Nickel-catalyzed intramolecular decarbonylative coupling of thioesters: from optimization to mechanistic insights into precatalyst activation and Ni(0) reactivity

Nickel complexes supported by dppa-type ligands efficiently catalyze the intramolecular decarbonylative coupling of a broad range of thioesters to thioethers. Under reductive conditions, robust and bench-stable nickel(II) precatalysts are readily converted in situ into catalytically competent nickel(0) species, as demonstrated by independent synthesis and spectroscopic investigations. A combination of catalytic studies, stoichiometric experiments, and in situ 31 P{ 1 H} NMR monitoring provides detailed mechanistic insights into the elementary steps of the transformation. Oxidative addition of the thioester and subsequent decarbonylation occur readily at room temperature, whereas reductive elimination requires elevated temperatures. The formation and accumulation of well-defined nickel(0) carbonyl complexes are observed under catalytic conditions and are proposed to be in equilibrium with catalytically active nickel(0) species, highlighting the key role of carbon monoxide in modulating the structure and reactivity of the nickel species. Additional ligand redistribution processes, leading to bis-thiolato and PPh 3 -containing species, are also evidenced and may contribute to catalyst deactivation. Altogether, these results support a catalytic cycle involving Ni 0 /Ni II intermediates and provide a comprehensive mechanistic framework for nickel-catalyzed decarbonylative transformations of thioesters.