Recent homogeneous catalytic applications of chelate and pincer N-heterocyclic carbenes

During the last decade, the use of N-heterocyclic carbene ligands (NHCs) based on imidazolium ions and related heterocycles has emerged as an alternative to phosphines in the design of new organometallic catalysts. We review catalysts with chelate and pincer NHC ligands, including complexes of palladium, ruthenium, rhodium and iridium. Transfer hydrogenation and Heck chemistry are given special attention. Also discussed are Suzuki and Sonogashira coupling and immobilization on clay supports. Synthetic aspects are covered as well as a discussion of structural features, catalytic properties and catalyst recovery and recycling. (C) 2004 Elsevier B.V. All rights reserved.