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Videos de Conceptos Relacionados

Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

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.
Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

Cycloaddition Reactions: MO Requirements for Thermal Activation

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+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

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.
Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

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.
Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation01:27

Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation

Robinson annulation is a base-catalyzed reaction for the synthesis of 2-cyclohexenone derivatives from 1,3-dicarbonyl donors (such as cyclic diketones, β-ketoesters, or β-diketones) and α,β-unsaturated carbonyl acceptors. Named after Sir Robert Robinson, who discovered it, this reaction yields a six-membered ring with three new C–C bonds (two σ bonds and one π bond).
C–C Bond Cleavage: Retro-Aldol Reaction00:57

C–C Bond Cleavage: Retro-Aldol Reaction

The reverse of the aldol addition reaction is called the retro-aldol reaction. Here, the carbon–carbon bond in the aldol product is cleaved under acidic or basic conditions to form two molecules of carbonyl compounds. The mechanism of the reaction consists of three steps.
In the first step, as depicted in Figure 1, the base deprotonates the β-hydroxy ketone at the hydroxyl group to form an alkoxide ion.

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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

Mecánicamente facilitado retro [4+2] cicloadditions adiciones mecánicas.

Kelly M Wiggins1, Jay A Syrett, David M Haddleton

  • 1Department of Chemistry and Biochemistry, The University of Texas, Austin, Texas 78712, USA.

Journal of the American Chemical Society
|April 21, 2011
PubMed
Resumen
Este resumen es generado por máquina.

La fuerza mecánica desencadena la escisión de la cadena de polímeros a través de la retrocicloadición. El ultrasonido aplicado a los poli (metilacrilatos) con adyuctos incrustados inició esta reacción, demostrando una nueva vía mecanoquímica.

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Área de la Ciencia:

  • Química de Polímeros La Química de Polímeros es la química de los polímeros.
  • Mecanoquímica es la mecanoquímica.
  • Ciencia de los materiales Ciencia de los materiales.

Sus antecedentes:

  • Las cadenas de polímeros se pueden funcionalizar utilizando reacciones de cicloadición.
  • Las reacciones mecanoquímicas ofrecen nuevas vías para la modificación de polímeros.

Objetivo del estudio:

  • Para investigar la activación mecánica de las reacciones de retro [4+2] cicloadición en polímeros.
  • Para explorar la escisión de la cadena de polímero inducida por ultrasonido.

Principales métodos:

  • Síntesis de poli (metilacrilatos) con añadidos de cicloadición [4+2] incrustados.
  • Tratamiento ultrasónico de polímeros a bajas temperaturas.
  • Análisis mediante cromatografía de permeación de gel (GPC) y espectroscopia UV-vis.
  • Etiquetado de las fracciones liberadas con cromóforos.

Principales resultados:

  • La retro [4+2] cicloadición inducida por ultrasonido en polímeros con M (n) > 30 kDa.
  • Se midió la tasa de adición retrocicloactivación activada mecánicamente utilizando aductos de antraceno.
  • La escisión de la cadena y la liberación de la porción dependieron de la ubicación del adducto y el peso molecular del polímero.

Conclusiones:

  • Los adductos de cicloadición incrustados dentro de las cadenas de polímeros son susceptibles a la fuerza mecánica.
  • La fuerza mecánica vectorialmente opuesta es clave para activar la retro-adición ciclónica.
  • Este estudio demuestra un nuevo método para la escisión de la cadena de polímeros a través de la mecanoquímica.