olefin metathesis
/ OH-leh-fin meh-TATH-eh-sis /
Imagine two dancing couples who, on a signal, swap partners: each person leaves their old partner and joins someone from the other pair. Olefin metathesis does exactly this with carbon-carbon double bonds. Take two alkenes, and a metal catalyst cuts each C=C double bond in the middle and reattaches the halves crosswise, so the two ends literally trade partners to give two new alkenes.
The reaction is run with a transition-metal carbene catalyst (a metal with a metal=carbon double bond), most famously the well-defined ruthenium catalysts developed by Grubbs. The mechanism works by a clever, reversible cycle: the metal carbene and an alkene form a four-membered ring (a metallacyclobutane), which then breaks apart the other way, swapping the carbon pieces and handing back a metal carbene with a different group, ready to do it again. The net effect is that the substituents on two double bonds are interchanged. Because each step is reversible, metathesis often relies on a driving force to push it forward, commonly the release of a small, volatile alkene such as ethylene (CH2=CH2) that escapes the flask. Several flavors exist: ring-closing metathesis stitches the two ends of one molecule into a ring (releasing ethylene), ring-opening metathesis polymerization opens strained rings into long polymers, and cross metathesis swaps partners between two different alkenes.
Olefin metathesis gave chemists a powerful new way to make and break C=C double bonds at will, especially to build rings (including large ones that are otherwise very hard to close) and specialty polymers, and it won the 2005 Nobel Prize in Chemistry. Its tidy, atom-economical partner-swapping makes it a flagship example of green chemistry, since well-designed metathesis wastes little and uses only a catalytic amount of metal.
Ring-closing metathesis of a molecule with two terminal alkenes at its ends uses a Grubbs catalyst to join those ends into a ring, expelling ethylene gas (CH2=CH2), whose escape drives the reaction forward.
Two alkene ends trade partners to close a ring; escaping ethylene pulls it forward.
Metathesis interchanges the partners across two C=C double bonds; it does not change the total number of double bonds, and because each step is reversible it usually needs a driving force (often loss of volatile ethylene) to give a single clean product.