Planar Cyclopenten‐4‐yl Cations: Highly Delocalized π Aromatics Stabilized by Hyperconjugation. Issue 42 (25th August 2020)

• Record Type: Journal Article
• Authors: Nees, Samuel; Kupfer, Thomas; Hofmann, Alexander; Braunschweig, Holger
• Is Part Of: Angewandte Chemie international edition. Volume 59:Issue 42(2020)
• Page Start: 18809
• Page End: 18815
• Publication Date: 2020-08-25
• Subjects: ACID -- carbocations -- cyclopenten-4-yl cation -- hyperconjugation -- π aromaticity; Chemistry -- Periodicals; 540
• DOI: 10.1002/anie.202009644
• Languages: English
• ISSNs: 1433-7851

Abstract

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 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 sp 2 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 sp 3 carbon atoms. Abstract : Delocalized, not classical: Despite its structural relationship to the non‐classical 7‐norbornenyl system earlier studies predicted the cyclopenten‐4‐yl cation to be a localized, classical carbocation. We have now been able to capture two stable derivatives of this elusive species by making use of anchoring aluminyl substituents and established a highly delocalized π aromatic structure.