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Alilación desacilativa: Alilación alilica a través de la activación retro-Claisen
Alexander J Grenning1, Jon A Tunge
1Department of Chemistry, The University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045-7528, USA.
Journal of the American Chemical Society
|August 12, 2011
Resumen
Un nuevo método de alilación desacilativa permite la formación de enlaces carbono-carbono utilizando alcoholes alílicos y pronucleófilos de cetonas. Este proceso catalizado por el paladio ofrece una alternativa intermolecular a las técnicas de alilación existentes, facilitando la síntesis de moléculas complejas.
Área de la Ciencia:
- Química orgánica es la química orgánica.
- La catálisis de la catálisis.
- Metodología sintética de la metodología sintética.
Sus antecedentes:
- La alquilación alilica es una reacción crucial en la síntesis orgánica para la formación de enlaces carbono-carbono.
- Los métodos existentes como la alilación descarboxilativa a menudo implican procesos unimoleculares y sustratos específicos.
- Se necesitan métodos intermoleculares eficientes que utilicen materiales de partida fácilmente disponibles.
Objetivo del estudio:
- Introducir un nuevo método catalítico para la alquilación alilica denominado "alilación desacilativa" utilizando alcoholes alilicos.
- Para demostrar la amplia aplicabilidad de este método para el acoplamiento con varios nucleófilos de carbono.
- Mostrar la utilidad de la alilación desacilativa en la construcción de arquitecturas moleculares complejas.
Principales métodos:
- La activación in situ de los alcoholes alílicos y los pronucleófilos cetónicos a través de la reacción retro-Claisen.
- Acoplamiento intermolecular catalizado por paladio de acetatos y carbaniones alilicos generados.
- Aplicación en acoplamientos de tres componentes para la síntesis rápida de 1,6-heptadienos.
Principales resultados:
- Alilación desacilativa exitosa de diversos nucleófilos de carbono relativamente no estabilizados.
- Formación de nuevos enlaces carbono-carbono a través de un ciclo catalítico que involucra al paladio.
- Síntesis eficiente de varios 1,6-heptadienos, lo que demuestra la utilidad del método.
Conclusiones:
- La alilación desacilativa proporciona una ruta intermolecular versátil y eficiente para la alquilación alílica.
- El método utiliza directamente los alcoholes alelicos accesibles, simplificando las estrategias sintéticas.
- Este enfoque amplía el conjunto de herramientas para la construcción de moléculas orgánicas complejas, incluidos los polienos.
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01:24[3,3] Sigmatropic Rearrangement of Allyl Vinyl Ethers: Claisen Rearrangement
The Claisen rearrangement is a [3,3] sigmatropic rearrangement of allyl vinyl ethers to unsaturated carbonyl compounds. The rearrangement is a concerted pericyclic reaction proceeding via a chair-like transition state.
00:57C–C Bond Cleavage: Retro-Aldol Reaction
The reverse of the aldol addition reaction is called the retro-aldol reaction. Here, the carbon–carbon bond in the aldol product is cleaved under acidic or basic conditions to form two molecules of carbonyl compounds. The mechanism of the reaction consists of three steps.
In the first step, as depicted in Figure 1, the base deprotonates the β-hydroxy ketone at the hydroxyl group to form an alkoxide ion.
In the first step, as depicted in Figure 1, the base deprotonates the β-hydroxy ketone at the hydroxyl group to form an alkoxide ion.
02:24Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
01:31Radical Substitution: Allylic Chlorination
Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic...
01:33Aldol Condensation vs Claisen Condensation
Aldol condensation is an acid or base-catalyzed condensation between aldehydes or ketones to give an α,ꞵ-unsaturated carbonyl compound. A base-promoted condensation between ester molecules to produce a ꞵ-ketoester is known as the Claisen condensation. In the presence of a base, both reactions involve deprotonation of the acidic α hydrogen to produce the corresponding enolates. The nucleophilic enolates attack their respective nonenolized carbonyl compound forming a tetrahedral intermediate.
02:27Preparation of Alkynes: Alkylation Reaction
Introduction
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.