Wang Zhe, Akisaka Rikuo, Yabumoto Sohshi, Nakagawa Tatsuo, Hatano Sayaka, Abe Manabu
Department of Chemistry, Graduate School of Science, Hiroshima University 1-3-1 Kagamiyama, Higashi-Hiroshima Hiroshima 739-8526 Japan
Unisoku Co., Ltd. 2-4-3 Kasugano, Hirakata Osaka 573-0131 Japan.
Chem Sci. 2020 Nov 10;12(2):613-625. doi: 10.1039/d0sc05311b.
Localised singlet diradicals are key intermediates in bond homolysis processes. Generally, these highly reactive species undergo radical-radical coupling reaction immediately after their generation. Therefore, their short-lived character hampers experimental investigations of their nature. In this study, we implemented the new concept of "stretch effect" to access a kinetically stabilised singlet diradicaloid. To this end, a macrocyclic structure was computationally designed to enable the experimental examination of a singlet diradicaloid with π-single bonding character. The kinetically stabilised diradicaloid exhibited a low carbon-carbon coupling reaction rate of 6.4 × 10 s (155.9 μs), approximately 11 and 1000 times slower than those of the first generation of macrocyclic system (7.0 × 10 s, 14.2 μs) and the parent system lacking the macrocycle (5 × 10 s, 200 ns) at 293 K in benzene, respectively. In addition, a significant dynamic solvent effect was observed for the first time in intramolecular radical-radical coupling reactions in viscous solvents such as glycerin triacetate. This theoretical and experimental study demonstrates that the stretch effect and solvent viscosity play important roles in retarding the σ-bond formation process, thus enabling a thorough examination of the nature of the singlet diradicaloid and paving the way toward a deeper understanding of reactive intermediates.
局域化单线态双自由基是键均裂过程中的关键中间体。一般来说，这些高反应活性物种在生成后会立即发生自由基-自由基偶联反应。因此，它们的短寿命特性阻碍了对其性质的实验研究。在本研究中，我们采用了“拉伸效应”这一新概念来获得动力学稳定的类单线态双自由基。为此，通过计算设计了一种大环结构，以便能够对具有π单键特征的类单线态双自由基进行实验研究。这种动力学稳定的类双自由基在293K的苯中表现出较低的碳-碳偶联反应速率，为6.4×10⁻⁵ s⁻¹（155.9 μs），分别比第一代大环体系（7.0×10⁻³ s⁻¹，14.2 μs）和不含大环的母体体系（5×10⁻¹ s⁻¹，200 ns）慢约11倍和1000倍。此外，首次在甘油三乙酸酯等粘性溶剂中的分子内自由基-自由基偶联反应中观察到显著的动态溶剂效应。这项理论和实验研究表明，拉伸效应和溶剂粘度在阻碍σ键形成过程中起着重要作用，从而能够对类单线态双自由基的性质进行全面研究，并为更深入理解反应中间体铺平道路。
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