Metal Carbenes in Cycloaddition Reactions

Two decades after E.O. Fischer’s first report of an isolated transition metal carbene complex (Fischer and Maasbol, 1964) this class of compounds represents a well-established research area in organometallic chemistry. We are now able to synthesize carbene complexes of nearly all transition metals. We can control the oxidation state, the ligand sphere and the stereochemistry of the metal as well as the reactivity of the metal to carbene bond (Dötz et al., 1983a). The original Fischer-type complexes contain a low-valent metal center connected to an electrophilic carbene carbon atom which results in a carbonyl-like reactivity. These properties are exactly reversed in compounds which have been discovered by R.R. Schrock ten years later which exhibit an ylid-like chemical behavior (Schrock, 1979).