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Reversible σ-Bond Formation in Bowl-Shaped π-Radical Cations: The Effects of Curved and Planar Structures. | LitMetric

Reversible σ-Bond Formation in Bowl-Shaped π-Radical Cations: The Effects of Curved and Planar Structures.

J Am Chem Soc

Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering , Nagoya University, Nagoya , Aichi 464-8603 , Japan.

Published: April 2018

AI Article Synopsis

  • The study explores the σ-dimerization of a unique bowl-shaped π-radical cation, highlighting its reversible ability to form σ-bonds, which has been less thoroughly researched compared to similar reactions with organic radicals.
  • Analysis methods like single-crystal X-ray diffraction and H NMR spectroscopy reveal that while the radical cation is mostly found in monomer form in room temperature solutions, cooling facilitates dimerization through σ-bond formation.
  • The findings indicate that the curved shape of the bowl-shaped π-radical cation not only promotes σ-dimerization at specific carbon atoms but also enhances reactivity with methanol, contrasting with planar π-conjugated systems.

Article Abstract

The reversible formation of σ-bonds between organic radicals has been widely investigated. However, reports on the formation of σ-dimers from delocalized π-radical cations are scarce. Herein, we report the reversible σ-dimerization behavior of a bowl-shaped π-radical cation generated from a nitrogen-embedded buckybowl, both in the crystalline state and in solution. The detailed structure of the σ-dimer in the crystalline state was determined by a single-crystal X-ray diffraction analysis. The monomeric radical cation exists predominantly in solution at room temperature, while dimerization of the radical cations occurs through carbon-carbon σ-bond formation upon reducing the temperature. H NMR and optical spectroscopy measurements confirmed the formation of a σ-dimer at low temperature. Comparative studies with a similar yet planar π-conjugated system suggested that the curved structure of the bowl-shaped π-radical cation facilitates the σ-dimerization at one of the internal sp-hybridized carbon atoms. This trend was also observed for the nucleophilic addition reaction of methanol to the π-radical cations. The methoxylation reaction proceeded only for the curved π-radical cation. Theoretical calculations revealed that the large relief of structural strain at the α-carbon atom during the dimerization or nucleophilic addition reactions accelerated the bond formation at the internal carbon atom of the curved radical cation.

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Source
http://dx.doi.org/10.1021/jacs.8b00798DOI Listing

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