Inorganic — organometallic
β-hydride elimination
How the β-hydride elimination 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 alkyl with a β-H and a vacant cis site. Conditions: Often spontaneous at 25 °C; heat speeds it. Bonds formed: M–H; C=C π; bonds broken: β C–H; M–C. Stereochemistry: Syn elimination: M and β-H must be coplanar. Rate and selectivity: Blocked by 18 e⁻ saturation or no β-H (e.g. M–CH₃, M–CH₂Ph).
At a glance
Step 1: β-H approaches the metal
A metal alkyl complex with an accessible β-hydrogen adopts a conformation where that hydrogen sits close to the empty orbital on the metal.
Step 2: Four-centre transition state
The β C–H bond's electrons shift onto the metal, forming a new M–H bond while a π bond forms between the alpha and beta carbons — the reverse of migratory insertion of an alkene.
Step 3: Metal hydride with a bound alkene
The product is a metal hydride with a coordinated alkene — this step commonly limits the lifetime of metal alkyls and terminates chain-growth processes like Ziegler-Natta polymerization.
Abbreviations and terms
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- M
- Metal. A generic metal centre.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| β-hydride elimination | Cross metathesis between two different alkenes | Migratory insertion | |
|---|---|---|---|
| Reagents | Metal alkyl with a β-H and a vacant cis site | Ru carbene (Grubbs II), two different alkenes | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO |
| Conditions | Often spontaneous at 25 °C; heat speeds it | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CO atmosphere, 25–100 °C |
| Stereochemistry | Syn elimination: M and β-H must be coplanar | Usually E-selective (thermodynamic) | Alkyl migrates; stereochemistry at carbon retained |
| Rate / selectivity | Blocked by 18 e⁻ saturation or no β-H (e.g. M–CH₃, M–CH₂Ph) | 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 β-hydride elimination?
Metal alkyl with a β-H and a vacant cis site. Conditions: Often spontaneous at 25 °C; heat speeds it.
Which bonds are formed and which are broken?
Formed: M–H; C=C π. Broken: β C–H; M–C.
What is the stereochemical outcome?
Syn elimination: M and β-H must be coplanar
What controls the rate or the selectivity?
Blocked by 18 e⁻ saturation or no β-H (e.g. M–CH₃, M–CH₂Ph)
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam