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Also known as: Grubbs Metathesis, Ring-Closing Metathesis (RCM), Cross Metathesis (CM)
Discovered by Robert Grubbs, Richard Schrock (1992)
Alkene bonds are broken and reformed — substituents on the double bonds are "scrambled." The key intermediate is a metallacyclobutane. Ring-closing metathesis (RCM) is especially powerful for making medium and large rings. Nobel Prize 2005.
The Ru=CH₂ carbene (from Grubbs catalyst) undergoes [2+2] cycloaddition with the alkene to form a metallacyclobutane.
[2+2] cycloaddition — allowed because metal is involved
Retro-[2+2] cycloreversion: the metallacyclobutane breaks in the other direction, forming a new alkene and a new Ru carbene.
New C=C bond formed, ethylene released
The new Ru carbene continues the catalytic cycle with another alkene molecule. For RCM, intramolecular reaction closes the ring.
Ethylene gas escapes — drives equilibrium forward
Diethyl diallylmalonate
Grubbs II (5 mol%), CH₂Cl₂
Diethyl cyclopent-3-ene-1,1-dicarboxylate
1-Hexene (self CM)
Grubbs I, toluene
5-Decene + Ethylene
Revolutionary for ring formation, especially medium and large rings that are otherwise difficult to make. Transformed total synthesis and polymer chemistry.