Experimental geometry of the epoxidation transition state
Experimental geometry of the epoxidation transition state
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DOI：
10.1021/ja963656u
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发表时间：
1997-04-09
影响因子：
15
通讯作者：
Houk, KN
中科院分区：
文献类型：
--
作者：
Singleton, DA;Merrigan, SR;Houk, KN
烯烃与过酸的环氧化反应是一种通用的合成方法，自从Bartlett提出该反应通过“蝴蝶”机理发生以来，它在物理有机化学年鉴中占有神圣的地位。[2]在大多数情况下，该机制的一致性已被接受，但关于两个CO键形成的时间和CO键形成和必要的氢转移所涉及的事件的顺序，有许多相反的建议。3-8取代基效应的经典研究表明，几乎同步形成的两个CO键。7例如，2-丁烯和异丁烯以几乎相同的速率环氧化。7 b相反，4-乙烯基联苯环氧化反应的2 H动力学同位素效应（KIE）被解释为涉及非常异步的过渡态。[3]这一观点最近得到了MP2/6- 31 G * 水平的从头计算的支持，它预测了高度不对称和异步的跃迁结构。5，6我们最近使用了大量的高精度实验KIE与高级别过渡结构/KIE计算的比较，明确定义了Diels-Alder过渡态的几何形状。[9]这里我们将这种方法应用于环氧化反应。结果提供了一个详细的，实验为基础的图片与几乎同步形成的CO键的环氧化过渡态一致，并确认环氧化过渡态的预测一般。8对于乙烯与过甲酸的反应，Becke 3-LYP/6- 31 G *（B3 LYP）10过渡结构（1）与先前在MP2/6- 31 G *（MP2）计算中获得的结构（2）显著不同。5，6它们都是螺环，但1具有Cs对称性，2中的两个新CO键的形成程度非常不同，过甲酸的粗糙平面相对于乙烯的CC轴在0.50处偏斜。在Becke 3-LYP/6- 31 G *+ ZPE水平上预测了1的活化势垒
The epoxidation of alkenes with peracids is a general synthetic method, 1 and has had a hallowed place in the annals of physical organic chemistry since Bartlett proposed that the reaction occurs through the “butterfly” mechanism. 2 For the most part the concerted nature of the mechanism has been accepted, but there have been many contrasting suggestions about the timing of formation of the two CO bonds and the sequence of events involved in CO bond formation and the requisite hydrogen transfer. 3-8 Classical studies of substituent effects suggested nearly synchronous formation of the two CO bonds. 7 For example, 2-butene and isobutene are epoxidized at nearly identical rates. 7b In contrast, 2H kinetic isotope effects (KIEs) for epoxidation of 4-vinylbiphenyl were interpreted as implicating a very asynchronous transition state. 3 This view has recently been supported by ab initio calculations at the MP2/6-31G* level which predict highly unsymmetrical and asynchronous transition structures. 5, 6We have recently used the comparison of a large set of highprecision experimental KIEs with high-level transition structure/KIE calculations to explicitly define the geometry of a Diels-Alder transition state. 9 Here we apply this methodology to epoxidation. The results provide a detailed, experimentally based picture of the epoxidation transition state consistent with nearly synchronous formation of the CO bonds and confirm predictions made about epoxidation transition states in general. 8 For the reaction of ethylene with performic acid, the Becke3-LYP/6-31G*(B3LYP) 10 transition structure (1) differs dramatically from that obtained previously in MP2/6-31G*(MP2) calculations (2). 5, 6 They are both spiro, but while 1 has Cs symmetry, the two new CO bonds in 2 have formed to very different extents and the rough plane of the performic acid is skewed at≈ 50 to the CC axis of the ethylene. The activation barrier predicted for 1 at the Becke3-LYP/6-31G*+ ZPE level