Inorganic — organometallic ring synthesis
Ring-closing metathesis (RCM)
How the Ring-closing metathesis (RCM) works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Terminal diene (1,6-heptadiene), Grubbs Ru=CH₂ carbene. Conditions: CH₂Cl₂ or toluene, 25–45 °C, dilute (0.01 M). Bonds formed: ring C=C; ethylene; bonds broken: two terminal C=C. Stereochemistry: Rings under 8 members form Z only; larger rings E/Z mixtures. Rate and selectivity: Irreversible because volatile ethylene escapes; dilution favours ring over polymer.
At a glance
Step 1: [2+2] with one terminal alkene
A metal carbene catalyst reacts with one of a diene's two terminal alkenes, forming a metallacyclobutane through the same [2+2] cycloaddition used in any olefin metathesis — the first step from the general metathesis mechanism covered earlier.
Step 2: Ring opens the other way
The ring reopens the other way, releasing a small alkene (commonly ethylene) and leaving a new metal carbene now tethered to the same molecule, positioned intramolecularly right next to the second alkene.
Step 3: Ring closes
A second [2+2]/retro-[2+2] cycle closes the ring intramolecularly, releasing the catalyst as a free carbene again — because both alkenes started on the same chain, this reaction builds a ring instead of joining two separate molecules, and the loss of volatile ethylene drives the equilibrium forward irreversibly.
Abbreviations and terms
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- M
- Metal. A generic metal centre.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- Grubbs
- Grubbs catalyst. Ruthenium carbene catalyst for olefin metathesis.
- RCM
- Ring-closing metathesis. Intramolecular olefin metathesis forming a ring.
How it compares with related reactions
| Ring-closing metathesis (RCM) | Cross metathesis between two different alkenes | Migratory insertion | |
|---|---|---|---|
| Reagents | Terminal diene (1,6-heptadiene), Grubbs Ru=CH₂ carbene | Ru carbene (Grubbs II), two different alkenes | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO |
| Conditions | CH₂Cl₂ or toluene, 25–45 °C, dilute (0.01 M) | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CO atmosphere, 25–100 °C |
| Stereochemistry | Rings under 8 members form Z only; larger rings E/Z mixtures | Usually E-selective (thermodynamic) | Alkyl migrates; stereochemistry at carbon retained |
| Rate / selectivity | Irreversible because volatile ethylene escapes; dilution favours ring over polymer | Initiation/phosphine-or-ether release often limits; [2+2] steps fast | Alkyl must be cis to CO; ligand then fills the vacant site |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Ring-closing metathesis (RCM)?
Terminal diene (1,6-heptadiene), Grubbs Ru=CH₂ carbene. Conditions: CH₂Cl₂ or toluene, 25–45 °C, dilute (0.01 M).
Which bonds are formed and which are broken?
Formed: ring C=C; ethylene. Broken: two terminal C=C.
What is the stereochemical outcome?
Rings under 8 members form Z only; larger rings E/Z mixtures
What controls the rate or the selectivity?
Irreversible because volatile ethylene escapes; dilution favours ring over polymer
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam