Nielsen Mogens Brøndsted
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen Ø, Denmark.
Chemphyschem. 2023 Aug 1;24(15):e202300236. doi: 10.1002/cphc.202300236. Epub 2023 Jun 16.
The [2+2] cycloaddition - retro-electrocyclization (CA-RE) reaction allows ready synthesis of redox-active donor-acceptor chromophores from an electron-rich alkyne and electron-poor olefins like tetracyanoethylene (TCNE). The detailed mechanism of the reaction has been subject of both computational and experimental studies. While several studies point towards a stepwise mechanism via a zwitterionic intermediate for the first step, the cycloaddition, the reaction follows neither simple second-order nor first-order kinetics. Recent studies have shown that the kinetics can be understood if an autocatalytic step is introduced in the mechanism, in which complex formation with the donor-substituted tetracyanobutadiene (TCBD) product possibly facilitates nucleophilic attack of the alkyne onto TCNE, generating the zwitterionic intermediate of the CA step. This Concept highlights the convenient use of the "click-like" CA-RE reaction to obtain elaborate donor-acceptor chromophores and the recent mechanistic results.
[2+2]环加成-逆电环化（CA-RE）反应可通过富电子炔烃与贫电子烯烃（如四氰基乙烯（TCNE））方便地合成氧化还原活性供体-受体发色团。该反应的详细机理一直是计算研究和实验研究的主题。虽然有几项研究表明第一步环加成反应通过两性离子中间体的逐步机理进行，但该反应既不遵循简单的二级动力学，也不遵循一级动力学。最近的研究表明，如果在机理中引入一个自催化步骤，就可以理解其动力学，其中与供体取代的四氰基丁二烯（TCBD）产物形成络合物可能促进炔烃对TCNE的亲核进攻，生成CA步骤的两性离子中间体。本概念强调了利用“类点击”CA-RE反应方便地获得精细的供体-受体发色团以及最近的机理研究结果。
