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Planar Cyclopenten-4-yl Cations: Highly Delocalized ? Aromatics Stabilized by Hyperconjugation.


ABSTRACT: Theoretical studies predicted the planar cyclopenten-4-yl cation to be a classical carbocation, and the highest-energy isomer of C5 H7 + . Hence, its existence has not been verified experimentally so far. We were now able to isolate two stable derivatives of the cyclopenten-4-yl cation by reaction of bulky alanes CpR AlBr2 with AlBr3 . Elucidation of their (electronic) structures by X-ray diffraction and quantum chemistry studies revealed planar geometries and strong hyperconjugation interactions primarily from the C-Al ? bonds to the empty p orbital of the cationic sp2 carbon center. A close inspection of the molecular orbitals (MOs) and of the anisotropy of current (induced) density (ACID), as well as the evaluation of various aromaticity descriptors indicated distinct aromaticity for these cyclopenten-4-yl derivatives, which strongly contrasts the classical description of this system. Here, strong delocalization of ? electrons spanning the whole carbocycle has been verified, thus providing rare examples of ? aromaticity involving saturated sp3 carbon atoms.

SUBMITTER: Nees S 

PROVIDER: S-EPMC7589364 | biostudies-literature | 2020 Jul

REPOSITORIES: biostudies-literature

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Planar Cyclopenten-4-yl Cations: Highly Delocalized π Aromatics Stabilized by Hyperconjugation.

Nees Samuel S   Kupfer Thomas T   Hofmann Alexander A   Braunschweig Holger H  

Angewandte Chemie (International ed. in English) 20200825 42


Theoretical studies predicted the planar cyclopenten-4-yl cation to be a classical carbocation, and the highest-energy isomer of C<sub>5</sub> H<sub>7</sub> <sup>+</sup> . Hence, its existence has not been verified experimentally so far. We were now able to isolate two stable derivatives of the cyclopenten-4-yl cation by reaction of bulky alanes Cp<sup>R</sup> AlBr<sub>2</sub> with AlBr<sub>3</sub> . Elucidation of their (electronic) structures by X-ray diffraction and quantum chemistry studies  ...[more]

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