Flash photolytic generation of ortho-quinone methide in aqueous solution and study of its chemistry in that medium
Flash photolytic generation of ortho-quinone methide in aqueous solution and study of its chemistry in that medium
复制标题
DOI：
10.1021/ja010826g
复制
发表时间：
2001-08-22
影响因子：
15
通讯作者：
Zhu, Y
中科院分区：
文献类型：
--
作者：
Chiang, Y;Kresge, AJ;Zhu, Y
邻羟基苯甲醇、邻羟基苯甲基对氰基苯基醚和（邻羟基苯甲基）三甲基碘化铵在高氯酸和氢氧化钠水溶液以及乙酸和磷酸氢二离子缓冲液中进行快速光解，产生作为短寿命瞬态物质的邻醌甲基化物，该物质在氢离子催化下水合回苯甲醇（k(H)+ = 8.4 x 10(5) M-1 s(-1)) 和氢氧离子催化 (k(HO)- = 3.0 x 10(4) M-1 s(-1)) 反应以及非催化 (k(UC) = 2.6 x 10(2) s(-1)) 过程。氢离子催化反应产生溶剂同位素效应 k(H)+/k(D)+ = 0.42，其逆性质表明该过程是通过醌甲基化物的羰基氧原子快速且可逆的平衡质子化发生的，然后是由水产生的碳正离子的限速捕获。另一方面，非催化反应的速率常数的大小表明该过程是通过水与醌甲基化物的亚甲基的简单亲核加成而发生的。乙酸缓冲液通过酸和碱催化途径也加速了醌甲基化物的衰变，对这些溶液中形成的反应产物的定量分析表明，这种加速是由乙酸根离子的亲核反应引起的，而不是由乙酸根离子辅助水合引起的。溴化物和硫氰酸根离子还通过氢离子催化和非催化途径加速醌甲基化物的衰变，并且溶剂同位素对氢离子催化反应的影响的逆性质表明，这些反应也通过醌甲基化物羰基氧的快速平衡质子化以及随后对随后的碳阳离子进行速率决定的亲核捕获而发生。将遭遇控制值分配给硫氰酸酯反应的速率决定步骤的速率常数导致羰基质子化醌甲基化物的酸度常数pK(a) = - 1.7。
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 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 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.