytic Generation of o-Quinone α-Phenylmethide and o-Quinone α-(p-Anisyl)methide in Aqueous Solution and Investigation of Their Reactions in that Medium. Saturation of Acid-Catalyzed Hydration

o-Quinone α-phenylmethide was generated as a short-lived transient species in aqueous solution by flash photolysis of o-hydroxy-α-phenylbenzyl alcohol, and its rate of decay was measured in HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4 -, and HCO3 - buffers. These data show that hydration of this quinone methide back to its benzyl alcohol precursor occurs by acid-, base-, and uncatalyzed routes. The acid-catalyzed reaction gives the solvent isotope effect k H + /k D + = 0.34, whose inverse nature indicates that this reaction occurs via rapid preequilibrium protonation of the quinone methide on its carbonyl oxygen atom followed by rate-determining capture of the ensuing carbocationic intermediate by water, a conclusion supported by the saturation of acid catalysis in concentrated HClO4 solution. o-Quinone α-(p-anisyl)methide was also generated by flash photolysis of the corresponding benzyl alcohol and of the p-cyanophenol ether of this alcohol as well, and its rate of decay was measured in HClO4 and NaOH solutions and in HCO2H, CH3CO2H, HN3, CF3CH2NH3 +, imidazolium ion, H2PO4 -, (CH2OH)3CNH3 +, (CH3)3CPO3H-, and HCO3 - buffers. Acid-, base-, and uncatalyzed hydration reaction routes were again found, and solvent isotope effects as well as saturation of acid catalysis, this time in dilute HClO4, confirmed a preequilibrium mechanism for the acid-catalyzed reaction. Analysis of the buffer data gave buffer-base rate constants that did not conform to the Brønsted relation, consistent with the expected nucleophilic nature of the buffer reactions.