Conjugation between 3D and 2D aromaticity: does it really exist? The case of carborane-fused heterocycles

Buzsáki, Dániel; Kovács, Máté; Humpfner, Evelyn; Harcsa-Pintér, Zsófia; Kelemen, Zsolt

doi:10.1039/d2sc03511a

Cited by 9 publications

(8 citation statements)

References 49 publications

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“…Once this takes place, the scaffolding of the o ‐carborane is unblocked, eliminating the C=C and therefore generating a non‐aromatic cyclic system and an aromatic cluster. Therefore, a full 3D/2D aromatic system can never occur, in contradiction to the recently reported experimental results of 3D/2D bonding [7,19a] . Thus we have studied again what could be the causes of the misinterpretation of the NICS and ASE in the o ‐carborane‐fused carbo‐ and heterocycles.…”

Section: Resultscontrasting

confidence: 66%

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Pioneering the Power of Twin Bonds in a Revolutionary Double Bond Formation. Unveiling the True Identity of o‐Carboryne as o‐Carborene

Poater,

Viñas,

Escayola

et al. 2023

Chemistry A European J

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The homolytic elimination of two H atoms from two adjacent carbons in benzene results in the aromatic product o‐benzyne. In a similar way, the homolytic elimination of two H atoms from the two adjacent carbons in 1,2‐C2B10H12 results in the aromatic product o‐carboryne. In this work, we provide experimental and computational evidences that despite the similarity of o‐carboryne and o‐benzyne, the nature of the C−C bond generated between two adjacent carbons that lose H atoms is different. While in o‐benzyne the C−C bond behaves as a triple bond, in o‐carboryne the C−C bond is a double bond. Therefore, we must stop naming 1,2‐dehydro‐o‐carboryne as o‐carboryne but instead call it o‐carborene.

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Section: Resultscontrasting

confidence: 66%

“…The latter is indicated in italics in the following pages in order to maintain the nomenclature adopted so far with respect to o‐ benzyne, but we will finally prove that it is in fact a carborene. Earlier work has already shown that 3D aromaticity does not match 2D aromaticity [2d,7] . And, why is that?…”

Section: Introductionmentioning

confidence: 96%

Pioneering the Power of Twin Bonds in a Revolutionary Double Bond Formation. Unveiling the True Identity of o‐Carboryne as o‐Carborene

Poater,

Viñas,

Escayola

et al. 2023

Chemistry A European J

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“…In addition to being computationally more challenging, excited-state (anti-)aromaticity also comes with the alert that aromaticity in an excited state is not necessarily Baird aromaticity; there are molecules which are Hückel aromatic in their excited states, [56][57][58] and there are those with excited states that are Hückel-and Baird-aromatic hybrids. [50,51] Finally, similar to the hexagonal hydrogen-bonded (HF) 3 trimer, some transition-metal cycles and carborane-fused heterocycles in the S 0 state, [59][60][61][62] one can encounter situations in excited states where negative NICS values are found in the center of rings without diatropic ring currents. Instead, localized circulations at the edges, or paratropic ring currents in adjacent rings, can lead to a negative NICS value in a certain ring.…”

Section: Henrik Ottosson Received His Phd In 1996 (University Of Goth...mentioning

confidence: 99%

Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief

Yan

Slanina

Bergman

et al. 2023

Chemistry A European J

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Gain of aromaticity or relief of antiaromaticity along a reaction path are important factors to consider in mechanism studies. Analysis of such changes along potential energy surfaces has historically focused on reactions in the electronic ground state (S 0 ), but can also be used for excited states. In the lowest ππ* states, the electron counts for aromaticity and antiaromaticity follow Baird's rule where 4n π-electrons indicate aromaticity and 4n + 2 π-electrons antiaromaticity. Yet, there are also cases where Hückel's rule plays a role in the excited state. The electron count reversals of Baird's rule compared to Hückel's rule explain many altered physicochemical properties upon excitation of (hetero)annulene derivatives. Here we illustrate how the gain of excited-state aromaticity (ESA) and relief of excited-state antiaromaticity (ESAA) have an impact on photoreactivity and photostability. Emphasis is placed on recent findings supported by the results of quantum chemical calculations, and photoreactions in a wide variety of areas are covered.

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“…Noticeably, the computed AICD plots allow us to observe how the ring current of this newly formed ring is interrupted (Figure and Figure S27). Thus, the hypothesized 2D–3D global aromaticity of these compounds does not take place but aromaticity is localized on the 3D carborane and on the 2D pyridyl ring, not on the intermediate-fused six-membered ring. ,, Then, what is the reason for the distinct NMR properties of these systems? In particular, what is the origin of the unprecedented 1 H NMR chemical shift of 7.46 ppm for the H atom of the fused six-membered ring δ(H*) of 3c ?…”

Section: Resultsmentioning

confidence: 99%

Facile Construction of New Hybrid Conjugation via Boron Cage Extension

et al. 2023

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Aromatic polycyclic systems have been extensively utilized as structural subunits for the preparation of various functional molecules. Currently, aromatics-based polycyclic systems are predominantly generated from the extension of two-dimensional (2D) aromatic rings. In contrast, polycyclic compounds based on the extension of three-dimensional (3D) aromatics such as boron clusters are less studied. Here, we report three types of boron cluster-cored tricyclic molecular systems, which are constructed from a 2D aromatic ring, a 3D aromatic nido-carborane, and an alkyne. These new tricyclic compounds can be facilely accessed by Pd-catalyzed B–H activation and the subsequent cascade heteroannulation of carborane and pyridine with an alkyne in an isolated yield of up to 85% under mild conditions without any additives. Computational results indicate that the newly generated ring from the fusion of the 3D carborane, the 2D pyridyl ring, and an alkyne is non-aromatic. However, such fusion not only leads to a 1H chemical shift considerably downfield shifted owing to the strong diatropic ring current of the embedded carborane but also devotes to new/improved physicochemical properties including increased thermal stability, the emergence of a new absorption band, and a largely red-shifted emission band and enhanced emission efficiency. Besides, a number of bright, color-tunable solid emitters spanning over all visible light are obtained with absolute luminescence efficiency of up to 61%, in contrast to aggregation-caused emission quenching of, e.g., Rhodamine B containing a 2D-aromatics-fused structure. This work demonstrates that the new hybrid conjugated tricyclic systems might be promising structural scaffolds for the construction of functional molecules.

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Conjugation between 3D and 2D aromaticity: does it really exist? The case of carborane-fused heterocycles

Abstract: Although several synthesized icosahedral carborane fused 2D π-ring systems are known, and even considerable conjugation has been noted between them in some cases, the phenomenon itself is not fully understood....

Cited by 9 publications

References 49 publications

Pioneering the Power of Twin Bonds in a Revolutionary Double Bond Formation. Unveiling the True Identity of o‐Carboryne as o‐Carborene

Pioneering the Power of Twin Bonds in a Revolutionary Double Bond Formation. Unveiling the True Identity of o‐Carboryne as o‐Carborene

Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief

Facile Construction of New Hybrid Conjugation via Boron Cage Extension

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