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

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.

Automated summary

The pyridone fragment in the ligand plays a crucial role in the C-H activation step of the oxidative Heck reaction. The presence of bipy-6-OH is required for the alkenylation of low acidity fluoroarenes, while it is detrimental for high acidity fluoroarenes. The use of catalytic amount of sodium molybdate as a base is optimal in all these reactions.