On the Mechanism of the Formal [2+2] Cycloaddition – Retro‐electrocyclization (CA‐RE) Reaction

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 1H NMR spectroscopy. By...

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Bibliographic Details
Published in:Chemistry : a European journal 2023-01, Vol.29 (3), p.e202202833-n/a
Main Authors: Hansen, Jonathan Kirschner Solberg, Tortzen, Christian G., Sørensen, Preben Graae, Brøndsted Nielsen, Mogens
Format: Article
Language:English
Subjects:
Alkynes
Alkynes - chemistry
Autocatalysis
Chemistry
Chromophores
Cyclization
Cycloaddition
Cycloaddition Reaction
Kinetics
Magnetic Resonance Spectroscopy
NMR
NMR spectroscopy
Nuclear magnetic resonance
retro-electrocyclization
Spectroscopy
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Summary: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 1H 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 13C NMR spectroscopy. For its final retro‐electrocyclization step, simple first‐order kinetics was observed and only minor solvent dependence, which indicates a concerted reaction. The “click‐like” [2+2] cycloaddition ‐ retro‐electrocyclization (CA‐RE) reaction was followed under a variety of conditions using NMR spectroscopy. Several mechanistic models were evaluated by considering multiple experiments simultaneously. Surprisingly, only a model in which the final 1,1,4,4‐tetracyanobuta‐1,3‐diene product promoted the first step described well all the data. The CA‐RE reaction is hence an autocatalytic reaction.
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.202202833