New Development of Biradical Chemistry : Construction of a Novel Structure

化学包括键的断裂和键的形成。单重态二自由基是均相分解过程中的关键中间体。一般来说，活性中间体是相当短暂的，因此，很大程度上是难以捉摸的，没有机会发生分子间反应。最近针对稳定单重态双自由基的研究提供了一些机会来考察这类电子和结构独特的分子的化学。在这一理论和实验相结合的研究中，我们发现氮原子对单重态2，2-二甲氧基环戊烷-1，3-二基的寿命以及分子内和分子间的反应性有显著的影响。在计算上，含氮的单重态双自由基比相应的闭环化合物在能量上更稳定。五元环体系中氮原子的引入有效地将单重态2，2-二甲氧基丙烷-1，3-二基的寿命提高到毫秒级。发现寿命极长的单重态1，3-二自由基与PTAD发生分子间反应，生成环加成产物。分子内反应为1，2-氧迁移反应。这些引人注目的发现应该会刺激未来关于极长寿命的单重态双自由基的新反应性的计算和实验，特别是单重态双自由基的分子间反应。

Chemistry involves bond-cleavage and bond formation. Singlet diradicals are key intermediates in homolytic processes. In general, the reactive intermediates are quite short-lived, and thus, largely elusive and have no chance to react intermolecularly. Recent studies, directed toward stabilizing singlet diradicals, have provided some opportunities to examine the chemistry of such electronically and structurally unique molecules.In this combined theoretical and experimental study, we found a notable effect of a nitrogen atom on the lifetime of singlet 2,2-dimethoxycyclopentane-1,3-diyls, and the intra-and intermolecular reactivity. Computationally, the nitrogen-containing singlet diradicals were calculated to be energetically more stable than the corresponding ring-closure compounds. The introduction of nitrogen atoms in the five-membered ring system effectively increased the lifetime of the singlet 2,2-dimethoxypropane-1,3-diyl to the millisecond time scale. The extremely long-lived singlet 1,3-diradical was found to react intermolecularly with PTAD to afford the cycloaddition product. A 1,2-oxygen migration reaction was revealed for intramolecular reactivity. These remarkable findings should stimulate future computations and experiments on the novel reactivity of extremely long-lived singlet diradicals, especially intermolecular reactions of singlet diradicals.