olefin metathesis as a catalytic process
/ OH-luh-fin meh-TATH-uh-sis /
Imagine two dancing couples on a floor, each pair holding hands. A caller shouts switch! and the partners swap: the left member of couple one now dances with the right member of couple two, and vice versa. Olefin metathesis does exactly this with carbon-carbon double bonds. Take two alkenes, each with a double bond; the catalyst breaks both double bonds in the middle and re-stitches the halves into two new alkenes with the partners exchanged. It is, quite literally, partner-swapping for double bonds, and it won Yves Chauvin, Robert Grubbs, and Richard Schrock the 2005 Nobel Prize in Chemistry.
The catalyst is a metal carbene — a complex with a metal-to-carbon double bond, M=CR2 — usually based on ruthenium (the robust, air-tolerant Grubbs catalysts) or molybdenum/tungsten (the highly active Schrock catalysts). The mechanism, proposed by Chauvin, runs through a four-membered ring. In plain steps: the metal carbene and one alkene join to form a metallacyclobutane (a square ring of metal, carbon, carbon, carbon); this ring then breaks the other way, releasing a new alkene and leaving a new metal carbene; that new carbene reacts with the next alkene the same way. The double bonds are repeatedly cut and reformed, shuffling the carbon fragments until the desired product accumulates. Useful variants include ring-closing metathesis (snipping a long molecule into a ring), cross metathesis (joining two different alkenes), and ring-opening metathesis polymerization (popping open strained rings to make polymer chains).
Metathesis matters because it gives chemists a clean, catalytic way to make carbon-carbon double bonds in places that were previously very hard to reach, often in a single step and with high atom economy — the only by-product is frequently a small, easily removed alkene like ethene. It transformed how complex molecules are built, from pharmaceuticals to advanced polymers and even renewable feedstocks from plant oils. As a catalytic process it is the green-chemistry poster child: mild conditions, few steps, little waste. The honest caveat is that the equilibrium can sit anywhere, so a clever metathesis is usually one where the product is removed (a volatile alkene boils off, or a ring forms entropically) to drive the swap the way you want — the catalyst speeds the shuffle but does not by itself decide the destination.
In ring-closing metathesis, a long open chain with double bonds at both ends is zipped into a ring by a Grubbs ruthenium catalyst, expelling a molecule of ethene gas that simply bubbles away and drags the equilibrium forward.
Swap the partners on double bonds — and let a volatile by-product escape to drive the reaction where you want.
Metathesis is an equilibrium, not a one-way street: the catalyst shuffles the double bonds both ways, so a good preparation works by removing one product (often gaseous ethene) to pull the swap in the desired direction.