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Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)01:16

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Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring...
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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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ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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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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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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[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

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The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
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Related Experiment Video

Updated: Sep 5, 2025

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
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Dearomative Ring Expansion of Polycyclic Arenes.

Paolo Piacentini1, Tanner W Bingham2, David Sarlah1,2

  • 1Department of Chemistry, University of Pavia, Viale Taramelli 12, 27100, Pavia, Italy.

Angewandte Chemie (International Ed. in English)
|July 8, 2022
PubMed
Summary

Researchers developed a new method for synthesizing benzocycloheptenes, crucial structures in natural products. This approach efficiently converts simple arenes into valuable substituted (aza)benzocycloheptatrienes for further chemical modifications.

Keywords:
ArenophilesBenzocycloheptatrienesBuchner ReactionDearomatizationRing Expansion

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

  • Organic Chemistry
  • Synthetic Chemistry
  • Medicinal Chemistry

Background:

  • Benzocycloheptenes are vital structural units in numerous natural products and biologically active molecules.
  • Existing synthetic methods for these compounds can be limited in scope or efficiency.

Purpose of the Study:

  • To develop a novel, concise, and versatile strategy for synthesizing substituted benzocycloheptenes.
  • To provide an alternative to traditional methods like the Buchner reaction, expanding access to these important scaffolds.

Main Methods:

  • A two-step sequence involving dearomative [4+2]-cycloaddition with an arenophile.
  • Subsequent palladium-catalyzed cyclopropanation followed by cycloreversion-initiated ring expansion.
  • Utilizing a methylene-insertion molecular editing approach.

Main Results:

  • Successful and direct preparation of a diverse range of substituted (aza)benzocycloheptatrienes from simple (hetero)arenes.
  • Demonstrated a new working alternative to the Buchner reaction, overcoming its limitations to monocyclic arenes.
  • The synthesized compounds are amenable to various downstream functionalizations.

Conclusions:

  • The reported strategy offers an efficient and broadly applicable method for constructing benzocycloheptene frameworks.
  • This molecular editing approach significantly advances the synthesis of complex polycyclic compounds.
  • The accessibility of these substituted (aza)benzocycloheptatrienes opens new avenues for drug discovery and materials science.