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Five-Membered Heterocyclic Aromatic Compounds: Overview

Five-Membered Heterocyclic Aromatic Compounds: Overview

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, respectively.

Criteria for Aromaticity and the Hückel 4n + 2 Rule

Criteria for Aromaticity and the Hückel 4n + 2 Rule

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 the 4n + 2 rule.

Aromatic Hydrocarbon Anions: Structural Overview

Aromatic Hydrocarbon Anions: Structural Overview

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 overlap of p...

Aromatic Hydrocarbon Cations: Structural Overview

Aromatic Hydrocarbon Cations: Structural Overview

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 with both...

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

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 annulenes. In...

Frost Circles for Different Conjugated Systems

Frost Circles for Different Conjugated Systems

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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Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

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Heteroaromatic rings of the future.

William R Pitt¹, David M Parry, Benjamin G Perry

¹UCB Celltech, Granta Park, Great Abington, Cambridge CB15 6GS, United Kingdom. will.pitt@ucb.com

Journal of Medicinal Chemistry

|April 8, 2009

Summary

Researchers created the Virtual Exploratory Heterocyclic Library (VEHICLe) of 24,847 small aromatic ring systems. Most are unsynthesized, presenting opportunities for novel synthetic protein ligand development.

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Area of Science:

Organic Chemistry
Medicinal Chemistry
Computational Chemistry

Background:

Small aromatic ring systems are crucial for designing new synthetic protein ligands.
A comprehensive catalog of these structures is essential for drug discovery and development.
Existing databases lack a complete inventory of potential heterocyclic scaffolds.

Purpose of the Study:

To create a comprehensive virtual library of small aromatic ring systems.
To identify and quantify the number of synthesized versus unsynthesized heterocyclic compounds.
To stimulate research by highlighting tractable, yet unexplored, heterocyclic structures.

Main Methods:

Generation of a complete list of 24,847 small aromatic ring systems, termed the Virtual Exploratory Heterocyclic Library (VEHICLe).
Substructure searches of literature and compound databases to identify synthesized compounds.
Application of a validated machine learning model to estimate the number of synthetically tractable, unpublished ring systems.

Main Results:

The VEHICLe database contains 24,847 unique small aromatic ring systems.
Only 1701 of these systems have been reported in the literature or databases.
Machine learning predicts over 3000 additional synthetically tractable, yet unpublished, ring systems.
The publication rate for novel examples is low, estimated at 5-10 per year.

Conclusions:

A vast majority of potential small aromatic ring systems remain unexplored.
Significant opportunities exist for the synthesis of novel heterocyclic compounds for protein ligand development.
This work provides a valuable resource and highlights untapped areas for synthetic chemists.