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Control estereoelectrónico en adición de nucleófilos a un ion de amidinio
1Department of Chemistry, University of California-San Diego, La Jolla, California 92093-0358, USA.
Journal of the American Chemical Society
|July 18, 2001
Resumen
El control estereoelectrónico en la adición nucleófila se cuantificó utilizando un ion tetrahidropirimidinio. Se favorece el ataque axial, pero las reacciones producen estereoisómeros cis/trans, revelando débiles efectos estereoelectrónicos en la reactividad.
Área de la Ciencia:
- Química orgánica es la química orgánica.
- La estereoquímica es la estereoquímica.
- Mecanismos de reacción Mecanismos de reacción
Sus antecedentes:
- Las reacciones nucleófilas de adición son fundamentales en la síntesis orgánica.
- Los efectos estereoelectrónicos influyen significativamente en las vías de reacción y la distribución del producto.
- Los iones de amidinium ofrecen características estructurales únicas para el estudio del control de la reacción.
Objetivo del estudio:
- Para medir cuantitativamente el control estereoelectrónico en reacciones de adición nucleófila.
- Investigar la influencia de los factores estereoelectrónicos y estéricos en los resultados de las reacciones.
- Para determinar las contribuciones relativas del control cinético y termodinámico en las reacciones de iones de amidinio.
Principales métodos:
- Síntesis y reacción del ion 1,3-dimetil-5-fenil-1,4,5,6-tetrahidropirimidinio.
- Adición nucleofílica con una amplia gama de carbaniones y cianuro.
- Análisis de las distribuciones de productos cinéticos y termodinámicos utilizando resonancia magnética nuclear de protones (1H NMR).
- Equilibrio catalizado por ácido para determinar las relaciones de los productos termodinámicos.
Principales resultados:
- La adición nucleofílica al ion amidinium se produjo a través de dos vías distintas, con el ataque axial favorecido.
- Las reacciones con varios nucleófilos produjeron mezclas de estereoisómeros cis y trans.
- Se encontró que los efectos estereoelectrónicos eran débiles, contribuyendo aproximadamente 1 kcal / mol a la reactividad.
Conclusiones:
- Los efectos estereoelectrónicos juegan un papel menor en la reactividad de este sistema de tetrahidropirimidinio.
- Tanto los factores estéricos como los estereoelectrónicos contribuyen a los resultados estereoquímicos observados.
- El estudio proporciona información cuantitativa sobre el equilibrio de fuerzas que gobiernan las reacciones nucleófilas de adición.
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