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Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

2.8K
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...
2.8K
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...
2.8K
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

3.9K
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,...
3.9K
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

2.7K
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.
2.7K
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.3K
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
2.3K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.0K
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
2.0K

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Cyclo[2]pyridine[8]pyrrole: An Expanded Porphyrinoid with Three Closed-Shell Oxidation States.

Ruiquan Li1, Chun-Hong Mak2,3, Ke Luo4

  • 1Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science, South-Central Minzu University, Wuhan 430074, China.

Journal of the American Chemical Society
|February 5, 2024
PubMed
Summary

A novel expanded porphyrinoid, cyclo[2]pyridine[8]pyrrole, exhibits three distinct oxidation states. This compound, macrocycle 1, can be reversibly reduced and oxidized, and its color change allows for biothiol detection.

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Area of Science:

  • Organic Chemistry
  • Supramolecular Chemistry
  • Analytical Chemistry

Background:

  • Expanded porphyrinoids are macrocyclic compounds with unique electronic and optical properties.
  • Controlling the oxidation states of macrocycles is crucial for their application in sensing and catalysis.
  • The development of new molecular probes for detecting biologically relevant molecules like biothiols is of significant interest.

Purpose of the Study:

  • To synthesize and characterize a novel expanded porphyrinoid, cyclo[2]pyridine[8]pyrrole (1).
  • To investigate the redox properties and multiple oxidation states of macrocycle 1.
  • To explore the potential of macrocycle 1 as a colorimetric sensor for biothiols.

Main Methods:

  • Synthesis of cyclo[2]pyridine[8]pyrrole (1) via oxidative coupling.
  • Characterization using NMR, UV-vis spectroscopy, Mass Spectrometry (MS), and X-ray crystallography.
  • Redox titrations and spectroscopic analysis to study oxidation and reduction processes.

Main Results:

  • Macrocycle 1 was successfully synthesized and confirmed to exist in three closed-shell oxidation levels.
  • Reversible redox transformations between the fully oxidized, half-oxidized, and fully reduced forms were achieved using various chemical agents.
  • A distinct color change was observed upon reduction of macrocycle 1 by thiol-containing compounds, enabling differentiation of cysteine, homocysteine, and glutathione.

Conclusions:

  • The novel expanded porphyrinoid cyclo[2]pyridine[8]pyrrole (1) demonstrates tunable redox properties.
  • The observed colorimetric response to biothiols highlights its potential as a sensitive and selective molecular probe.
  • This work expands the scope of porphyrinoid chemistry and offers a new platform for developing redox-responsive sensors.