MacMillan Joshua W M, Marczenko Katherine M, Johnson Erin R, Chitnis Saurabh S
Department of Chemistry, Dalhousie University, 6274 Coburg Road, B3H 4R2, Halifax, Canada.
Chemistry. 2020 Dec 18;26(71):17134-17142. doi: 10.1002/chem.202003153. Epub 2020 Nov 11.
The addition of Sb-H bonds to alkynes was reported recently as a new hydroelementation reaction that exclusively yields anti-Markovnikov Z-olefins from terminal acetylenes. We examine four possible mechanisms that are consistent with the observed stereochemical and regiochemical outcomes. A comprehensive analysis of solvent, substituent, isotope, additive, and temperature effects on hydrostibination reaction rates definitively refutes three ionic mechanisms involving closed-shell charged intermediates. Instead the data support a fourth pathway featuring open-shell neutral intermediates. Density-functional theory (DFT) calculations are consistent with this model, predicting an activation barrier that is in agreement with the experimental value (Eyring analysis) and a rate limiting step that is congruent with the experimental kinetic isotope effect. We therefore conclude that hydrostibination of arylacetylenes is initiated by the generation of stibinyl radicals, which then participate in a cycle featuring Sb and Sb intermediates to yield the observed Z-olefins as products. This mechanistic understanding will enable rational evolution of hydrostibination as a synthetic methodology.
最近有报道称，向炔烃中添加Sb-H键是一种新的加氢元素化反应，该反应仅从末端乙炔生成反马氏规则的Z-烯烃。我们研究了四种与观察到的立体化学和区域化学结果一致的可能机理。对溶剂、取代基、同位素、添加剂和温度对氢化锑化反应速率的影响进行的全面分析明确驳斥了三种涉及闭壳带电中间体的离子机理。相反，数据支持了以开壳中性中间体为特征的第四条途径。密度泛函理论（DFT）计算与该模型一致，预测的活化能垒与实验值（艾林分析）一致，速率限制步骤与实验动力学同位素效应一致。因此，我们得出结论，芳基乙炔的氢化锑化反应是由锑基自由基的产生引发的，这些自由基随后参与一个以Sb和Sb中间体为特征的循环，生成观察到的Z-烯烃作为产物。这种机理理解将有助于氢化锑化作为一种合成方法的合理发展。
