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Related Concept Videos

Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.7K
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.
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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.3K
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.
2.3K
Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

2.4K
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
2.4K
Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

Cycloaddition Reactions: MO Requirements for Thermal Activation

4.0K
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
4.0K
Cationic Chain-Growth Polymerization: Mechanism00:57

Cationic Chain-Growth Polymerization: Mechanism

2.6K
The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the...
2.6K
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

2.1K
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
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A Simple and Efficient Protocol for the Catalytic Insertion Polymerization of Functional Norbornenes
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Pnictogen-bonding catalysis: brevetoxin-type polyether cyclizations.

Andrea Gini1, Miguel Paraja1, Bartomeu Galmés2

  • 1Department of Organic Chemistry , University of Geneva , Geneva , Switzerland . Email: stefan.matile@unige.ch ; http://www.unige.ch/sciences/chiorg/matile/ ; Tel: +41 22 379 6523.

Chemical Science
|November 30, 2020
PubMed
Summary
This summary is machine-generated.

Pnictogen-bonding catalysis, using antimony compounds, enables the synthesis of complex polyether structures by overcoming traditional chemical rules. This novel approach offers a unique supramolecular alternative to Lewis acid catalysis.

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Area of Science:

  • Organometallic chemistry
  • Supramolecular chemistry
  • Catalysis

Background:

  • Pnictogen-bond donors offer unique catalytic properties due to their electronic and structural features.
  • Natural product synthesis often requires overcoming challenging stereochemical pathways.

Purpose of the Study:

  • To explore the catalytic potential of fluoroarylated pnictogen compounds in epoxide-opening polyether cyclizations.
  • To investigate the ability of pnictogen-bonding catalysis to access stereochemically restricted cyclic structures.

Main Methods:

  • Catalysis using fluoroarylated antimony(V) and antimony(III) compounds, alongside bismuth, tin, and germanium analogues.
  • Analysis of reaction products to determine stereoselectivity and adherence to or deviation from established cyclization rules.

Main Results:

  • Selective *endo* cyclization into *trans*-fused ladder oligomers, defying the Baldwin rules, was achieved.
  • Tris(3,4,5-trifluorophenyl)stibines and hypervalent stiborane catecholates exhibited distinct *anti*-Baldwin stereoselectivity.
  • Lewis acids, Brønsted acids, and π acids did not yield similar cyclic products.

Conclusions:

  • Pnictogen-bonding catalysis provides a novel supramolecular approach to complex molecule synthesis.
  • This method offers a unique alternative to traditional covalent Lewis acid catalysis.
  • The ability to break Baldwin rules opens new avenues in stereoselective synthesis.