[(IPent) PdCl2(morpholine)]: A Readily Activated Precatalyst for Room-Temperature, Additive-Free Carbon-Sulfur Coupling

A series of new, easily activated NHC-PdII precatalysts featuring a trans-oriented morpholine ligand were prepared and evaluated for activity in carbon-sulfur cross-coupling chemistry. [(IPent) PdCl2(morpholine)] (IPent= 1,3bis(2,6-di(3-pentyl) phenyl) imidazol-2-ylidene) was identified as the most active precatalyst and was shown to effectively couple a wide variety of deactivated aryl halides with both aryl and alkyl thiols at or near ambient temperature, without the need for additives, external activators, or pre-activation steps. Mechanistic studies revealed that, in contrast to other common NHC-PdII precatalysts, these complexes are rapidly reduced to the active NHC-Pd0 species at ambient temperature in the presence of KOtBu, thus avoiding the formation of deleterious off-cycle PdII-thiolate resting states.