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El paso que limita la velocidad precede a la formación de enlaces C-C en la reacción de aldol intramolecular
Hui Zhu1, Fernando R Clemente, K N Houk
1University of California, Merced, School of Natural Sciences, P.O. Box 2039, Merced, California 95344, USA.
Journal of the American Chemical Society
|February 5, 2009
Resumen
Se estudió la reacción Hajos-Parrish-Eder-Sauer-Wiechert, un modelo para la catálisis de la prolina. La formación de enlaces carbono-carbono no es el paso que limita la velocidad en esta reacción orgánica clave.
Área de la Ciencia:
- Química orgánica es la química orgánica.
- La catálisis de la catálisis.
- Mecanismos de reacción Mecanismos de reacción
Sus antecedentes:
- La reacción de Hajos-Parrish-Eder-Sauer-Wiechert es un ejemplo fundamental de las reacciones de aldol intramoleculares catalizadas por prolina.
- Las reacciones catalizadas por prolina son un área de rápida expansión en la síntesis orgánica.
- Comprender los mecanismos de estas reacciones es crucial para desarrollar nuevas metodologías sintéticas.
Objetivo del estudio:
- Para investigar el paso limitador de velocidad en la reacción Hajos-Parrish-Eder-Sauer-Wiechert.
- Proporcionar evidencia experimental definitiva con respecto al mecanismo de las reacciones de aldol catalizadas por prolina.
Principales métodos:
- Se utilizaron mediciones experimentales de los efectos del isótopo cinético (13) C.
- Aplicó técnicas establecidas para elucidación mecanicista en química orgánica.
Principales resultados:
- Se demostró de manera concluyente que la formación de enlaces carbono-carbono no es el paso que limita la tasa.
- Proporcionó datos cuantitativos que apoyan una comprensión mecanicista revisada.
Conclusiones:
- Los hallazgos desafían las suposiciones anteriores sobre el paso que determina la velocidad en esta reacción arquetípica.
- Ofrece una visión crítica de la clase más amplia de transformaciones catalizadas por prolina.
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01:10C–C Bond Formation: Aldol Condensation Overview
Aldol condensation is an important route in synthetic organic chemistry used to generate a new carbon–carbon bond under basic or acidic conditions. The aldol condensation reaction presented in Figure 1 constitutes an aldol addition reaction followed by the dehydration process.
01:08Base-Catalyzed Aldol Addition Reaction
As depicted in Figure 1, base-catalyzed aldol addition involves adding two carbonyl compounds in aqueous sodium hydroxide to form a β-hydroxy carbonyl compound.
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.
01:15Acid-Catalyzed Aldol Addition Reaction
The aldol reaction of a ketone under acidic conditions successfully forms an unsaturated carbonyl as the final product instead of an aldol. The acid-catalyzed aldol reaction is depicted in Figure 1.
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.
01:18Intramolecular Aldol Reaction
Intramolecular aldol reaction occurs in dicarbonyl compounds such as dialdehydes, diketones, and keto-aldehydes. The dicarbonyl compounds possess more than one nucleophilic ⍺ carbon for the base to deprotonate and form the enolates. For example, in symmetrical diketones, there are four ⍺ carbons. Hence, four types of enolates are possible when treated with a base. However, since the molecule is symmetrical, the enolates formed on either side of one carbonyl group are equivalent to those formed...