Caminos homolíticos frente a heterolíticos en la fotoquímica de las haloanilinas
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Synthesis of Antiviral Tetrahydrocarbazole Derivatives by Photochemical and Acid-catalyzed C-H Functionalization via Intermediate Peroxides CHIPS
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Mauro Freccero1, Maurizio Fagnoni, Angelo Albini
1Department of Organic Chemistry, University of Pavia, via Taramelli 10, 27100 Pavia, Italy.
Journal of the American Chemical Society
|October 23, 2003
Resumen
La fotoquímica de las haloanilinas depende del tipo de disolvente y halógeno. Los disolventes polares permiten una eficiente escisión heterolítica de los enlaces C-Cl y C-Br, mientras que los disolventes próticos facilitan la escisión de los enlaces C-F a través de cationes fenilo fotogenerados.
Área de la Ciencia:
- La fotoquímica es la fotoquímica.
- Química orgánica es la química orgánica.
- Química Física es la química física.
Sus antecedentes:
- La fotoquímica de las 4-haloanilinas y las 4-halo-N,N-dimetilanilinas se investiga a través de varias polaridades de disolventes.
- Las reacciones se inician desde el estado triplet, con una reactividad variable observada dependiendo del entorno halógeno y solvente.
Objetivo del estudio:
- Para dilucidar los mecanismos dependientes del disolvente y del halógeno de la fotoquímica de la haloanilina.
- Caracterizar los estados triplet excitados y los perfiles de energía para las vías de disociación homolítica y heterolítica.
- Definir las condiciones para una deshalogenación eficiente a través de cationes fenólicos fotogenerados para aplicaciones sintéticas y ambientales.
Principales métodos:
- Se emplearon experimentos fotofísicos y fotólisis de flash para estudiar la cinética de la reacción y los rendimientos cuánticos.
- Los estudios computacionales a nivel UB3LYP/6-31G(d) caracterizaron estados triplet y paisajes energéticos.
- Los experimentos de captura selectiva con aliltrimetilsilano monitorearon la formación de cationes de fenilo.
Principales resultados:
- La reactividad en disolventes apolares es baja, con rendimientos cuánticos que aumentan significativamente en medios polares para cloro y derivados de flúor.
- La escisión homolítica de los enlaces C-X es endotérmica, mientras que la escisión heterolítica de C-Cl y C-Br es exotérmica en el acetonitrilo.
- La descloración heterolítica eficiente se produjo en disolventes polares (Phi = 0,77 en MeCN), mientras que la desfluoración fue eficiente solo en disolventes próticos (Phi = 0,48 en MeOH).
Conclusiones:
- La polaridad del disolvente y la identidad del halógeno determinan críticamente la eficiencia y el mecanismo (homolítico vs. heterolítico) de la deshalogenación.
- Los cationes fenólicos fotogenerados pueden utilizarse eficazmente en metodologías sintéticas.
- La comprensión de estas vías fotoquímicas es crucial para abordar la degradación de los contaminantes aromáticos halogenados y la fototoxicidad de los medicamentos fluorados.
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