Reaction monitoring reveals poisoning mechanism of Pd2(dba)3 and guides catalyst selection

We have discovered that the dba ligand in the commonly used Pd2(dba)3 .CHCl3 cross-coupling pre-catalyst is susceptible to bisarylation when used in the presence of aryl iodides. The in situ formed dbaAr2 ligands result in Pd-species with altered catalytic activity. In the case of study, the room temperature C3 arylation of benzo[b]thiophenes with aryl iodides, we have observed a marked catalyst deactivation when dba is arylated with electrondeficient aryl iodides, accounting for the poor yields obtained in the C3 arylation reactions with these aryl iodides. Based on these studies, we report a new catalytic system, employing a dba-free Pd catalyst, which allows for the first time the direct C3-arylation of benzo[b]thiophenes with electron-deficient aryl iodides at room temperature.