Yoshidomi Shohei, Abe Manabu
JST-CREST , K's Gobancho 6F, 7, Gobancho , Chiyoda-ku, Tokyo 102-0075 , Japan.
J Am Chem Soc. 2019 Mar 6;141(9):3920-3933. doi: 10.1021/jacs.8b12254. Epub 2019 Feb 20.
Localized singlet diradicals are key intermediates in bond homolysis. A thorough study of the reactive species is needed to clarify the mechanisms of the homolytic bond cleavage and formation processes. In general, the singlet diradicals are quite short-lived because of the fast radical-radical coupling reactions. The short-lived characteristic has retarded the thorough study on bond homolysis. In this study, a new series of long-lived singlet diradicals, viz., 1,2-diazacyclopentane-3,5-diyl, were identified, and their electronic structures and novel reactivities were thoroughly studied using laser-flash photolysis (LFP), product analysis, and computational studies. A direct observation of the thermal equilibration (fast process) between the singlet diradicals and the corresponding ring-closing compounds was undertaken on the submicrosecond time scale. The solvent and substituent effects on the equilibration constant and rate constants for the ring-closing reaction and ring-opening reaction clarify the novel nitrogen-atom effect on the localized singlet 1,3-diyl diradicals. Two types of alkoxy-migrated compounds, 9 and 10, were isolated with high yields as the final products. Crossover, spin-trapping, and LFP experiments for the formation of alkoxy-group migration products (i.e., 9 versus 10) revealed the unique temperature effect on the product ratio of the two types of alkoxy-migration products. The temperature-insensitive intersystem crossing process (slow process, millisecond time scale) was found to be a key step in the formation of 9, which is an entropy-controlled pathway. An intramolecular migration process was identified for the formation of 10 that was accelerated by a polar solvent in an enthalpy-controlled process. This unique heteroatom effect has opened up a new series of localized singlet diradicals that are crucial intermediates in bond homolysis.
局域单重态双自由基是键均裂的关键中间体。需要对这些反应性物种进行深入研究，以阐明均裂键断裂和形成过程的机制。一般来说，单重态双自由基寿命很短，因为自由基 - 自由基耦合反应很快。这种短寿命特性阻碍了对键均裂的深入研究。在本研究中，鉴定出了一系列新的长寿命单重态双自由基，即1,2 - 二氮杂环戊烷 - 3,5 - 二基，并使用激光闪光光解（LFP）、产物分析和计算研究对其电子结构和新颖的反应性进行了深入研究。在亚微秒时间尺度上对单重态双自由基与相应闭环化合物之间的热平衡（快速过程）进行了直接观察。溶剂和取代基对闭环反应和开环反应的平衡常数和速率常数的影响阐明了局域单重态1,3 - 二基双自由基上新颖的氮原子效应。两种类型的烷氧基迁移化合物9和10以高产率作为最终产物被分离出来。关于烷氧基迁移产物（即9与10）形成的交叉、自旋捕获和LFP实验揭示了两种类型烷氧基迁移产物的产物比例对温度的独特依赖性。发现温度不敏感的系间窜越过程（缓慢过程，毫秒时间尺度）是形成9的关键步骤，这是一个熵控制的途径。确定了分子内迁移过程导致10的形成，该过程在极性溶剂中由焓控制过程加速。这种独特的杂原子效应开辟了一系列新的局域单重态双自由基，它们是键均裂中的关键中间体。
