Department of Chemistry, Graduate School of Science, Hiroshima University, Japan.
Beilstein J Org Chem. 2013 May 14;9:925-33. doi: 10.3762/bjoc.9.106. Print 2013.
Localized singlet diradicals are in general quite short-lived intermediates in processes involving homolytic bond-cleavage and formation reactions. In the past decade, long-lived singlet diradicals have been reported in cyclic systems such as cyclobutane-1,3-diyls and cyclopentane-1,3-diyls. Experimental investigation of the chemistry of singlet diradicals has become possible. The present study explores the substituents and the effect of their substitution pattern at the C(1)-C(3) positions on the lifetime of singlet octahydropentalene-1,3-diyls to understand the role of the substituents on the reactivity of the localized singlet diradicals.
A series of singlet 2,2-dialkoxy-1,3-diaryloctahydropentalene-1,3-diyls DR were generated in the photochemical denitrogenation of the corresponding azoalkanes AZ. The ring-closed products CP, i.e., 3,3-dialkoxy-2,4-diphenyltricyclo[3.3.0.0(2,4)]octanes, were quantitatively obtained in the denitrogenation reaction. The first-order decay process (k = 1/τ) was observed for the fate of the singlet diradicals DR (λmax ≈ 580-590 nm). The activation parameters, ΔH (‡) and ΔS (‡), for the ring-closing reaction (σ-bond formation process) were determined by the temperature-dependent change of the lifetime. The energy barrier was found to be largely dependent upon the substituents Ar and Ar'. The singlet diradical DRf (Ar = 3,5-dimethoxyphenyl, OCH2Ar' = OCH2(3,5-dimethoxyphenyl)) was the longest-lived, τ293 = 5394 ± 59 ns, among the diradicals studied here. The lifetime of the parent diradical DR (Ar = Ph, OCH2Ar' = OCH3) was 299 ± 2 ns at 293 K.
The lifetimes of the singlet 1,3-diyls are found to be largely dependent on the substituent pattern of Ar and Ar' at the C(1)-C(3) positions. Both the enthalpy and entropy effect were found to play crucial roles in increasing the lifetime.
局部单重态二自由基通常是涉及均裂键断裂和形成反应过程中相当短暂的中间体。在过去的十年中，已经报道了环状体系如环丁烷-1,3-二基和环戊烷-1,3-二基中的长寿命单重态二自由基。单重态二自由基化学的实验研究成为可能。本研究探讨了 C(1)-C(3)位置取代基及其取代模式对单重态八氢苉-1,3-二基寿命的影响，以了解取代基对局部单重态二自由基反应性的作用。
通过相应偶氮烷的光致脱氮反应生成了一系列单重态 2,2-二烷氧基-1,3-二芳基八氢苉-1,3-二基 DR。在脱氮反应中定量获得了闭环产物 CP，即 3,3-二烷氧基-2,4-二苯基三环[3.3.0.0(2,4)]辛烷。单重态二自由基 DR 的命运（k = 1/τ）观察到一级衰减过程（λmax ≈ 580-590nm）。通过寿命随温度的变化，确定了环封闭反应（σ键形成过程）的活化参数ΔH（‡）和ΔS（‡）。发现能垒主要取决于取代基 Ar 和 Ar'。单重态二自由基 DRf（Ar = 3,5-二甲氧基苯基，OCH2Ar' = OCH2(3,5-二甲氧基苯基））是研究的二自由基中寿命最长的，τ293 = 5394 ± 59 ns。母体二自由基 DR（Ar = Ph，OCH2Ar' = OCH3）在 293 K 时的寿命为 299 ± 2 ns。
单重态 1,3-二基的寿命发现主要取决于 C(1)-C(3)位置取代基 Ar 和 Ar'的取代模式。焓和熵效应都被发现对延长寿命起着至关重要的作用。
