Department of Chemistry and ‡Research Centre for Toxic Compounds in the Environment, Faculty of Science, Masaryk University , Kamenice 5, 625 00, Brno, Czech Republic.
J Am Chem Soc. 2013 Oct 9;135(40):15209-15. doi: 10.1021/ja407588p. Epub 2013 Sep 30.
The photochemistry of the hydroxybenzocycloalkanonyl derivatives 6b-e provides the triplet oxyallyl diradicals (3)9 that decay via intersystem crossing to their more stable singlet isomers (1)9. Vibrationally resolved transient spectra of (3)9 were recorded by pump-probe spectroscopy and laser flash photolysis. It was found that the ring strain dependent rate of intersystem crossing is the rate-limiting step in the formation of photo-Favorskii or solvolysis reaction products in water. The reactivities of open-shell singlet oxyallyls (1)9a-e determine the product ratios due to their relative abilities to form the corresponding cyclopropanones 10. The smallest five-membered derivative, (1)9b, represents the first example of an oxyallyl diradicaloid that cannot form cyclopropanone 10b or the isomeric allene oxide 13b; instead, it is eventually trapped by water to form the sole solvolysis product 12b. Our observations provide a comprehensive overview of the role of oxyallyl diradicals in reaction mechanisms and offer a new strategy to stabilize open-shell singlet diradicals.
羟基苯并环烷酮衍生物 6b-e 的光化学提供了三重态氧烯丙基二自由基 (3)9，它们通过系间窜越衰减为更稳定的单重态异构体 (1)9。通过泵浦-探测光谱法和激光闪光光解记录了 (3)9 的振动分辨瞬态光谱。研究发现，环应变依赖性的系间窜越速率是在水中形成光 Favorskii 或溶剂解反应产物的限速步骤。由于其形成相应的环丙酮 10 的相对能力，开壳单重态氧烯丙基 (1)9a-e 的反应性决定了产物的比例。最小的五元衍生物 (1)9b 代表了第一个不能形成环丙酮 10b 或异构的烯丙基氧化物 13b 的氧烯丙基二自由基样化合物的例子；相反，它最终被水捕获，形成唯一的溶剂解产物 12b。我们的观察结果提供了对氧烯丙基二自由基在反应机制中的作用的全面概述，并为稳定开壳单重态二自由基提供了一种新策略。
