Heterocyclic carbenes (NHCs), especially monodentate ones, have become the ligand of choice for many transition-metal-catalyzed transformations. They generally form highly stable complexes, have strong sigma-donor character, and have a unique shape that can be used to generate sterically demanding ligands.In this Account, we survey recent developments in the design and synthesis of some sterically demanding NHCs with a particularly strong influence on the metal's coordination sphere. We show the successful and insightful application of these ligands in transition-metal catalysis. First, we discuss methods for determining and classifying the electronic and steric properties of NHCs. In addition, we present data on the most important NHC ligands.The selective variation of either electronic or steric parameters of NHCs, and therefore of the catalyst, allows for the optimization of the reaction. Thus, we prepared several series of differentially substituted NHC derivatives. However, because the substituents varied were not directly connected to the carbene carbon, it was difficult to induce a large electronic variation. In contrast, an independent variation of the ligands' steric properties was more straightforward. We highlight three different classes of very sterically demanding NHCs that allow this kind of a steric variation: imidazo[1,5-a]pyridine-3-ylidenes, bioxazoline-derived carbenes (IBiox), and cyclic (alkyl)(amino)carbenes (CAAC).These latter NHC ligands can facilitate a number of challenging cross-coupling reactions. Successful transformations often require a monoligated palladium complex as the catalytically active species, and the sterically demanding NHC ligand favors this monoligated complex. In addition, the electron-rich NHC facilitates difficult oxidative addition steps. Moreover, the conformational flexibility of the ligands can facilitate the formation of catalytically active species and hemilabile interactions, such as agostic or anagostic bonds, as well as stabilize coordinatively unsaturated catalyst species. The increasing level of understanding of the role of NHC ligands in transition-metal catalysis will soon allow the design of even more sophisticated ligand systems.

The special advantage of the title reaction to form substituted indoles 2 lies within the broad scope of the transformation: A multitude of N‐aryl enaminones 1 can be prepared readily in one step from commercially available anilines. Furthermore, anilines can be converted directly in a one‐pot process into the indole products. R1=H, Me, OMe, Cl, F, carbonyl functionality, CN, fused aryl; R2=alkyl, aryl; R3=alkyl, Oalkyl.

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An exceedingly sterically demanding, rigid, and chiral NHC ligand, IBiox[(-)-menthyl] (1), was prepared and structurally characterized. With a buried volume of approximately 50%, this ligand arguably represents one of the most sterically demanding monodentate ligands. The ability to use aryl chloride substrates in intramolecular palladium-catalyzed alpha-arylations reveals its unique reactivity. Moreover, C(2)-symmetric 1 allows the highly enantioselective formation of oxindoles with up to 99% ee.

The direct Pd-catalyzed oxidative coupling of two C-H-bonds within N-aryl-enamines 1 allows the efficient formation of differently substituted indoles 2. In this cross-dehydrogenative coupling, many different functional groups are tolerated and the starting material N-aryl-enamines 1 can be easily prepared in one step from commercially available anilines. In addition, the whole sequence can also be run in a one-pot fashion. Optimization data, mechanistic insight, substrate scope, and applications are reported in this full paper.