TY - JOUR

T1 - On the Mechanism of the Formal [2+2] Cycloaddition - Retro-electrocyclization (CA-RE) Reaction

AU - Hansen, Jonathan Kirschner Solberg

AU - Tortzen, Christian G.

AU - Sorensen, Preben Graae

AU - Brondsted Nielsen, Mogens

PY - 2023

Y1 - 2023

N2 - 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-1 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-13 NMR spectroscopy. For its final retro-electrocyclization step, simple first-order kinetics was observed and only minor solvent dependence, which indicates a concerted reaction.

AB - 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-1 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-13 NMR spectroscopy. For its final retro-electrocyclization step, simple first-order kinetics was observed and only minor solvent dependence, which indicates a concerted reaction.

KW - autocatalysis

KW - cycloaddition

KW - kinetics

KW - NMR spectroscopy

KW - retro-electrocyclization

KW - CHARGE-TRANSFER INTERACTIONS

KW - MOLECULAR-COMPLEXES

KW - TETRACYANOETHYLENE

KW - CHROMOPHORES

KW - TCNE

KW - ALKYNES

KW - RETROELECTROCYCLIZATION

KW - REGIOSELECTIVITY

KW - FERROCENYL

KW - CHEMISTRY

U2 - 10.1002/chem.202202833

DO - 10.1002/chem.202202833

M3 - Journal article

C2 - 36217899

VL - 29

JO - Chemistry: A European Journal

JF - Chemistry: A European Journal

SN - 0947-6539

IS - 3

M1 - e202202833

ER -