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Related Concept Videos

Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

2.2K
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.
2.2K
Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

2.9K
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
2.9K
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

1.9K
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
1.9K
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.5K
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.
2.5K
Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

Cycloaddition Reactions: MO Requirements for Thermal Activation

3.8K
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.
3.8K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.1K
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.
2.1K

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy.

Jiao Yu J Wang1, Mitchell T Blyth1, Michael S Sherburn1

  • 1Research School of Chemistry, Australian National University, Canberra, Australian Capital Territory 2601, Australia.

Journal of the American Chemical Society
|January 7, 2022
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Summary

This study explores the photoenolization/Diels-Alder (PEDA) reaction sequence using computational and experimental methods. The PEDA sequence shows broad applicability with carbonyl prodienes, while thiocarbonyls and imines present unique reactivity challenges and opportunities.

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Area of Science:

  • Organic Photochemistry
  • Computational Chemistry
  • Reaction Mechanism Studies

Background:

  • The photoenolization/Diels-Alder (PEDA) reaction is a powerful synthetic tool.
  • Understanding the scope and limitations of the PEDA sequence is crucial for its application.
  • Previous studies have not comprehensively investigated the factors influencing PEDA reactivity.

Purpose of the Study:

  • To conduct a broad spectrum investigation of the PEDA sequence.
  • To examine the influence of various substituents and reaction partners on the PEDA reaction.
  • To explore the potential of related reaction pathways like photoenolization/cheletropic addition (PECA).

Main Methods:

  • Utilized M06-2X/6-31+G(d,p) calculations with SMD solvation.
  • Supported computational findings with experimental UV-vis spectroscopy.
  • Investigated a test set of 20 prodienes and 20 dienophiles with diverse electronic and structural properties.

Main Results:

  • The PEDA sequence is tolerant of most substitution patterns in carbonyl-based prodienes.
  • Thiocarbonyl derivatives react slower due to inefficient intersystem crossing and competing hydrogen atom transfer pathways.
  • Ortho-alkyl imines exhibit higher barriers in the excited state but can undergo facile PEDA if overcome.
  • The Diels-Alder reaction scope is broader than previously reported, with potential for reactions involving ethylene and electron-rich alkenes.
  • Carbon monoxide (CO) is predicted to undergo a facile (4+1)cheletropic addition (PECA) instead of a [4+2]cycloaddition.

Conclusions:

  • The PEDA sequence is a versatile reaction with broad applicability, particularly for carbonyl prodienes.
  • Thiocarbonyl and imine derivatives offer unique reactivity profiles that can be exploited under specific conditions.
  • The novel photoenolization/cheletropic addition (PECA) sequence provides a metal-free route to benzannelated cyclopentanones.