Pick your Pd partners: A number of catalytic systems have been developed for palladium‐catalyzed CH activation/CC bond formation. Recent studies concerning the palladium(II)‐catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle are discussed (see scheme), and the versatility and practicality of this new mode of catalysis are presented. Unaddressed questions and the potential for development in the field are also addressed. 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: PdII/Pd0, PdII/PdIV, Pd0/PdII/PdIV, and Pd0/PdII 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 PdII/Pd0 catalytic cycle. Despite the progress made to date, improving the versatility and practicality of this new reaction remains a tremendous challenge.

The synthesis of a [2]catenane with 87‐membered rings was improved and extended to [2]catenanes with 63‐ and 147‐membered rings. One of the key features is the carbonate linkage between phenols with tolane substituents in the 2‐ and 6‐positions, which serves as a covalent template for the geometrical arrangement of a macrocycle and a ring precursor. Subsequent cyclization of the threaded ring precursor gives the precatenane as the main product, and this is converted into the catenane by carbonate hydrolysis. As well as the precatenane, its dumbbell shaped isomer is formed in the cyclization step. From the known conformation of the templating carbonate moiety and the strong dependence of the ratio of precatenane and dumbbell on the ring size, the dumbbell's origin is attributed to the conformational flexibility of the large rings and not to geometrical ambiguity of the carbonate moiety. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

A facile access to monodisperse ultralarge rings counting 126, 174, and 294 ring atoms is described. It follows a reaction sequence that is well suited for the preparation of [2]catenanes but altered just in the sequence of the two steps cyclization and carbonate formation. The carbonate acts as a covalent template that is easily formed and later cleaved. The obtained monocyclic products are constitutional isomers of the catenanes.