Su MD, Chu SY
School of Chemistry, Kaoshiung Medical University, Taiwan, Republic of China.
Chemistry. 2000 Oct 16;6(20):3777-87. doi: 10.1002/1521-3765(20001016)6:20<3777::aid-chem3777>3.0.co;2-5.
The potential-energy surfaces for the abstraction reactions of carbenes with oxirane and thiirane have been characterized in detail by using density functional theory (B3LYP/6-31G*), which include zero-point corrections. Six carbene species: dimethylcarbene, cyclobutylidene, cyclohexylidene, phenylchlorocarbene, methoxyphenylcarbene, and dimethoxycarbene have been chosen in this work is model reactants. All the interactions involve the initial formation of a loose donor-acceptor complex followed by a heteroatom shift via a two-center transition state. The complexation energies, activation barriers, and enthalpies of the reactions were used comparatively to determine the relative carbenic reactivity, as well as the influence of substituents on the reaction potential-energy surface. As a result, our theoretical investigations indicate that, irrespective of deoxygenation and desulfurization, the relative carbenic reactivity decreases in the order: cyclobutylidene > dimethylcarbene approximately equals cyclohexylidene > phenylchlorocarbene > methoxyphenylcarbene >> dimethoxycarbene. Namely, the alkyl-substituted carbene abstractions are much more favorable than those of the pi-donor-substituted carbenes. Moreover, for a given carbene, while both deoxygenation and desulfurization are facile processes, the deoxygenation reaction is more exothermic but less kinetically favorable. Furthermore, a configuration-mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained are in good agreement with available experimental observations, and allow a number of predictions to be made.
利用密度泛函理论（B3LYP/6 - 31G*）对卡宾与环氧乙烷和硫杂环丙烷的抽象反应势能面进行了详细表征，其中包括零点校正。本工作选取了六种卡宾物种：二甲基卡宾、环丁烯卡宾、环己烯卡宾、苯基氯卡宾、甲氧基苯基卡宾和二甲氧基卡宾作为模型反应物。所有相互作用均涉及首先形成一个松散的供体 - 受体复合物，随后通过一个双中心过渡态发生杂原子迁移。通过比较反应的络合能、活化能垒和反应焓来确定相对的卡宾反应活性，以及取代基对反应势能面的影响。结果表明，我们的理论研究表明，无论脱氧和脱硫情况如何，相对卡宾反应活性按以下顺序降低：环丁烯卡宾＞二甲基卡宾≈环己烯卡宾＞苯基氯卡宾＞甲氧基苯基卡宾＞＞二甲氧基卡宾。也就是说，烷基取代的卡宾抽象反应比π供体取代的卡宾抽象反应更有利。此外，对于给定的卡宾，虽然脱氧和脱硫都是容易发生的过程，但脱氧反应放热更多，但动力学上不太有利。此外，基于Pross和Shaik的工作建立了一个构型混合模型来解释计算结果。所得结果与现有的实验观察结果吻合良好，并可做出一些预测。
