Inorganic — organometallic
Cross metathesis between two different alkenes
How the Cross metathesis between two different alkenes 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: Ru carbene (Grubbs II), two different alkenes. Conditions: CH₂Cl₂, 25–40 °C; excess partner or ethylene removal. Bonds formed: new C=C (cross product); bonds broken: C=C of both partners. Stereochemistry: Usually E-selective (thermodynamic). Rate and selectivity: Initiation/phosphine-or-ether release often limits; [2+2] steps fast.
At a glance
Step 1: [2+2] with alkene A, then retro-[2+2]
A metal carbene catalyst reacts with one alkene partner, forming a metallacyclobutane and then releasing half of it as a new metal carbene — the identical [2+2]/retro-[2+2] logic covered in the general olefin metathesis entry.
Step 2: Second cycle with alkene B
This new metal carbene, now carrying a fragment of the first alkene, reacts with a different alkene partner in a second metathesis cycle.
Step 3: Equilibrium and selectivity
The product is a new, unsymmetrical alkene combining fragments from both starting partners — because metathesis is an equilibrium process, a large excess of one partner (or removal of a volatile byproduct like ethylene) is typically used to push the statistical mixture of possible products toward the desired cross product.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- 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.
How it compares with related reactions
| Cross metathesis between two different alkenes | Migratory insertion | Olefin metathesis via metallacyclobutane | |
|---|---|---|---|
| Reagents | Ru carbene (Grubbs II), two different alkenes | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO | Metal alkylidene (Grubbs Ru=CHR) + alkene |
| Conditions | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CO atmosphere, 25–100 °C | CH₂Cl₂ or toluene, 25–40 °C |
| Stereochemistry | Usually E-selective (thermodynamic) | Alkyl migrates; stereochemistry at carbon retained | Mostly E (thermodynamic) for internal alkenes |
| Rate / selectivity | Initiation/phosphine-or-ether release often limits; [2+2] steps fast | Alkyl must be cis to CO; ligand then fills the vacant site | Driven by release of volatile ethylene (RCM, ROMP strain) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Cross metathesis between two different alkenes?
Ru carbene (Grubbs II), two different alkenes. Conditions: CH₂Cl₂, 25–40 °C; excess partner or ethylene removal.
Which bonds are formed and which are broken?
Formed: new C=C (cross product). Broken: C=C of both partners.
What is the stereochemical outcome?
Usually E-selective (thermodynamic)
What controls the rate or the selectivity?
Initiation/phosphine-or-ether release often limits; [2+2] steps fast
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam