Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100, Copenhagen, Denmark.
Chemistry. 2023 Jan 12;29(3):e202202833. doi: 10.1002/chem.202202833. Epub 2022 Nov 22.
The [2+2] cycloaddition - retro-electrocyclization (CA-RE) reaction is a "click-like" protocol for facile synthesis of donor-acceptor chromophores from an alkyne and tetracyanoethylene. Herein we shed light on the mechanism of this reaction by detailed kinetics studies using H NMR spectroscopy. By considering several experiments simultaneously, a variety of mechanistic models was evaluated. Surprisingly, a model in which the final 1,1,4,4-tetracyanobuta-1,3-diene product promoted the first step was the only one that described well the experimental data. This autocatalysis model also involved a non-concerted, stepwise formation of the cyclobutene cycloaddition adduct. By proper choice of conditions, we were able to generate the transient cyclobutene in sufficient amount to verify it as an intermediate using C NMR spectroscopy. For its final retro-electrocyclization step, simple first-order kinetics was observed and only minor solvent dependence, which indicates a concerted reaction.
[2+2]环加成-反电环化（CA-RE）反应是一种简便的从炔烃和四氰乙烯合成给体-受体发色团的“点击样”协议。在此，我们通过使用 1 H NMR 光谱学进行详细的动力学研究来阐明该反应的机理。通过同时考虑几种实验，评估了多种机理模型。令人惊讶的是，最终 1,1,4,4-四氰基丁-1,3-二烯产物促进第一步的模型是唯一能够很好地描述实验数据的模型。这种自动催化模型还涉及非协同的、逐步形成环丁烯环加成加合物。通过适当选择条件，我们能够生成足够量的瞬态环丁烯，并用 13 C NMR 光谱学将其验证为中间体。对于其最终的反电环化步骤，观察到简单的一级动力学，并且溶剂依赖性很小，这表明是协同反应。
