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Videos de Conceptos Relacionados

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation02:47

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
Regioselectivity and Stereochemistry of Hydroboration02:36

Regioselectivity and Stereochemistry of Hydroboration

A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Hydroboration-Oxidation of Alkenes03:08

Hydroboration-Oxidation of Alkenes

In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration02:34

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The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
Nucleophilic Aromatic Substitution: Elimination–Addition01:11

Nucleophilic Aromatic Substitution: Elimination–Addition

Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is confirmed through isotopic...
Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule02:17

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

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.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...

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A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis
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Hidroboración de alquenos: productos intermedios calientes que reaccionan mientras se enfrían.

David R Glowacki1, C H Liang, Stephen P Marsden

  • 1School of Chemistry, University of Leeds, Leeds LS2 9JT, United Kingdom. david.r.glowacki@bristol.ac.uk

Journal of the American Chemical Society
|September 14, 2010
PubMed
Resumen
Este resumen es generado por máquina.

El comportamiento no estadístico en las reacciones químicas desafía los modelos tradicionales. Este estudio utiliza un modelo de ecuación maestra para demostrar que incluso en solución, los intermediarios calientes pueden reaccionar antes de la termalización completa, preservando los modelos estadísticos.

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Área de la Ciencia:

  • Química Física es la química física.
  • Química orgánica es la química orgánica.
  • La Dinámica Química es la Dinámica Química.

Sus antecedentes:

  • El comportamiento no estadístico en las reacciones químicas está ganando atención.
  • Las desviaciones de las predicciones de la Teoría del Estado de Transición (TST) desafían los modelos estándar de reactividad química.

Objetivo del estudio:

  • Proponer una interpretación alternativa de los resultados experimentales para los mecanismos de hidroboración.
  • Para investigar el papel de los efectos dinámicos no estadísticos frente a los modelos de reacción estadística.

Principales métodos:

  • Utilizó un modelo de colisión débil Rice-Ramsperger-Kassel-Marcus (RRKM) -Ecuación maestra (ME) para los cálculos.
  • Metodología de ecuación maestra aplicada a una reacción térmica en la fase de solución.

Principales resultados:

  • Se demostró que los intermediarios "calientes" pueden reaccionar durante la relajación gradual a través de colisiones débiles en solución.
  • Proporcionó una interpretación estadística de los resultados experimentales que se desvían de las predicciones de TST.
  • Mostró la ecuación maestra como un modelo computacionalmente tratable para las reacciones de la fase de solución.

Conclusiones:

  • La ecuación maestra ofrece una alternativa viable a la TST convencional para explicar ciertos mecanismos de reacción orgánica.
  • La relajación por colisión gradual de los intermediarios puede ser frecuente en varios sistemas poliatómicos.
  • Este trabajo proporciona un nuevo enfoque computacional para estudiar la dinámica de las reacciones químicas en solución.