Chiang Yvonne, Kresge A Jerry, Sadovski Oleg, Zhan Hao-Qiang
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
J Org Chem. 2005 Mar 4;70(5):1643-6. doi: 10.1021/jo0402677.
o-Thioquinone methide, 2, was generated in aqueous solution by flash photolysis of benzothiete, 1, and rates of hydration of this quinone methide to o-mercaptobenzyl alcohol, 3, were measured in perchloric acid solutions, using H2O and D2O as the solvent, and also in acetic acid and tris(hydroxymethyl)methylammonium ion buffers, using H2O as the solvent. The rate profiles constructed from these data show hydronium-ion-catalyzed and uncatalyzed hydration reaction regions, just like the rate profiles based on literature data for hydration of the oxygen analogue, o-quinone methide, of the presently examined substrate. Solvent isotope effects on hydronium-ion catalysis of hydration for the two substrates, however, are quite different: k(H)/k(D) = 0.42 for the oxygen quinone methide, whereas k(H)/k(D) = 1.66 for the sulfur substrate. The inverse nature (k(H)/k(D) < 1) of the isotope effect in the oxygen system indicates that this reaction occurs by a preequilibrium proton-transfer reaction mechanism, with protonation of the substrate on its oxygen atom being fast and reversible and capture of the benzyl-type carbocationic intermediate so formed being rate-determining. The normal direction (k(H)/k(D) > 1) of the isotope effect in the sulfur system, on the other hand, suggests that protonation of the substrate on its sulfur atom is in this case rate-determining, with carbocation capture a fast following step. A semiquantitative argument supporting this hypothesis is presented.
通过苯并噻硫酮（1）的闪光光解在水溶液中生成邻硫代苯醌甲基化物（2），并在高氯酸溶液中，以H₂O和D₂O作为溶剂，以及在乙酸和三（羟甲基）甲基铵离子缓冲液中，以H₂O作为溶剂，测量了该醌甲基化物水合生成邻巯基苄醇（3）的速率。根据这些数据构建的速率分布图显示了水合氢离子催化和非催化水合反应区域，就像基于目前所研究底物的氧类似物邻醌甲基化物水合的文献数据构建的速率分布图一样。然而，两种底物水合的水合氢离子催化的溶剂同位素效应却大不相同：氧醌甲基化物的k(H)/k(D) = 0.42，而硫底物的k(H)/k(D) = 1.66。氧体系中同位素效应的反向性质（k(H)/k(D) < 1）表明该反应是通过预平衡质子转移反应机制发生的，底物在其氧原子上的质子化快速且可逆，而捕获由此形成的苄基型碳正离子中间体是速率决定步骤。另一方面，硫体系中同位素效应的正常方向（k(H)/k(D) > 1）表明，在这种情况下底物在其硫原子上的质子化是速率决定步骤，随后碳正离子的捕获是快速步骤。本文提出了支持这一假设的半定量论据。
