TRANSITION-STATE STRUCTURAL VARIATION AND MECHANISTIC CHANGE
TRANSITION-STATE STRUCTURAL VARIATION AND MECHANISTIC CHANGE
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DOI：
10.1039/ft9949001709
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发表时间：
1994-06-21
期刊：
影响因子：
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通讯作者：
WILLIAMS, IH
中科院分区：
文献类型：
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作者：
BARNES, JA;WILKIE, J;WILLIAMS, IH
莫尔 - 奥费拉尔 - 詹克斯图（More O'Ferrall - Jencks diagrams）在有机反应性的理论观点和实验观点之间提供了一个重要的联系，它通常用于对反应机理以及过渡态结构变化的经验性讨论。我们对α - 取代的烷基底物上的S_N2取代反应以及羰基的一般酸催化氢化物还原反应进行了半经验的AM1分子轨道计算，这些计算表明过渡态结构的变化与这些图的预测相符。我们提供了一个在考虑机理变化时使用这些图可能存在危险的例子。我们研究了在简并反应H₂O + CHR'R''OH₂⁺ → +H₂OCHR'R'' + OH₂中，随着α - 取代基R' 和R'' 的改变，反应机理从S_N2到S_N1的转变。甲基（R' = R'' = H）和乙基（R' = H，R'' = Me）底物按S_N2机理反应，而异丙基（R' = R'' = Me）、甲氧基甲基（R' = H，R'' = OMe）和甲氧基乙基（R' = Me，R'' = OMe）底物遵循S_N1机理。根据多林（Doering）和蔡斯（Zeiss）的经典模型，这些机理相互融合，S₁中间体出现时的结构和能量与消失的S_N2过渡态基本相同。
An important link between theoretical and experimental views of organic reactivity is provided by More O'Ferrall-Jencks diagrams, which are commonly used in empirical discussions of reaction mechanisms and of changes in transition-state structure. We have performed semiempirical AM1 molecular-orbital calculations for S(N)2 displacements on alpha-substituted alkyl substrates and for general acid-catalysed hydride reduction of a carbonyl group which demonstrate transition-state structural variations in accord with the predictions of these diagrams. We provide an example of a possible danger associated with the use of these diagrams in considerations of mechanistic change. We have examined the changeover in mechanism from S(N)2 to S(N)1 occurring in the degenerate reaction H2O + CHR'R''OH2+ --> +H2OCHR'R'' + OH2 as the alpha-substituents R' and R'' are altered. The methyl (R' = R'' = H) and ethyl (R' = H, R'' = Me) substrates react by the S(N)2 mechanism, whereas the isopropyl (R' = R'' = Me), methoxymethyl (R' = H, R'' = OMe) and methoxyethyl (R' = Me, R'' = OMe) substrates follow the S(N)1 mechanism. The mechanisms merge according to the classical model of Doering and Zeiss, with the Sl intermediate appearing with essentially the same structure and energy as the disappearing S(N)2 transition state.