Competing solvolytic elimination and substitution reactions via very short-lived ion-pair intermediates
作者:Xiaofeng Zeng、Alf Thibblin
DOI:10.1039/b102869n
日期:——
The solvolysis of 9-methyl-9-(2-X-2-propyl)fluorene (X = Cl, Br, or OOCCF3) (1-X) in aqueous acetonitrile or aqueous 1,1,1-trifluoroethanol (TFE) yields the alkene, 9-methyl-9-(propen-2-yl)fluorene (2), and the alcohol, 9-methyl-9-(2-hydroxy-2-propyl)fluorene (1-OH), accompanied by a small amount of substitution product from the reaction with the organic component of the solvent mixture. The fraction of elimination product increases with decreasing fraction of water in the solvent mixture as well as by addition of general bases, which can be expressed by a Brønsted parameter of β = 0.07 for the reaction of 1-Cl, measured with substituted acetate anions. Also, the addition of chloride ions increases the fraction of alkene. The kinetic
deuterium isotope effects on the reactions of the hexadeuterated substrates vary with solvent composition in a way which is not consistent with a common carbocation intermediate which has time to choose between dehydronation and addition of a solvent water molecule. A stepwise preassociation mechanism is proposed for the elimination reaction.
9-甲基-9-(2-X-2-丙基)芴 (X = Cl、Br 或 OOCCF3) (1-X) 在乙腈水溶液或 1,1,1-三氟乙醇 (TFE) 水溶液中溶剂解,得到烯烃,9-甲基-9-(丙烯-2-基)芴(2),和醇,9-甲基-9-(2-羟基-2-丙基)芴(1-OH),并伴有与溶剂混合物的有机组分反应产生少量取代产物。消除产物的分数随着溶剂混合物中水分数的减少以及一般碱的添加而增加,这可以通过 1-Cl 反应的 β = 0.07 的布朗斯台德参数来表示,用取代的乙酸根阴离子测量。此外,氯离子的添加增加了烯烃的分数。动力学
氘同位素对六氘化底物反应的影响随溶剂组成的不同而变化,这与常见的碳正离子中间体不一致,后者有时间在脱水和添加溶剂水分子之间进行选择。提出了用于消除反应的逐步预缔合机制。