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Abstract
Theoretical studies predicted the planar cyclopenten‐4‐yl cation to be a classical carbocation, and the highest‐energy isomer of C5H7 +. 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 CpRAlBr2 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.
Highlights
Recent interest on cyclopentenyl cations 1 stems from their intermediacy in important organic (Nazarov cyclization –formation of cyclopentenones)[1,2,3,4,5] and industrial processes.[6,7,8,9] Historically it was the presence of a C=C double bond in close proximity to a cationic sp2 carbon center, and the possibility of gaining “extra stabilization” by allylic or homoaromatic delocalization, that put this system into the focus of carbocation chemistry.[10]
Solvolysis studies argue against any degree of bishomoaromatic delocalization in cyclopenten-4-yl cations (1B); a classical description as 1C is usually preferred here
We have examined the anisotropy of the current density (ACID)[40,41] and different aromaticity indicators, which revealed that the cyclopenten-4-yl cation should be considered a highly delocalized p aromatic system rather than a classical carbocation
Summary
Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10. 1002/anie.202009644. This is in stark contrast to the results obtained for structurally related 7-norbornenyl systems; the solvolysis rates are enhanced by a factor of up to 1011 over those of the saturated 7-norbornyl analogs, affording carbocations of exceptional stability.[20] These 7-norbornenyl cations are dramatically stabilized by homoaromatic 3-center-2-electron bonding,[10] making them readily amenable for structural characterization even in the solid state.[21,22,23,24] solvolysis studies argue against any degree of bishomoaromatic delocalization in cyclopenten-4-yl cations (1B); a classical description as 1C is usually preferred here This view was further supported by low values for the calculated delocalization indices (DI = 0.09) of the transannular CÀC bonds in planar 1C.[25] It should be noted, that a different theoretical approach recently indicated significant bishomoaromatic delocalization in 1C.[26] Overall, it is not surprising that stable derivatives of the cyclopentenyl cation are only known for allylic isomer 1A, including a small number of structurally characterized molecules.[27,28,29,30,31,32,33] By contrast, derivatives of isomers 1B and 1C are still absent in the literature; only a dianionic boron analog of bishomoaromatic 1B has been realized so far.[34]. Our results demonstrate that delocalization of p electrons can involve sp carbon centers via hyperconjugation effects, creating an alternative to homoaromaticity for electronic stabilization in cyclopenten-4-yl cations of the type 1C
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