Cyclization by Catalytic Ruthenium Carbene Insertion into C sp 3H Bonds

In the past decade, palladium-catalyzed C-H activation/C-C bond-forming reactions have emerged as promising new catalytic transformations; however, development in this field is still at an early stage compared to the state of the art in cross-coupling reactions using aryl and alkyl halides. This Review begins with a brief introduction of four extensively investigated modes of catalysis for forming C-C bonds from C-H bonds: Pd(II)/Pd(0), Pd(II)/Pd(IV), Pd(0)/Pd(II)/Pd(IV), and Pd(0)/Pd(II) catalysis. A more detailed discussion is then directed towards the recent development of palladium(II)-catalyzed coupling of C-H bonds with organometallic reagents through a Pd(II)/Pd(0) catalytic cycle. Despite the progress made to date, improving the versatility and practicality of this new reaction remains a tremendous challenge.