[Pd(NHC)(mu-Cl)Cl](2) : versatile and highly reactive complexes for cross-coupling reactions that avoid formation of inactive Pd(I) off-cycle products

The development of more reactive, general, easily accessible, and readily avail-able Pd(II)-NHC precatalysts remains a key challenge in homogeneous catalysis. In this study, we establish air-stable NHC-Pd(II) chloro-dimers, [Pd(NHC)(mu-Cl) Cl](2), as the most reactive Pd(II)-NHC catalysts developed to date. Most crucially, compared with [Pd(NHC)(allyl)Cl] complexes, replacement of the allyl throw-away ligand with chloride allows for a more facile activation step, while effec-tively preventing the formation of off-cycle [Pd-2(mu-allyl)(mu-Cl)(NHC)(2)] products. The utility is demonstrated via broad compatibility with amide cross-coupling, Suzuki cross-coupling, and the direct, late-stage functionalization of pharmaceu-ticals. Computational studies provide key insight into the NHC-Pd(II) chloro-dimer activation pathway. A facile synthesis of NHC-Pd(II) chloro-dimers in one-pot from NHC salts is reported. Considering the tremendous utility of Pd-catalyzed cross-coupling reactions and the overwhelming success of [Pd(NHC)(allyl)Cl] precatalysts, we believe that NHC-Pd(II) chloro-dimers, [Pd(NHC)(mu-Cl)Cl](2), should be considered as go-to precatalysts of choice in cross-coupling processes