Zhang Zheng-Feng, Su Ming-Der
Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan.
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan.
ACS Omega. 2021 Aug 20;6(34):22272-22283. doi: 10.1021/acsomega.1c02958. eCollection 2021 Aug 31.
The mechanisms of C-H bond insertion and alkene cycloaddition were investigated theoretically using five model systems: group 13 analogues of the four-membered nucleophilic -heterocyclic carbenes (NHCs) (; E = group 13 element). The theoretical findings indicate that, except for with HC=CH, these four-membered NHCs undergo insertion and [1 + 2] cycloaddition reactions with difficulty because their activation barriers are quite high (31 kcal/mol). The theoretically confirmed chemical inertness of the four-membered NHCs and might explain why they have been experimentally detected at room temperature. Additionally, our theoretical observations indicate that the reactivity of these four-membered NHCs featuring a central group 13 element follows the order ≫ > > > . The theoretical examination suggests that the smaller the atomic radius of the central group 13 element in the four-membered NHC analogue is, the larger the aromaticity of this carbenic molecule is, the higher the basicity of this carbenic molecule in nature is, the larger its nucleophilic attack on other oncoming molecules is, the smaller the barrier heights of its C-H bond insertion and [1 + 2] cycloaddition reactions will be, the higher its exothermicities for these products will be, and thus, the greater its reactivity will be. Moreover, the present theoretical findings reveal that the reactivity of is governed by its highest occupied molecular orbital, a nonbonding sp lone pair orbital. In contrast, the reactivity of the four heavier (E' = Al, Ga, In, and Tl) molecules is mainly determined by their lowest unoccupied molecular orbital, a vacant p-π orbital. The conclusions gained from this study allow many predictions to be made.
使用五个模型体系从理论上研究了C-H键插入和烯烃环加成的机理：四元亲核杂环卡宾（NHCs）的13族类似物（；E = 13族元素）。理论研究结果表明，除了与HC=CH的情况外，这些四元NHCs难以发生插入反应和[1 + 2]环加成反应，因为它们的活化能垒相当高（31 kcal/mol）。理论上证实的四元NHCs 和 的化学惰性或许可以解释为什么它们能在室温下通过实验检测到。此外，我们的理论观察表明，这些以13族元素为中心的四元NHCs的反应活性顺序为 ≫ > > > 。理论研究表明，四元NHC类似物中中心13族元素的原子半径越小，该卡宾分子的芳香性越大，其本质上的碱性越强，对其他即将到来分子的亲核攻击越大，其C-H键插入和[1 + 2]环加成反应的能垒高度越小，这些产物的放热性越高，因此，其反应活性越大。此外，目前的理论研究结果表明， 的反应活性由其最高占据分子轨道（一个非键sp孤对轨道）决定。相比之下，四种较重的 （E' = Al、Ga、In和Tl）分子的反应活性主要由其最低未占据分子轨道（一个空的p-π轨道）决定。从这项研究中得出的结论使得能够做出许多预测。
