Isotope effects and the mechanism of chlorotrimethylsilane-mediated addition of cuprates to enones
Isotope effects and the mechanism of chlorotrimethylsilane-mediated addition of cuprates to enones
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DOI：
10.1021/ja993373c
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发表时间：
2000-04-12
影响因子：
15
通讯作者：
Singleton, DA
中科院分区：
文献类型：
--
作者：
Frantz, DE;Singleton, DA
测定了三甲基氯硅烷介导的环己烯酮与二丁基铜酸锂在四氢呋喃中以及与丁基(叔丁基乙炔基)铜酸锂在乙醚中的反应的动力学同位素效应。对于四氢呋喃中的反应，观察到显着的羰基氧同位素效应（(16)k/(17)k = 1.018-1.019）和小的烯属碳同位素效应（(12)k/(13)k = 1.003-1.008）与中间π配合物的限速硅烷化一致。理论上预测的模型反应的同位素效应支持了这一结论。三甲基氯硅烷与二甲基苯基氯硅烷的相对反应性研究也支持限速硅烷化。丁基碳同位素对产物形成不存在显着影响，表明铜酸丁基在导致产物确定步骤的中间体中是不等价的。在乙醚中，同位素效应恢复到与先前在不存在三甲基氯硅烷的情况下环己烯酮与二丁基铜酸锂反应中发现的值类似的值，这与该溶剂中的总体机理没有变化一致。提出了 TMSCl 随着溶剂和底物变化的不同影响的机制假设。
Kinetic isotope effects were determined for the chlorotrimethylsilane-mediated reactions of cyclohexenone with lithium dibutylcuprate in tetrahydrofuran and with lithium butyl(tert-butylethynyl)cuprate in ether. For the reaction in tetrahydrofuran, the observation of a significant carbonyl oxygen isotope effect ((16)k/(17)k = 1.018-1.019) and small olefinic carbon isotope effects ((12)k/(13)k = 1.003-1.008) is consistent with rate-limiting silylation of an intermediate pi-complex. Theoretically predicted isotope effects for model reactions support this conclusion. Rate-limiting silylation is also supported by relative reactivity studies of chlorotrimethylsilane versus chlorodimethylphenylsilane. The absence of a significant butyl-group carbon isotope effect on product formation indicates that the cuprate butyl groups are nonequivalent in the intermediate leading to the product-determining step. In diethyl ether the isotope effects revert to values similar to those found previously in reactions of cyclohexenone with Lithium dibutylcuprate in the absence of chlorotrimethylsilane, consistent with no change in the overall mechanism in this solvent. A mechanistic hypothesis For the differing effects of TMSCl with changes in solvent and substrate is presented.