"Concerted" transition state, stepwise mechanism. dynamics effects in C2-C6 enyne allene cyclizations
"Concerted" transition state, stepwise mechanism. dynamics effects in C2-C6 enyne allene cyclizations
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DOI：
10.1021/ja0508673
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发表时间：
2005-06-29
影响因子：
15
通讯作者：
Singleton, DA
中科院分区：
文献类型：
--
作者：
Bekele, T;Christian, CF;Singleton, DA
结合动力学同位素效应、理论计算和动力学轨迹研究了烯炔-联烯的C-2-C-6（Schmittel）/烯环化反应。对于联烯醇乙酸酯9的环化，同位素效应（k（CH 3）/k（CD 3））约为1.43。同位素效应解释在一个高度异步的过渡态附近的协调/逐步的边界，这是支持密度泛函理论计算，定位一个高度异步的过渡结构的协调烯反应。然而，实验系统和模型反应的计算都无法找到一个过渡结构形成的双自由基中间体的逐步机制，逐步机制和异步协调机制开始几何相似，这两个途径似乎已经合并到初始过渡结构。对于模型反应，从初始过渡结构发出的准经典直接动力学轨迹在101个轨迹中的29个中提供了双自由基中间体。大部分剩余的轨道完成氢转移之前，碳-碳键的形成，尽管先进的碳碳键形成的异步过渡结构。总的来说，从起始材料到产物的单一最小能量路径不足以描述反应，并且考虑动力学效应对于理解机制是必要的。这些意见对其他反应的音乐会的问题的影响进行了讨论。
The C-2-C-6 (Schmittel)/ene cyclization of enyne-allenes is studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. For the cyclization of allenol acetate 9, the isotope effect (k(CH3)/k(CD3)) is approximately 1.43. The isotope effect is interpreted in terms of a highly asynchronous transition state near the concerted/stepwise boundary, This is supported by density functional theory calculations that locate a highly asynchronous transition structure for the concerted ene reaction. However, calculations of both the experimental system and a model reaction were unable to locate a transition structure for formation of the diradical intermediate of a stepwise mechanism, The stepwise mechanism and the asynchronous concerted mechanism start out geometrically similar, and the two pathways appear to have merged as far as the initial transition structure. For the model reaction, quasiclassical direct dynamics trajectories emanating from the initial transition structure afforded the diradical intermediate in 29 out of 101 trajectories. A large portion of the remaining trajectories completes hydrogen transfer before carbon-carbon bond formation, despite the advanced carbon carbon bond formation in the asynchronous transition structure. Overall, the single minimum-energy path from starting material to product is inadequate to describe the reaction, and a consideration of dynamic effects is necessary to understand the mechanism. The implications of these observations toward questions of concert in other reactions are discussed.