Arylazoformamide palladium(II) complexes as effective precatalysts for the Heck reaction

Arylazoformamide (AAF) ligands coordinate to Pd(II)Cl2 creating Pd(II)Cl2(AAF)2 complexes. When subjected to typical Heck conditions at 1 mol% of complex loading, a broad substrate scope revealed the need for high heat (150 ˚C) and an inorganic base (Na2CO3) to give good to excellent yields (60 - 99%). Not surprisingly, aryliodides outperformed arylbromides when coupling alkenes with electron-donating group (EDG) substituents yet showed very limited selectivity for alkenes with electron-withdrawing groups (EWGs). Mechanistically, it is believed that the complex behaves as a precatalyst and, under reaction conditions, reduces to a Pd(0) complex before proceeding through the catalytic cycle. The reactive species is suggested to be either a bis-AAF coordinated Pd(0) complex or as a formal azoenolate coordinated Pd(II) complex and both neutral and cationic mechanistic pathways were evaluated. Overall, this report provides a glimpse at exploring ligand properties as seen through a known reaction