Study of the palladium reduction step of the photo-catalyzed Heck reaction through NMR methods

João Pedro Brussolo da Silva (Universidade Estadual Campinas, Brazil)

LinkedIn: João Pedro Brussolo da Silva

Abstract: Palladium is one of the most extensively employed transition metals in organic catalysis, particularly in cross-coupling transformations. In many Heck reactions, phosphine ligands coordinate to palladium, while a Pd(II) precatalyst is reduced to Pd(0), generating the catalytically active species that enters the Heck catalytic cycle. More recently, the incorporation of photocatalysis has enabled a much faster reduction of palladium under light irradiation, leading to significant improvements in reaction rates. Nevertheless, the precise role of light throughout the catalytic cycle, as well as the identity of the active palladium species, remains incompletely understood. Because the performance of transition-metal-catalyzed cross-coupling reactions is highly dependent on the ligand environment, identifying the catalytic complex is essential for obtaining a deeper mechanistic understanding.

To address these questions, this work investigates the palladium reduction step using in situ NMR spectroscopy. Kinetic 31P NMR experiments were carried out to compare the reduction process under irradiated and dark conditions, allowing the influence of light on catalyst formation to be assessed. In addition, diffusion-ordered spectroscopy (DOSY) experiments were employed to estimate the molecular size of the species present in solution, providing complementary evidence for the identification of the catalytic complexes.