Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies
Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies
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DOI：
10.1021/jacs.1c07282
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发表时间：
2021-11-24
影响因子：
15
通讯作者：
Hayes, Dugan
中科院分区：
文献类型：
--
作者：
Jayasekara, Gethmini K.;Antolini, Cali;Hayes, Dugan
[2 + 2]光环加成提供了一种简单、单步的环丁烷部分路线，否则由于环应变和/或空间相互作用，环丁烷部分将不受欢迎或不可能。我们结合光学和 X 射线瞬态吸收光谱来阐明以降冰片烯和环己烯为模型底物的 Cu(I) 催化分子间光环加成反应的机理。我们发现，对于降冰片烯，反应通过初始金属到配体电荷转移 (MLCT) 状态进行，该状态持续 18 ns，然后金属返回到单价氧化状态。 Cu K 边光谱持续演化直至接近 5 μs，然后在 50 μs 的测量时间内保持不变，反映产物形成和配体解离。我们假设 MLCT 跃迁和反向电子转移有助于敏化其中一个降冰片烯配体的三重激发态，然后与另一个降冰片烯配体二聚化生成产物。然而，对于环己烯的情况，我们没有观察到光激发后的电荷转移状态，而是找到了金属-配体键强度增加的证据，这种强度在产物形成发生之前持续了几个纳秒。这与以配体光异构化为初始步骤的机制一致，该机制由 Salomon 和 Kochi 于 1974 年首次提出，用于解释反应的立体选择性。我们的研究揭示了如何仅通过基材结构的微小变化来引导这种光催化反应沿着截然不同的轨迹进行。
The [2 + 2] photocycloaddition provides a simple, single-step route to cyclobutane moieties that would otherwise be disfavored or impossible due to ring strain and/or steric interactions. We have used a combination of optical and X-ray transient absorption spectroscopies to elucidate the mechanism of the Cu(I)-catalyzed intermolecular photocycloaddition reaction using norbornene and cyclohexene as model substrates. We find that for norbornene the reaction proceeds through an initial metal-to-ligand charge transfer (MLCT) state that persists for 18 ns before the metal returns to the monovalent oxidation state. The Cu K-edge spectrum continues to evolve until similar to 5 mu s and then remains unchanged for the 50 mu s duration of the measurement, reflecting product formation and ligand dissociation. We hypothesize that the MLCT transition and reverse electron transfer serve to sensitize the triplet excited state of one of the norbornene ligands, which then dimerizes with the other to give the product. For the case of cyclohexene, however, we do not observe a charge transfer state following photoexcitation and instead find evidence for an increase in the metal-ligand bond strength that persists for several ns before product formation occurs. This is consistent with a mechanism in which ligand photoisomerization is the initial step, which was first proposed by Salomon and Kochi in 1974 to explain the stereoselectivity of the reaction. Our investigation reveals how this photocatalytic reaction may be directed along strikingly disparate trajectories by only very minor changes to the structure of the substrate.