Chiang Y, Kresge A J, Zhu Y
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
J Am Chem Soc. 2001 Aug 22;123(33):8089-94. doi: 10.1021/ja010826g.
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 that underwent hydration back to benzyl alcohol in hydrogen-ion catalyzed (k(H+) = 8.4 x 10(5) M(-1) s(-1)) and hydroxide-ion 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 effect k(H+)/k(D)+ = 0.42, whose inverse nature indicates that this process occurs by rapid and reversible equilibrium protonation of the carbonyl oxygen atom of the quinone methide, followed by rate-determining capture of the carbocation 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 quinone methide. Decay of the quinone methide is also accelerated by acetic acid buffers through both acid- and base-catalyzed pathways, and quantitative analysis of the reaction products formed in these solutions shows that this acceleration is caused by nucleophilic reactions of acetate ion rather than by acetate ion assisted hydration. Bromide and thiocyanate ions also accelerate decay of the quinone methide through both hydrogen-ion catalyzed and uncatalyzed pathways, and the inverse nature of solvent isotope effects on the hydrogen-ion catalyzed reactions shows that these reactions also occur by rapid equilibrium protonation of the quinone methide carbonyl oxygen followed 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 for the acidity constant of the carbonyl-protonated quinone methide.
在高氯酸水溶液、氢氧化钠溶液、乙酸和磷酸二氢根离子缓冲溶液中，对邻羟基苯甲醇、邻羟基苄基对氰基苯基醚和（邻羟基苄基）三甲基碘化铵进行闪光光解，生成邻醌甲基化物，它是一种短寿命的瞬态物种，会在氢离子催化（k(H+) = 8.4×10⁵ M⁻¹ s⁻¹）、氢氧根离子催化（k(HO⁻) = 3.0×10⁴ M⁻¹ s⁻¹）的反应以及一个无催化（k(UC) = 2.6×10² s⁻¹）的过程中发生水合反应回到苄醇。氢离子催化的反应给出溶剂同位素效应k(H+)/k(D⁺) = 0.42，其相反的性质表明该过程是通过醌甲基化物羰基氧原子的快速且可逆的平衡质子化，随后是由水对如此产生的碳正离子进行速率决定步骤的捕获而发生的。另一方面，无催化反应速率常数的大小表明该过程是通过水对醌甲基化物亚甲基的简单亲核加成而发生的。醌甲基化物的衰变也会被乙酸缓冲液通过酸催化和碱催化途径加速，对这些溶液中形成的反应产物进行定量分析表明，这种加速是由醋酸根离子的亲核反应引起的，而不是由醋酸根离子辅助的水合作用引起的。溴离子和硫氰酸根离子也会通过氢离子催化和无催化途径加速醌甲基化物的衰变，并且溶剂同位素效应在氢离子催化反应中的相反性质表明这些反应也是通过醌甲基化物羰基氧的快速平衡质子化，随后是对产生的碳正离子进行速率决定步骤的亲核捕获而发生的。给硫氰酸反应的速率决定步骤的速率常数赋予一个遭遇控制值，得到羰基质子化醌甲基化物的酸度常数pK(a) = -1.7。
