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Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

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Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous...
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The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
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π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

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In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
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Aromatic Hydrocarbon Cations: Structural Overview01:18

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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
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Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom,...
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Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?  
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Aromaticity in Fully π-Conjugated Open-Cage Molecules.

Shaofei Wu1,2, Yong Ni1, Yi Han1

  • 1Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.

Journal of the American Chemical Society
|December 8, 2022
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Researchers synthesized a novel 3D π-conjugated molecular cage, exploring its aromaticity. The cage and its oxidized forms exhibit unique 2D and 3D aromatic and antiaromatic properties, challenging previous understandings.

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Area of Science:

  • Organic Chemistry
  • Supramolecular Chemistry
  • Materials Science

Background:

  • Aromaticity in 2D π-conjugated systems is well-established.
  • Aromaticity in 3D π-conjugated molecular cages remains underexplored due to synthetic difficulties.
  • Triphenylamine derivatives are key building blocks in π-conjugated materials.

Purpose of the Study:

  • To report the facile synthesis of a novel 3D π-conjugated molecular cage.
  • To investigate the aromaticity and electronic properties of the cage and its oxidized derivatives.
  • To compare the aromaticity of the cage with related macrocyclic trimers.

Main Methods:

  • Platinum-mediated assembly of a pinacol borate trisubstituted dimethylmethylene-bridged triphenylamine (DTPA) derivative.
  • Reductive elimination to form the π-conjugated molecular cage.
  • Oxidation of the cage and model compounds, followed by experimental measurements and theoretical calculations (e.g., DFT).

Main Results:

  • A π-conjugated molecular cage (1) with an open-cage structure was successfully synthesized.
  • Neutral cage 1 exhibits localized aromaticity in its benzene rings.
  • Dication 1^2+ shows bicyclic (anti)aromaticity with both aromatic (38π) and antiaromatic (28π) macrocycles.
  • Tetracation 1^4+ displays dominant 2D Hückel antiaromaticity (36π) in one macrocycle.
  • Model compounds 2 and 3 show global antiaromatic and nonaromatic properties, respectively, in their dicationic forms.
  • Several oxidized species exhibit open-shell singlet ground states with diradical character, while one adopts a triplet ground state.

Conclusions:

  • The study presents a facile synthetic route to a 3D π-conjugated molecular cage.
  • The cage and its derivatives demonstrate complex 2D and 3D aromaticity and antiaromaticity.
  • This work expands the understanding of aromaticity in three-dimensional π-conjugated systems.