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

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 Cp^R 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 σ 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 π electrons spanning the whole carbocycle has been verified, thus providing rare examples of π aromaticity involving saturated sp³ carbon atoms.