Inorganic — organometallic (Grubbs-type)
Olefin metathesis via metallacyclobutane
How the Olefin metathesis via metallacyclobutane 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: Metal alkylidene (Grubbs Ru=CHR) + alkene. Conditions: CH₂Cl₂ or toluene, 25–40 °C. Bonds formed: new C=C; new M=C; bonds broken: old C=C; old M=C. Stereochemistry: Mostly E (thermodynamic) for internal alkenes. Rate and selectivity: Driven by release of volatile ethylene (RCM, ROMP strain).
At a glance
Step 1: Alkene approaches the M=C bond
A metal carbene catalyst (like a Grubbs Ru complex) coordinates an alkene; the metal-carbon double bond and the alkene's π bond align for a [2+2] cycloaddition.
Step 2: Metallacyclobutane
This forms a strained four-membered metallacyclobutane ring — the two former double bonds are now single bonds within the ring.
Step 3: Ring opens the other way
The ring can reopen the other way, breaking different bonds than it formed — swapping alkylidene partners and releasing a new alkene plus a new metal carbene to continue the cycle.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- 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.
- ROMP
- Ring-opening metathesis polymerisation. Polymerisation of strained cycloalkenes with a metal carbene.
- RCM
- Ring-closing metathesis. Intramolecular olefin metathesis forming a ring.
How it compares with related reactions
| Olefin metathesis via metallacyclobutane | Cross metathesis between two different alkenes | Migratory insertion | |
|---|---|---|---|
| Reagents | Metal alkylidene (Grubbs Ru=CHR) + alkene | Ru carbene (Grubbs II), two different alkenes | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO |
| Conditions | CH₂Cl₂ or toluene, 25–40 °C | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CO atmosphere, 25–100 °C |
| Stereochemistry | Mostly E (thermodynamic) for internal alkenes | Usually E-selective (thermodynamic) | Alkyl migrates; stereochemistry at carbon retained |
| Rate / selectivity | Driven by release of volatile ethylene (RCM, ROMP strain) | 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 Olefin metathesis via metallacyclobutane?
Metal alkylidene (Grubbs Ru=CHR) + alkene. Conditions: CH₂Cl₂ or toluene, 25–40 °C.
Which bonds are formed and which are broken?
Formed: new C=C; new M=C. Broken: old C=C; old M=C.
What is the stereochemical outcome?
Mostly E (thermodynamic) for internal alkenes
What controls the rate or the selectivity?
Driven by release of volatile ethylene (RCM, ROMP strain)
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam