Palladacyclic N-heterocyclic carbene precatalysts for transition metal catalysis

Palladacycles remain a challenging and significant research field in organic chemistry and have emerged as a type of powerful and versatile precatalysts or key active intermediates for transition metal catalysis.These achievements in this area are correlated to the design and development of useful ancillary ligands,such as N-heterocyclic carbenes(NHCs),which not only stabilize the actual catalytic active species facilitating the transformations,but also provide additional control over the selectivity of reactions.In this context,NHCs-ligated palladacycles(NHCP_(dcycles))with different electronic and steric properties have been synthesized and applied as green precatalysts(high stability and activity,low catalyst loading and mild reaction conditions)to accelerate transition metalcatalyzed reactions.Therefore,this review focuses mainly on the strategy of NHC-Pd_(cycles) design and catalytic results obtained from representative transition metal catalysis,such as Suzuki-Miyaura,Heck-Mizoroki and Sonogashira cross-coupling reactions,Buchwald-Hartwig amination,carbonylation as well as arylation.At last,the current limitations and potential trends for further development of NHC-Pdcycles are also highlighted.