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

Abstract

Theoretical studies predicted the planar cyclopenten-4-yl cation to be aclassical carbocation, and the highest-energy isomer of C 5 H 7 + .H ence,i ts existence has not been verified experimentally so far.W ew ere now able to isolate two stable derivatives of the cyclopenten-4-yl cation by reaction of bulky alanes Cp R AlBr 2 with AlBr 3 .E lucidation of their (electronic) structures by X-raydiffraction and quantum chemistry studies revealed planar geometries and strong hyperconjugation interactions primarily from the CÀAl s bonds to the empty p orbital of the cationic sp 2 carbon center.Aclose inspection of the molecular orbitals (MOs) and of the anisotropyofcurrent (induced) density (ACID), as well as the evaluation of various aromaticity descriptors indicated distinct aromaticity for these cyclopenten-4-yl derivatives,w hich strongly contrasts the classical description of this system. Here,strong delocalization of p electrons spanning the whole carbocycle has been verified, thus providing rare examples of p aromaticity involving saturated sp 3 carbon atoms.

Isolable Phospha- and Arsaalumenes

“…With mc -DI values of 97.2 ( 6 ) and 102.6 ( 7 ), and MCI values of 773.1 ( 6 ) and 233.0 ( 7 ), respectively, the aromaticity of 6 and 7 outmatches that of common organic aromatics ( cf . mc -DI: 20.5 (C 6 H 6 ), 19.8 (NC 5 H 5 ); MCI: 72.1 (C 6 H 6 ), 66.0 (NC 5 H 5 )) . Similar findings have been reported for other all inorganic aromatics such as the Al 4 2– dianion (MCI: 258.3). , We also calculated the nucleus-independent chemical shifts 1 Å above and orthogonal to the Al 2 P ring plane (NICS(1) zz ) and the original isotropic NICS(0) iso indices of 6 and 7 as magnetic aromaticity descriptors.…”

“…mc-DI: 20.5 (C 6 H 6 ), 19.8 (NC 5 H 5 ); MCI: 72.1 (C 6 H 6 ), 66.0 (NC 5 H 5 )). 63 Similar findings have been reported for other all inorganic aromatics such as the Al 4…”

Isolation and Reactivity of an Antiaromatic s‐Block Metal Compound

“…Finally, we probed the antiaromaticity of CAAC‐stabilized BeC 4 rings by using the anisotropy of the induced current density (ACID) method, as proposed by Herges and co‐workers [75, 76] . ACID has been widely used for assessing delocalization, conjugation, and aromaticity in distinct molecular systems [76, 77] . Briefly, by plotting the ACID scalar field isosurfaces together with the current density vectors, it is possible to distinguish between diatropic (clockwise) and paratropic (counterclockwise) π‐electron circulation, which are characteristic of aromatic and antiaromatic molecules, respectively.…”

Isolierung und Reaktivität eines s‐Block‐Metall‐Antiaromaten

“…Schließlich untersuchten wir die Antiaromatizität von CAAC‐stabilisierten BeC 4 ‐Ringen, indem wir die Anisotropie nach der Methode der induzierten Stromdichte (ACID) verwendeten, wie von Herges und Mitarbeitern empfohlen [75, 76] . ACID wurde häufig zur Beurteilung von Delokalisierung, Konjugation und Aromatizität in verschiedenen molekularen Systemen verwendet [76, 77] . Durch das Auftragen der ACID‐Skalarfeldisoflächen zusammen mit den Stromdichtevektoren ist es möglich, zwischen diatroper (im Uhrzeigersinn) und paratroper (gegen den Uhrzeigersinn) π‐Elektronenzirkulation zu unterscheiden, die für aromatische bzw.…”