Olefin Metathesis and Ring-Closing Strategy
Metathesis exchanges the ends of two alkenes through a metallacyclobutane, and the equilibrium is usually driven by loss of a small volatile alkene. That single reaction turns a long diene into a ring, which changes how a synthesis is planned.
What a learner can do afterwards
- Draws the metallacyclobutane and shows how it accounts for the exchanged fragments
- Predicts the ring formed from a stated diene and names what drives the reaction forwards
- Explains why high dilution favours ring closing over chain extension
- Chooses between a metathesis disconnection and a carbonyl disconnection for a macrocyclic target
1 · Read
Metathesis swaps the ends of two alkenes. Each alkene splits and the halves trade partners, and the trade passes through a four-membered ring that holds the metal: the metallacyclobutane. If you can draw that ring, you can account for every fragment in the products. You need no handle on either bond; the metal does the swapping.
Picture a long chain with an alkene at each end. The metal joins the two ends, the chain becomes a ring, and the leftover ends pair up into a small alkene that escapes as a gas. That escape matters: with product leaving, the back reaction starves and the mixture moves forward. To find the ring size, count the chain atoms and subtract the atoms carried away by the small fragment.
Now put one diene in a big flask of solvent. Its two ends meet each other far more often than two separate molecules meet, so the molecule closes into a ring. Crowd the flask and the picture flips: molecules bump into neighbours, chains grow, and rings lose. When you want rings, dilute; when you want chains, concentrate.
When you plan a large ring, look at its ends. If you can cut the ring at a bond that leaves an alkene on each side, metathesis can stitch it shut, while a carbonyl route with its matching polar handles may fit worse. Choose the cut whose ends your reaction can actually join.
You trade alkene ends through a four-membered ring, let the escaping fragment pull you forward, and dilute when you want a ring.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.