Non‐Chelate‐Assisted Palladium‐Catalyzed Aerobic Oxidative Heck Reaction of Fluorobenzenes and Other Arenes: When Does the C−H Activation Need Help?

Abstract

The pyridone fragment in the ligand [2, 2’‐bipyridin]‐6(1H)‐one (bipy‐6‐OH) enables the oxidative Heck reaction of simple arenes with oxygen as the sole oxidant and no redox mediator. Arenes with either electron‐donating or electron‐withdrawing groups can be functionalized in this way. Experimental data on the reaction with toluene as the model arene shows that the C−H activation step is turnover limiting and that the ligand structure is crucial to facilitate the reaction, which supports the involvement of the pyridone fragment in the C−H activation step. In the case of fluoroarenes, the alkenylation of mono and 1,2‐difluoro benzenes requires the presence of bipy‐6‐OH. In contrast, this ligand is detrimental for the alkenylation of 1,3‐difluoro, tri, tetra and pentafluoro benzenes which can be carried out using just [Pd(OAc)2]. This correlates with the acidity of the fluoroarenes, the most acidic undergoing easier C−H activation so other steps of the reaction such as the coordination‐insertion of the olefin become kinetically important for polyfluorinated arenes. The use of just a catalytic amount of sodium molybdate as a base proved to be optimal in all these reactions.

Hidrocarburos como reactivos en reacciones de acoplamiento cruzado C–C catalizadas por paladio: Ventajas y retos con una mirada mecanística

“…Nuestro grupo introdujo ligandos quelato de tipo bipiridona para acelerar notablemente reacciones de arilación directa de piridina, 30 para permitir la arilacion de arenos simples, 31 y también de reacciones de Heck oxidativas con oxígeno como oxidante que no ocurren en ausencia de ligando cooperativo (Esquema 8). 32 Otros ligandos quelato derivados de la piridona se han empleado en los últimos años. 33 El Esquema 9 muestra los dos modos de coordinación de estos ligandos, neutro y monoaniónico, en complejos modelo sintetizados independientemente.…”

Selectivity Switch in Non‐Directed Palladium‐Catalyzed C−H Olefination of Chlorobenzene Derivatives

“…Since these pioneering works, several ligands have been designed to improve and modulate the reactivity and selectivity of the Fujiwara‐Moritani reaction. Among them, the 3,5‐dichloropyridine L2 , [11] bidentate 2‐hydroxyphenanthroline L3a [12a] and [2,2’‐bipyridin]‐6(1 H )‐one L3b , [12b] pyrazole‐substituted 2‐pyridone L4 , [13] or the weakly coordinating bidentate thioethercarboxylic acid ligand L5 [14] are particularly efficient. Thanks to these major advances, arene‐limited transformations were then possible using for instance the 2‐pyridone L6 [ 15] or a combination of the pyridine L7 and N ‐acetylglycine [16] …”

“… Ligand‐accelerated non‐directed Fujiwara‐Moritani olefinations. L1 : Yu et al ., 2009 ; [10a] L2 : Sanford et al ., 2012 ; [11] L3a : Duan et al ., 2014 ; [12a] L3b : Albéniz and Villalba, 2021 ; [12b] L4 : Joo et al ., 2023 ; [13] L5 : Fernández‐Ibáñez et al ., 2017 ; [14] L6 : Yu et al ., 2017 ; [15] L6 : Van Gemmeren et al ., 2018 [16] …”