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Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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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.
Removing one hydrogen from the intervening CH2 group...
4.4K
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...
4.4K
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

6.1K
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,...
6.1K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

15.3K
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?
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or...
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Radicals: Electronic Structure and Geometry01:07

Radicals: Electronic Structure and Geometry

5.4K
This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
Accordingly, the structure of a trivalent radical lies between the geometries of carbocations and carbanions. An sp2-hybridized carbocation is trigonal planar, while an sp3-hybridized carbanion is trigonal pyramidal. Here, the difference in geometry is...
5.4K
Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

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Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Chimerical pyrene-based [7]helicenes as twisted polycondensed aromatics.

Michal Buchta1, Jiří Rybáček1, Andrej Jančařík1

  • 1Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 166 10 Prague 6 (Czech Republic), Fax: (+420) 220-183-133.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|May 1, 2015
PubMed
Summary

Researchers synthesized novel pyrene-based helicenes using metal-catalyzed cycloisomerization. These compounds exhibit unique helical structures and fluorescence properties, with potential for advanced material applications.

Keywords:
alkynesasymmetric synthesischiralitycycloisomerisationhelical structures

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Area of Science:

  • Organic Chemistry
  • Supramolecular Chemistry
  • Materials Science

Background:

  • Helicenes are chiral aromatic compounds with unique three-dimensional structures.
  • Pyrene-containing molecules offer interesting photophysical properties.
  • Developing novel synthetic routes for complex helical structures is crucial.

Purpose of the Study:

  • To synthesize novel pyrene-based dibenzo[7]helicene and 2H-pyran[7]helicene derivatives.
  • To investigate the enantioselective synthesis and properties of these new helicenes.
  • To explore their structural, photophysical, and chiroptical characteristics.

Main Methods:

  • Synthesis via Ni(0)- or Co(I)-mediated [2+2+2] cycloisomerization of dipyrenyl-acetylene-derived triynes.
  • Enantioselective cycloisomerization using Ni(0)/QUINAP catalysis for enantioenriched products.
  • Racemate resolution by chiral liquid chromatography and diastereoselective cycloisomerization.
  • Characterization using X-ray crystallography, UV/Vis spectroscopy, electronic circular dichroism (ECD), and fluorescence spectroscopy.
  • Computational studies (DFT) for structural analysis.

Main Results:

  • Successful synthesis of pyrene-based dibenzo[7]helicene (1) and 2H-pyran[7]helicene (2).
  • Achieved enantioenriched (1) via asymmetric catalysis (57% ee) and enantiopure (2) via diastereoselective synthesis.
  • X-ray crystallography revealed a slightly over-flattened helical backbone in (2) due to dispersion forces.
  • Both compounds exhibit fluorescence in dichloromethane (ΦF = 0.10 for 1, 0.17 for 2).
  • Demonstrated formation of stabilized intramolecular excimer states with large Stokes shifts (296 nm for 1, 203 nm for 2).

Conclusions:

  • Novel pyrene-fused helicenes were synthesized and characterized.
  • The study highlights effective strategies for enantioselective synthesis of complex helical architectures.
  • The synthesized helicenes possess significant fluorescence and excimer formation capabilities, suggesting potential in materials science.