High-turnover copper-catalyzed amination of aryl bromides: exploring catalyst and ligand degradation pathways

Copper-catalyzed Ullmann-type amination has emerged as a cost-effective and sustainable alternative to palladium-based C-N coupling, yet its broader adoption is often limited by high catalyst loadings. These high loadings arise in part from catalyst deactivation pathways that are still not fully understood. In this study, we examine the mechanism and stability of a homogeneous copper-oxalamide catalytic system for the coupling of aryl bromides with primary amines. As well as revealing mechanistic insight into the catalytic process, these kinetic studies show that under these conditions (EtOH solvent and KOH base) the copper centre is remarkably robust, but the oxalamide ligand undergoes rapid base-mediated hydrolysis, thus establishing ligand decomposition as a key limitation to catalyst longevity. By compensating for this ligand instability through controlled excess, we are able to achieve exceptionally low copper loadings of 5-50 ppm, delivering turnover numbers in copper of up to 7 × 10⁴ for aryl bromides and 2 × 10⁵ for aryl iodides. These findings further highlight copper's potential as a greener alternative to palladium in pharmaceutical and agrochemical synthesis and provide a foundation for further ligand design taking into account both catalyst stability and activity.