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Complejos de carbenos de rutenio de medio sándwich enlazan la trans-hidrogenación y la gem-hidrogenación de alquinas
Alexandre Guthertz1, Markus Leutzsch1, Lawrence M Wolf1
1Max-Planck-Institut für Kohlenforschung , Mülheim/Ruhr 45470, Germany.
Journal of the American Chemical Society
|February 13, 2018
Resumen
Los catalizadores de rutenio logran la síntesis de E-alqueno a través de la transhidrogenación de las alquinas internas. Un estudio mecanicista revela metaciclopropeno y carbenos intermedios, lo que explica las vías de hidrogenación de gemas y la formación de productos.
Área de la Ciencia:
- Química organometálica
- Catálisis
- Mecanismos de reacción
Sus antecedentes:
- La hidrogenación de alquinas internas generalmente produce alquenos Z.
- Los catalizadores basados en [Cp*Ru] exhiben una estereoselectividad inusual, formando E-alquenos.
- El mecanismo preciso para esta trans-hidrogenación poco ortodoxa sigue sin estar claro.
Objetivo del estudio:
- Elucidar la vía mecanicista completa de la hidrogenación interna del alquino utilizando catalizadores basados en [Cp*Ru].
- Investigar la formación y el destino de los intermedios clave, incluidos los metaciclopropenos y los carbenos de rutenio.
- Comprender los factores que rigen la competencia entre la transhidrogenación y la gemhidrogenación.
Principales métodos:
- Investigaciones experimentales combinadas y estudios computacionales (teoría funcional de la densidad - DFT, grupo acoplado - CCSD).
- Espectroscopia de resonancia magnética nuclear (RMN) por transferencia de polarización inducida por parahidrógeno (PHIP).
- Caracterización por aislamiento y difracción de rayos X de los carbenos intermedios, en particular con los grupos -OR propargílicos.
Principales resultados:
- Se forma principalmente un intermediario de metaciclopropeno, que conduce al E-alqueno a través de la entrega trans concertada de H2.
- Una vía competidora forma intermedios de rutenio carbeno a través de la hidrogenación de gemas formal del enlace π-alquina.
- Los carbenos intermedios pueden dar lugar a E-alquenos, isómeros posicionales, alcanos o sufrir reacciones adicionales como la ciclopropanación y la expansión de anillos.
Conclusiones:
- El estudio proporciona una comprensión mecanicista detallada de la hidrogenación de alquino catalizada por [Cp*Ru], reconciliando las observaciones experimentales con las predicciones teóricas.
- La formación de carbenos intermedios a través de la hidrogenación de gemas explica las vías de competencia y permite nuevas transformaciones sintéticas.
- Se obtienen trans-hidrogenaciones altamente selectivas de los alcoholes propárgicos, lo que demuestra su utilidad preparativa.
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