Flash photolytic generation of ortho-quinone methide in aqueous solution and study of its chemistry in that medium

Flash photolysis of o-hydroxybenzyl alcohol, o-hydroxybenzyl p-cyanophenyl ether, and (o-hydroxybenzyl)trimethylammonium iodide in aqueous perchloric acid and sodium hydroxide solutions, and in acetic acid and biphosphate ion buffers, produced o-quinone methide as a short-lived transient species tha t underwent hydration back to benzyl alcohol in hydrogen-ion catalyzed (k(H )+ = 8.4 x 10(5) M-1 s(-1)) and hydroxideion catalyzed (k(HO)- = 3.0 x 10(4 ) M-1 s(-1)) reactions as well as an uncatalyzed (k(UC) = 2.6 x 10(2) s(-1) ) process. The hydrogen-ion catalyzed reaction gave the solvent isotope eff ect k(H)+/k(D)+ = 0.42, whose inverse nature indicates that this process oc curs by rapid and reversible equilibrium protonation of the carbonyl oxygen atom of the quinone methide, followed by rate-determining capture of the c arbocation so produced by water. The magnitude of the rate constant of the uncatalyzed reaction, on the other hand, indicates that this process occurs by simple nucleophilic addition of water to the methylene group of the qui none methide. Decay of the quinone methide is also accelerated by acetic ac id buffers through both acid- and base-catalyzed pathways, and quantitative analysis of the reaction products formed in these solutions shows that thi s acceleration is caused by nucleophilic reactions of acetate ion rather th an by acetate ion assisted hydration. Bromide and thiocyanate ions also acc elerate decay of the quinone methide through both hydrogen-ion catalyzed an d uncatalyzed pathways, and the inverse nature of solvent isotope effects o n the hydrogen-ion catalyzed reactions shows that these reactions also occu r by rapid equilibrium protonation of the quinone methide carbonyl oxygen f ollowed by rate-determining nucleophilic capture of the ensuing carbocation . Assignment of an encounter-controlled value to the rate constant for the rate-determining step of the thiocyanate reaction leads to pK(a) = - 1.7 fo r the acidity constant of the carbonyl-protonated quinone methide.