Olefin metathesis is a reaction in which two molecules containing carbon–carbon double bonds exchange parts and form new double bonds. Yves Chauvin solved a long-standing puzzle by proposing that metal carbenes form a four-membered ring intermediate that rearranges the bonds. Richard Schrock created highly active catalysts based on molybdenum or tungsten; they react explosively fast but are sensitive to air and water. Robert Grubbs designed ruthenium catalysts that are much more stable, allowing the reaction to be used widely in laboratories and factories. By shortening synthetic routes and reducing by-products, metathesis has become a flagship example of environmentally friendly “green chemistry.”

The metathesis reaction proceeds via a sequence in which a transition-metal alkylidene bearing a carbene ligand undergoes [2+2] cycloaddition with an olefin to form a metallacyclobutane, followed by cycloreversion that yields a new alkylidene and olefin. The Chauvin mechanism frames this reversible [2+2] cycle and explains why the reaction is enthalpically and entropically efficient, leading to high catalytic turnover. Schrock catalysts enhance π-back-donation by placing strongly donating imido or alkoxy ligands on high-valent Mo or W centers, dramatically increasing activity. Grubbs catalysts balance air- and moisture-stability with broad substrate scope by combining phosphine or N-heterocyclic carbene ligands on Ru, enabling applications such as ring-opening metathesis polymerization (ROMP) and cross metathesis. These advances revolutionized complex natural-product synthesis, functional polymer construction, and the manufacture of agro- and pharmaceutical intermediates, with quantitative green-chemistry metrics (E-factor, PMI) confirming their sustainability.

The Chauvin metallacyclobutane pathway invokes σ–π interconversion with Fukui-type frontier-orbital interactions, accounting for its exceptionally low activation enthalpy. Schrock’s high-valent Mo(VI)/W(VI) alkylidenes employ strongly σ-donating imido and tub-shaped tert-butoxy ligands that tune electron density and suppress β-hydride elimination in the transition state. Conversely, Grubbs’ Ru(II) NHC–phosphine catalysts moderately weaken the Ru–C alkylidene bond, leveraging PPh3 dissociation–association equilibria to generate active species even in the presence of proton donors. Second- and third-generation Ru complexes now exploit ligand-regulated re-growth and precise E/Z control, with quantitative analyses of π-steric electronic parameters and remote-ligand effects guiding design. Metathesis has expanded into asymmetric induction, micro-flow synthesis, and mechanochemical polymerization, and its interplay with palladium cross-coupling is a central parameter in modern synthetic strategy.