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Videos de Conceptos Relacionados

Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

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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.
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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

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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.
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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.4K
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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Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation01:27

Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation

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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).
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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.
10.3K
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
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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Acoplamientos electrofílicos enantioselectivos catalizados por cobalto: Síntesis estereoselectivas de azaciclos de 5

Zhaoming Ma1, Wenqiang Xu2, Yun-Dong Wu2

  • 1State Key Laboratory of Chemical Oncogenomics, Guangdong Provincial Key Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, 2199 Lishui Road, Nanshan District, Shenzhen 518055, China.

Journal of the American Chemical Society
|July 21, 2023
PubMed
Resumen

Los catalizadores de cobalto quiral permiten acoplamientos reductores enantioselectivos, creando valiosos anillos de 5 a 7 miembros que contienen nitrógeno con estereocentros cuaternarios. Este método ofrece una ruta estereoselectiva para estructuras cíclicas complejas.

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Área de la Ciencia:

  • Química organometálica
  • Catálisis asimétrica
  • Síntesis orgánica

Sus antecedentes:

  • Los ligandos piroxíricos quirales son efectivos en la catálisis asimétrica.
  • Los acoplamientos reductivos son importantes para la formación de enlaces C-C.
  • La síntesis de heterociclos que contienen nitrógeno con estereocentros cuaternarios sigue siendo un reto.

Objetivo del estudio:

  • Desarrollar una reacción de acoplamiento reductor enantioselectivo.
  • Para sintetizar azaciclos de 5 a 7 miembros con estereocentros cuaternarios.
  • Elucidar el mecanismo de la selectividad electrofila cruzada.

Principales métodos:

  • Los complejos de cobalto de los ligandos piroxíricos quirales se utilizaron como catalizadores.
  • Los iododienos no conjugados se acoplaron con yoduros de arilo o bromuros de alquenilo.
  • Se realizaron estudios mecanicistas para comprender la selectividad.

Principales resultados:

  • La reacción catalizó con éxito los acoplamientos reductores enantioselectivos.
  • Se logró una síntesis estereoselectiva de azaciclos de 5 a 7 miembros con estereocentros cuaternarios.
  • La selectividad electrofílica cruzada se atribuyó al acoplamiento de los intermediarios de alquilcobalto (I) con yoduros de arilo.

Conclusiones:

  • El acoplamiento reductor enantioselectivo catalizado por cobalto proporciona un método poderoso para sintetizar azaciclos complejos.
  • El estudio aclara el mecanismo de estereoselectividad en estas reacciones.
  • Este enfoque ofrece una nueva estrategia para acceder a bloques de construcción quirales valiosos.