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Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN101:14

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Treating arylamines with nitrous acid gives aryldiazonium salts that are effective substrates in nucleophilic aromatic substitution reactions. The diazonio group in these salts can be easily displaced by different nucleophiles, yielding a wide variety of substituted benzenes. The leaving group departs as nitrogen gas, and this easy elimination is the driving force for the substitution reaction.
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo,...
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If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
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A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
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In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
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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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Nucleophilic Aromatic Substitution: Addition–Elimination (SNAr)01:30

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Nucleophilic substitution in aromatic compounds is feasible in substrates bearing strong electron-withdrawing substituents positioned ortho or para to the leaving group. The reaction proceeds via two steps: the addition of the nucleophile and the elimination of the leaving group.
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Ciclización de Nazarov dirigida por silicio enantioselectiva

Jin Cao1, Meng-Yang Hu1, Si-Yuan Liu1

  • 1The State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China.

Journal of the American Chemical Society
|April 28, 2021
PubMed
Resumen

Este estudio introduce una nueva reacción de Nazarov dirigida por silicio para sintetizar ciclopentenonas quirales. Este método catalítico ofrece una nueva ruta a moléculas complejas no accesibles a través de otras reacciones enantioselectivas.

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Área de la Ciencia:

  • Química orgánica
  • Química sintética

Sus antecedentes:

  • La electrociclación de Nazarov es una reacción clave para la síntesis de ciclopentenona.
  • Los métodos catalíticos asimétricos existentes tienen limitaciones en los patrones de sustitución de productos debido al control de la posición del enlace doble.

Objetivo del estudio:

  • Desarrollar una nueva reacción de Nazarov dirigida por silicio altamente enantioselectiva.
  • Superar las limitaciones de los métodos anteriores y ampliar las aplicaciones sintéticas.

Principales métodos:

  • Catalización cooperativa con un ácido de Lewis y un ácido quiral de Brønsted.
  • Utilización de grupos de silicio en sustratos de dienona para dirigir la regioselectividad.
  • Estudios mecanicistas relacionados con la sinergia ácido-Brønsted de Lewis y la transferencia de protones intermedios de enol.

Principales resultados:

  • Síntesis exitosa de ciclopentenonas quirales con patrones de sustitución únicos.
  • Logró una alta enantioselectividad en la reacción de Nazarov dirigida por silicio.
  • Se demostró que el grupo de silicio dicta la posición del doble enlace mediante la estabilización de un intermediario beta-carbocatión.

Conclusiones:

  • La reacción de Nazarov dirigida por silicio desarrollada proporciona acceso a valiosas ciclopentenonas quirales.
  • La catálisis cooperativa de ácido de Lewis y quiral de Brønsted ofrece una vía de activación sinérgica.
  • La enantioselectividad es impulsada principalmente por la transferencia quiral de protones catalizada por ácido de Brønsted en el intermedio enol.