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Evidencia estructural de que los sustituyentes alkoxi adoptan orientaciones pseudoaxiales preferidas electrónicamente
Stephen Chamberland1, Joseph W Ziller, K A Woerpel
1Department of Chemistry, University of California, Irvine, California 92697-2025, USA.
Journal of the American Chemical Society
|April 14, 2005
Resumen
Los sustituyentes remotos en los acetalos de tetrahidropirano influyen en la selectividad de la sustitución nucleofílica. Los grupos alkoxi favorecen las conformaciones pseudoaxiales en los intermediarios de iones de oxocarbenio, lo que afecta los resultados de la reacción.
Área de la Ciencia:
- Química orgánica es la química orgánica.
- Química de los carbohidratos Química de los carbohidratos
- Química computacional es la química computacional.
Sus antecedentes:
- Las reacciones de sustitución nucleófila son fundamentales en la síntesis orgánica.
- Los acetalos de tetrahidropirano son intermediarios clave en las reacciones de glicosilación.
- Las preferencias conformamentales de los intermediarios afectan significativamente la selectividad de la reacción.
Objetivo del estudio:
- Investigar la influencia de los sustituyentes remotos de C-4 en la selectividad de las reacciones de sustitución nucleófila de los acetalos de tetrahidropirano.
- Para aclarar las preferencias conformacionales de los intermediarios de iones de oxocarbenio en la glicosilación.
- Proporcionar evidencia teórica y experimental de las preferencias conformacionales observadas.
Principales métodos:
- Reacciones de sustitución nucleófila con acetalos de tetrahidropirano monosubstituidos.
- Estudios computacionales sobre los intermediarios de reacción.
- Análisis espectroscópico (NMR, IR). análisis espectroscópico.
- Cristalografía de rayos X de iones análogos de dioxocarbenio.
Principales resultados:
- Se observaron selectividades opuestas en las reacciones de sustitución nucleófila basadas en sustituyentes alquilo o alkoxi C-4 remotos.
- Los acetalos de tetrahidropirano sustituidos por alkoxi favorecen las conformaciones pseudoaxiales de los intermediarios de iones de oxocarbenio.
- Los datos teóricos, espectroscópicos y cristalográficos apoyan la preferencia conformacional por los sustituyentes alcoxi.
Conclusiones:
- Los sustituyentes de alkoxi remotos dirigen a los intermediarios de iones de oxocarbenio para que adopten una conformación pseudoaxial.
- Esta preferencia conformacional explica las selectividades opuestas observadas en las reacciones de sustitución nucleófila.
- La orientación pseudoaxial favorecida electrónicamente de los sustituyentes electronegativos en anillos de seis miembros con déficit de electrones impulsa esta preferencia de contraste.
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