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Related Concept Videos

Conformations of Cycloalkanes02:29

Conformations of Cycloalkanes

Adolf von Baeyer attempted to explain the instabilities of small and large cycloalkane rings using the concept of angle strain — the strain caused by the deviation of bond angles from the ideal 109.5° tetrahedral value for sp3  hybridized carbons. However, while cyclopropane and cyclobutane are strained, as expected from their highly compressed bond angles, cyclopentane is more strained than predicted, and cyclohexane is virtually strain-free. Hence, Baeyer’s theory that was based on the...
Aromatic Hydrocarbon Anions: Structural Overview01:18

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...
Frost Circles for Different Conjugated Systems01:18

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.
Aromatic Hydrocarbon Cations: Structural Overview01:18

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...
Thermal Sigmatropic Reactions: Overview01:16

Thermal Sigmatropic Reactions: Overview

Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred to as...
Cycloalkanes02:28

Cycloalkanes

Cycloalkanes are saturated cyclic hydrocarbons with carbon atoms arranged in the form of rings. They have two fewer hydrogen atoms than the corresponding acyclic alkane; therefore, their general formula is CnH2n. The structural formulas of cycloalkanes are simplified using the line-angle representation. The regular polygons are used to represent the cycloalkane rings, with each side representing a carbon-carbon bond.
The IUPAC nomenclature of cycloalkanes follows similar rules that apply to...

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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones
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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones

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Is cyclopropane really the sigma-aromatic paradigm?

Wei Wu1, Ben Ma, Judy I-Chia Wu

  • 1Department of Chemistry, College of Chemistry and Chemical Engineering, The State Key Laboratory for Physical Chemistry of Solid States, Center for Theoretical Chemistry, Xiamen University, Xiamen, Fujian 361005, China. weiwu@xmu.edu.cn

Chemistry (Weinheim an Der Bergstrasse, Germany)
|June 30, 2009
PubMed
Summary

This study directly evaluates sigma-aromatic stabilization in cyclopropane, finding minimal energetic effects. Computational results suggest alternative explanations for cyclopropane's behavior, challenging the sigma-aromaticity concept.

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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

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Solid-phase Synthesis of [4.4] Spirocyclic Oximes
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Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Published on: February 6, 2019

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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones
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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Solid-phase Synthesis of [4.4] Spirocyclic Oximes
05:15

Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Published on: February 6, 2019

Area of Science:

  • Theoretical Chemistry
  • Computational Chemistry
  • Physical Chemistry

Background:

  • The concept of sigma-aromaticity was proposed to explain cyclopropane's unique properties.
  • Previous indirect energetic evaluations of sigma-aromaticity in cyclopropane yielded conflicting results.
  • Recent studies have questioned the existence of sigma-ring currents in cyclopropane.

Purpose of the Study:

  • To directly evaluate the sigma-aromatic stabilization energy of cyclopropane for the first time.
  • To investigate the sigma-aromatic stabilization in trisilacyclopropane.
  • To provide computational evidence supporting alternative interpretations of cyclopropane's energetic behavior.

Main Methods:

  • Ab initio valence bond (VB) computations.
  • Configuration interaction calculations at the VBSCF/cc-PVTZ level.
  • Comparison of cyclopropane and trisilacyclopropane with acyclic alkane and silane references.

Main Results:

  • The sigma-aromatic stabilization energy of cyclopropane is estimated to be at most 3.5 kcal mol(-1) relative to propane, and near zero relative to n-butane.
  • Trisilacyclopropane exhibits limited sigma-aromatic stabilization compared to its acyclic counterparts.
  • The findings challenge the significance of sigma-aromaticity as an explanation for cyclopropane's energetic properties.

Conclusions:

  • Direct computational evaluation suggests that sigma-aromatic stabilization in cyclopropane is not a significant energetic factor.
  • The study supports alternative interpretations for the energetic behavior of cyclopropane and related silicon compounds.
  • The concept of sigma-aromaticity may not be necessary to explain the observed properties of cyclopropane.