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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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Turnover Number and Catalytic Efficiency01:19

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The turnover number of an enzyme is the maximum number of substrate molecules it can transform per unit time. Turnover numbers for most enzymes range from 1 to 1000 molecules per second. Catalase has the known highest turnover number, capable of converting up to 2.8×106 molecules of hydrogen peroxide into water and oxygen per second. Lysozyme has the lowest known turnover number of half a molecule per second.
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Catalytically Perfect Enzymes01:07

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The theory of catalytically perfect enzymes was first proposed by W.J. Albery and J. R. Knowles in 1976. These enzymes catalyze biochemical reactions at high-speed. Their catalytic efficiency values range from 108-109 M-1s-1. These enzymes are also called 'diffusion-controlled' as the only rate-limiting step in the catalysis is that of the substrate diffusion into the active site. Examples include triose phosphate isomerase, fumarase, and superoxide dismutase.
 
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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

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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.
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Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

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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.
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Reduction of Alkenes: Catalytic Hydrogenation02:13

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Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
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Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions
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Adición catalítica de ciclo de paladio-oxalilo

Barry M Trost1, Zhongxing Huang2, Ganesh M Murhade2

  • 1Department of Chemistry, Stanford University, Stanford, CA 94305-5080, USA. bmtrost@stanford.edu.

Science (New York, N.Y.)
|November 3, 2018
PubMed
Resumen
Este resumen es generado por máquina.

Los investigadores desarrollaron una nueva cicloadición (3+2) catalizada por el paladio utilizando un intermediario Pd-oxalilo. Este método sintetiza de manera eficiente diversos esqueletos de tetrahidrofurano, superando las limitaciones de la química tradicional de los cationes de oxyallyl.

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Área de la Ciencia:

  • Síntesis orgánica
  • Catálisis
  • Química medicinal

Sus antecedentes:

  • Las reacciones de cicloadición quimioselectivas son cruciales para la construcción de sistemas complejos de anillos en la síntesis orgánica.
  • Los cationes oxalilo suelen someterse a cicloadiciones (4 + 3), lo que limita su utilidad en la formación de anillos de cinco miembros.
  • El desarrollo de nuevas vías sintéticas para acceder a esqueletos de cinco miembros sigue siendo un desafío importante.

Objetivo del estudio:

  • Explorar nuevas reacciones de cicloadición para sintetizar sistemas de anillos de cinco miembros.
  • Investigar el uso de la catálisis del paladio en la química de los cationes de oxalilo.
  • Desarrollar un método para acceder a esqueletos de tetrahidrofurano a través de una vía de cicloadición (3+2).

Principales métodos:

  • Generación de un intermediario de paladio-oxalilo a partir de un precursor adaptado mediante el uso de un catalizador Pd (0).
  • Reacción del intermediario Pd-oxalilo con dienos conjugados en una cicloadición (3+2).
  • Conversión posterior catalizada por el paladio de los aductos de tetrahidrofurano a las ciclopentanonas.

Principales resultados:

  • Se logró una nueva reacción de cicloadición catalizada por Pd (3+2), que produjo diversos esqueletos de tetrahidrofurano.
  • La reacción procede por una vía escalonada que implica la transferencia de Pd-alilo y el cierre del anillo, prevaleciendo sobre la selectividad convencional (4 + 3).
  • Los heterociclos resultantes pueden transformarse fácilmente en ciclopentanonas carbocíclicas.

Conclusiones:

  • Este estudio presenta una nueva estrategia para acceder a los anillos de cinco miembros utilizando catálisis de paladio.
  • El método desarrollado ofrece una vía versátil para los tetrahidrofuranos y las ciclopentanonas sustituidos.
  • Este enfoque amplía la utilidad sintética de los intermediarios de oxyallyl en la química orgánica.