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Sobre la selectividad del 6-endo en las ciclizaciones del radical 4-penten-1-óxilo
Jens Hartung1, Rainer Kneuer, Christian Rummey
1Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany. hartung@chemie.uni-kl.de
Journal of the American Chemical Society
|September 24, 2004
Resumen
Se estudiaron los efectos de los sustitutos en las ciclizaciones radicales. El aumento de la masa del sustituto aumentó la selectividad de 6-endo-trig, favoreciendo la formación de tetrahidropirano debido a las interacciones favorables de FMO y la reducción de la tensión de torsión.
Área de la Ciencia:
- Química orgánica es la química orgánica.
- Química computacional es la química computacional.
- Mecanismos de reacción Mecanismos de reacción
Sus antecedentes:
- Las ciclizaciones radicales son reacciones clave en la síntesis orgánica.
- Comprender la regioselectividad es crucial para controlar los resultados de las reacciones.
- Los efectos de sustitución pueden influir significativamente en las vías de reacción.
Objetivo del estudio:
- Investigar las regioselectividades de las ciclizaciones de radicales 4-penten-1-óxilo 4-sustituidas.
- Para dilucidar los factores que rigen la formación de anillos de diferentes tamaños (tetrahidrofurano frente a tetrahidropirano).
- Para correlacionar las observaciones experimentales con predicciones computacionales.
Principales métodos:
- Investigación experimental combinada y estudio computacional utilizando la teoría funcional de densidad (DFT).
- Variación sistemática de los sustituyentes en la posición 4 del radical pentenilo.
- Análisis de las energías de estado de transición y parámetros geométricos.
Principales resultados:
- Se observó un aumento progresivo de la selectividad de 6-endo-trig con el aumento del tamaño del sustituto (H < CH3 < C(CH3) 3 < C6H5).
- Se favoreció la formación de tetrahidrofuranos (5-exo-trig) para sustituyentes más pequeños.
- Se prefirió la formación de tetrahidropirano (6-endo-trig) para los sustituyentes de tert-butilo y fenilo.
Conclusiones:
- La regioselectividad se rige por un equilibrio entre la tensión de torsión y las interacciones orbitales moleculares de frontera (FMO).
- Las interacciones FMO favorecen la ciclización 6-endo-trig para sustitutos voluminosos al reducir las energías del estado de transición.
- La tensión de torsión juega un papel en las vías 6-endo-trig, pero no en las vías 5-exo-trig.
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