Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, P. R. China.
University of Chinese Academy of Sciences , Beijing 100049, P. R. China.
J Am Chem Soc. 2017 Nov 8;139(44):15564-15567. doi: 10.1021/jacs.7b06086. Epub 2017 Oct 30.
Iron-catalyzed alkene [2+2] cycloaddition reactions represent a promising stepwise pathway to effect the kinetically hindered concerted [2+2] cycloaddition. However, the fundamental reactivity paradigm of these reactions remains unclear. Based on high level combined CASPT2/DFT modelings, herein we reveal an unprecedented substrate-dependent two-state reactivity scenario for the key C-C coupling in this iron catalysis, in which the representative substrates of mono-olefins only and mono-olefin plus 1,3-diene exhibit different reactivity paradigms. The role of the redox-active ligand is found to generate a ferric oxidation state for the metallacyclic intermediate of C-C coupling, thereby rendering a thermodynamically more accessible Fe/Fe reductive elimination process compared with the otherwise Fe/Fe one. The enhancement of the spin state transition efficiency between the singlet and triplet states is predicted as an alternative way to increase the C-C coupling reactivity in the cross [2+2] cycloaddition reactions between mono-olefins and dienes. This work highlights the ab initio multi-reference method in describing very complicated open-shell iron catalysis.
铁催化的烯烃 [2+2] 环加成反应代表了一种很有前途的逐步途径，可以实现动力学上受阻的协同 [2+2] 环加成。然而，这些反应的基本反应性模式仍不清楚。基于高水平的组合 CASPT2/DFT 建模，本文揭示了铁催化中关键 C-C 偶联的前所未有的底物依赖性两态反应性情景，其中单烯烃和单烯烃加 1,3-二烯的代表性底物表现出不同的反应性模式。发现氧化还原活性配体的作用是为 C-C 偶联的金属环中间体生成铁的氧化态，从而使 Fe/Fe 还原消除过程在热力学上比其他情况下的 Fe/Fe 更易接近。预测增加单烯烃和二烯之间交叉 [2+2] 环加成反应中单重态和三重态之间的自旋态跃迁效率可以作为增加 C-C 偶联反应性的另一种方法。这项工作突出了从头算多参考方法在描述非常复杂的开壳层铁催化中的作用。
