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Videos de Conceptos Relacionados

Sharpless Epoxidation02:57

Sharpless Epoxidation

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The conversion of allylic alcohols into epoxides using the chiral catalyst was discovered by K. Barry Sharpless and is known as Sharpless epoxidation. The use of a chiral catalyst enables the formation of one enantiomer of the product in excess. This chiral catalyst is mainly a chiral complex of titanium tetraisopropoxide and tartrate ester (specific stereoisomer). The stereoisomer used in the chiral catalyst dictates the formation of the enantiomer of the product. In other words, the use of...
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Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide02:44

Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide

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Alkenes are converted to 1,2-diols or glycols through a process called dihydroxylation. It involves the addition of two hydroxyl groups across the double bond with two different stereochemical approaches, namely anti and syn. Dihydroxylation using osmium tetroxide progresses with syn stereochemistry.
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Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids02:04

Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids

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Diols are compounds with two hydroxyl groups. In addition to syn dihydroxylation, diols can also be synthesized through the process of anti dihydroxylation. The process involves treating an alkene with a peroxycarboxylic acid to form an epoxide. Epoxides are highly strained three-membered rings with oxygen and two carbons occupying the corners of an equilateral triangle. This step is followed by ring-opening of the epoxide in the presence of an aqueous acid to give a trans diol.
6.9K
Preparation of Epoxides03:00

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8.8K
Overview
Epoxides result from alkene oxidation, which can be achieved by a) air, b) peroxy acids, c) hypochlorous acids, and d) halohydrin cyclization.
Epoxidation with Peroxy Acids
Epoxidation of alkenes via oxidation with peroxy acids involves the conversion of a carbon–carbon double bond to an epoxide using the oxidizing agent meta-chloroperoxybenzoic acid, commonly known as MCPBA. Since the O–O bond of peroxy acids is very weak, the addition of electrophilic oxygen of peroxy acids to...
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Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction01:22

Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction

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The radical dimerization of ketones or aldehydes gives vicinal diols through a pinacol coupling reaction. However, the behavior of titanium metals used for the reaction as a source of electrons is unusual. When the reaction is carried out in the presence of titanium, diols can be isolated at low temperatures. Else titanium further reacts with diols, forming alkenes through the McMurry reaction.
2.2K
Acid-Catalyzed Ring-Opening of Epoxides02:24

Acid-Catalyzed Ring-Opening of Epoxides

8.3K
Epoxides that are three-membered ring systems are more reactive than other cyclic and acyclic ethers. The high reactivity of epoxides originates from the strain present in the ring. This ring strain acts as a driving force for epoxides to undergo ring-opening reactions either with halogen acids or weak nucleophiles in the presence of mild acid. The acid catalyst converts the epoxide oxygen, a poor leaving group, into an oxonium ion, a better leaving group, making the reaction feasible. The...
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The Effect of Interfacial Chemical Bonding in TiO2-SiO2 Composites on Their Photocatalytic NOx Abatement Performance
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Epoxidación eficiente sobre los sitios dinucleares en la silicalita de titanio-1

Christopher P Gordon1, Hauke Engler2, Amadeus Samuel Tragl3

  • 1Department of Chemistry and Applied Biosciences, ETH Zürich, Zurich, Switzerland.

Nature
|October 29, 2020
PubMed
Resumen

Los sitios de titanio dinuclear, no los átomos aislados, son la clave para la catálisis de silicita de titanio-1 (TS-1) para la epoxidación de olefinas utilizando peróxido de hidrógeno (H2O2). Este hallazgo mejora la comprensión de TS-1

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Área de la Ciencia:

  • Ciencias de los materiales
  • Catálisis
  • Química inorgánica

Sus antecedentes:

  • La silicalita de titanio-1 (TS-1) es crucial para la epoxidación industrial de las olefinas utilizando peróxido de hidrógeno (H2O2).
  • Su actividad catalítica está tradicionalmente vinculada a sitios aislados de Ti{IV} dentro del marco de las IFM.
  • La estructura precisa de estos sitios activos sigue sin confirmarse a pesar de una extensa investigación.

Objetivo del estudio:

  • Caracterizar los sitios activos de titanio en los catalizadores TS-1 para la epoxidación del propileno.
  • Para aclarar la base estructural de la alta eficiencia catalítica y la selectividad de TS-1.
  • Proponer un modelo revisado para los sitios activos en la catálisis TS-1.

Principales métodos:

  • Se empleó espectroscopia avanzada (por ejemplo, RMN 170) y microscopia.
  • Caracterización detallada de los catalizadores TS-1 altamente activos y selectivos.
  • Se realizaron cálculos de la teoría funcional de la densidad (DFT) para modelar las vías de reacción.

Principales resultados:

  • El análisis espectroscópico reveló la formación de especies puentes de peroxo en sitios de titanio dinuclear al reaccionar con H217O2.
  • Los cálculos de DFT confirmaron una vía de reacción de baja energía facilitada por la cooperatividad entre dos átomos de titanio.
  • Se identificó un estado clave de transición de transferencia de oxígeno, similar a la epoxidación peracida.

Conclusiones:

  • Se proponen sitios de titanio dinuclear, no aislados, como los sitios activos responsables de la alta eficiencia de TS-1 en la epoxidación del propileno.
  • Esta comprensión revisada de la estructura del sitio activo ofrece potencial para una mayor optimización de los catalizadores TS-1 y los procesos de epoxidación industrial.