Chelated η2-alkene- and η3-allyl-carbene complexes of late transition metals: structure–reactivity relations and preparative use: Part 10. The chemistry of metallacyclic alkenylcarbene complexes

Chelated alkene–carbene complexes of Group VI metals typically undergo intramolecular [2+2]-cycloaddition reactions, whereas the iron analogues and related (η3-allyl)–ironcarbene complexes show a more diverse behaviour. They are amenable to reactions at the carbene carbon atom, at the terminus of the π-ligand and even at the tether depending on intrinsic stereoelectronic parameters as well as external factors like solvent polarity and ‘softness’ of attacking nucleophiles. Cascades with extensive reorganization of the organic ligand leading to oligo-enynes, δ-lactones and cyclopentenones may ensue. They are discussed together with some new findings allowing a generalized mechanistic description. The chemical and configurational stabilities of planar-chiral alkene–carbene and allyl–carbene complexes of iron, chromium, manganese and cobalt are compared.