1,2-Diazacyclopentane-3,5-diyl Diradicals: Electronic Structure and Reactivity
1,2-Diazacyclopentane-3,5-diyl Diradicals: Electronic Structure and Reactivity
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1,2-二氮杂环戊烷-3,5-二基二基：电子结构和反应性
DOI：
10.1021/jacs.8b12254
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发表时间：
2019
影响因子：
15
通讯作者：
Abe Manabu
中科院分区：
文献类型：
--
作者：
Yoshidomi Shohei;Abe Manabu
定域单线态双自由基是键均裂的关键中间体。需要对反应性物种进行深入研究，以澄清均裂键断裂和形成过程的机制。一般来说，单线态双自由基是相当短的寿命，因为快速的自由基-自由基偶联反应。这种短寿命特性阻碍了对键均裂的深入研究。在这项研究中，一系列新的长寿命单线态双自由基，即，1，2-二氮杂环戊烷-3，5-二基，并利用激光闪光光解（LFP）、产物分析和计算研究了它们的电子结构和新颖的反应活性。在亚微秒时间尺度上直接观察了单线态双自由基和相应的闭环化合物之间的热平衡（快速过程）。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氮原子对局域单线态1，3-二基双自由基的新效应。两种类型的烷氧基迁移的化合物，9和10，分离与高收率的最终产品。用于形成烷氧基迁移产物（即，9versus 10）揭示了温度对两种类型的烷氧基迁移产物的产物比的独特影响。温度不敏感的系统间穿越过程（慢过程，毫秒时间尺度）被发现是9形成的关键步骤，这是一个熵控制的途径。分子内的迁移过程被确定为10的形成，这是加速由极性溶剂在一个控制的过程。这种独特的杂原子效应开辟了一系列新的本地化单线态双自由基，是关键的中间体键均裂。
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,9and10, 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.,9versus10) 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 of9, which is an entropy-controlled pathway. An intramolecular migration process was identified for the formation of10that 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.