THEORETICAL-STUDY OF THE ALLENE EFFECT IN [1,N] SIGMATROPIC HYDROGEN SHIFTS
THEORETICAL-STUDY OF THE ALLENE EFFECT IN [1,N] SIGMATROPIC HYDROGEN SHIFTS
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DOI：
10.1021/ja00133a021
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发表时间：
1995-07-19
影响因子：
15
通讯作者：
JENSEN, F
中科院分区：
文献类型：
--
作者：
JENSEN, F
用从头算方法研究了CH_3-（CH=CH）（（n-3）/2）- CH= CH_2体系中[1，n]氢转移反应（n = 3，5，7，9和11），并与末端双键被丙二烯取代的相应反应进行了比较. MP2/6- 31 G ~*//HF/3- 21 G水平上的活化能相当准确地再现了[1，5]和[1，7]反应的实验结果。对于[1，3]和[1，5]反应，引入丙二烯使活化能降低10- 1 - 2 kcal/mol，而对于[1，7]和[1，9]反应的影响仅为1-2 kcal/mol。通过将过渡结构视为两个相互作用的自由基片段，可以合理化母体系统活化能的变化以及母体和丙二烯系统之间活化能的差异。计算表明，只有一个非常弱的依赖性的主要动力学同位素效应的过渡态几何形状（线性和对称性的氢转移），计算值是远远小于实验数据，表明隧道是重要的。
The [1,n] hydrogen shift reactions (n = 3, 5, 7, 9, and 11) in CH3 - (CH=CH)((n-3)/2) - CH=CH2 have been investigated by ab initio calculations, and the results are compared with the corresponding reaction in systems where the terminal double bond is substituted by an allene unit. Activation energies at the MP2/6-31G*//HF/3-21G level reproduce experimental results for the [1,5] and [1,7] reactions quite accurately. For the [1,3] and [1,5] reactions the introduction of an allene moiety lowers the activation energy by 10-12 kcal/mol, while the effect for the [1,7] and [1,9] reactions is only 1-2 kcal/mol. By considering the transition structures as two interacting radical fragments it is possible to rationalize both the variation in activation energy for the parent systems and the difference in activation energy between the parent and allene systems. The calculations indicate only a very weak dependence of primary kinetic isotope effects on transition state geometries (linearity and symmetry of the hydrogen transfer), and the calculated values are much smaller than experimental data, suggesting that tunneling is important.