Metathesis is a chemical transformation in which two molecules undergo an exchange of atoms or groupings of atoms to form new molecules. Metathesis processes mediated by metal alkylidenes or alkylidynes that redistribute π-systems of unsaturated molecules are classified as diene, enyne, or diyne metathesis depending on the types of π-systems involved (Scheme 1). Although these metathesis processes involve different combinations of unsaturated functionalities, they all share identical elementary steps: [2 + 2] cycloaddition, to form a metallacyclobutane (diene metathesis), metallacyclobutene (enyne metathesis), or metallacyclobutadiene (diyne metathesis) intermediate, followed by cycloreversion. Both diene and enyne metatheses require metal alkylidenes as the mediators for π-system exchange, whereas alkyne metathesis is only promoted by metal alkylidynes.
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