N‐Heterocyclic Carbenes

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Abstract
The chemistry of N‐heterocyclic carbenes has long been limited to metal coordination compounds derived from azolium precursors, a development that was started by Öfele and Wanzlick in 1968. Since free carbenes are now available through the work of Arduengo (1991), a renaissance in this little‐recognized area of chemistry has occurred. A leading motive is the advantages of N‐heterocyclic carbenes as ligands in organometallic catalysts, where they extend the scope of application reached by phosphanes (functionalized, chiral, water‐soluble, and immobilized derivatives). The present review summarizes the state of the art with regard to synthesis, structure, bonding theory, metal coordination chemistry, and catalysis. Chelating, functionalized, chiral, and immobilized ligands can be generated and attached to metal centers in straightforward procedures under mild conditions. A wealth of new chemistry is thus opened. It is also shown how carbenes derived from imidazoles and triazoles behave as ligands in catalysis. It is reasonable to assume that N‐heterocyclic carbenes surpass the ubiquitous phosphanes as ligands in a number of organometallic catalytic reactions.