10 Carbenes: reactivity and catalysis

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Abstract
Since the first report of a stable carbene by Arduengo in 1991, the use of N-heterocyclic carbenes (NHC) in organic synthesis has increased dramatically. This is principally due to their unique properties. NHC are known to form strong bonds to late transition metals since they are excellent σ-donors and possess (in general) minimal π-back donation capabilities. In view of this, NHCs have rapidly been used as a new class of ligand, capable of mimicking the well-known phosphines without the classical problems associated with phosphine use. The recent use of NHC is not restricted to catalysis using late transition metals, but they are also used as organic catalysts or as reactants in multi-component reactions, as is described in later sections of this report. Studies have shown that nucleophilic N-heterocyclic carbenes represent a very versatile class of ligands due to their interesting steric and electronic properties. These ligands have several advantages over the commonly utilized phosphines: (i) stabilizing effect, (ii) high thermal stability, (iii) resistance to dissociation from the metal center. Therefore an excess of the ligand is not required in order to prevent aggregation of the catalyst to yield bulk metal. As a consequence, an increasing number of catalytic reactions make use of nucleophilic carbenes as catalyst modifiers. Specific examples include the use of metal–carbene complexes in hydrosilylation, Ru-catalysed furan synthesis and olefin metathesis. In this report, attention will be focused on the latest highlights in the field of use of carbenes. Undoubtedly, NHC ligands are currently important in a wide range of reactions and the reader will find here a summary of current applications.

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