Inorganic — organometallic
Reductive elimination: forming a C–C bond at a metal center
How the Reductive 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: cis-(CH₃)(Ph)Pd(II)(PPh₃)₂. Conditions: Mild heat, THF or toluene; ligands must be cis. Bonds formed: C(sp³)–C(sp²) σ bond; bonds broken: Pd–CH₃ and Pd–Ph σ bonds. Stereochemistry: Retention at carbon; requires cis arrangement. Rate and selectivity: Faster with bulky ligands, electron-poor metals; the product-forming step of Suzuki/Negishi.
At a glance
Step 1: Two cis organic ligands on Pd(II)
Two organic groups — say, methyl and phenyl — sit cis to each other on a Pd(II) center, positioned close enough to interact directly.
Step 2: Pd–C bonds become the new C–C bond
Electron density from the metal-carbon bonds flows to form a new C–C σ bond directly between the two organic fragments, while both M–C bonds break at once.
Step 3: Coupled product + Pd(0)
The metal's oxidation state and coordination number both drop by two (the reverse of oxidative addition), releasing the coupled organic product. This is the bond-forming step that finishes catalytic cycles like Suzuki and Negishi cross-coupling.
Abbreviations and terms
- M
- Metal. A generic metal centre.
- Ph
- Phenyl. –C₆H₅.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- PPh3
- Triphenylphosphine. Nucleophilic phosphine; forms very strong P=O bonds, driving Wittig, Appel and Mitsunobu reactions.
How it compares with related reactions
| Reductive elimination | Cross metathesis between two different alkenes | Migratory insertion | |
|---|---|---|---|
| Reagents | cis-(CH₃)(Ph)Pd(II)(PPh₃)₂ | Ru carbene (Grubbs II), two different alkenes | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO |
| Conditions | Mild heat, THF or toluene; ligands must be cis | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CO atmosphere, 25–100 °C |
| Stereochemistry | Retention at carbon; requires cis arrangement | Usually E-selective (thermodynamic) | Alkyl migrates; stereochemistry at carbon retained |
| Rate / selectivity | Faster with bulky ligands, electron-poor metals; the product-forming step of Suzuki/Negishi | 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 Reductive elimination?
cis-(CH₃)(Ph)Pd(II)(PPh₃)₂. Conditions: Mild heat, THF or toluene; ligands must be cis.
Which bonds are formed and which are broken?
Formed: C(sp³)–C(sp²) σ bond. Broken: Pd–CH₃ and Pd–Ph σ bonds.
What is the stereochemical outcome?
Retention at carbon; requires cis arrangement
What controls the rate or the selectivity?
Faster with bulky ligands, electron-poor metals; the product-forming step of Suzuki/Negishi
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam