Swiping Right on Palladium: Matching Precursors, Ligands, and Reaction Conditions

While ligand design has dominated efforts to optimize palladium-catalyzed cross-coupling reactions, the choice of palladium source is often driven by convenience rather than systematic evaluation. To address whether a universally effective Pd precursor exists or if performance depends on ligand class and reaction conditions, we employed high-throughput experimentation (HTE) to generate a dataset of >450 reactions systematically comparing precursor effects across diverse ligand families. Using Buchwald–Hartwig amination of a sterically demanding substrate as a model reaction, we find that the palladium source can have tremendous influence on reaction outcome— comparable in magnitude to ligand choice itself. However, precursor sensitivity varies dramatically across four distinct ligand classes: biarylphosphines exhibit robust, source-insensitive performance, while monophosphines and certain bulky ligands show pronounced sensitivity to the Pd source, with a fourth class remaining inactive regardless of precursor. These trends are rationalized through a kinetic framework emphasizing precursor stability under reaction conditions and competing off- cycle deactivation pathways. Along the way, we discovered an unprecedented base-promoted decomposition pathway for MeNAP-type precatalysts, explaining discrepancies between in situ use and preformed catalysts. Extension to Suzuki–Miyaura coupling confirms analogous liganddependent trends. For HTE applications with in situ precatalyst formation, [Pd(tBu-indenyl)Cl]₂ emerges as the most robust choice, while commonly used Pd(OAc)₂ and Pd₂(dba)₃ require caution. MeNAP precursors can be e=ectively used in high-throughput screening workflows, provided that precautions are taken to mitigate their instability under basic conditions. Beyond catalytic efficiency, these findings have important implications for the reproducibility and comparability of ligand screening studies, where precursor choice can mask or amplify ligand effects.