Monoligated Palladium Species as Catalysts in Cross-Coupling Reactions

Palladium-mediated cross-coupling reactions are attractive organometallic transformations for the generation of C--C, C--N, C--O, and C--S bonds. Despite being widely employed in small-scale syntheses, cross-coupling reactions have not found important industrial applications because until recently, only reactive aryl bromides and iodides could be used as substrates. These substrates are generally more expensive and less widely available than their chloride counterparts. Over the past few years, new catalytic systems with the ability to activate unreactive and sterically hindered aryl chlorides have been developed. The new catalysts are based on palladium complexes that contain electron-rich and bulky phosphine or carbene ligands. The enhanced reactivity observed with these new systems has been attributed to the formation of unsaturated and reactive [PdL] species which can readily undergo oxidative addition reactions with ArX to yield [Pd(Ar)X(L)].