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Reacciones de hidroboración de anillos aromáticos pobres en electrones por boranos de carbenos N-heterocíclicos

Wen Dai¹, Steven J Geib¹, Dennis P Curran¹

¹Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15208, United States.

Journal of the American Chemical Society

|February 27, 2020

Resumen

Los boranos N-heterocíclicos sufren reacciones fotorredóxicas con anillos aromáticos, dando lugar a ciclohexadienos borilados desaromatizados. Este proceso favorece la formación de productos estéricamente obstaculizados a través de un mecanismo propuesto mediado por radicales.

Área de la Ciencia:

Química orgánica
La fotoquímica
Química del organoboron

Sus antecedentes:

Los boranos de carbenos N-heterocíclicos (boranos NHC) son reactivos versátiles en la síntesis orgánica.
La catálisis fotorredóxica permite nuevas vías de reacción en condiciones suaves.
Los anillos aromáticos pobres en electrones son motivos comunes en productos farmacéuticos y materiales.

Objetivo del estudio:

Investigar la reacción de los boranos NHC con anillos aromáticos pobres en electrones en condiciones de fotorreducción.
Para aclarar el mecanismo de desaromatización y borilación.
Comprender la regioselectividad de los productos observados.

Principales métodos:

Catalización por fotorreducción con luz visible.
Reacción de los boranos NHC con varios sustratos aromáticos pobres en electrones.
Análisis espectroscópico (NMR, espectrometría de masas) para la caracterización del producto.
Estudios mecanicistas que incluyen experimentos de captura de radicales (implicados).

Principales resultados:

Los 3-NHC-boril-1,5-ciclohexadienos aromatizados fueron sintetizados con éxito.
Los productos se derivan formalmente de las reacciones de 1,4-hidroboración.
Se observó la formación regioselectiva del producto más estéricamente obstaculizado (doblemente ortosubstituido).
Se propone un mecanismo que involucra la oxidación de NHC-borano, la reducción del anillo aromático, el acoplamiento radical y la protonación.

Conclusiones:

La catálisis fotorredóxica proporciona una ruta eficiente a los ciclohexadienos borilados desaromatizados de los boranos NHC y los aromáticos pobres en electrones.
La reacción exhibe una alta regioselectividad, favoreciendo productos estéricamente exigentes.
El mecanismo basado en radicales propuesto explica los resultados observados y ofrece información sobre la reactividad del NHC-borano.

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