Bekele Tefsit, Christian Chad F, Lipton Mark A, Singleton Daniel A
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
J Am Chem Soc. 2005 Jun 29;127(25):9216-23. doi: 10.1021/ja0508673.
The C2-C6 (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.
通过动力学同位素效应、理论计算和动力学轨迹相结合的方法，研究了烯炔丙二烯的C2-C6（施密特尔）/烯环化反应。对于乙酸烯丙酯9的环化反应，同位素效应（k(CH3)/k(CD3)约为1.43。该同位素效应可根据接近协同/逐步边界的高度异步过渡态来解释。密度泛函理论计算支持了这一点，该计算确定了协同烯反应的高度异步过渡结构。然而，对实验体系和模型反应的计算均未能找到逐步机理中双自由基中间体形成的过渡结构。逐步机理和异步协同机理在几何结构上开始时相似，并且就初始过渡结构而言，这两条途径似乎已经合并。对于模型反应，从初始过渡结构出发的准经典直接动力学轨迹在101条轨迹中有29条产生了双自由基中间体。尽管在异步过渡结构中碳-碳键形成已提前，但其余大部分轨迹在碳-碳键形成之前就完成了氢转移。总体而言，从起始原料到产物的单一最低能量路径不足以描述该反应，必须考虑动力学效应才能理解其机理。讨论了这些观察结果对其他反应中协同问题的影响。
