Abnormal NHC Palladium Complexes: Synthesis, Structure, and Reactivity

Developments in palladium chemistry have been spurred predominantly by the outstanding application potential of this metal in catalysis. The quest for new ligands in order to modulate the catalytic activity and selectivity of the palladium center has been greatly stimulated by the discovery of N-heterocyclic carbenes as formally neutral, strongly donating, and covalently binding ligands. Abnormal variations of N-heterocyclic carbenes, even though known (yet not recognized) for 30 years, have received very little attention until recently. In parts this may have been due to the fact that the free abnormal carbene ligand is much less stable than the normal carbene analogues. In the last decade, significant progress has been made in abnormal carbene palladium chemistry and reliable synthetic routes as well as promising catalytic applications have been developed. As a consequence, these types of complexes have gradually transformed from laboratory curiosities to unique formally neutral ligands with exceptional donor ability. Here, the advances in abnormal carbene palladium chemistry are summarized. In an attempt to stimulate the entry of newcomers in this fascinating field of research, elementary aspects of synthesis are discussed as well as progress in characterization of the complexes. Most recent (catalytic) applications may highlight the potential of this rapidly growing area of palladium chemistry.