New Development of Biradical Chemistry : Construction of a Novel Structure

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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.